Java编程思想 第21章 并发

这是在2013年的笔记整理。现在重新拿出来,放在网上,重新总结下。

两种基本的线程实现方式 以及中断

 

package thread;

 

 

/**

*

* @author zjf

* @create_time 2013-12-18

* @use测试基本的两种线程的实现方式

*         测试中断

*/

public class BasicThreadTest {

 

    public static void main(String[] args) {

        Counter c1 = new Counter();

        c1.start();

        Thread c2 = new Thread(new Countable());

        c2.start();

        try {

            Thread.sleep(1000);

        } catch (InterruptedException e) {

            e.printStackTrace();

        }

        //中断

        c1.interrupt();

        c2.interrupt();

          

        

        try {

            Thread.sleep(1000);

        } catch (InterruptedException e) {

            e.printStackTrace();

        }

        //此时c1线程已经终止不能再次start 多次启动一个线程是非法的。java.lang.IllegalThreadStateException

        c1.start();

    }

      

    

    /**

     *

     * @author zjf

     * @create_time 2013-12-18

     * @use Runnable接口方式的实现

     */

    static class Countable implements Runnable{

        public void run() {

            int i = 0;

            while(!Thread.currentThread().isInterrupted())

            {

                System.out.println(this.toString() + "-------------" + i);

                i ++;

            }

        }

    }

    /**

     *

     * @author zjf

     * @create_time 2013-12-18

     * @use Thread继承方式的实现

     */

    static class Counter extends Thread{

        public void run() {

            int i = 0;

            while(!Thread.currentThread().isInterrupted())

            {

                System.out.println(this.toString() + "-------------" + i);

                i ++;

            }

        }

    }

}

中断
  • public void interrupt()

中断线程。

如果线程在调用 Object 类的 wait()wait(long)wait(long, int) 方法,或者该类的 join()join(long)join(long, int)sleep(long)sleep(long, int) 方法过程中受阻,则其中断状态将被清除,它还将收到一个 InterruptedException

  • public static boolean interrupted()

测试当前线程是否已经中断。线程的中断状态 由该方法清除。

  • public boolean isInterrupted()

测试线程是否已经中断。线程的中断状态 不受该方法的影响。

测试睡眠被中断

sleep是静态方法。

 

package thread;

 

 

/**

*

* @author zjf

* @create_time 2013-12-18

* @use测试Sleep方法被中断

*/

public class SleepTest {

 

    /**

     * @author zjf

     * @create_time 2013-12-18

     * @use测试目的:睡眠时是否可以被中断

     * @param args

     */

    public static void main(String[] args) {

        Thread t = new Thread(new Sleepable());

        t.start();

        try {

            Thread.sleep(1000);

        } catch (InterruptedException e) {

            e.printStackTrace();

        }

        t.interrupt();

    }

 

    

    static class Sleepable implements Runnable{

        public void run() {

            try {

                //睡眠10但是线程开始1秒后被中断当前线程在睡眠时能够接收到中断然后退出

                Thread.sleep(10000);

            } catch (InterruptedException e) {

                //如果被中断就退出

                System.out.println("exit");//一秒后退出

            }

        }

    }

}

 

测试使用yield让步

yield是静态方法。

 

package thread;

 

public class YieldTest {

 

    /**

     * @author zjf

     * @create_time 2013-12-18

     * @use测试yield方法

     * @param args

     */

    public static void main(String[] args) {

        new Thread() {

            @Override

            public void run() {

                for(int i = 1; i < 100; i++)

                {

                    System.out.println("yield-----" + i);

                    //测试结果显示使用yield让步与不使用差别不大

                    Thread.yield();

                }

            }

        }.start();

        

        new Thread() {

            @Override

            public void run() {

                for(int i = 1; i < 100; i++)

                {

                    System.out.println("notyield-----" + i);

                }

            }

        }.start();

    }

 

}

 

测试cached线程池

 

newCachedThreadPool:创建一个可根据需要创建新线程的线程池,但是在以前构造的线程可用时将重用它们。对于执行很多短期异步任务的程序而言,这些线程池通常可提高程序性能。调用 execute 将重用以前构造的线程(如果线程可用)。如果现有线程没有可用的,则创建一个新线程并添加到池中。终止并从缓存中移除那些已有 60 秒钟未被使用的线程。因此,长时间保持空闲的线程池不会使用任何资源。

 

CachedThreadPool一般会创建所需数量的线程,并且会复用,这是选择的首选。

 

package thread;

 

import java.util.concurrent.ExecutorService;

import java.util.concurrent.Executors;

 

public class CachedThreadPoolTest {

 

    /**

     * @author zjf

     * @create_time 2013-12-18

     * @use测试Cached线程池

     * @param args

     */

    public static void main(String[] args) {

 

        /*

         * cached线程池不能设置拥有线程的数量

         */

        ExecutorService es = Executors.newCachedThreadPool();

        for (int i = 0; i < 10; i++) {

            es.execute(new Countable(i));

        }

        

        /*

         * 因为要复用线程所以线程执行完任务之后不会立刻关闭而是等待一分钟(可配置)

         * 的时间如果在这一分钟期间没有新的任务要执行会自动关闭

         * shutdown标明不会再有新的任务加入所以加入shutdown代码之后任务执行之后就会关闭线程

         * 不会等待一分钟

         */

        es.shutdown();

 

    }

 

    static class Countable implements Runnable {

 

        private int i;

 

        public Countable(int i) {

            this.i = i;

        }

 

        public void run() {

 

            System.out.println("" + i + "启动的线程的ID"

                    + Thread.currentThread().getId());

            

            /**

             *输出为

                 0启动的线程的ID7

                2启动的线程的ID9

                1启动的线程的ID8

                3启动的线程的ID10

                4启动的线程的ID11

                5启动的线程的ID12

                6启动的线程的ID13

                8启动的线程的ID8

                7启动的线程的ID9

                9启动的线程的ID10

                

                可见在地8 9 10个线程的时候复用了第1 2 3个线程。

                这建立在第1 2 3个线程已经运行完的基础上。

             */

 

        }

    }

 

}

shutdown和shutdownnow

shutdown:

  • 阻止加入新的任务。
  • 结束已经完成任务的空闲线程,直到所有任务执行完毕,关闭所有线程为止。

shutdownnow:

  1. 完成shutdown的功能。
  2. 向每个未完成的线程发布中断命令。
  3. 返回未执行的任务列表
shutdownnow

package thread;

 

import java.util.List;

import java.util.concurrent.ExecutorService;

import java.util.concurrent.Executors;

import java.util.concurrent.TimeUnit;

 

public class ShutdownNowTest {

 

    /**

     * @author zjf

     * @create_time 2014-2-18

     * @use

     * @param args

     * @throws InterruptedException

     */

    public static void main(String[] args) throws InterruptedException {

 

        ExecutorService es = Executors.newFixedThreadPool(3);

        for(int i = 0; i < 10; i ++)

        {

            es.execute(new Countable(i));

        }

        TimeUnit.SECONDS.sleep(1);

        //返回等待的任务列表

        List<Runnable> countList = es.shutdownNow();

        for(Runnable r : countList)

        {

            System.out.println(r.toString() + " is not done...");

        }

    }

 

}

 

class Countable implements Runnable{

    private int i;

    public Countable(int i) {

        this.i = i;

    }

    

    public int getI() {

        return i;

    }

 

    @Override

    public String toString() {

        

        return "thread, id : " + i;

    }

    

    public void run() {

        

        try {

            System.out.println(this.toString() + " is start to run...");

            TimeUnit.MILLISECONDS.sleep(500);

            System.out.println(this.toString() + " is done...");

        } catch (InterruptedException e) {

            System.out.println(this.toString() + " is interrupted...");

        }

    }

    

}

 

 

/**输出

 

thread, id : 0 is start to run...

thread, id : 1 is start to run...

thread, id : 2 is start to run...

thread, id : 0 is done...

thread, id : 1 is done...

thread, id : 2 is done...

thread, id : 3 is start to run...

thread, id : 4 is start to run...

thread, id : 5 is start to run...

thread, id : 5 is done...

thread, id : 3 is done...

thread, id : 4 is done...

thread, id : 6 is start to run...

thread, id : 6 is interrupted...

thread, id : 7 is not done...

thread, id : 8 is not done...

thread, id : 9 is not done...

 

*/

测试ThreadFactory

package thread;

 

import java.util.concurrent.ExecutorService;

import java.util.concurrent.Executors;

import java.util.concurrent.ThreadFactory;

 

public class ThreadFactoryTest {

 

    /**

     * @author zjf

     * @create_time 2013-12-18

     * @use测试Cached线程池

     * @param args

     */

    public static void main(String[] args) {

 

        ThreadFactory threadFactory = new MyThreadFactory();

        ExecutorService es = Executors.newCachedThreadPool(threadFactory);

        for (int i = 0; i < 10; i++) {

            es.execute(new Countable(i));

        }

        es.shutdown();

    }

 

    static class Countable implements Runnable {

 

        private int i;

        public Countable(int i) {

            this.i = i;

        }

        public void run() {

            System.out.println("" + i + "个任务正在运行!");

        }

    }

 

    static class MyThreadFactory implements ThreadFactory {

        private static int count = 0;

        public Thread newThread(Runnable r) {

            return new MyThread(r,count++);

        }

    };

    

    static class MyThread extends Thread

    {

        private Runnable target;

        private int count;

        public MyThread(Runnable target, int count) {

            super();

            this.target = target;

            this.count = count;

        }

        @Override

        public void run() {

            System.out.println("" + count + "个线程启动!" );

            if(target != null)

            {

                target.run();

            }

            System.out.println("" + count + "个线程结束!" );

        }

    }

}

/*

* 输出结果

        0个线程启动!

        1个线程启动!

        2个线程启动!

        3个线程启动!

        0个任务正在运行!

        1个任务正在运行!

        2个任务正在运行!

        4个线程启动!

        3个任务正在运行!

        5个线程启动!

        4个任务正在运行!

        5个任务正在运行!

        8个任务正在运行!

        6个线程启动!

        7个任务正在运行!

        7个线程启动!

        6个任务正在运行!

        9个任务正在运行!

        7个线程结束!

        0个线程结束!

        3个线程结束!

        6个线程结束!

        5个线程结束!

        1个线程结束!

        4个线程结束!

        2个线程结束!

        

    证明:    Countable中的run方法被执行了10

            MyThread中的run方法只被执行了9

            原因:CachedThreadPool在需要的时候会调用ThreadFactorynewThread方法但是也会用到缓存

            */

测试FixedThreadPool

 

newFixedThreadPool:创建一个可重用固定线程集合的线程池,以共享的无界队列方式来运行这些线程。如果在关闭前的执行期间由于失败而导致任何线程终止,那么一个新线程将代替它执行后续的任务(如果需要)。在某个线程被显式地关闭之前,池中的线程将一直存在。(这与cacheThreadPool不一样)

 

 

 

package thread;

 

import java.util.concurrent.ExecutorService;

import java.util.concurrent.Executors;

 

public class FixedThreadPoolTest {

 

    /**

     * @author zjf

     * @create_time 2013-12-18

     * @use测试Fixed线程池

     * @param args

     */

    public static void main(String[] args) {

 

        

        ExecutorService es = Executors.newFixedThreadPool(3);

        for (int i = 0; i < 5; i++) {

            es.execute(new Countable(i));

        }

        es.shutdown();

 

    }

 

    static class Countable implements Runnable {

 

        private int i;

 

        public Countable(int i) {

            this.i = i;

        }

 

        public void run() {

 

            System.out.println("" + i + "启动的线程的ID"

                    + Thread.currentThread().getId());

 

        }

    }

 

}

/*

0启动的线程的ID7

2启动的线程的ID9

1启动的线程的ID8

3启动的线程的ID7

4启动的线程的ID9

*/

 

SingleThreadExecutor

newSingleThreadExecutor():

创建一个使用单个 worker 线程的 Executor,以无界队列方式来运行该线程。(注意,如果因为在关闭前的执行期间出现失败而终止了此单个线程,那么如果需要,一个新线程将代替它执行后续的任务)。可保证顺序地执行各个任务,并且在任意给定的时间不会有多个线程是活动的。与其他等效的 newFixedThreadPool(1) 不同,可保证无需重新配置此方法所返回的执行程序即可使用其他的线程(备注:应该是内部实现的差异 外部的使用没什么差异)。

 

因为一个任务执行完毕之后,线程才会空闲下来去执行另外一个任务,所以可以保证顺序执行任务。

测试ScheduledExecutorService

scheduled

adj. 预定的;已排程的

v. 把…列表;把…列入计划;安排(schedule的过去分词)

 

上面演示的线程执行器或者线程池都是ExecutorService,下面看看ScheduledExecutorService。ScheduledExecutorService集成并且扩展了ExecutorService,可安排在给定的延迟后运行或定期执行的命令。

 

package thread;

 

import java.util.concurrent.Executors;

import java.util.concurrent.ScheduledExecutorService;

import java.util.concurrent.TimeUnit;

 

public class SingleThreadScheduledTest {

 

    /**

     * @author zjf

     * @create_time 2013-12-23

     * @use测试SingleThreadScheduled线程池

     * @param args

     * @throws InterruptedException

     */

    public static void main(String[] args) throws InterruptedException {

 

        ScheduledExecutorService es = Executors

                .newSingleThreadScheduledExecutor();

        //ScheduledThreadPool需要传参控制池中所保存的线程数(即使线程是空闲的也包括在内)

        //ScheduledExecutorService es = Executors.newScheduledThreadPool(1);

        // 给定时间延迟后执行

        // es.schedule(new Countable(), 1, TimeUnit.SECONDS);

        // 传入一个任务然后按照给定频率循环执行在每次任务开始执行的时间点之间存在固定间隔

        //es.scheduleAtFixedRate(new Countable(), 2, 1, TimeUnit.SECONDS);

        // 传入一个任务然后按照给定频率循环执行每一次执行终止和下一次执行开始之间都存在给定的间隔

        es.scheduleWithFixedDelay(new Countable(), 2, 1, TimeUnit.SECONDS);

        // 如果没有这句代码将没有任何反应,因为----|

        // 下面的shutdown代码将会阻止执行新加入任务包含延迟执行而未执行的任务

        TimeUnit.SECONDS.sleep(10);

        es.shutdown();

    }

 

    static class Countable implements Runnable {

 

        public void run() {

            System.out.println("一个任务运行开始!");

            try {

                TimeUnit.SECONDS.sleep(1);

            } catch (InterruptedException e) {

            }

            System.out.println("一个任务运行结束!");

        }

    }

 

}

 

package thread;

 

import java.util.concurrent.Executors;

import java.util.concurrent.ScheduledExecutorService;

import java.util.concurrent.TimeUnit;

 

public class ScheduledThreadPoolTest {

 

    /**

     * @author zjf

     * @create_time 2013-12-23

     * @use测试SingleThreadScheduled线程池

     * @param args

     * @throws InterruptedException

     */

    public static void main(String[] args) throws InterruptedException {

 

        //ScheduledThreadPool需要传参控制池中所保存的线程数(即使线程是空闲的也包括在内)

        ScheduledExecutorService es = Executors.newScheduledThreadPool(1);

        es.scheduleAtFixedRate(new Countable(), 0, 1, TimeUnit.SECONDS);

        TimeUnit.SECONDS.sleep(10);

        es.shutdown();

    }

 

    static class Countable implements Runnable {

 

        public void run() {

            System.out.println("一个任务运行开始!");

            try {

                TimeUnit.SECONDS.sleep(3);

            } catch (InterruptedException e) {

            }

            System.out.println("一个任务运行结束!");

        }

    }

}

/*

* 线程池中只有一个线程 + 每隔1秒要执行一个任务 + 一个任务要运行3秒才结束

* 结果是每隔3秒才能执行一次

*/

 

优先级

package thread;

 

import java.util.concurrent.ExecutorService;

import java.util.concurrent.Executors;

import java.util.concurrent.TimeUnit;

 

public class PriorTest {

 

    /**

     * @author zjf

     * @create_time 2013-12-23

     * @use测试优先级

     * @param args

     * @throws InterruptedException

     */

    public static void main(String[] args) throws InterruptedException {

 

        ExecutorService es = Executors.newCachedThreadPool();

        es.execute(new Runnable() {

            public void run() {

                Thread.currentThread().setPriority(Thread.MAX_PRIORITY);

                int i = 0;

                while (!Thread.currentThread().isInterrupted()) {

                    System.out.println("MAX_PRIORITY" + i);

                    i++;

                }

            }

        });

        

        es.execute(new Runnable() {

            public void run() {

                Thread.currentThread().setPriority(Thread.MIN_PRIORITY);

                int i = 0;

                while (!Thread.currentThread().isInterrupted()) {

                    System.out.println("MIN_PRIORITY" + i);

                    i++;

                }

            }

        });

        TimeUnit.SECONDS.sleep(1);

        es.shutdownNow();

    }

}

/*

* 最后一次输出结果是

*     MAX_PRIORITY32525

*    MIN_PRIORITY31289

* 差别并不大调整优先级适用于做适当的强弱调整不能用来控制流程走势

* windows7个优先级 java可以设置10个优先级

*/

测试Callable

package thread;

 

import java.util.concurrent.Callable;

import java.util.concurrent.ExecutionException;

import java.util.concurrent.ExecutorService;

import java.util.concurrent.Executors;

import java.util.concurrent.Future;

import java.util.concurrent.TimeUnit;

 

public class CallableTest {

 

    /**

     * @author zjf

     * @create_time 2013-12-23

     * @use

     * @param args

     */

    public static void main(String[] args) {

        ExecutorService es = Executors.newCachedThreadPool();

        //CallableFuture都是泛型设计的 T代表返回值的类型

        Future<String> future = es.submit(new Callable<String>() {

            //call方法返回T 并且可以抛出异常到主线程

            public String call() throws Exception {

                System.out.println("running.");

                TimeUnit.SECONDS.sleep(1);

                return "hello world!";

            }

        });

 

        es.shutdown();

        

        //如果被调用线程还没有完成 get方法将阻塞也可以使用isDone()方法来判断是否完成

        try {

            System.out.println(future.get());

        } catch (InterruptedException e) {

            e.printStackTrace();//异常处理

        } catch (ExecutionException e) {

            e.printStackTrace();//异常处理

        }

    }

 

}

submit解析

package thread;

 

import java.util.concurrent.Callable;

import java.util.concurrent.ExecutionException;

import java.util.concurrent.ExecutorService;

import java.util.concurrent.Executors;

import java.util.concurrent.Future;

 

public class SubmitTest {

 

    /**

     * @author zjf

     * @create_time 2014-2-18

     * @use

     * @param args

     */

 

    /*

     * submit方法提交了一个任务给es去执行 es将分配一个线程来执行如果遇到ruturn或者抛出了异常信息都将记录到Future对象中

     * 注意异常不会立刻抛出只是记录到future 在调用futureget方法时候才抛出

     */

 

    public static void main(String[] args) {

        ExecutorService es = Executors.newCachedThreadPool();

 

        // 方式1

        Future<String> future1 = es.submit(new Callable<String>() {

            public String call() throws Exception {

                return "done";

            }

        });

 

        try {

            future1.get();

        } catch (InterruptedException e) {

            e.printStackTrace();

        } catch (ExecutionException e) {

            e.printStackTrace();

        }

 

        // 方式2 这种方式的get永远为null 但是可以抛出异常

        Future<?> future2 = es.submit(new Runnable() {

 

            public void run() {

 

            }

        });

 

        try {

            future2.get();

        } catch (InterruptedException e) {

 

            e.printStackTrace();

        } catch (ExecutionException e) {

 

            e.printStackTrace();

        }

 

        // 方式3 因为run方法是void即使加上String.class 只能获得一个 Future<String>对象而已 get的结果仍然是String型的null

        es.submit(new Runnable() {

            public void run() {

                

            }

        }, String.class);

    }

 

}

join方法

如果在一个线程的run方法中调用t.join,那么将会在t执行结束之后才会继续当前线程。

 void

join()
          等待该线程终止。

 void

join(long millis)
          等待该线程终止的时间最长为 millis 毫秒。

Join抛出InterruptedException。可以被中断。

package thread;

 

public class JoinTest {

 

    public static void main(String[] args) {

        Thrd thrd = new Thrd();

        thrd.start();

        try {

            thrd.join();

        } catch (InterruptedException e) {

            e.printStackTrace();

        }

        System.out.println("after join");

        System.out.println("exit");

    }

 

    static class Thrd extends Thread {

        @Override

        public void run() {

            for (int i = 0; i < 10; i++) {

                System.out.println("running " + i);

                try {

                    Thread.sleep(1000);

                } catch (InterruptedException e) {

                    e.printStackTrace();

                }

            }

        }

    }

}

 

未捕捉异常处理器

通过futureget方法也可以获取异常不知道这两种方式有何差别?

 

package thread;

 

import java.lang.Thread.UncaughtExceptionHandler;

import java.util.concurrent.ExecutorService;

import java.util.concurrent.Executors;

import java.util.concurrent.ThreadFactory;

 

public class UncaughtExceptionHandlerTest {

 

    /**

     * @author zjf

     * @create_time 2013-12-23

     * @use

     * @param args

     */

    public static void main(String[] args) {

 

        ExecutorService es = Executors.newCachedThreadPool(new ThreadFactory() {

            public Thread newThread(Runnable r) {

                Thread t = new Thread(r);

                t.setUncaughtExceptionHandler(new UncaughtExceptionHandler() {

                    public void uncaughtException(Thread t, Throwable e) {

                        System.out.println("线程" + t.getId() + "发生了异常:"

                                + e.getMessage());

                    }

                });

                return t;

            }

        });

 

        es.execute(new Runnable() {

            public void run() {

                throw new RuntimeException("自定义异常");

            }

        });

 

        es.shutdown();

    }

}

通过Funture来捕捉异常:

package thread;

 

import java.util.concurrent.ExecutionException;

import java.util.concurrent.ExecutorService;

import java.util.concurrent.Executors;

 

public class ExceptionTest {

 

    /**

     * @author zjf

     * @create_time 2014-2-18

     * @use

     * @param args

     */

    public static void main(String[] args) {

 

        ExecutorService es = Executors.newCachedThreadPool();

        try {

            es.submit(new Runnable() {

                public void run() {

                    throw new RuntimeException("error");

                }

            }).get();

        } catch (InterruptedException e) {

            

            e.printStackTrace();

        } catch (ExecutionException e) {

            e.printStackTrace();

            System.out.println("-------------------------------------------------------------------------------------------------------------");

            e.getCause().printStackTrace();

        }

        es.shutdown();

    }

}

java.util.concurrent.ExecutionException: java.lang.RuntimeException: error

    at java.util.concurrent.FutureTask$Sync.innerGet(FutureTask.java:205)

    at java.util.concurrent.FutureTask.get(FutureTask.java:80)

    at thread.ExceptionTest.main(ExceptionTest.java:23)

Caused by: java.lang.RuntimeException: error

    at thread.ExceptionTest$1.run(ExceptionTest.java:21)

    at java.util.concurrent.Executors$RunnableAdapter.call(Executors.java:417)

    at java.util.concurrent.FutureTask$Sync.innerRun(FutureTask.java:269)

    at java.util.concurrent.FutureTask.run(FutureTask.java:123)

    at java.util.concurrent.ThreadPoolExecutor$Worker.runTask(ThreadPoolExecutor.java:650)

    at java.util.concurrent.ThreadPoolExecutor$Worker.run(ThreadPoolExecutor.java:675)

    at java.lang.Thread.run(Thread.java:595)

-------------------------------------------------------------------------------------------------------------

java.lang.RuntimeException: error

    at thread.ExceptionTest$1.run(ExceptionTest.java:21)

    at java.util.concurrent.Executors$RunnableAdapter.call(Executors.java:417)

    at java.util.concurrent.FutureTask$Sync.innerRun(FutureTask.java:269)

    at java.util.concurrent.FutureTask.run(FutureTask.java:123)

    at java.util.concurrent.ThreadPoolExecutor$Worker.runTask(ThreadPoolExecutor.java:650)

    at java.util.concurrent.ThreadPoolExecutor$Worker.run(ThreadPoolExecutor.java:675)

    at java.lang.Thread.run(Thread.java:595)

 

20131224 周二

java编程思想 第21章 并发

监视器

内部的默认锁:对象内部的锁 用于对象synchronized方法。class内部的锁用于类的static synchronized方法。

什么时候使用同步

对于共享资源的访问:

如果一个线程正在写入一个数据,而这个数据有可能会被另外一个线程读取,那么当前写入操作应该同步。

如果一个线程正在读取一个数据,而这个数据有可能之前被另外一个线程写入,那么当前读取操作应该同步。

ReentrantLock

可重入锁,累计进入锁定数值累计,需要多次解锁才能彻底解锁。

保持计数:当前线程重入锁的次数。

默认为非公平锁:有一个构造函数可以接受公平参数,如果设置为true,将会尽力保证将锁资源分配给等待时间最长的线程以保证公平。如果使用无参构造函数,那么将采用非公平锁。

lock():尝试获取锁,成功后将保持计数+1,如果锁被另外一个线程持有,将等待。

lockInterruptibly():和lock用法意一样,区别是它在获取锁的等待过程中可以被中断。

tryLock():和lock的差别:1.它将忽略公平设置。永远不公平。2. 如果锁被另外一个线程持有,立即返回或者按照传递的等待时间超时后返回。

unlock():如果当前线程是此锁定所有者,则将保持计数减 1。如果保持计数现在为 0,则释放该锁定。

 

什么时候需要使用锁

使用默认的synchronized方法,或者synchronized块,是基于默认的ReentrantLock实现的。但是ReentrantLock的功能要更多。而且不带参数的synchronized是针对当前对象或者当前类的默认锁的,如果一个类有多个方法要同步,但是不是每个方法都相互牵制,那么应该使用lock来区别对待。

原子操作

对非long和double之外的基本类型的读取和赋值操作时原子操作

package thread;

 

import java.util.concurrent.ExecutorService;

import java.util.concurrent.Executors;

import java.util.concurrent.TimeUnit;

import java.util.concurrent.locks.Lock;

import java.util.concurrent.locks.ReentrantLock;

 

public class AtomiTest {

 

    /**

     * @author zjf

     * @create_time 2013-12-25

     * @use

     * @param args

     * @throws InterruptedException

     */

    public static void main(String[] args) throws InterruptedException {

        ExecutorService es = Executors.newCachedThreadPool();

        for(int i =0; i <runCount; i++)

        {

            //新建100个线程来执行countAdd的操作

            es.execute(new Runnable(){

                public void run() {

                    for(int i =0; i <100; i++)

                    {

                        countAdd();

                    }

                    //标示当前线程已经运行完毕

                    iAmDone();

                }});

        }

        es.shutdown();

        //所有线程运行完毕后打印出结果

        while(!isAllDone())

        {

            TimeUnit.MILLISECONDS.sleep(500);

        }

        System.out.println(getCount());

    }

 

    private static int count = 0;

    

    private static int done = 0;

    

    private static int runCount = 100;

    

    private static Lock countLock = new ReentrantLock();

    

    //此处不加上synchronized 结果将不是10000(比10000小)

    public synchronized static void iAmDone()

    {

        done ++;

    }

    

    public synchronized static boolean isAllDone()

    {

        return done == 100;

    }

    

    public static void countAdd()

    {

        //使用lock 不在方法中加synchronized 这样不会与上面的done方法公用thislock 可以提升性能

        countLock.lock();

        count ++;

        countLock.unlock();

    }

    

    public static int getCount()

    {

        return count;

    }

}

 

原子性和可视性

在多核处理器中,多任务被分配到多核上去处理,一个核上的更改不会即可刷新到其他核。加上volatile关键字可以保证可视性。

java可以保证对除了long和double之外的基本类型的简单的赋值和读取操作的原子性。但是不能保证可视性。

如果加上了volatile关键字,不仅可以保证可视性,同时也可以保证long和double的原子性。

synchronized可以保证可视性。

如果对于基本类型(不包含long和double)执行简单的(一句代码的)读写操作(比较抽象,++就不是),可以保证原子性和可视性。但是既然是一句话那么不如使用sychronized,也不会耗费多少资源,而且稳定。

对原子性的误用

import java.util.concurrent.ExecutorService;

import java.util.concurrent.Executors;

 

public class UseAtomiWrong {

 

    /**

     * @author zjf

     * @create_time 2013-12-25

     * @use

     * @param args

     */

    public static void main(String[] args) {

        ExecutorService es = Executors.newCachedThreadPool();

        for (int i = 0; i < 100; i++) {

            es.execute(new Runnable() {

                public void run() {

                    for (int i = 0; i < 100; i++) {

                        add();

                    }

                }

            });

        }

        es.execute(new Runnable(){

            public void run() {

                int i = getEvent();

                while(i%2 == 0)

                {

                    i = getEvent();

                }

                System.out.println(i);//打印出了奇数

            }});

 

        es.shutdown();

    }

 

    public volatile static int even = 0;

 

    //虽然对even加上了volatile 但是可能会读到只执行了一次even++之后的不稳定状态还是要加synchronized

    public static int getEvent()

    {

        return even;

    }

    public synchronized static void add() {

        even++;

        even++;

    }

}

原子类

AtomicBooleanAtomicIntegerAtomicLongAtomicReference 的实例各自提供对相应类型单个变量的访问和更新。每个类也为该类型提供适当的实用工具方法。例如,类 AtomicLong 和 AtomicInteger 提供了原子增量方法。一个应用程序将按以下方式生成序列号:

class Sequencer {

private AtomicLong sequenceNumber = new AtomicLong(0);

public long next() { return sequenceNumber.getAndIncrement(); }

}

原子访问和更新的内存效果一般遵循以下可变规则:

get 具有读取 volatile 变量的内存效果。

set 具有写入(分配) volatile 变量的内存效果。

weakCompareAndSet 以原子方式读取和有条件地写入变量,并对于该变量上的其他内存操作进行排序,否则将充当普通的非可变内存操作。

compareAndSet 和所有其他的读取和更新操作(如 getAndIncrement)都有读取和写入 volatile 变量的内存效果。

临界区

Object obj = new Object();

        Lock lock = new ReentrantLock();

        //使用当前对象的默认所

        synchronized (this) {

            //代码

        }

        //使用其他对象的默认锁

        synchronized (obj) {

            //代码

        }

        //使用指定锁

        synchronized (lock) {

            //代码

        }

判断ExecutorService中的所有任务是否完成

ExecutorService.isTerminated():

Returns true if all tasks have completed following shut down. Note that isTerminated is never true unless either shutdown or shutdownNow was called first.

 

ExecutorService.awaitTermination(long timeout, TimeUnit unit) throws InterruptedException:

Blocks until all tasks have completed execution after a shutdown request, or the timeout occurs, or the current thread is interrupted, whichever happens first.

 

线程状态

新建:最初建立需要经历的一个极其短暂的临时状态。

就绪:只要给了时间片就可以运行。

阻塞:sleep wait或者等待锁资源的时候处于的状态。

死亡。运行完成的线程进入死亡状态。不可以再运行其他任务。

关闭线程的方法:

通过interrupt来结束线程。es.shutdownnow可以给所有线程发送中断指令。如果需要对某一个线程中断,可以使用Future。如:

package thread;

 

import java.util.concurrent.ExecutorService;

import java.util.concurrent.Executors;

import java.util.concurrent.Future;

import java.util.concurrent.TimeUnit;

import java.util.concurrent.locks.Lock;

import java.util.concurrent.locks.ReentrantLock;

 

public class FutureInterruptTest {

 

    public static Lock lock = new ReentrantLock();

    

    /**

     * @author zjf

     * @create_time 2013-12-25

     * @use

     * @param args

     * @throws InterruptedException

     */

    public static void main(String[] args) throws InterruptedException {

 

        ExecutorService es = Executors.newCachedThreadPool();

        es.execute(new Runnable() {

            public void run() {

                lock.lock();

                System.out.println("locking...");

                try {

                    //锁定5秒钟

                    TimeUnit.SECONDS.sleep(5);

                } catch (InterruptedException e) {

                    

                }

                lock.unlock();

            }

        });

        

        Future<?> future = es.submit(new Runnable() {

            public void run() {

                try {

                    //陷入等待锁的状态

                    lock.lockInterruptibly();

                    lock.unlock();

                } catch (InterruptedException e) {

                    

                }

                System.out.println("done...");

            }

        });

        

        TimeUnit.SECONDS.sleep(1);

        //cancel方法如果线程时新建未运行状态那么就结束它如果已经运行那么中断它

        //如果把上面的lock.lockInterruptibly();改为lock那么将接收不到中断响应,直到获得锁。

        future.cancel(true);

    }

}

 

使用volatile的boolean变量:

 

package thread;

 

import java.util.concurrent.ExecutorService;

import java.util.concurrent.Executors;

import java.util.concurrent.TimeUnit;

 

public class TerminateThreadUserBoolean {

 

    public static volatile boolean stop = false;

 

    /**

     * @author zjf

     * @create_time 2013-12-25

     * @use

     * @param args

     * @throws InterruptedException

     * @throws InterruptedException

     */

    public static void main(String[] args) throws InterruptedException {

        ExecutorService es = Executors.newCachedThreadPool();

        for (int i = 0; i < 10; i++) {

            es.execute(new Runnable() {

                public void run() {

                    

                    

                    while(!stop)

                    {

                        System.out.println("running...");

                        try {

                            TimeUnit.SECONDS.sleep(1);

                        } catch (InterruptedException e) {

                            System.out.println("InterruptedException");

                        }

                        System.out.println("runed");

                    }

                    

                }

            });

        }

        TimeUnit.SECONDS.sleep(3);

        stop = true;

        System.out.println("try to shutdown");

        // shutdown之后才能测试awaitTermination

        es.shutdown();

        boolean success = es.awaitTermination(1, TimeUnit.SECONDS);

        if(success)

        {

            System.out.println("所有线程已经关闭..");

        }

        else {

            System.out.println("部分线程没有关闭..");

        }

        

    }

 

}

 

中断循环线程:

 

package thread;

 

import java.util.concurrent.ExecutorService;

import java.util.concurrent.Executors;

import java.util.concurrent.TimeUnit;

 

public class ThreadCloseTest {

 

    /**

     * @author zjf

     * @create_time 2014-2-21

     * @use

     * @param args

     * @throws InterruptedException

     */

    public static void main(String[] args) throws InterruptedException {

 

        ExecutorService es = Executors.newCachedThreadPool();

        es.execute(new Runnable() {

            public void run() {

                //try放在while的外层

                try {

                    //while中检测中断

                    while(!Thread.interrupted())

                    {

                        System.out.println("i am running");

                        TimeUnit.MILLISECONDS.sleep(500);

                    }

                } catch (InterruptedException e) {

                    

                }

                finally

                {

                    System.out.println("i am interrupted");

                }

            }

        });

        TimeUnit.SECONDS.sleep(3);

        es.shutdownNow();

    }

 

}

 

synchronized使用默认lock.lock;在等待默认锁的过程中不能被中断。
interrupt

public void interrupt()

中断线程。

public static boolean interrupted()

测试当前线程是否已经中断。线程的中断状态 由该方法清除。换句话说,如果连续两次调用该方法,则第二次调用将返回 false(在第一次调用已清除了其中断状态之后,且第二次调用检验完中断状态前,当前线程再次中断的情况除外)。

public boolean isInterrupted()

测试线程是否已经中断。线程的中断状态 不受该方法的影响。

备注:如果使用执行器,在每个任务结束之后 ,将会自动将线程的中断状态清除,然后再去执行下一个任务。示例代码:

package thread;

 

import java.util.concurrent.ExecutorService;

import java.util.concurrent.Executors;

import java.util.concurrent.Future;

import java.util.concurrent.TimeUnit;

 

public class InterruptTest {

 

    public static void main(String[] args) throws InterruptedException {

        ExecutorService es = Executors.newSingleThreadExecutor();

        Future<?> future = es.submit(new Runnable() {

 

            public void run() {

                //只判断不清除中断

                while(!Thread.currentThread().isInterrupted())

                {

                    System.out.println("1st running");

                }

            }

        });

        TimeUnit.SECONDS.sleep(1);

        //发送中断

        future.cancel(true);

        es.execute(new Runnable() {

 

            public void run() {

                while(!Thread.currentThread().isInterrupted())

                {

                    //如果中断标志没有清楚将不会打印出下面代码

                    System.out.println("2st running");

                }

            }

        });

        es.shutdown();

    }

 

}

Wait和notify

锁和同步块是用来解决线程互斥的问题。wait和notify是用来解决线程协作的问题。

wait和notify是针对一个锁的,wait和notify的对象是这个锁上的其他等待对象。

wait方法可以被中断。notify不会等待,所以不需要被中断。

sleep和yield不会释放锁(它们跟锁没有关系,只跟线程有关),而wait和notify必须放在sychronized块内,因为跟锁是关联在一起的。wait将释放锁,然后等待。

notify会唤醒在这个锁上等待的其他程序。但是不会释放锁。直到当前的线程放弃此对象上的锁定,才能继续执行被唤醒的线程。被唤醒的线程将以常规方式与在该对象上主动同步的其他所有线程进行竞争;例如,唤醒的线程在获取notify线程释放的锁方面没有特权。

 

wait(long timeout):此方法导致当前线程(称之为 T将其自身放置在对象的等待集中,然后放弃此对象上的所有同步要求(包含放弃锁)。出于线程调度目的,线程 T 被禁用,且处于休眠状态,直到发生以下四种情况之一:
			
  • 其他某个线程调用此对象的 notify 方法,并且线程 T 碰巧被任选为被唤醒的线程。
  • 其他某个线程调用此对象的 notifyAll 方法。
  • 其他某个线程中断线程 T。
  • 已经到达指定的实际时间。但是,如果 timeout 为零,则不考虑实际时间,该线程将一直等待,直到获得通知。

然后,从对象的等待集中删除线程 T,并重新进行线程调度。然后,该线程以常规方式与其他线程竞争,以获得在该对象上同步的权利;一旦获得对该对象的控制权,该对象上的所有其同步声明都将被还原到以前的状态 - 这就是调用 wait 方法时的情况。然后,线程 T 从 wait 方法的调用中返回。所以,从 wait 方法返回时,该对象和线程 T 的同步状态与调用 wait 方法时的情况完全相同。

如:

package thread;

 

import java.util.concurrent.ExecutorService;

import java.util.concurrent.Executors;

import java.util.concurrent.TimeUnit;

 

public class NotifyTest {

 

    /**

     * @author zjf

     * @create_time 2013-12-30

     * @use

     * @param args

     * @throws InterruptedException

     */

    public static void main(String[] args) throws InterruptedException {

        final NotifyTest t = new NotifyTest();

        ExecutorService es = Executors.newCachedThreadPool();

        es.execute(new Runnable() {

 

            public void run() {

                try {

                    t.testWait();

                } catch (InterruptedException e) {

                    

                    e.printStackTrace();

                }

            }

        });

          

        

        es.execute(new Runnable() {

 

            public void run() {

                t.testNotify();

            }

        });

        

    }

 

    public synchronized void testWait() throws InterruptedException {

        System.out.println("pre wait");

        wait();

        System.out.println("after wait");

    }

 

    public synchronized void testNotify() {

        System.out.println("pre notify");

        //虽然唤醒了testWait 但是没有释放所资源 testWait仍然无法运行

        notify();

        try {

            //20秒之后程序执行完毕然后释放了锁这时才会输出 "after wait"

            TimeUnit.SECONDS.sleep(20);

        } catch (InterruptedException e) {

            

            e.printStackTrace();

        }

        System.out.println("after notify");

    }

}

Wait的简单例子

package thread;

 

import java.util.concurrent.ExecutorService;

import java.util.concurrent.Executors;

import java.util.concurrent.TimeUnit;

 

public class WaxTest {

 

    /**

     * @author zjf

     * @create_time 2013-12-30

     * @use

     * @param args

     * @throws InterruptedException

     */

    public static void main(String[] args) throws InterruptedException {

        Car car = new Car();

        ExecutorService es = Executors.newCachedThreadPool();

        es.execute(new Waxing(car));

        es.execute(new Buffering(car));

        TimeUnit.SECONDS.sleep(6);

        es.shutdownNow();

    }

 

    static class Car{

        private boolean isWaxOn = false;

        

        //一般将InterruptedException抛出到任务的run方法中去控制

        //这里不加synchronized也可以应为它是在synchronizedwaxing方法中被调用的

        public synchronized void waitForWaxing() throws InterruptedException

        {

            //使用while

            while(isWaxOn == true)

            {

                wait();

            }

        }

        

        public synchronized void waitForBuffing() throws InterruptedException

        {

            //使用while

            while(isWaxOn == false)

            {

                wait();

            }

        }

          

        

        public synchronized void waxing() throws InterruptedException

        {

            waitForWaxing();

            TimeUnit.MILLISECONDS.sleep(200);

            System.out.println("waxing on");

            isWaxOn = true;

            notifyAll();

        }

        

        public synchronized void buffing() throws InterruptedException

        {

            waitForBuffing();

            TimeUnit.MILLISECONDS.sleep(200);

            System.out.println("buffing over");

            isWaxOn = false;

            notifyAll();

        }

    }

    

    //涂蜡任务

    static class Waxing implements Runnable{

 

        private Car car;

        

        public Waxing(Car car) {

            super();

            this.car = car;

        }

 

        public void run() {

            try {

                while(!Thread.interrupted())

                {

                    car.waxing();

                }

            } catch (InterruptedException e) {

                

            }

        }

        

    }

    

    //抛光任务

    static class Buffering implements Runnable{

 

        private Car car;

        

        public Buffering(Car car) {

            super();

            this.car = car;

        }

 

        public void run() {

            try {

                while(!Thread.interrupted())

                {

                    car.buffing();

                }

            } catch (InterruptedException e) {

                

            }

        }

    }

    

}

 

package thread;

 

import java.util.concurrent.ExecutorService;

import java.util.concurrent.Executors;

import java.util.concurrent.TimeUnit;

 

public class RestanurantTest {

 

    /**

     * @author zjf

     * @create_time 2013-12-31

     * @use

     * @param args

     */

    public static void main(String[] args) {

        new Restanurant();

    }

 

    static class Meal {

        private int orderNumber;

 

        public Meal(int orderNumber) {

            super();

            this.orderNumber = orderNumber;

        }

 

        @Override

        public String toString() {

 

            return "Meal:" + orderNumber;

        }

    }

 

    static class Restanurant {

        Meal meal;

        WaitPerson waiter = new WaitPerson(this);

        Chef chef = new Chef(this);

        ExecutorService es = Executors.newCachedThreadPool();

 

        public Restanurant() {

            es.execute(waiter);

            es.execute(chef);

            es.shutdown();

        }

    }

 

    static class WaitPerson implements Runnable {

 

        private Restanurant restanurant;

 

        public WaitPerson(Restanurant restanurant) {

            super();

            this.restanurant = restanurant;

        }

 

        public void run() {

            try {

                while (!Thread.interrupted()) {

                    //使用共有的restanurant来控制同步

                    synchronized (restanurant) {

                        while (restanurant.meal == null) {

                            //因为是synchronizedrestanurant 所以wait方法也是调用restanurant

                            restanurant.wait();

                        }

                        TimeUnit.MILLISECONDS.sleep(200);

                        restanurant.meal = null;

                        System.out.println("服务员上餐结束!");

                        restanurant.notifyAll();

                    }

 

                }

            } catch (InterruptedException e) {

 

            }

 

        }

 

    }

 

    static class Chef implements Runnable {

 

        private Restanurant restanurant;

 

        public Chef(Restanurant restanurant) {

            super();

            this.restanurant = restanurant;

        }

 

        private int orderNumber = 0;

 

        public void run() {

            try {

                while (!Thread.interrupted()) {

                    synchronized (restanurant) {

                        while (restanurant.meal != null) {

                            restanurant.wait();

                        }

                        TimeUnit.MILLISECONDS.sleep(200);

                        restanurant.meal = new Meal(++orderNumber);

                        System.out.println("厨师做饭完毕!");

                        restanurant.notifyAll();

                        if(orderNumber >= 10)

                        {

                            restanurant.es.shutdownNow();

                        }

                    }

                }

            } catch (InterruptedException e) {

 

            }

        }

 

    }

}

 

使用显式的condition

package thread;

 

import java.util.concurrent.ExecutorService;

import java.util.concurrent.Executors;

import java.util.concurrent.TimeUnit;

import java.util.concurrent.locks.Condition;

import java.util.concurrent.locks.Lock;

import java.util.concurrent.locks.ReentrantLock;

 

public class RestanurantTest1 {

 

    /**

     * @author zjf

     * @create_time 2013-12-31

     * @use

     * @param args

     */

    public static void main(String[] args) {

        new Restanurant();

    }

 

    static class Meal {

        private int orderNumber;

 

        public Meal(int orderNumber) {

            super();

            this.orderNumber = orderNumber;

        }

 

        @Override

        public String toString() {

 

            return "Meal:" + orderNumber;

        }

    }

 

    static class Restanurant {

        Lock mealLock = new ReentrantLock();

        Condition condition = mealLock.newCondition();

        Meal meal;

        WaitPerson waiter = new WaitPerson(this);

        Chef chef = new Chef(this);

        ExecutorService es = Executors.newCachedThreadPool();

 

        public Restanurant() {

            es.execute(waiter);

            es.execute(chef);

            es.shutdown();

        }

    }

 

    static class WaitPerson implements Runnable {

 

        private Restanurant restanurant;

 

        public WaitPerson(Restanurant restanurant) {

            super();

            this.restanurant = restanurant;

        }

 

        public void run() {

            try {

                while (!Thread.interrupted()) {

                    restanurant.mealLock.lockInterruptibly();

                        while (restanurant.meal == null) {

                            restanurant.condition.await();

                        }

                        TimeUnit.MILLISECONDS.sleep(200);

                        restanurant.meal = null;

                        System.out.println("服务员上餐结束!");

                        restanurant.condition.signalAll();

                        restanurant.mealLock.unlock();

 

                }

            } catch (InterruptedException e) {

 

            }

 

        }

 

    }

 

    static class Chef implements Runnable {

 

        private Restanurant restanurant;

 

        public Chef(Restanurant restanurant) {

            super();

            this.restanurant = restanurant;

        }

 

        private int orderNumber = 0;

 

        public void run() {

            try {

                while (!Thread.interrupted()) {

                    restanurant.mealLock.lockInterruptibly();

                        while (restanurant.meal != null) {

                            restanurant.condition.await();

                        }

                        TimeUnit.MILLISECONDS.sleep(200);

                        restanurant.meal = new Meal(++orderNumber);

                        System.out.println("厨师做饭完毕!");

                        restanurant.condition.signalAll();

                        if(orderNumber >= 10)

                        {

                            restanurant.es.shutdownNow();

                        }

                        restanurant.mealLock.unlock();

                }

            } catch (InterruptedException e) {

 

            }

        }

 

    }

}

BlockingQueue

接口 BlockingQueue<E>

E take()

throws InterruptedException

检索并移除此队列的头部,如果此队列不存在任何元素,则一直等待。

void put(E o)

throws InterruptedException

将指定元素添加到此队列尾部,如果没有可用空间,将一直等待(如果有必要)。

实现类:ArrayBlockingQueue<E> 固定数目, LinkedBlockingQueue<E>不固定数目。

20140224 周一

java编程思想 第21章 并发

 

使用管道在线程间传递数据

package thread;

 

import java.io.IOException;

import java.io.PipedReader;

import java.io.PipedWriter;

import java.util.concurrent.ExecutorService;

import java.util.concurrent.Executors;

import java.util.concurrent.TimeUnit;

 

public class PipedReaderTest {

 

    /**

     * @author zjf

     * @create_time 2014-2-24

     * @use

     * @param args

     * @throws IOException

     * @throws InterruptedException

     */

    public static void main(String[] args) throws IOException,

            InterruptedException {

 

        ExecutorService es = Executors.newCachedThreadPool();

        final PipedWriter writer = new PipedWriter();

        final PipedReader reader = new PipedReader();

        //两个关联上

        writer.connect(reader);

        es.execute(new Runnable() {

            public void run() {

                char[] strs = "my name is zjf".toCharArray();

                try {

                    for (char c : strs) {

                        writer.write(c);

                        TimeUnit.MILLISECONDS.sleep(300);

                    }

 

                } catch (IOException e) {

 

                    e.printStackTrace();

                } catch (InterruptedException e) {

 

                    e.printStackTrace();

                } finally {

                    try {

                        //输出完成后关闭writer

                        writer.close();

                    } catch (IOException e) {

                        e.printStackTrace();

                    }

                }

            }

        });

 

        es.execute(new Runnable() {

 

            public void run() {

                try {

                    while (!Thread.interrupted()) {

                        int c;

                        //writer关闭之后将会获取-1 循环被终止陷入外层interruptedwhile循环中

                        while ((c = reader.read()) != -1) {

                            System.out.println((char) c);

                        }

                    }

                } catch (IOException e) {

                    e.printStackTrace();

                }

            }

        });

 

        //5秒后发送中断指令

        TimeUnit.SECONDS.sleep(5);

        es.shutdownNow();

    }

}

 

20140225 周二

java编程思想 第21章 并发

CountDownLatch

用给定的计数 初始化 CountDownLatch。由于调用了 countDown() 方法,所以在当前计数到达零之前,await 方法会一直受阻塞。之后,会释放所有等待的线程,await 的所有后续调用都将立即返回。这种现象只出现一次——计数无法被重置。如果需要重置计数,请考虑使用 CyclicBarrier

CountDownLatch 是一个通用同步工具,它有很多用途。将计数 1 初始化的 CountDownLatch 用作一个简单的开/关锁存器,或入口:在通过调用 countDown() 的线程打开入口前,所有调用 await 的线程都一直在入口处等待。用 N 初始化的 CountDownLatch 可以使一个线程在 N 个线程完成某项操作之前一直等待,或者使其在某项操作完成 N 次之前一直等待。

 

示例用法: 下面给出了两个类,其中一组 worker 线程使用了两个倒计数锁存器:

第一个类是一个启动信号,在 driver 为继续执行 worker 做好准备之前,它会阻止所有的 worker 继续执行。

第二个类是一个完成信号,它允许 driver 在完成所有 worker 之前一直等待。

class Driver { // ...

void main() throws InterruptedException {

CountDownLatch startSignal = new CountDownLatch(1);

CountDownLatch doneSignal = new CountDownLatch(N);

 

for (int i = 0; i < N; ++i) // create and start threads

new Thread(new Worker(startSignal, doneSignal)).start();

 

doSomethingElse(); // don't let run yet

startSignal.countDown(); // let all threads proceed

doSomethingElse();

doneSignal.await(); // wait for all to finish

}

}

 

class Worker implements Runnable {

private final CountDownLatch startSignal;

private final CountDownLatch doneSignal;

Worker(CountDownLatch startSignal, CountDownLatch doneSignal) {

this.startSignal = startSignal;

this.doneSignal = doneSignal;

}

public void run() {

try {

startSignal.await();

doWork();

doneSignal.countDown();

} catch (InterruptedException ex) {} // return;

}

 

void doWork() { ... }

}

 

 

另一种典型用法是,将一个问题分成 N 个部分,用执行每个部分并让锁存器倒计数的 Runnable 来描述每个部分,然后将所有 Runnable 加入到 Executor 队列。当所有的子部分完成后,协调线程就能够通过 await。(当线程必须用这种方法反复倒计数时,可改为使用 CyclicBarrier。)

class Driver2 { // ...

void main() throws InterruptedException {

CountDownLatch doneSignal = new CountDownLatch(N);

Executor e = ...

 

for (int i = 0; i < N; ++i) // create and start threads

e.execute(new WorkerRunnable(doneSignal, i));

 

doneSignal.await(); // wait for all to finish

}

}

 

class WorkerRunnable implements Runnable {

private final CountDownLatch doneSignal;

private final int i;

WorkerRunnable(CountDownLatch doneSignal, int i) {

this.doneSignal = doneSignal;

this.i = i;

}

public void run() {

try {

doWork(i);

doneSignal.countDown();

} catch (InterruptedException ex) {} // return;

}

 

void doWork() { ... }

}

 

 

我的例子:

 

package thread;

 

import java.util.concurrent.CountDownLatch;

import java.util.concurrent.ExecutorService;

import java.util.concurrent.Executors;

 

public class CountDownLatchTest {

 

    /**

     * @author zjf

     * @create_time 2014-2-25

     * @use计算1 + 2 + 3 + ... + 1000000000

     * @param args

     */

    public static void main(String[] args) {

        long startTime = System.currentTimeMillis();

        long sum = sum(1, 1000000000, 10);

        long endTime = System.currentTimeMillis();

        System.out.println("计算结果是" + sum + ",耗时" + (endTime - startTime) + "毫秒!");

    }

 

    public static long sum(int start, int end, int concurrentSize) {

        ConcurrentSumer sumer = new ConcurrentSumer(start, end, concurrentSize);

        return sumer.sum();

    }

}

 

class ConcurrentSumer {

    private long sum = 0;

    private int start;

    private int end;

    private int concurrentSize;

    CountDownLatch countDownLatch;

    

    public ConcurrentSumer(int start, int end, int concurrentSize) {

        super();

        this.start = start;

        this.end = end;

        this.concurrentSize = concurrentSize;

        countDownLatch = new CountDownLatch(concurrentSize);

    }

    

    private synchronized void addSum(long add)

    {

        sum += add;

    }

    

    public long sum() {

        ExecutorService es = Executors.newCachedThreadPool();

        int extend = (end - start)/concurrentSize +1;

        while(start <= end)

        {

            es.execute(new SumTask(start,(start + extend) > end ? end : (start + extend)));

            start = start + extend + 1;

        }

        es.shutdown();

        try {

            //等待所有任务完成

            countDownLatch.await();

            System.out.println("所有任务已经完成...");

        } catch (InterruptedException e) {

            //如果没有等到所有任务都完成就被中断那么返回0

            sum = 0;

        }

        return sum;

    }

 

    class SumTask implements Runnable {

        private int st;

        private int ed;

        

        public SumTask(int st, int ed) {

            super();

            this.st = st;

            this.ed = ed;

        }

 

        public void run() {

            long s = 0;

            for(int i = st; i <= ed; i++ )

            {

                s += i;

            }

            addSum(s);

            System.out.println("一个线程已经完成...");

            countDownLatch.countDown();

        }

    }

 

}

 

CyclicBarrier

cyclic ['saiklik]

adj.

1. 周期的,构成周期的

2. 循环的,轮转的;往复运动的

 

barrier ['bæriə]

n.

1. (阻碍通道的)障碍物,屏障(如栅栏、挡板、挡墙、壁垒、障壁、十字转门等)

 

 

一个同步辅助类,它允许一组线程互相等待,直到到达某个公共屏障点 (common barrier point)。

public CyclicBarrier(int parties,

				Runnable barrierAction)

创建一个新的 CyclicBarrier

parties:并行运行的任务数。

barrierAction:每当并行任务的任务调用的barrier的await方法的次数到达parties此的时候,那么barrierAction方法将会被执行一次,此时所有的并行任务处于等待状态,等待barrierAction执行完毕,所有卡在await方法的并行任务得以继续执行。

public int await()
          throws InterruptedException,

				BrokenBarrierException
			

在所有参与者都已经在此 barrier 上调用 await 方法之前,将一直等待。

 

赛马例子:

package thread;

 

import java.util.ArrayList;

import java.util.List;

import java.util.Random;

import java.util.concurrent.BrokenBarrierException;

import java.util.concurrent.CyclicBarrier;

import java.util.concurrent.ExecutorService;

import java.util.concurrent.Executors;

import java.util.concurrent.TimeUnit;

/**

*

* @author zjf

* @create_time 2014-2-26

* @use赛马模拟小程序

*/

public class CyclicBarrierTest {

    public static void main(String[] args) {

        //赛马数量

        int horseCount = 10;

        //目标距离

        final int targetLine = 20;

        final List<Horse> horses = new ArrayList<Horse>();

        final ExecutorService es = Executors.newCachedThreadPool();

        //每一组马匹走完一步之后统计是否已经到达终点

        CyclicBarrier barrier = new CyclicBarrier(horseCount, new Runnable() {

            public void run() {

                System.out.println("");

                for (Horse horse : horses) {

                    if (horse.getComplete() >= targetLine) {

                        System.out.println(horse + " wone!");

                        //如果有到达终点的发送中断

                        es.shutdownNow();

                        break;

                    }

 

                }

            }

        });

 

        for (int i = 0; i < horseCount; i++) {

            Horse horse = new Horse(i, barrier);

            horses.add(horse);

            es.execute(horse);

        }

    }

}

 

class Horse implements Runnable {

    private int id;

    private CyclicBarrier barrier;

    private int complete = 0;

    private Random random = new Random();

 

    public synchronized int getComplete() {

        return complete;

    }

 

    public synchronized void oneStep() {

        //模拟一步的距离

        complete += random.nextInt(3);

        System.out.print(this + " : " + complete + "--");

    }

 

    public Horse(int i, CyclicBarrier barrier) {

        this.id = i;

        this.barrier = barrier;

    }

 

    public void run() {

        try {

            while (!Thread.interrupted()) {

                oneStep();

                //每执行一步之后等待并行的都执行完await之后才能继续执行

                barrier.await();

                TimeUnit.MILLISECONDS.sleep(300);

            }

        } catch (InterruptedException e) {

 

        } catch (BrokenBarrierException e) {

 

        }

    }

 

    @Override

    public String toString() {

        return "horse" + id;

    }

}

 

20140226 周三

java编程思想 第21章 并发

PriorityBlockingQueue

一个无界的阻塞队列,它使用与类 PriorityQueue 相同的顺序规则,并且提供了阻塞检索的操作。虽然此队列逻辑上是无界的,但是由于资源被耗尽,所以试图执行添加操作可能会失败(导致 OutOfMemoryError)。此类不允许使用 null 元素。

 

例子:

package thread;

 

import java.util.concurrent.ExecutorService;

import java.util.concurrent.Executors;

import java.util.concurrent.PriorityBlockingQueue;

import java.util.concurrent.TimeUnit;

 

public class PriorityBlockingQueueTest {

 

    /**

     * @author zjf

     * @create_time 2014-2-26

     * @use

     * @param args

     * @throws InterruptedException

     */

    public static void main(String[] args) throws InterruptedException {

        //Task类实现了Comparable接口,按照排序来决定优先级。

        final PriorityBlockingQueue<Task> taskQueue = new PriorityBlockingQueue<Task>();

        ExecutorService es = Executors.newCachedThreadPool();

        es.execute(new Runnable() {

            public void run() {

                try {

                    while(!Thread.interrupted())

                    {

                        Task task = taskQueue.take();

                        System.out.println(task);

                        //每隔200秒取出一个

                        TimeUnit.MILLISECONDS.sleep(200);

                    }

                } catch (InterruptedException e) {

                    

                }

            }

        });

        for (final TASKLEVLE taskLevel : TASKLEVLE.values()) {

            es.execute(new Runnable() {

                public void run() {

                    try {

                        for (int i = 0; i < 5; i++) {

                            //5个任务每隔300秒放入一个

                            taskQueue.put(new Task(taskLevel, i));

                            TimeUnit.MILLISECONDS.sleep(300);

                        }

                    } catch (Exception e) {

 

                    }

                }

            });

        }

        

        TimeUnit.SECONDS.sleep(10);

        es.shutdownNow();

    }

 

}

/*

* Task要实现Comparable接口

*/

class Task implements Comparable<Task> {

    private TASKLEVLE taskLevel = TASKLEVLE.MIDDLE;

 

    private final long id;

 

    public Task(TASKLEVLE taskLevel, long id) {

        super();

        this.taskLevel = taskLevel;

        this.id = id;

    }

 

    public int compareTo(Task o) {

        return o.taskLevel.compareTo(taskLevel);

    }

 

    @Override

    public String toString() {

        return "task-" + id + "level-" + taskLevel;

    }

 

}

 

enum TASKLEVLE {

    LOW, MIDDLE, HIGH, SUPER

};

并发

优先权
  • 线程的"优先权"priority)能告诉调度程序其重要性如何。尽管处理器处理现有线程集的顺序是不确定的,但是如果有许多线程被阻塞并在等待运行,那么调度程序将倾向于让优先权最高的线程先执行。然而,这并不是意味着优先权较低的线程将得不到执行(也就是说,优先权不会导致死锁)。优先级较低的线程仅仅是执行的频率较低。
  • 对于已存在的线程,你可以用getPriority( )方法得到其优先权,也可以在任何时候使用setPriority( )方法更改其优先权
  • 尽管JDK10个优先级别,但它与多数操作系统都不能映射得很好。比如,Windows 20007个优先级且不是固定的,所以这种映射关系也是不确定的(尽管SunSolaris231个优先级)。唯一可移植的策略是当你调整优先级的时候,只使用MAX_PRIORITYNORM_PRIORITY,和MIN_PRIORITY三种级别。
来自JDK API

线程 是程序中的执行线程。Java 虚拟机允许应用程序并发地运行多个执行线程。

每个线程都有一个优先级,高优先级线程的执行优先于低优先级线程。每个线程都可以或不可以标记为一个守护程序。当某个线程中运行的代码创建一个新 Thread 对象时,该新线程的初始优先级被设定为创建线程的优先级,并且当且仅当创建线程是守护线程时,新线程才是守护程序。

当 Java 虚拟机启动时,通常都会有单个非守护线程(它通常会调用某个指定类的 main 方法)。Java 虚拟机会继续执行线程,直到下列任一情况出现时为止: 调用了 Runtime 类的 exit 方法,并且安全管理器允许退出操作发生。

非守护线程的所有线程都已停止运行,无论是通过从对 run 方法的调用中返回,还是通过抛出一个传播到 run 方法之外的异常。

守护线程

public final void setDaemon(boolean on)

将该线程标记为守护线程或用户线程。当正在运行的线程都是守护线程时,Java 虚拟机退出。

该方法必须在启动线程前调用。

Thread和Runnable
  • 你的类也许已经继承了其它的类,在这种情况下,就不可能同时继承ThreadJava并不支持多重继承)。这时,你可以使用"实现Runnable接口"的方法作为替代。
  • Thread也是从Runnable接口实现而来的。
  • Runnable类型的类只需一个run( )方法,但是如果你想要对这个Thread对象做点别的事情(比如在toString( )里调用getName( )),那么你就必须通过调用Thread.currentThread( )方法明确得到对此线程的引用。

posted on 2017-06-20 23:01  张小贱1987  阅读(600)  评论(0编辑  收藏  举报

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