ThreadLocal 和 Countdaowlatch

ThreadLocal 和 Countdaowlatch

ThreadLocal

ThreadLocal提供了线程内存储变量的能力，这些变量不同之处在于每一个线程读取的变量是对应的互相独立的。通过get和set方法就可以得到当前线程对应的值。

    public void set(T var1) {
        Thread var2 = Thread.currentThread();
        ThreadLocal.ThreadLocalMap var3 = this.getMap(var2);
        if (var3 != null) {
            var3.set(this, var1);
        } else {
            this.createMap(var2, var1);
        }
    }

 public T get() {
        Thread var1 = Thread.currentThread();
        ThreadLocal.ThreadLocalMap var2 = this.getMap(var1);
        if (var2 != null) {
            ThreadLocal.ThreadLocalMap.Entry var3 = var2.getEntry(this);
            if (var3 != null) {
                Object var4 = var3.value;
                return var4;
            }
        }
        return this.setInitialValue();
    }

public void remove() {
        ThreadLocal.ThreadLocalMap var1 = this.getMap(Thread.currentThread());
        if (var1 != null) {
            var1.remove(this);
        }

    }

ThreadLocal往往用来实现变量在线程之间的隔离

创建一个ThreadLocal对象，指定存储的数据为integer，初始值为100

package Thread;

public class ThreadLocalTest {

    static ThreadLocal<Integer> threadLocal = new ThreadLocal<Integer>(){
        @Override
        protected Integer initialValue() {
            return 100;
        }
    };

    static class ADDThread extends Thread{
        @Override
        public void run() {
            Integer integer = threadLocal.get();
            System.out.println(Thread.currentThread().getName()+"----初始值----"+integer);

            integer +=100;
            System.out.println(Thread.currentThread().getName()+"----线程 操作int+100 ----");

            threadLocal.set(integer);
            System.out.println(Thread.currentThread().getName()+"----调用set存放新的integer到threadlocal，保存修改");

            System.out.println(Thread.currentThread().getName()+"----当前线程的副本值"+threadLocal.get());

        }
    }

    static class SubThread extends Thread{
        @Override
        public void run() {
            Integer integer = threadLocal.get();
            System.out.println(Thread.currentThread().getName()+"----初始值----"+integer);
        }
    }
    public static void main(String[] args) {
        new ADDThread().start();
        new SubThread().start();
    }
}
================================================================
Thread-0----初始值----100
Thread-0----线程 操作int+100 ----
Thread-0----调用set存放新的integer到threadlocal，保存修改
Thread-0----当前线程的副本值200
Thread-1----初始值----100

ThreadLocal原理：

ThreadLocal内部有一个内部类ThreadLocalMap；ThreadLocalMap的内部有一个Entry内部类，ThreadLocalMap内部类维护了一个Entry数组，Entry（类似Map.Entry）key是ThreadLocal，value是Object；每一个线程都有一个ThreadLocalMap的实例，这个ThreadLocalMap内部又有一个Entry数组，将threadLocal作为key获取每个线程中独立的副本，因为threadLocal可以有多个，所以Entry以数组的形式存放；

static class Entry extends WeakReference<ThreadLocal<?>> {
            /** The value associated with this ThreadLocal. */
            Object value;

            Entry(ThreadLocal<?> k, Object v) {
                super(k);
                value = v;
            }
        }

Countdaowlatch

• countDownLatch是在java1.5被引入，跟它一起被引入的工具类还有CyclicBarrier、Semaphore、concurrentHashMap和BlockingQueue。
• 存在于java.util.cucurrent包下。

countDownLatch这个类使一个线程等待其他线程各自执行完毕后再执行。

是通过一个计数器来实现的，计数器的初始值是线程的数量。每当一个线程执行完毕后，计数器的值就-1，当计数器的值为0时，表示所有线程都执行完毕，然后在闭锁上等待的线程就可以恢复工作了。

//调用await()方法的线程会被挂起，它会等待直到count值为0才继续执行
public void await() throws InterruptedException { };   
//和await()类似，只不过等待一定的时间后count值还没变为0的话就会继续执行
public boolean await(long timeout, TimeUnit unit) throws InterruptedException { };  
//将count值减1
public void countDown() { };  

实例：

package Thread;

import java.util.concurrent.CountDownLatch;
import java.util.concurrent.ExecutorService;
import java.util.concurrent.Executors;

public class CountDownLatchTest {
    public static void main(String[] args) {
        final CountDownLatch latch = new CountDownLatch(2);
        System.out.println("Main Thread start run....");
        // first thread run
        ExecutorService es1 = Executors.newSingleThreadExecutor();
        es1.execute(new Runnable() {
            @Override
            public void run() {
                try {
                    Thread.sleep(3000);
                    System.out.println("fist thread :" + Thread.currentThread().getName() + "run ..");
                } catch (InterruptedException e) {
                    e.printStackTrace();
                }
                latch.countDown();
            }
        });
        es1.shutdown();
        // second thread run
        ExecutorService es2 = Executors.newSingleThreadExecutor();
        es2.execute(new Runnable() {
            @Override
            public void run() {
                try {
                    Thread.sleep(3000);
                } catch (InterruptedException e) {
                    e.printStackTrace();
                }
                System.out.println("second Thread : " + Thread.currentThread().getName() + " run ");
                latch.countDown();
            }
        });
        es2.shutdown();
        System.out.println("wait two thread finished");
        try {
            latch.await();
        } catch (InterruptedException e) {
            e.printStackTrace();
        }
        System.out.println("main thread run !!");
    }
}
===============================================
Main Thread start run....
wait two thread finished
fist thread :pool-1-thread-1run ..
second Thread : pool-2-thread-1 run 
main thread run !!

CountDownLatch的用法
CountDownLatch典型用法1：某一线程在开始运行前等待n个线程执行完毕。将CountDownLatch的计数器初始化为n new CountDownLatch(n) ，每当一个任务线程执行完毕，就将计数器减1 countdownlatch.countDown()，当计数器的值变为0时，在CountDownLatch上 await() 的线程就会被唤醒。一个典型应用场景就是启动一个服务时，主线程需要等待多个组件加载完毕，之后再继续执行。

CountDownLatch典型用法2：实现多个线程开始执行任务的最大并行性。注意是并行性，不是并发，强调的是多个线程在某一时刻同时开始执行。类似于赛跑，将多个线程放到起点，等待发令枪响，然后同时开跑。做法是初始化一个共享的CountDownLatch(1)，将其计数器初始化为1，多个线程在开始执行任务前首先 coundownlatch.await()，当主线程调用 countDown() 时，计数器变为0，多个线程同时被唤醒。