java多线程

Java多线程笔记

1 定义

程序
- 程序是指令和数据的有序集合，没有任何运行的含义，是个静态的概念
进程（process）
- 进程是执行程序的一次执行过程，是动态的概念
- 进程是系统资源分配的单位
线程（thread）
- 一个进程中可以包含若干个线程
- 线程是CPU调度和执行的单位
多线程
- 真正的多线程是指有多个CPU，即多核
- 在一个CPU的情况下，在一个时间点，CPU只能执行一个代码，只是切换很快，有同时进行的错觉

2 线程创建

2.1 继承Thread类

自定义线程类继承Thread类
重写run()方法，编写线程执行体
创建线程对象，调用start()方法启动线程

public class TestThread1 extends Thread {
    //run方法线程体
    @Override
    public void run() {
        for (int i = 0; i < 20; i++) {
            System.out.println("the number in sub thread is " + i);
        }
    }
    
    //main线程，主线程
    public static void main(String[] args) {
        //创建一个子线程
        TestThread1 testThread = new TestThread1();
        //调用start方法开启线程
        testThread.start();
        
        for (int i = 0; i < 20; i++) {
            System.out.println("the number in main thread is " + i);
        }
    }
}

运行之后，我们能看到两个线程的输出是交错的，说明两个线程在同时运行。如果不明显，可以将循环次数调大

如果在testThread直接调用run方法，那么不会出现多线程的情况，输出结果是由上至下依次输出的。调用start方法才会产生多线程的情况

另外，线程开启后不一定立即执行，由CPU调度执行

实现多线程同步下载图片

使用commons.io包

Copypublic class TestThread2 extends Thread {
    
    private String url;		//网络文件地址
    private String fileName;	//保存的文件
    
    public TestThread2(String url, String fileName) {
        this.url = url;
        this.fileName = fileName;
    }
    
    @Override
    public void run() {
        WebDownloader webDownloader = new WebDownloader();
        webDownloader.download(url, fileName);
        System.out.println("over, and the file is " + fileName);
    }
    
    public static void main(String[] args) {
        TestThread2 t1 = new TestThread2("https://img1.doubanio.com/view/photo/raw/public/p1413828179.jpg", "1.jpg");
        TestThread2 t2 = new TestThread2("https://img9.doubanio.com/view/photo/raw/public/p2162992026.jpg", "2.jpg");
        TestThread2 t3 = new TestThread2("https://img1.doubanio.com/view/photo/raw/public/p1413828209.jpg", "3.jpg");
        t1.start();
        t2.start();
        t3.start();
    }
}

//下载器
class WebDownloader {
    //下载方法
    public void download(String url, String fileName) {
        try {
            FileUtils.copyURLToFile(new URL(url), new File(fileName));
        } catch (IOException e) {
            e.printStackTrace();
        }
    }
}

2.2 实现Runnable接口

定义线程类实现Runnable接口
实现run()方法，编写线程执行体
创建线程对象，调用start()方法启动线程

Copypublic class TestThread3 implements Runnable {
    //run方法线程体
    @Override
    public void run() {
        for (int i = 0; i < 20; i++) {
            System.out.println("the number in sub thread is " + i);
        }
    }
    
    //main线程，主线程
    public static void main(String[] args) {
        //创建Runnable接口的实现类对象
        TestThread3 testThread = new TestThread3();
        //创建线程对象，通过线程对象开启线程
        //Thread thread = new Thread(testThread);
        //thread.start();
        new Thread(testThread).start();
        
        for (int i = 0; i < 20; i++) {
            System.out.println("the number in main thread is " + i);
        }
    }
}

建议使用实现Runnable接口的做法，避免单继承的局限性，方便一个对象被多个线程使用

多个线程操作同一个对象

Copypublic class TestThread4 implements Runnable {
    
    private int ticketNum = 10;
    
    @Override
    public void run() {
        while (true) {
            if (ticketNum <= 0)
                break;
            //模拟延时
            try {
                Thread.sleep(200);
            } catch (InterruptedException e) {
                e.printStackTrace();
            }
            System.out.println(Thread.currentThread().getName() + "拿到了第" + ticketNum-- + "张票");
        }
    }
    
    public static void main(String[] args) {
        TestThread4 thread = new TestThread4();
        new Thread(thread, "test1").start();
        new Thread(thread, "test2").start();
        new Thread(thread, "test3").start();
    }
}

输出结果显示会产生数据紊乱，不符合常理的问题，多个线程操作同一个资源的情况下，线程不安全

其实，new Thread(thread).start() 这种方式是静态代理实现的，Thread类也是实现了Runnable接口，把实现了Runnable接口的thread对象作为参数传进去，就是静态代理的过程

此外，对于thread对象可以用匿名内部类代替，比如

Copynew Thread(testThread).start();

其中，testThread实现了Runnable接口，可以写为

Copynew Thread(new Runnable {
    @Override
    public void run() {
        for (int i = 0; i < 20; i++) {
            System.out.println("the number in sub thread is " + i);
        }
    }
}).start();

还可以用lambda表达式进一步简化，这要求jdk的版本为8以上

Copynew Thread(() -> {
    for (int i = 0; i < 20; i++) {
        System.out.println("the number in sub thread is " + i);
    }
}).start();

2.3 实现Callable接口

实现Callable接口，需要返回值类型
重写call方法，需要抛出异常
创建目标对象

创建执行服务

CopyExecutorService service = Executors.newFixedThreadPool(1);

提交执行

CopyFuture<Boolean> result = service.submit(thread);

获取结果
```
Copyboolean r = result.get();
```
关闭服务
```
Copyservice.shutdownNow();
```

Copypublic class TestCallable implements Callable<Boolean> {
    
    @Override
    public Boolean call() {
        for (int i = 0; i < 20; i++) {
            System.out.println("the number in sub thread is " + i);
        }
        return true;
    }
    
    public static void main(String[] args) throws ExecutionException, InterruptedException {
        TestCallable thread = new TestCallable();
        ExecutorService service = Executors.newFixedThreadPool(1);
        Future<Boolean> result = service.submit(thread);
        boolean r = result.get();
        service.shutdownNow();
        
        for (int i = 0; i < 20; i++) {
            System.out.println("the number in main thread is " + i);
        }
    }
}

3 线程状态

3.1 停止线程

不推荐使用jdk提供的stop()，destroy()方法
推荐线程自己停止
- 使用一个标志位进行终止变量，当flag=false时，则终止线程运行

Copypublic class TestStop implements Runnable {
    
    //设置一个标志位
    private boolean flag = true;
    
    @Override
    public void run() {
        int i = 0;
        while (flag) {
            System.out.println("thread is running... i is " + i++);
        }
    }
    
    //设置公开的方法停止线程
    public void stop() {
        flag = false;
    }
    
    public static void main(String[] args) {
        TestStop testStop = new TestStop();
        new Thread(testStop).start();
        for (int i = 0; i < 100; i++) {
            System.out.println("main thread..." + i);
            if (i == 90) {
                testStop.stop();
                System.out.println("thread stop");w
            }
        }
    }
}

3.2 线程休眠

sleep方法指定当前线程阻塞的毫秒数
sleep存在异常InterruptedException
sleep时间达到后线程进入就绪状态
每个对象都有一个锁，sleep不会释放锁

Copypublic class TestSleep implements Runnable {
        
    private int ticketNum = 10;
    
    @Override
    public void run() {
        while (true) {
            if (ticketNum <= 0)
                break;
            //模拟延时
            try {
                Thread.sleep(100);
            } catch (InterruptedException e) {
                e.printStackTrace();
            }
            System.out.println(Thread.currentThread().getName() + "拿到了第" + ticketNum-- + "张票");
        }
    }
    
    public static void main(String[] args) {
        TestThread4 thread = new TestThread4();
        new Thread(thread, "test1").start();
        new Thread(thread, "test2").start();
        new Thread(thread, "test3").start();
    }
}

模拟网络延时可以放大问题的发生性

Copy//模拟倒计时
public class CountDown {
    public static void countDown() throws InterruptedException {
        int num = 10;
        while (true) {
            Thread.sleep(1000);
            System.out.println(num--);
            if (num <= 0) {
                break;
            }
        }
    }
    
    pubilc static void main(String[] args) {
        try {
            countDown();
        } catch {
            e.printStackTrace();
        }
    }
}

3.3 线程yield

yield让当前正在执行的线程暂停，但不阻塞
将线程从运行状态转为就绪状态
让CPU重新调度，yield不一定成功

Copypublic class TestYield {
    public static void main(String[] args) {
        new Thread(new MyThread(), "a").start();
        new Thread(new MyThread(), "b").start();
    }
}

class MyThread implements Runnable {
    @Override
    public void run() {
        System.out.println(Thread.currentThread().getName() + "线程开始执行");
        Thread.yield();
        System.out.println("线程停止执行");
    }
}

3.4 线程强制执行

join合并线程，其他线程阻塞，待此线程执行完成后，再执行其他线程，相当于插队

Copypublic class TestJoin implements Runnable {
    
    @Override
    public void run() {
        for (int i = 0; i < 100; i++) {
            System.out.println("sub thread is running..." + i);
        }
    }
    
    public static void main(String[] args) throws InterruptedException {
        Thread thread = new Thread(new TestJoin());
        thread.start();
        
        
        //main thread
        for (int i = 0; i < 100; i++) {
            if (i == 20) {
                thread.join();
            }
            System.out.println("main thread is running..." + i);
        }
    }
}

3.5 观测线程状态

Thread.State
- NEW：尚未启动的线程处于此状态
- RUNNABLE：在Java虚拟机中执行的线程处于此状态
- BLOCKED：被阻塞等待监视器锁定的线程处于此状态
- WAITING：正在等待另一个线程执行特定动作的线程处于此状态
- TIMED_WAITING：正在等待另一个线程执行动作达到指定等待时间的线程处于此状态
- TERMINATED：已退出的线程处于此状态

Copypublic class TestState {
    public static void main(String[] args) throws InterruptedException {
        Thread thread = new Thread(() -> {
            try {
                for (int i = 0; i < 5; i++) {
                	Thread.sleep(1000);
            	}
            } catch (InterruptedException e) {
                e.printStackTrace();
            }
            System.out.println("sub thread is running...");
        });
        
        //观察状态
        Thread.State state = thread.getState();
        System.out.println(state);
        
        //观察启动后状态
        thread.start();
        state = thread.getState();
        System.out.println(state);
        
        while (state != Thread.State.TERMINATED) {
            Thread.sleep(100);
            state = thread.getState();
            System.out.println(stated);
        }
    }
}

3.6 线程优先级

线程调度器监控程序中启动后进入就绪状态的所有线程，按照优先级决定应该调度哪个线程来执行

线程的优先级用数字表示，范围从1到10，数字越大代表优先级越高，通过getPriority()和setPriority(int)来获取和改变优先级

优先级的设定一定要在线程启动之前

此外，线程优先级低只是意味着获得调度的概率低，并不是优先级低就不会被调用，得看CPU的调度

Copypublic class TestPriority {
    public static void main(String[] args) {
        //主线程优先级
        System.out.println(Thread.currentThread().getName() + "-->" + Thread.currentThread().getPriority());
        
        Thread t1 = new Thread(new MyThread());
        Thread t2 = new Thread(new MyThread());
        Thread t3 = new Thread(new MyThread());
        Thread t4 = new Thread(new MyThread());
        Thread t5 = new Thread(new MyThread());
        Thread t6 = new Thread(new MyThread());
        
        //设置优先级
        t1.start();
        t2.setPriority(1);
        t2.start();
        t3.setPriority(4);
        t3.start();
        t4.setPriority(Thread.MAX_PRIORITY);
        t4.start();
        t5.setPriority(-1);	//报错
        t5.start();
        t6.setPriority(11);	//报错
        t6.start();
    }
}

class MyThread implements Runnable {
    @Override
    public void run() {
        System.out.println(Thread.currentThread().getName() + "-->" + Thread.currentThread().getPriority());
    }
}

3.7 守护（daemon）线程

线程分为用户线程和守护线程，守护线程如后台记录操作日志，监控内存，垃圾回收等

虚拟机必须确保用户线程执行完毕，不用等待守护线程执行完毕

Copypublic class TestDaemon {
    public static void main(String[] args) {
        Thread thread = new Thread(new MonitorThread);
        //默认为false表示用户线程
        thread.setDaemon(true);
        thread.start();
        
        new Thread(new MyThread()).start();
    }
}

class MyThread implements Runnable {
    @Override
    public void run() {
        for (int i = 0; i < 100; i++) {
            System.out.println("sub thread is running...");
        }
        System.out.println("over");
    }
}

class MonitorThread implements Runnable {
    @Override
    public void run() {
        while (true) {
            System.out.println("monitor is running...");
        }
    }
}

结果显示monitor线程会在mythread线程执行完毕后再执行一段时间，最终停止

4 线程同步

线程同步发生在多个线程操作同一个资源，也就是并发的情况下

线程同步其实是一种等待机制，多个需要同时访问对象的线程进入此对象的等待池形成队列，等待前面线程使用完毕。

4.1 锁机制

为了保证数据在方法中被访问时的正确性，在访问时加入锁机制synchronized，当一个线程获得对象的排它锁，独占资源，其他线程必须等待，使用后释放锁即可

但也存在问题

一个线程持有锁会导致其他所有需要此锁的线程挂起
在多线程竞争下，加锁释放锁会导致比较多的上下文切换和调度延时，引发性能问题
如果一个优先级高的线程等待一个优先级低的线程释放锁，会导致优先级倒置，引发性能问题

4.2 线程不安全案例

Copypublic class UnsafeBuyTicket {
    public static void main(String[] args) {
        new Thread(new BuyTicket(), "test1").start();
        new Thread(new BuyTicket(), "test2").start();
        new Thread(new BuyTicket(), "test3").start();
    }
}

class BuyTicket implements Runnable {
    
    private int ticketNum = 10;
    private boolean flag = true;
    
    private void buy() throws InterruptedException {
        if (ticketNum <= 0) {
            flag = false;
            return;
        }
        Thread.sleep(100);
        System.out.println(Thread.currentThread().getName() + "get " + ticketNum--);
    }
    
    @Override
    public void run() {
        while (flag) {
            try {
                buy();
            } catch (InterruptedException e) {
                e.printStackTrace();
            }
        }
    }
}
Copypublic class UnsafeBank {
    public static void main(String[] args) {
        Account account = new Account("testAccount", 100);
        Withdraw withdraw1 = new Withdraw(account, 50, "test1");
        Withdraw withdraw2 = new Withdraw(account, 100, "test2");
        
        withdraw1.start();
        withdraw2.start();
    }
}

class Account {
    String name;
    int money;
    
    public Account(String name, int money) {
        this.name = name;
        this.money = money;
    }
}

class Withdraw extends Thread {
    Account account;
    int withdrawMoney;
    int moneyInHand;
    
    public Withdraw(Account account, int withdrawMoney, String name) {
        super(name);
        this.account = account;
        this.withdrawMoney = withdrawMoney;
    }
    
    @Override
    public void run() {
        if (account.money < withdrawMoney) {
            System.out.println(Thread.currentThread().getName() + "no enough money");
            return;
        }
        
        //sleep增加问题发生的可能性
        try {
            Thread.sleep(1000);
        } catch (InterruptedException e) {
            e.printStackTrace();
        }
        
        account.money -= withdrawMoney;
        moneyInHand += withdrawMoney;
        System.out.println(account.name + "余额为：" + account.money);
        System.out.println(Thread.currentThread().getName() + "手中的钱：" + moneyInHand);
    }
}
Copypublic class UnsafeList {
    public static void main(String[] args) {
        List<String> list = new ArrayList<>();
        for (int i = 0; i < 10000; i++) {
            new Thread(() -> {
                list.add(Thread.currnetThread().getName());
            }).start();
        }
        
        try {
            Thread.sleep(3000);
        } catch (InterruptedException e) {
            e.printStackTrace();
        }
        System.out.println(list.size());
    }
}

4.2 同步方法

通过private关键字保证数据对象只能被方法访问，使用synchronized关键字

synchronized方法
- public synchronized void method(int args) {}
- synchronized方法控制对象的访问，每个对象对应一把锁，每个synchronized方法都必须获得调用该方法的对象的锁才能执行，否则线程阻塞。方法一旦执行，就独占该锁，直到方法返回才释放锁
- 将一个大的方法声明为synchronized会影响效率
  - 方法内需要修改的资源才需要锁，锁的范围太大会影响执行速度
synchronized块
- synchronized(Obj) {}
- Obj为同步监视器
  - 可以是任何对象，但推荐使用共享资源作为同步监视器
  - 同步方法中无需指定同步监视器，因为同步方法的同步监视器就是this
- 执行过程和synchronized方法一样

使用同步方法或者同步代码块修改不安全案例

Copypublic class SafeBuyTicket {
    public static void main(String[] args) {
        new Thread(new BuyTicket(), "test1").start();
        new Thread(new BuyTicket(), "test2").start();
        new Thread(new BuyTicket(), "test3").start();
    }
}

class BuyTicket implements Runnable {
    
    private int ticketNum = 10;
    private boolean flag = true;
    
    //synchronized同步方法，锁的是this
    private synchronized void buy() throws InterruptedException {
        if (ticketNum <= 0) {
            flag = false;
            return;
        }
        Thread.sleep(100);
        System.out.println(Thread.currentThread().getName() + "get " + ticketNum--);
    }
    
    @Override
    public void run() {
        while (flag) {
            try {
                buy();
            } catch (InterruptedException e) {
                e.printStackTrace();
            }
        }
    }
}
Copypublic class SafeBank {
    public static void main(String[] args) {
        Account account = new Account("testAccount", 100);
        Withdraw withdraw1 = new Withdraw(account, 50, "test1");
        Withdraw withdraw2 = new Withdraw(account, 100, "test2");
        
        withdraw1.start();
        withdraw2.start();
    }
}

class Account {
    String name;
    int money;
    
    public Account(String name, int money) {
        this.name = name;
        this.money = money;
    }
}

class Withdraw extends Thread {
    Account account;
    int withdrawMoney;
    int moneyInHand;
    
    public Withdraw(Account account, int withdrawMoney, String name) {
        super(name);
        this.account = account;
        this.withdrawMoney = withdrawMoney;
    }
    
    @Override
    public void run() {
        //锁的是共享资源account
        //如果synchronized加载run方法上，那么锁的是this，也就是Withdraw对象，但这个不是共享资源
        synchronized (account) {
            if (account.money < withdrawMoney) {
                System.out.println(Thread.currentThread().getName() + "no enough money");
                return;
            }
        
            //sleep增加问题发生的可能性
            try {
                Thread.sleep(1000);
            } catch (InterruptedException e) {
                e.printStackTrace();
            }
        
            account.money -= withdrawMoney;
            moneyInHand += withdrawMoney;
            System.out.println(account.name + "余额为：" + account.money);
            System.out.println(Thread.currentThread().getName() + "手中的钱：" + moneyInHand);
        }
    }
}
Copypublic class UnsafeList {
    public static void main(String[] args) {
        List<String> list = new ArrayList<>();
        for (int i = 0; i < 10000; i++) {
            new Thread(() -> {
                synchronized (list) {
                    list.add(Thread.currnetThread().getName());
                }
            }).start();
        }
        
        try {
            Thread.sleep(3000);
        } catch (InterruptedException e) {
            e.printStackTrace();
        }
        System.out.println(list.size());
    }
}

对于第三个案例，我们还可以使用JUC安全类型内的集合OnWriteArrayList

Copypublic class TestJUC {
    public static void main(String[] args) {
        CopyOnWriteArrayList<String> list = new CopyOnWriteArrayList<>();
        for (int i = 0; i < 10000; i++) {
            new Thread(() -> {
                list.add(Thread.currentThread().getName());
            }).start();
        }
        
        try {
            Thread.sleep(3000);
        } catch (InterruptedException e) {
            e.printStackTrace();
        }
        System.out.println(list.size());
    }
}

4.3 死锁

多个线程各自占有一些共享资源，并且互相等待其他线程占有的资源才能运行，导致两个或多个线程都在等待对方释放资源，都停止执行的情形

一个同步块同时拥有两个以上对象的锁时，就有可能会发生死锁问题

Copypublic class TestDeadlock {
    public static void main(String[] args) {
        MyThread t1 = new MyThread("test1", 0);
        MyThread t2 = new MyThread("test2", 1);
        
        t1.start();
        t2.start();
    }
}

class A {
    
}

class B {
    
}

class MyThread extends Thread {
    //用static来保证资源只有一份
    static A a = new A();
    static B b = new B();
    
    String name;
    int choice;
    
    public MyThread(String name, int choice) {
        this.name = name;
        this.choice = choice;
    }
    
    @Override
    public void run() {
        try {
            do();
        } catch (InterruptedException e) {
            e.printStackTrace();
        }
    }
    
    private void do() throws InterruptedException {
        if (choice == 0) {
            synchronized (a) {
                System.out.println(name + "get a");
                Thread.sleep(1000);
                synchronized (b) {
                    System.out.println(name + "get b");
                }
            }
        } else {
            synchronized (b) {
                System.out.println(name + "get b");
                Thread.sleep(2000);
                synchronized (a) {
                    System.out.println(name + "get a");
                }
            }
        }
    }
}

程序会产生死锁，因为一个同步块同时拥有了两个锁a和b，而且a和b的资源只有一份（用static修饰）。如果把程序修改一下，就不会死锁

Copyprivate void do() throws InterruptedException {
        if (choice == 0) {
            synchronized (a) {
                System.out.println(name + "get a");
                Thread.sleep(1000);
            }
            synchronized (b) {
                System.out.println(name + "get b");
            }
        } else {
            synchronized (b) {
                System.out.println(name + "get b");
                Thread.sleep(2000);
            }
            synchronized (a) {
                System.out.println(name + "get a");
            }
        }
    }

死锁产生的必要条件

互斥条件：一个资源每次只能被一个进程使用
请求与保持条件：一个进程因请求资源而阻塞时，对已获得的资源保持不妨
不剥夺条件：进程已获得的资源，在未使用完之前，不能强行剥夺
循环等待条件：若干进程之间形成一种头尾相接的循环等待资源关系

只要想办法打破其中任一条件，就可以避免死锁的发生

4.4 Lock

synchronized关键字是隐式定义锁来实现同步，而Lock对象显示定义同步锁实现同步

Lock接口位于JUC包，是多个线程对共享资源进行访问的工具，作用和synchronized一样

ReentrantLock 可重入锁
- 实现了Lock
- 与synchronized有相同的并发性和内存语义
- 可以显示加锁和释放锁
- 可以使用tryLock()方法尝试获得锁，如果无法获得，程序不会无限等待下去，而是做一些额外处理

Copypublic class TestLock {
    public static void main(String[] args) {
        MyThread thread = new MyThread();
        
        new Thread(thread).start();
        new Thread(thread).start();
        new Thread(thread).start();
    }
}

class MyThread implements Runnable {
    
    int ticketNum = 10;
    
    private final ReentrantLock lock = new ReentrantLock();
    
    @Override
    public void run() {
        while (true) {
            //加锁
            lock.lock();
            try {
                if (ticketNum <= 0) {
                    break;
                }
            
                try {
                    Thread.sleep(1000);
                } catch (InterruptedException e) {
                    e.printStackTrace();
                }
                System.out.println(ticketNum--);
            } finally {
                //释放锁
                lock.unlock();
            }
            
        }
    }
}

Lock和synchronized对比

Lock是显示锁，需要手动开启和关闭，synchronized是隐式锁，出了作用域自动释放
Lock只有代码块锁，synchronized有代码块锁和方法锁
使用Lock锁，JVM将花费较少的时间来调度线程，性能更好
优先使用顺序：Lock > 同步代码块 > 同步方法

5 线程协作

5.1 线程通信

Java提供了几个方法解决线程通信的问题

wait()：表示线程一直等待，直到其他线程通知，与sleep不同，会释放锁
wait(long timeout)：指定等待的毫秒数
notify()：唤醒一个处于等待状态的线程
notifyAll()：唤醒同一个对象上所有调用wait()方法的线程，优先级高的优先调度

这几个方法都是Object类的方法，都只能在同步方法或同步代码块中使用，否则会抛出异常

5.2 生产者消费者问题

解决方法一：管程法

生产者将生产好的数据放入缓冲区，消费者从缓冲区获取数据

Copypublic class TestPC {
    public static void main(String[] args) {
        SynBuffer buffer = new SynBuffer();
        
        new Producer(buffer).start();
        new Consumer(buffer).start();
    }
}

class Producer extends Thread {
    SynBuffer buffer;
    
    public Producer(SynBuffer buffer) {
        this.buffer = buffer;
    }
    
    @Override
    public void run() {
        for (int i = 0; i < 100; i++) {
            buffer.push(new Product(i));
            System.out.println("生产了 " + i + "个产品");
        }
    }
}

class Consumer extends Thread {
    SynBuffer buffer;
    
    public Consumer(SynBuffer buffer) {
        this.buffer = buffer;
    }
    
    @Override
    public void run() {
        for (int i = 0; i < 100; i++) {
            System.out.println("消费了 " + buffer.pop().id + "个产品");
        }
    }
}

class Product {
    int id;
    
    public Product(int id) {
        this.id = id;
    }
}

class SynBuffer {
    Product[] products = new Product[10];
    int count = 0;
    
    public synchronized void push(Product product) {
        if (count == products.length) {
            try {
                this.wait();
            } catch (InterruptedException e) {
                e.printStackTrace();
            }
        }
        products[count] = product;
        count++;
        this.notifyAll();
    }
    
    public synchronized Product pop() {
        if (count == 0) {
            try {
                this.wait();
            } catch (InterruptedException e) {
                e.printStackTrace();
            }
        }
        
        count--;
        Product product = products[count];
        this.notifyAll();
        return product;
    }
}

解决方法二：信号灯法

Copypublic class TestPC {
    public static void main(String[] args) {
        Show show = new Show();
        
        new Actor(show).start();
        new Audience(show).start();
    }
}

class Actor extends Thread {
    Show show;
    
    public Actor(Show show) {
        this.show = show;
    }
    
    @Override
    public void run() {
        for (int i = 0; i < 20; i++) {
            if (i % 2 == 0) {
                show.play("show1");
            } else {
                show.play("show2");
            }
        }
    }
}

class Audience extends Thread {
    Show show;
    
    public Audience(Show show) {
        this.show = show;
    }
    
    @Override
    public void run() {
        for (int i = 0; i < 20; i++) {
            show.watch();
        }
    }
}

class Show {
    String item;
    boolean flag = true;
    
    public synchronized void play(String item) {
        if (!flag) {
            try {
                this.wait();
            } catch (InterruptedException e) {
                e.printStackTrace();
            }
        }
        System.out.println("演员表演了" + item);
        
        this.notifyAll();
        this.item = item;
        this.flag = !this.flag;
    }
    
    public synchronized void watch() {
        if (flag) {
            try {
                this.wait();
            } catch (IterruptedException e) {
                e.printStackTrace();
            }
        }
        System.out.println("观看了" + item);
        
        this.notifyAll();
        this.flag = !this.flag;
    }
}

6 线程池

经常创建和销毁使用量特别大的资源对性能影响很大，一般要使用线程池，提前创建好多个线程，放入线程池中，使用时直接获取，使用完放回池中，避免频繁创建销毁，可以重复利用

6.1 使用线程池

线程池相关的API：ExecutorService和Executors

ExecutorService：真正的线程池接口，常见子类ThreadPoolExecutor
- void execute(Runnable command)：执行任务/命令。无返回值，一般用来执行Runnable
- Future submit(Callable tast)：执行任务，有返回值，一般用来执行Callable
- void shutdown()：关闭连接池
Executors：工具类，线程池的工厂类，用于创建并返回不同类型的线程池

Copypublic class TestPool {
    public static void main(String[] args) {
        //创建服务，创建线程池
        ExecutorService service = Executors.newFixedThreadPool(10);
        
        //执行
        service.execute(new MyThread());
        service.execute(new MyThread());
        service.execute(new MyThread());
        service.execute(new MyThread());
        
        //关闭连接
        service.shutdown();
}

class MyThread implements Runnbale {
    @Override
    public void run() {
        System.out.println(Thread.currentThread().getName() + i);
    }
}

posted @ 2021-02-04 16:31 初出茅庐小菜鸡阅读(68) 评论(0) 编辑收藏举报

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