Java多线程技术三：锁的使用——使用ReentrantLock类-2

接上篇：Java多线程技术三：锁的使用——使用ReentrantLock类-1

9 公平锁与非公平锁

? ? ? ? 公平锁采用先到先得的策略，每次获取锁之前都会检查队列里面有没有排队等待的线程，如果没有才会尝试获取锁，如果有就将当前线程追加到队列中。

? ? ? ? 非公平锁采用“有机会插队”的策略，一个线程获取锁之前，要先去尝试获取锁，而不是在队列中等待，如果成功获取锁，说明线程虽然是后启动的，但先获得了锁，这就是“插队”效果，如果获取锁没有成功，那么将自己追加到队列中进行等待。

public class MyService {
    public Lock lock;
    
    public MyService(boolean fair){
        lock = new ReentrantLock();
    }
    
    public void testMethod(){
        try {
            lock.lock();
            System.out.println("testMethod 方法的线程名 = " + Thread.currentThread().getName());
            Thread.sleep(500);
        }catch (InterruptedException e){
            e.printStackTrace();
        }finally {
            lock.unlock();
        }
    }
    
}

public class MyThread extends Thread{
    private MyService service;

    public MyThread(MyService service) {
        this.service = service;
    }
    
    @Override
    public void run(){
        service.testMethod();
    }
}

? ? ? ? 创建公平锁测试的运行类

public class Run1 {
    public static void main(String[] args) throws InterruptedException {
        MyService service = new MyService(true);
        MyThread[] arr1 = new MyThread[10];
        MyThread[] arr2 = new MyThread[10];
        for (int i = 0; i < arr1.length; i++) {
            arr1[i] = new MyThread(service);
            arr1[i].setName("arr1+++"+(i+1));
        }
        for (int i = 0; i < arr1.length; i++) {
            arr1[i].start();
        }
        for (int i = 0; i < arr2.length; i++) {
            arr2[i] = new MyThread(service);
            arr2[i].setName("arr2---"+(i+1));
        }
        Thread.sleep(500);
        for (int i = 0; i < arr2.length; i++) {
            arr2[i].start();
        }
    }
}

? ? ? ? 打印的结果是arr1+++在前，arr2---在后，说明arr2---没有任何机会抢到锁，这就是公平锁的特点。

? ? ? ? 下面创建非公平锁测试的运行类

public class Run2 {
    public static void main(String[] args) throws InterruptedException {
        MyService service = new MyService(false);
        MyThread[] arr1 = new MyThread[20];
        MyThread[] arr2 = new MyThread[20];
        for (int i = 0; i < arr1.length; i++) {
            arr1[i] = new MyThread(service);
            arr1[i].setName("arr1+++"+(i+1));
        }
        for (int i = 0; i < arr1.length; i++) {
            arr1[i].start();
        }
        for (int i = 0; i < arr2.length; i++) {
            arr2[i] = new MyThread(service);
            arr2[i].setName("arr2---"+(i+1));
        }
        Thread.sleep(500);
        for (int i = 0; i < arr2.length; i++) {
            arr2[i].start();
        }
    }
}

?

在多次运行程序后，使用非公平锁又看见额能在第2次arr2---，说明启动的线程先抢到锁，这就是非公平锁的特点。

10 getHoldCount()方法

? ? ? ? public int?getHoldCount()方法的作用是查询“当前线程”保持此锁定的个数，也就是调用lock()方法的次数。

public class MyService {
    private ReentrantLock lock = new ReentrantLock(true);
    public void testMethod1(){
        System.out.println("A" + lock.getHoldCount());
        lock.lock();
        System.out.println("B" + lock.getHoldCount());
        testMethod2();
        System.out.println("F" + lock.getHoldCount());
        lock.unlock();
        System.out.println("G" + lock.getHoldCount());
    }

    public void testMethod2(){
        System.out.println("C" + lock.getHoldCount());
        lock.lock();
        System.out.println("D" + lock.getHoldCount());
        lock.unlock();
        System.out.println("E" + lock.getHoldCount());
    }
}

public class Run1 {
    public static void main(String[] args) {
        MyService service = new MyService();
        service.testMethod1();
    }
}

? ? ? ? ?执行lock方法进行锁重入，导致count计数加1的效果，执行unlock方法会使count呈减1的效果。

11 getQueueLength()方法

? ? ? ? public final int getQueueLength()方法 的作用是返回正等待获取此锁线程的估计数，比如有5个线程，1个线程长时间占有锁，那么在调用getQueueLength()方法后的返回值是4，说明有4个线程同时在等待锁的释放。

public class Service {
    public ReentrantLock lock = new ReentrantLock();
    public void serviceMethod1(){
        try {
            lock.lock();
            System.out.println("ThreadName = " +Thread.currentThread().getName());
            Thread.sleep(Integer.MAX_VALUE);
        }catch (InterruptedException e){
            e.printStackTrace();
        }finally {
            lock.unlock();
        }
    }
}

public class Run1 {
    public static void main(String[] args) throws InterruptedException {
        final Service service = new Service();
        Runnable runnable = new Runnable() {
            @Override
            public void run() {
                service.serviceMethod1();
            }
        };
        Thread[] threads = new Thread[10];
        for (int i = 0; i < 10; i++) {
            threads[i] = new Thread(runnable);
        }
        for (int i = 0; i < 10; i++) {
            threads[i].start();
        }
        Thread.sleep(2000);
        System.out.println("有线程数 ：" + service.lock.getQueueLength() + "在等待获取锁");

    }
}

12 getWaitQueueLength(Condition condition)方法

? ? ? ? public int getWaitQueueLength(Condition condition)方法的所用是返回等待与此锁相关的给定条件Condition的线程估计数，比如有5个线程，每个线程都执行了同一个Condition对象的await方法，则调用getWaitQueueLength方法的返回值是5。?

public class Service {
    private ReentrantLock lock = new ReentrantLock();
    private Condition newCondition = lock.newCondition();
    public void waitMethod(){
        try {
            lock.lock();
            newCondition.await();
        }catch (InterruptedException e){
            e.printStackTrace();
        }finally {
            lock.unlock();
        }
    }
    
    public void notifyMethod(){
        try {
            lock.lock();
            System.out.println("有"+lock.getWaitQueueLength(newCondition) + "个线程正在等待newCondition");
            newCondition.signal();
        }finally {
            lock.unlock();
        }
    }
    
}

public class Run1 {
    public static void main(String[] args) throws InterruptedException {
        final  Service service = new Service();
        Runnable runnable = new Runnable() {
            @Override
            public void run() {
                service.waitMethod();
            }
        };
        Thread[] threads = new Thread[10];
        for (int i = 0; i < 10; i++) {
            threads[i] = new Thread(runnable);
        }
        for (int i = 0; i < 10; i++) {
            threads[i].start();
        }
        Thread.sleep(2000);
        service.notifyMethod();
    }
}

13 hasQueueThread(Thread thread)方法

? ? ? ? public final boolean hasQueueThread(Thread thread)方法的作用是查询指定的线程是否正在等待获取此锁，也就是判断参数中的线程是否在等待队列中。

public class Service {
    public ReentrantLock lock = new ReentrantLock();
    public Condition condition = lock.newCondition();
    public void waitMethod(){
        try {
            lock.lock();
            Thread.sleep(Integer.MAX_VALUE);
        }catch (InterruptedException e){
            e.printStackTrace();
        }finally {
            lock.unlock();
        }
    }
}

public class Run1 {
    public static void main(String[] args) throws InterruptedException {
        final Service service = new Service();
        Runnable runnable = new Runnable() {
            @Override
            public void run() {
                service.waitMethod();
            }
        };
        Thread a = new Thread(runnable);
        a.start();
        Thread.sleep(500);
        Thread b = new Thread(runnable);
        b.start();
        Thread.sleep(500);
        System.out.println(service.lock.hasQueuedThread(a));
        System.out.println(service.lock.hasQueuedThread(b));
    }
}

14 isLocked()方法

? ? ? ? public boolean isLocked()方法的作用是查询此锁定是否由任意线程持有。

public class Service {
    private ReentrantLock lock = new ReentrantLock();
    public void serviceMethod(){
        try {
            System.out.println(lock.isLocked());
            lock.lock();
            System.out.println(lock.isLocked());
        }finally {
            lock.unlock();
        }
    }
}

public class Run1 {
    public static void main(String[] args) {
        final Service service = new Service();
        Runnable runnable = new Runnable() {
            @Override
            public void run() {
                service.serviceMethod();
            }
        };
        Thread thread = new Thread(runnable);
        thread.start();
    }
}

15 lockInterruptibly()方法

? ? ? ? public void lockInterruptibly()方法的作用是当某个线程尝试获得锁并且阻塞在lock-Interruptibly()方法时，该线程可以被中断。?

public class MyService {
    private ReentrantLock lock = new ReentrantLock();
    public void testMethod() throws InterruptedException {
        lock.lockInterruptibly();
        System.out.println("开始执行 testMethod方法，线程名= " + Thread.currentThread().getName()+ ";执行时间 = " + Utils.data(System.currentTimeMillis()));
        for (int i = 0; i < Integer.MAX_VALUE / 10; i++) {
            String s = new String();
            Math.random();
            Thread.currentThread().yield();
        }
        System.out.println("执行完毕 testMethod方法，线程名=" + Thread.currentThread().getName()+ ";执行时间 = " + Utils.data(System.currentTimeMillis()));
        lock.unlock();
    }
}

public class ThreadA extends Thread{
    private MyService service;

    public ThreadA(MyService service) {
        this.service = service;
    }
    @Override
    public void run(){
        try {
            service.testMethod();
        } catch (InterruptedException e) {
            throw new RuntimeException(e);
        }
    }
}

public class Run1 {
    public static void main(String[] args) throws InterruptedException {
        MyService service = new MyService();
        ThreadA a = new ThreadA(service);
        a.setName("A");
        a.start();
        Thread.sleep(500);
        ThreadA b = new ThreadA(service);
        b.setName("B");
        b.start();
        Thread.sleep(500);
        b.interrupt();
        System.out.println("main中断b，但并没有成功");
    }
}

16 tryLock()方法

? ? ? ? public boolean tryLock()方法的作用是嗅探拿锁，如果当前线程发现锁被其他线程持有了，则返回false，那么程序继续执行后面的代码，而不是呈阻塞等待锁的状态。?

public class MyService {
    public ReentrantLock lock = new ReentrantLock();
    public void waitMethod(){
        if(lock.tryLock()){
            System.out.println(Thread.currentThread().getName() + "获得锁");
        }else {
            System.out.println(Thread.currentThread().getName() + "没有获得锁");
        }
    }
}

public class Run1 {
    public static void main(String[] args) {
        final MyService service = new MyService();
        Runnable runnable = new Runnable() {
            @Override
            public void run() {
                service.waitMethod();
            }
        };
        Thread a = new Thread(runnable);
        a.setName("A");
        a.start();
        Thread b = new Thread(runnable);
        b.setName("B");
        b.start();
    }
}

17 实现线程按顺序执行业务

public class MyService {
    private ReentrantLock lock = new ReentrantLock();
    private Condition condition = lock.newCondition();
    volatile private int nextWhoPrint = 1;

    public void testMethod1(){
        try {
            lock.lock();
            while(nextWhoPrint != 1){
                condition.await();
            }
            System.out.println("AAA");
            nextWhoPrint = 2;
            condition.signalAll();
        }catch (InterruptedException e){
            e.printStackTrace();
        }finally {
            lock.unlock();
        }
    }

    public void testMethod2(){
        try {
            lock.lock();
            while(nextWhoPrint != 2){
                condition.await();
            }
            System.out.println("   BBB");
            nextWhoPrint = 3;
            condition.signalAll();
        }catch (InterruptedException e){
            e.printStackTrace();
        }finally {
            lock.unlock();
        }
    }

    public void testMethod3(){
        try {
            lock.lock();
            while(nextWhoPrint != 3){
                condition.await();
            }
            System.out.println("       CCC");
            nextWhoPrint = 1;
            condition.signalAll();
        }catch (InterruptedException e){
            e.printStackTrace();
        }finally {
            lock.unlock();
        }
    }
}

public class ThreadA extends Thread{
    private MyService service;

    public ThreadA(MyService service) {
        this.service = service;
    }
    @Override
    public void run(){
        service.testMethod1();
    }
}

public class ThreadB extends Thread{
    private MyService service;

    public ThreadB(MyService service) {
        this.service = service;
    }
    @Override
    public void run(){
        service.testMethod2();
    }
}

public class ThreadC extends Thread{
    private MyService service;

    public ThreadC(MyService service) {
        this.service = service;
    }
    @Override
    public void run(){
        service.testMethod3();
    }
}

public class Run1 {
    public static void main(String[] args) {
        MyService service = new MyService();
        for (int i = 0; i < 5; i++) {
            ThreadA a = new ThreadA(service);
            a.start();
            ThreadB b = new ThreadB(service);
            b.start();
            ThreadC c = new ThreadC(service);
            c.start();

        }
    }
}