Java多线程（七）：ReentrantLock

加锁和解锁

我们来看下ReentrantLock的基本用法
ThreadDomain35类

public class ThreadDomain35 {

    private Lock lock = new ReentrantLock();

    public void testMethod()
    {
        try
        {
            lock.lock();
            for (int i = 0; i < 2; i++)
            {
                System.out.println("ThreadName = " + Thread.currentThread().getName() + ", i  = " + i);
            }
        }
        finally
        {
            lock.unlock();
        }
    }
}

线程和main方法

public class MyThread35 extends Thread {

    private ThreadDomain35 td;

    public MyThread35(ThreadDomain35 td)
    {
        this.td = td;
    }

    public void run()
    {
        td.testMethod();
    }

    public static void main(String[] args)
    {
        ThreadDomain35 td = new ThreadDomain35();
        MyThread35 mt0 = new MyThread35(td);
        MyThread35 mt1 = new MyThread35(td);
        MyThread35 mt2 = new MyThread35(td);
        mt0.start();
        mt1.start();
        mt2.start();
    }
}

输出结果

ThreadName = Thread-2, i  = 0
ThreadName = Thread-2, i  = 1
ThreadName = Thread-0, i  = 0
ThreadName = Thread-0, i  = 1
ThreadName = Thread-1, i  = 0
ThreadName = Thread-1, i  = 1

一个线程必须执行完才能执行下一个线程，说明ReentrantLock可以加锁。

ReentrantLock持有的对象监视器和synchronized不同

ThreadDomain37类,methodB用synchronized修饰

public class ThreadDomain37 {
    private Lock lock = new ReentrantLock();

    public void methodA()
    {
        try
        {
            lock.lock();
            System.out.println("MethodA begin ThreadName = " + Thread.currentThread().getName());
            Thread.sleep(5000);
            System.out.println("MethodA end ThreadName = " + Thread.currentThread().getName());
        }
        catch (InterruptedException e)
        {
            e.printStackTrace();
        }
        finally
        {
            lock.unlock();
        }

    }

    public synchronized void methodB()
    {
        System.out.println("MethodB begin ThreadName = " + Thread.currentThread().getName());
        System.out.println("MethodB begin ThreadName = " + Thread.currentThread().getName());
    }
}

MyThread37_0类

public class MyThread37_0 extends Thread {

    private ThreadDomain37 td;

    public MyThread37_0(ThreadDomain37 td)
    {
        this.td = td;
    }

    public void run()
    {
        td.methodA();
    }
}

MyThread37_1类

public class MyThread37_1 extends Thread {
    private ThreadDomain37 td;

    public MyThread37_1(ThreadDomain37 td)
    {
        this.td = td;
    }

    public void run()
    {
        td.methodB();
    }
}

MyThread37_main方法

public class MyThread37_main {

    public static void main(String[] args)
    {
        ThreadDomain37 td = new ThreadDomain37();
        MyThread37_0 mt0 = new MyThread37_0(td);
        MyThread37_1 mt1 = new MyThread37_1(td);
        mt0.start();
        mt1.start();
    }

}

运行结果如下

MethodA begin ThreadName = Thread-0
MethodB begin ThreadName = Thread-1
MethodB begin ThreadName = Thread-1
MethodA end ThreadName = Thread-0

加了synchronized依然是异步执行，说明ReentrantLock和synchronized持有的对象监视器不同。ReentrantLock需要手动加锁和释放锁。

Condition

基本用法

synchronized与wait()和nitofy()/notifyAll()方法可以实现等待/唤醒模型，ReentrantLock同样可以，需要借助Condition的await()和signal/signalAll()，await()释放锁。
ThreadDomain38类

public class ThreadDomain38 {
    private Lock lock = new ReentrantLock();
    private Condition condition = lock.newCondition();

    public void await()
    {
        try
        {
            lock.lock();
            System.out.println("await时间为：" + System.currentTimeMillis());
            condition.await();
            System.out.println("await等待结束");
        }
        catch (InterruptedException e)
        {
            e.printStackTrace();
        }
        finally
        {
            lock.unlock();
        }
    }

    public void signal()
    {
        try
        {
            lock.lock();
            System.out.println("signal时间为：" + System.currentTimeMillis());
            condition.signal();
            System.out.println("signal等待结束");
        }
        finally
        {
            lock.unlock();
        }
    }
}

MyThread38类，线程和main方法

public class MyThread38 extends Thread
{
    private ThreadDomain38 td;

    public MyThread38(ThreadDomain38 td)
    {
        this.td = td;
    }

    public void run()
    {
        td.await();
    }

    public static void main(String[] args) throws Exception
    {
        ThreadDomain38 td = new ThreadDomain38();
        MyThread38 mt = new MyThread38(td);
        mt.start();
        Thread.sleep(3000);
        td.signal();
    }
}

运行结果如下

await时间为：1563505465346
signal时间为：1563505468345
signal等待结束
await等待结束

可以看到，ReentrantLock和Condition实现了等待/通知模型。
一个Lock可以创建多个Condition;
notify()唤醒的线程是随机的，signal()可以有选择性地唤醒。

Condition选择 唤醒/等待

现在看一个利用Condition选择等待和唤醒的例子
ThreadDomain47类，定义add和sub方法

public class ThreadDomain47 {
    private final Lock lock = new ReentrantLock();

    private final Condition addCondition = lock.newCondition();

    private final Condition subCondition = lock.newCondition();


    private static int num = 0;
    private List<String> lists = new LinkedList<String>();

    public void add() {
        lock.lock();

        try {
            while(lists.size() == 10) {//当集合已满,则"添加"线程等待
                addCondition.await();
            }

            num++;
            lists.add("add Banana" + num);
            System.out.println("The Lists Size is " + lists.size());
            System.out.println("The Current Thread is " + "增加线程");
            System.out.println("==============================");
            this.subCondition.signal();

        } catch (InterruptedException e) {
            e.printStackTrace();
        } finally {//释放锁
            lock.unlock();
        }
    }


    public void sub() {
        lock.lock();

        try {
            while(lists.size() == 0) {//当集合为空时,"减少"线程等待
                subCondition.await();
            }

            String str = lists.get(0);
            lists.remove(0);
            System.out.println("The Token Banana is [" + str + "]");
            System.out.println("The Current Thread is " + "减少线程");
            System.out.println("==============================");
            num--;
            addCondition.signal();

        } catch (InterruptedException e) {
            e.printStackTrace();
        } finally {
            lock.unlock();
        }
    }
}

MyThread40_0类，增加线程

public class MyThread40_0 implements Runnable {

    private ThreadDomain47 task;

    public MyThread40_0(ThreadDomain47 task) {
        this.task = task;
    }

    @Override
    public void run() {
        task.add();
    }

}

MyThread40_1类，减少线程

public class MyThread40_1 implements Runnable {
    private ThreadDomain47 task;

    public MyThread40_1(ThreadDomain47 task) {
        this.task = task;
    }

    @Override
    public void run() {
        task.sub();
    }

}

main方法，启动线程

public class MyThread40_main {
    public static void main(String[] args) {
        ThreadDomain47 task = new ThreadDomain47();

        Thread t1=new Thread(new MyThread40_0(task));
        Thread t3=new Thread(new MyThread40_0(task));
        Thread t7=new Thread(new MyThread40_0(task));
        Thread t8=new Thread(new MyThread40_0(task));
        Thread t2 = new Thread(new MyThread40_1(task));
        Thread t4 = new Thread(new MyThread40_1(task));
        Thread t5 = new Thread(new MyThread40_1(task));
        Thread t6 = new Thread(new MyThread40_1(task));

        t1.start();
        t2.start();
        t3.start();
        t4.start();
        t5.start();
        t6.start();
        t7.start();
        t8.start();
    }
}

输出结果如下

The Lists Size is 1
The Current Thread is 增加线程
==============================
The Lists Size is 2
The Current Thread is 增加线程
==============================
The Token Banana is [add Banana1]
The Current Thread is 减少线程
==============================
The Token Banana is [add Banana2]
The Current Thread is 减少线程
==============================
The Lists Size is 1
The Current Thread is 增加线程
==============================
The Token Banana is [add Banana1]
The Current Thread is 减少线程
==============================
The Lists Size is 1
The Current Thread is 增加线程
==============================
The Token Banana is [add Banana1]
The Current Thread is 减少线程
==============================

可以看到，lists的数量不会增加太多，也不会减少太多。当集合满，使增加线程等待，唤醒减少线程；当集合空，使减少线程等待，唤醒增加线程。我们用wait()/notify()机制无法实现该效果，这里体现了Condition的强大之处。

ReentrantLock中的方法

公平锁和非公平锁
ReentrantLock可以指定公平锁和非公平锁，公平锁根据线程运行的顺序获取锁，非公平锁则通过抢占获得锁，不按线程运行顺序。synchronized是非公平锁。在ReentrantLock(boolean fair)构造函数传入true/false来指定公平锁/非公平锁。
看个例子
ThreadDomain39类和main方法

public class ThreadDomain39 {
    private Lock lock = new ReentrantLock(true);

    public void testMethod()
    {
        try
        {
            lock.lock();
            System.out.println("ThreadName" + Thread.currentThread().getName() + "获得锁");
        }
        finally
        {
            lock.unlock();
        }
    }

    public static void main(String[] args) throws Exception
    {
        final ThreadDomain39 td = new ThreadDomain39();
        Runnable runnable = new Runnable()
        {
            public void run()
            {
                System.out.println("线程" + Thread.currentThread().getName() + "运行了");
                td.testMethod();
            }
        };
        Thread[] threads = new Thread[5];
        for (int i = 0; i < 5; i++)
            threads[i] = new Thread(runnable);
        for (int i = 0; i < 5; i++)
            threads[i].start();
    }
}

输出结果如下

线程Thread-0运行了
ThreadNameThread-0获得锁
线程Thread-1运行了
线程Thread-2运行了
ThreadNameThread-1获得锁
线程Thread-3运行了
线程Thread-4运行了
ThreadNameThread-2获得锁
ThreadNameThread-3获得锁
ThreadNameThread-4获得锁

可以看到公平锁获得锁的顺序和线程运行的顺序相同。公平锁尽可能地让线程获取锁的顺序和线程运行顺序保持一致，再执行几次，可能不一致。
ReentrantLock构造函数传入false，输出结果如下：

线程Thread-0运行了
线程Thread-2运行了
线程Thread-4运行了
线程Thread-3运行了
ThreadNameThread-0获得锁
线程Thread-1运行了
ThreadNameThread-1获得锁
ThreadNameThread-2获得锁
ThreadNameThread-4获得锁
ThreadNameThread-3获得锁

非公平锁获得锁的顺序和线程运行的顺序不同

getHoldCount()

获取当前线程调用lock()的次数，一般debug使用。
看个例子

public class ThreadDomain40 {
    private ReentrantLock lock = new ReentrantLock();

    public void testMethod1()
    {
        try
        {
            lock.lock();
            System.out.println("testMethod1 getHoldCount = " + lock.getHoldCount());
            testMethod2();
        }
        finally
        {
            lock.unlock();
        }
    }

    public void testMethod2()
    {
        try
        {
            lock.lock();
            System.out.println("testMethod2 getHoldCount = " + lock.getHoldCount());
        }
        finally
        {
            lock.unlock();
        }
    }



    public static void main(String[] args)
    {
        ThreadDomain40 td = new ThreadDomain40();
        td.testMethod1();
    }


}

输出结果如下

testMethod1 getHoldCount = 1
testMethod2 getHoldCount = 2

可以看到，testMethod1()被调用了一次，testMethod2()被调用了两次，ReentrantLock和synchronized一样，锁都是可重入的。

getQueueLength()和isFair()

getQueueLength()获取等待的线程数量，isFair()判断是否是公平锁。
ThreadDomain41类和main方法,Thread.sleep(2000)使第一个线程之后的线程都来不及启动，Thread.sleep(Integer.MAX_VALUE)使线程无法unlock()。

public class ThreadDomain41 {
    public ReentrantLock lock = new ReentrantLock();

    public void testMethod()
    {
        try
        {
            lock.lock();
            System.out.println("ThreadName = " + Thread.currentThread().getName() + "进入方法！");
            System.out.println("是否公平锁？" + lock.isFair());
            Thread.sleep(Integer.MAX_VALUE);
        }
        catch (InterruptedException e)
        {
            e.printStackTrace();
        }
        finally
        {
            lock.unlock();
        }
    }

    public static void main(String[] args) throws InterruptedException
    {
        final ThreadDomain41 td = new ThreadDomain41();
        Runnable runnable = new Runnable()
        {
            public void run()
            {
                td.testMethod();
            }
        };
        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" + td.lock.getQueueLength() + "个线程正在等待！");
    }
}

输出结果如下

ThreadName = Thread-1进入方法！
是否公平锁？false
有9个线程正在等待！

ReentrantLock默认是非公平锁，只有一个线程lock()，9个线程在等待。

hasQueuedThread()和hasQueuedThreads()

hasQueuedThread(Thread thread)查询指定线程是否在等待锁，hasQueuedThreads()查询是否有线程在等待锁。
看个例子
ThreadDomain41类和main方法，和上面例子类似，Thread.sleep(Integer.MAX_VALUE); 让线程不释放锁，Thread.sleep(2000);让第一个线程之后的线程都无法启动。

public class ThreadDomain42 extends ReentrantLock {
    public void waitMethod()
    {
        try
        {
            lock();
            Thread.sleep(Integer.MAX_VALUE);
        }
        catch (InterruptedException e)
        {
            e.printStackTrace();
        }
        finally
        {
            unlock();
        }
    }

    public static void main(String[] args) throws InterruptedException
    {
        final ThreadDomain42 td = new ThreadDomain42();
        Runnable runnable = new Runnable()
        {
            public void run()
            {
                td.waitMethod();
            }
        };
        Thread t0 = new Thread(runnable);
        t0.start();
        Thread.sleep(500);
        Thread t1 = new Thread(runnable);
        t1.start();
        Thread.sleep(500);
        Thread t2 = new Thread(runnable);
        t2.start();
        Thread.sleep(500);
        System.out.println("t0 is waiting？" + td.hasQueuedThread(t0));
        System.out.println("t1 is waiting？" + td.hasQueuedThread(t1));
        System.out.println("t2 is waiting？" + td.hasQueuedThread(t2));
        System.out.println("Is any thread waiting？" + td.hasQueuedThreads());
    }
}

输出结果如下

t0 is waiting？false
t1 is waiting？true
t2 is waiting？true
Is any thread waiting？true

t0线程获得了锁，t0没有释放锁，导致t1,t2等待锁。

isHeldByCurrentThread()和isLocked()

isHeldByCurrentThread()判断锁是否由当前线程持有，isLocked()判断锁是否由任意线程持有。
请看示例
ThreadDomain43类和main方法

public class ThreadDomain43 extends ReentrantLock {
    public void testMethod()
    {
        try
        {
            lock();
            System.out.println(Thread.currentThread().getName() + "线程持有了锁！");
            System.out.println(Thread.currentThread().getName() + "线程是否持有锁？" +
                    isHeldByCurrentThread());
            System.out.println("是否任意线程持有了锁？" + isLocked());
        } finally
        {
            unlock();
        }
    }

    public void testHoldLock()
    {
        System.out.println(Thread.currentThread().getName() + "线程是否持有锁？" +
                isHeldByCurrentThread());
        System.out.println("是否任意线程持有了锁？" + isLocked());
    }

    public static void main(String[] args)
    {
        final ThreadDomain43 td = new ThreadDomain43();
        Runnable runnable0 = new Runnable()
        {
            public void run()
            {
                td.testMethod();
            }
        };
        Runnable runnable1 = new Runnable()
        {
            public void run()
            {
                td.testHoldLock();
            }
        };
        Thread t0 = new Thread(runnable0);
        Thread t1 = new Thread(runnable1);
        t0.start();
        t1.start();
    }
}

输出结果如下

Thread-0线程持有了锁！
Thread-1线程是否持有锁？false
Thread-0线程是否持有锁？true
是否任意线程持有了锁？true
是否任意线程持有了锁？true

Thread-0线程testMethod方法持有锁，Thread-1线程testHoldLock方法没有lock操作，所以不持有锁。

tryLock()和tryLock(long timeout, TimeUnit unit)

tryLock()有加锁的功能，获得了锁且锁没有被另外一个线程持有，此时返回true，否则返回false，可以有效避免死锁。tryLock(long timeout, TimeUnit unit)表示在给定的时间内获得了锁，锁没有被其他线程持有，且不处于中断状态。返回true,否则返回false；
看个例子

public class MyThread39 {
    public static void main(String[] args) {

        System.out.println("开始");
        final Lock lock = new ReentrantLock();
        new Thread() {
            @Override
            public void run() {
                String tName = Thread.currentThread().getName();
                if (lock.tryLock()) {
                    System.out.println(tName + "获取到锁！");
                } else {
                    System.out.println(tName + "获取不到锁！");
                    return;
                }
                try {
                    for (int i = 0; i < 5; i++) {
                        System.out.println(tName + ":" + i);
                    }
                    Thread.sleep(5000);
                } catch (Exception e) {
                    System.out.println(tName + "出错了！");
                } finally {
                    System.out.println(tName + "释放锁！");
                    lock.unlock();
                }

            }
        }.start();

        new Thread() {
            @Override
            public void run() {
                String tName = Thread.currentThread().getName();

                try {
                    if (lock.tryLock(1,TimeUnit.SECONDS)) {
                        System.out.println(tName + "获取到锁！");
                    } else {
                        System.out.println(tName + "获取不到锁！");
                        return;
                    }
                } catch (InterruptedException e) {
                    e.printStackTrace();
                }

                try {
                    for (int i = 0; i < 5; i++) {
                        System.out.println(tName + ":" + i);
                    }

                } catch (Exception e) {
                    System.out.println(tName + "出错");
                } finally {
                    System.out.println(tName + "释放锁！");
                    lock.unlock();
                }
            }
        }.start();

        System.out.println("结束");
    }
}

输出结果如下

开始
Thread-0获取到锁！
Thread-0:0
Thread-0:1
Thread-0:2
Thread-0:3
Thread-0:4
结束
Thread-1获取不到锁！
Thread-0释放锁！

Thread-0先获得了锁，且sleep了5秒，导致Thread-1获取不到锁，我们给Thread-1的tryLock设置1秒，一秒内获取不到锁就会返回false。
如果Thread.sleep(0)，那么Thread-0和Thread-1都可以获得锁，园友可以自己试下。

synchronized和ReentrantLock的比较

1.synchronized关键字是语法层面的实现，ReentrantLock要手动lock()和unlock();
2.synchronized是不公平锁，ReentrantLock可以指定是公平锁还是非公平锁；
3.synchronized等待/唤醒机制是随机的，ReentrantLock借助Condition的等待/唤醒机制可以自行选择等待/唤醒；