ReenTrantLock和Synchronized

1.jdk中常用的独占锁有两种:Synchronized,ReenTrantLock。两种锁在性能上差别不大,但ReenTrantLock手动加锁，解锁，更加灵活，功能更加丰富

1)独占锁,可重入锁

public class TestReenTrantLock {

    static Lock lock = new ReentrantLock(true);

    public static void main(String[] args) {
        //可重入锁:一个线程多次获取同一个对象上的锁。可重入锁的意义之一在于防止死锁 
        实现原理:当线程获取锁时，jvm将记录锁的占有者，并将锁中的计数器加1，同一个线程再次获得锁，计数器再次加1。未被占用的锁，计数器为0。当线程退出同步块，计数器值将递减，直到计数器值为0，锁被释放。其他线程才能获得锁
        独占锁:一个锁一次只能被一个线程占有
        method();
        method1();

        lock.unlock();
        lock.unlock();
    }


    public static void method(){
        lock.lock();
        System.out.println("第一次获取锁");
    }

    public static void method1(){
        lock.lock();
        System.out.println("第二次获取锁");
    }
}

这里在method中获取了TestReenTrantLock类锁,调用method1时再一次获取了TestReenTrantLock类锁,这就是可重入锁

2)可以实现公平锁

public class FairReenTrantLock {

    /**
     * 公平锁：当锁可用时,在锁上等待时间最长的线程获取锁的使用权
     * 无参数或为fasle，为非公平锁
     * 非公平锁：随机获得锁的使用权
     */
    static Lock lock = new ReentrantLock(true);

    public static void main(String[] args) {
        for (int i=0; i<5; i++){
            new Thread(new ThreadDemo()).start();
        }
    }


    static class ThreadDemo implements Runnable{

        public ThreadDemo(){

        }

        @Override
        public void run() {
            try {
                TimeUnit.SECONDS.sleep(2);
            }catch (Exception e){
                e.printStackTrace();
            }
            for(int i=0; i<2; i++){
                lock.lock();
                System.out.println("获得锁的线程"+Thread.currentThread().getName());
                lock.unlock();
            }
        }
    }

}

3)可以响应线程中断

• 首先了解一下 public void interrupt();(属于ThreadL类)
  

public class TestInterrupt {

    public static void main(String[] args) throws InterruptedException {
        Thread testThread = new TestThread();
        testThread.start();
        testThread.interrupt();
    }

    /*
    *下面测试的三个方法都会阻塞线程,如果没有使用这个三个方法，线程会直接走完，
    *而且isInterrupted()为true
    *使用了其中一个,会抛出InterruptedException，isInterrupted()为false
    */
    static class TestThread extends Thread {

        @Override
        public void run() {
            System.out.println("线程开始运行");
            try {
                //TimeUnit.SECONDS.sleep(5);
                //wait();
                join();
            }catch (Exception e){
                System.out.println("线程发生异常"+e);
                System.out.println(Thread.currentThread().isInterrupted());
            }
            System.out.println("线程结束运行");
        }
    }
}

• 响应中断案例

public class LockInterruptibly {

    static Lock firstLock = new ReentrantLock();
    static Lock secondLock = new ReentrantLock();

    public static void main(String[] args) {
        Thread firstthread = new Thread(new ThreadDemo(firstLock,secondLock));
        Thread secondThread = new Thread(new ThreadDemo(secondLock,firstLock));
        firstthread.start();
        secondThread.start();
        firstthread.interrupt();
    }

    static class ThreadDemo implements Runnable {
        private Lock firstLock;
        private Lock secondLock;

        public ThreadDemo(Lock firstLock,Lock secondLock){
            this.firstLock = firstLock;
            this.secondLock = secondLock;
        }

        //会中断处于等待的线程,释放锁，然后另一个线程获取锁,继续走
        @Override
        public void run() {
            try {
                firstLock.lockInterruptibly();
                TimeUnit.SECONDS.sleep(1);
                secondLock.lockInterruptibly();
            }catch (Exception e){
                e.printStackTrace();
            }finally {
                firstLock.unlock();
                secondLock.unlock();
                System.out.println(Thread.currentThread().getName()+"正常结束");
            }
        }
    }
}

• 不响应中断案例

public class TestSynchronized {

    static Object resource1 = new Object();
    static Object resource2 = new Object();

    public static void main(String[] args) {
        Thread thread1 = new TestSynchronizedThread(resource1,resource2);
        Thread thread2 = new TestSynchronizedThread(resource2,resource1);

        thread1.start();
        thread2.start();
        thread1.interrupt();//中断线程，
    }

    static class TestSynchronizedThread extends Thread {
        Object resource1;
        Object resource2;
        public TestSynchronizedThread(Object resource1,Object resource2){
            this.resource1 = resource1;
            this.resource2 = resource2;
        }

        //即使跑出异常，也不会中断线程,两个线程继续互相等待资源，造成死锁
        @Override
        public void run() {
            synchronized (resource1){
                System.out.println(Thread.currentThread().getName()+"--111");
                try {
                    TimeUnit.SECONDS.sleep(1);
                    System.out.println(Thread.currentThread().getName()+"--222");
                } catch (InterruptedException e) {
                    System.out.println(Thread.currentThread().getName()+"--333");
                }

                System.out.println(Thread.currentThread().getName()+"--444");
                synchronized (resource2){
                    System.out.println(555);
                }
            }

        }
    }
}

4)获取锁时,限时等待
5)利用condition实现等待通知机制,
6)condition实现阻塞队列

synchronized