详解Java中的锁Lock和synchronized

Lock lock = new ReentrantLock(); //这里可以是自己实现Lock接口的实现类，也可以是jdk提供的同步组件
lock.lock();//一般不将锁的获取放在try语句块中，因为如果发生异常，在抛出异常的同时，也会导致锁的无故释放
try {
}finally {
    lock.unlock(); //放在finally代码块中，保证锁一定会被释放
}

public interface Lock {

    
    void lock();

    
    void lockInterruptibly() throws InterruptedException;

    
    boolean tryLock();

    
    boolean tryLock(long time, TimeUnit unit) throws InterruptedException;

    
    void unlock();

    
    Condition newCondition();
}

import java.util.concurrent.locks.Lock;
 import java.util.concurrent.locks.ReentrantLock;
 
 public class TestCR {
     Lock lock = new ReentrantLock();
     
     void m1(){
         try{
             lock.lock(); // 加锁
             for(int i = 0; i < 4; i++){
                 TimeUnit.SECONDS.sleep(1);
                 System.out.println("m1() method " + i);
             }
             m2(); //在释放锁之前，调用m2方法
         }catch(InterruptedException e){
             e.printStackTrace();
         }finally{
             lock.unlock(); // 解锁
         }
     }
     
     void m2(){
         lock.lock();
         System.out.println("m2() method");
         lock.unlock();
     }
     
     public static void main(String[] args) {
         final TestCR t = new TestCR();
         new Thread(new Runnable() {
             @Override
             public void run() {
                 t.m1();
             }
         }).start();
 
         new Thread(new Runnable() {
             @Override
             public void run() {
                 t.m2();
             }
         }).start();
     }
 }

abstract static class Sync extends AbstractQueuedSynchronizer {
    
    abstract void lock();

    
    final boolean nonfairTryAcquire(int acquires) {
        final Thread current = Thread.currentThread();//获取当前线程
        int c = getState(); //获取当前同步状态的值
        if (c == 0) { //如果当前的同步状态还没有被任何线程获取
            if (compareAndSetState(0, acquires)) { //就更新同步状态的值，因为已经有线程获取到同步装填
                setExclusiveOwnerThread(current);//设置同步状态的线程拥有者为当前获取的线程
                return true;
            }
        }
        else if (current == getExclusiveOwnerThread()) {//增加再次获取同步状态的处理逻辑
            int nextc = c + acquires; //如果再次尝试获取同步状态的线程就是当前已经占有同步状态的线程,那么就更新同步状态的值(进行增加操作)
            if (nextc < 0) // 对同步状态的值进行非法判断
                throw new Error("Maximum lock count exceeded");
            setState(nextc); //更新state的值
            return true;
        }
        return false;
    }

    
    protected final boolean tryRelease(int releases) {
        int c = getState() - releases; //对同一线程而言，就是减去相应的获取次数
        if (Thread.currentThread() != getExclusiveOwnerThread())
            throw new IllegalMonitorStateException();
        boolean free = false; //返回值
        if (c == 0) { //只有该线程将获取的次数全部释放之后，才会返回true，并且将当前同步状态的持有者设置为null
            free = true;
            setExclusiveOwnerThread(null);
        }
        setState(c); //更新state
        return free;
    }

        
    protected final boolean isHeldExclusively() {
        return getExclusiveOwnerThread() == Thread.currentThread();
    }

    final ConditionObject newCondition() {
        return new ConditionObject();
    }

        
    final Thread getOwner() {
        return getState() == 0 ? null : getExclusiveOwnerThread();
    }

        
    final int getHoldCount() {
        return isHeldExclusively() ? getState() : 0;
    }

        
    final boolean isLocked() {
        return getState() != 0;
    }

    private void readObject(java.io.ObjectInputStream s)
        throws java.io.IOException, ClassNotFoundException {
        s.defaultReadObject();
        setState(0); // reset to unlocked state
    }
}

//非公平锁实现
static final class NonfairSync extends Sync {

    
    final void lock() {
        if (compareAndSetState(0, 1))
            setExclusiveOwnerThread(Thread.currentThread());
        else
            acquire(1);
    }

    
    protected final boolean tryAcquire(int acquires) {
        return nonfairTryAcquire(acquires);
    }
}

static final class FairSync extends Sync {

    final void lock() {
        acquire(1); //调用AQS的模板方法实现锁的获取
    }

    
    protected final boolean tryAcquire(int acquires) {
        final Thread current = Thread.currentThread(); //获取当前线程
        int c = getState(); //获取当前同步状态的值
        if (c == 0) { //当前同步状态没有被任何线程获取的时候
            if (!hasQueuedPredecessors() &&
                compareAndSetState(0, acquires)) { //这个点的主要处理逻辑就是：hasQueuedPredecessors判断当前线程所在的结点是否含有前驱结点,　　　　　　　　　　　　　　　　　　　　　　　　　　　　　　　　　 如果返回值为true表示有前驱结点，那么当前线程需要等待前驱结点中的线程获取并释放锁之后才能获取锁，保证了FIFO
                setExclusiveOwnerThread(current);
                return true;
            }
        }
        else if (current == getExclusiveOwnerThread()) { //支持重入的逻辑，和非公平锁的实现原理相同
            int nextc = c + acquires;
            if (nextc < 0)
                throw new Error("Maximum lock count exceeded");
            setState(nextc);
            return true;
        }
        return false;
    }
}
//hasQueuedPredecessors的处理逻辑
public final boolean hasQueuedPredecessors() {
    // 简单而言，就是判断当前线程是否有前驱结点
    // 当前结点含有前驱结点时候返回true；当前结点为头结点挥着队列为空的时候返回false
    Node t = tail; // Read fields in reverse initialization order
    Node h = head;
    Node s;
    return h != t &&
        ((s = h.next) == null || s.thread != Thread.currentThread());
}

import java.util.ArrayList;
import java.util.Collection;
import java.util.Collections;
import java.util.List;
import java.util.concurrent.locks.Lock;
import java.util.concurrent.locks.ReentrantLock;

import org.junit.Test;

public class TestReentrantLock {
    
    private Lock fairLock = new ReentrantLock2(true);
    private Lock unFairLock = new ReentrantLock2(false);

    @Test
    public void testFair() throws InterruptedException {
        testLock(fairLock); //测试公平锁
    }

    @Test
    public void testUnFair() throws InterruptedException {
        testLock(unFairLock); //测试非公平锁
    }

    private void testLock(Lock lock) throws InterruptedException {
        for (int i = 0; i < 5; i++) {
            Thread thread = new Thread(new Job(lock)) {
                public String toString() {
                        return getName();
                }
            };
            thread.setName(i+"");
            thread.start();
        }
        Thread.sleep(12000);
    }

    private static class Job extends Thread {
        private Lock lock;
        public Job(Lock lock) {
            this.lock = lock;
        }
        @Override
        public void run() {
            //两次打印当前线程和等待队列中的Threads
            for (int i = 0; i < 2; i++) {
                lock.lock(); //获取锁
                try {
                    Thread.sleep(1000);
                    System.out.println("当前线程=>" + Thread.currentThread().getName() + " " +
                            "等待队列中的线程=>" + ((ReentrantLock2)lock).getQueuedThreads());
                } catch (InterruptedException e) {
                    e.printStackTrace();
                } finally {
                    lock.unlock(); //释放锁
                }
            }
        }

    }

    private static class ReentrantLock2 extends ReentrantLock {
        public ReentrantLock2(boolean fair) {
            super(fair);
        }
        public Collection<Thread> getQueuedThreads() { //逆序打印等待队列中的线程
            List<Thread> list = new ArrayList<Thread>(super.getQueuedThreads());
            Collections.reverse(list);
            return list;
        }
    }


}

package cn.source.sync;

public class TestSync01 {
    private static int count = 0;
    private Object object = new Object();

    public void testSyn1() {
        //同步代码块（这里面是锁临界资源，即括号中的对象）
        synchronized (object) {
            System.out.println(Thread.currentThread().getName()
                +" count =" + count++);
        }
    }

    public void testSyn2() {
        //锁当前对象（相当于普通同步方法）
        synchronized (this) {
            System.out.println(Thread.currentThread().getName()
                    +" count =" + count++);
        }
    }

    //普通同步方法：锁当前对象
    public synchronized void testSyn3() {
        System.out.println(Thread.currentThread().getName()
                +" count =" + count++);
    }

    //静态同步方法，锁的是当前类型的类对象（即TestSync01.class）
    public static synchronized void testSyn4() {
        System.out.println(Thread.currentThread().getName()
                +" count =" + count++);
    }

    //下面的这种方式也是锁当前类型的类对象
    public static void testSyn5() {
        synchronized (TestSync01.class) {
            System.out.println(Thread.currentThread().getName()
                    +" count =" + count ++);
        }
    }
}