java多线程lock详解

    Lock是JDK1.5种新增的同步工具,其实真正的实现Lock接口的类就三个,ReentrantLockReentrantReadWriteLock的两个内部类(ReadLockWriteLock实现了Lock的接口);

          ReentrantLock 实现了标准的互斥操作,也就是一次只能有一个线程持有锁,也即所谓独占锁的概念。我们也一直在强调这个特点。显然这个特点在一定程度上面减低了吞吐量,实际上独占锁是一种保守的锁策略,在这种情况下任何“读/读”,“写/读”,“写/写”操作都不能同时发生。

public class ReentrantLockTest {
	// 公平锁
	private Lock lock = new ReentrantLock(true);

	// 资源
	private Resource resource = new Resource();

	public static void main(String[] args) {
		final ReentrantLockTest test = new ReentrantLockTest();
		ExecutorService service = Executors.newCachedThreadPool();
		for (int i = 0; i < 20; i++) {
			service.submit(new Runnable() {

				@Override
				public void run() {
					Random random = ThreadLocalRandom.current();

					test.write(random.nextInt());
				}
			});

			service.submit(new Runnable() {

				@Override
				public void run() {
					test.read();
				}
			});
		}
		service.shutdown();
	}

	private void write(final int value) {
		// 如果可以获取锁
		if (lock.tryLock()) {
			try {
				// 执行业务逻辑
				System.out.println(Thread.currentThread().getName() + "获取了锁 写入 value=" + value);

				resource.setValue(String.valueOf(value));

			} finally {
				// 释放锁
				System.out.println(Thread.currentThread().getName() + "释放了锁");
				lock.unlock();
			}
		}
	}

	void read() {
		if (lock.tryLock()) {
			try {
				System.out.println(Thread.currentThread().getName() + "获取了锁 value=" + resource.getValue());
			} finally {
				System.out.println(Thread.currentThread().getName() + "释放了锁");
				lock.unlock();
			}
		}
	}
}

 

 

 

   测试的结果是:

 

pool-1-thread-1获取了锁 写入 value=2111184282
pool-1-thread-1释放了锁
pool-1-thread-2获取了锁 value=2111184282
pool-1-thread-2释放了锁
pool-1-thread-2获取了锁 写入 value=-1679499784
pool-1-thread-2释放了锁
pool-1-thread-2获取了锁 写入 value=1678350750
pool-1-thread-2释放了锁
pool-1-thread-2获取了锁 value=1678350750
pool-1-thread-2释放了锁
pool-1-thread-2获取了锁 value=1678350750
pool-1-thread-2释放了锁
pool-1-thread-2获取了锁 写入 value=152101866
pool-1-thread-2释放了锁
pool-1-thread-2获取了锁 value=152101866
pool-1-thread-2释放了锁
pool-1-thread-2获取了锁 写入 value=1147384711
pool-1-thread-2释放了锁
pool-1-thread-2获取了锁 value=1147384711
pool-1-thread-2释放了锁
pool-1-thread-2获取了锁 写入 value=-2130483959
pool-1-thread-2释放了锁
pool-1-thread-2获取了锁 value=-2130483959
pool-1-thread-2释放了锁
pool-1-thread-2获取了锁 写入 value=417404714
pool-1-thread-2释放了锁
pool-1-thread-2获取了锁 value=417404714
pool-1-thread-2释放了锁
pool-1-thread-2获取了锁 写入 value=-405288963
pool-1-thread-2释放了锁
pool-1-thread-2获取了锁 value=-405288963
pool-1-thread-2释放了锁
pool-1-thread-2获取了锁 写入 value=-139338780
pool-1-thread-2释放了锁
pool-1-thread-2获取了锁 value=-139338780
pool-1-thread-2释放了锁
pool-1-thread-2获取了锁 写入 value=-745627488
pool-1-thread-2释放了锁
pool-1-thread-2获取了锁 value=-745627488
pool-1-thread-2释放了锁
pool-1-thread-2获取了锁 写入 value=-1455461843
pool-1-thread-2释放了锁
pool-1-thread-2获取了锁 value=-1455461843
pool-1-thread-2释放了锁
pool-1-thread-2获取了锁 写入 value=-1129478741
pool-1-thread-2释放了锁
pool-1-thread-2获取了锁 写入 value=-1560021948
pool-1-thread-2释放了锁

     但实际应用场景中我们会经常遇到这样的情况:某些资源需要并发访问,并且大部分时间是用来进行读操作的,写操作比较少,而锁是有一定的开销的,当并发比较大 的时候,锁的开销就比较可观了。所以如果可能的话就尽量少用锁,如果非要用锁的话就尝试看能否能实现读写分离,将其改造为读写锁。

	// 公平锁
	private ReentrantReadWriteLock readWriteLock = new ReentrantReadWriteLock();

	// 写锁
	private Lock writeLock = readWriteLock.writeLock();

	// 读锁
	private Lock readLock = readWriteLock.readLock();

	// 资源
	private Resource resource = new Resource();

	public static void main(String[] args) {
		final ReentrantReadWriteLockTest test = new ReentrantReadWriteLockTest();
		ExecutorService service = Executors.newCachedThreadPool();
		for (int i = 0; i < 10; i++) {
			service.submit(new Runnable() {

				@Override
				public void run() {
					Random random = ThreadLocalRandom.current();

					test.write(random.nextInt());
				}
			});

			service.submit(new Runnable() {

				@Override
				public void run() {
					test.read();
				}
			});
		}
		service.shutdown();
	}

	private void write(final int value) {
		// 如果可以获取锁
		if (writeLock.tryLock()) {

			try {
				// 执行业务逻辑
				System.out.println(Thread.currentThread().getName() + "获取了锁 写入 value=" + value);

				resource.setValue(String.valueOf(value));

			} finally {
				// 释放锁
				System.out.println(Thread.currentThread().getName() + "释放了锁");
				writeLock.unlock();
			}
		}
	}

	private void read() {
		if (readLock.tryLock()) {
			try {

				System.out.println(Thread.currentThread().getName() + "获取了锁 value=" + resource.getValue());
			} finally {
				System.out.println(Thread.currentThread().getName() + "释放了锁");
				readLock.unlock();
			}
		}
	}

}

 测试结果是:

pool-1-thread-1获取了锁 写入 value=-702575113
pool-1-thread-1释放了锁
pool-1-thread-1获取了锁 value=-702575113
pool-1-thread-1释放了锁
pool-1-thread-1获取了锁 写入 value=-1619148924
pool-1-thread-1释放了锁
pool-1-thread-1获取了锁 value=-1619148924
pool-1-thread-1释放了锁
pool-1-thread-1获取了锁 写入 value=-1469315956
pool-1-thread-1释放了锁
pool-1-thread-1获取了锁 value=-1469315956
pool-1-thread-1释放了锁
pool-1-thread-1获取了锁 写入 value=1667915800
pool-1-thread-1释放了锁
pool-1-thread-1获取了锁 value=1667915800
pool-1-thread-1释放了锁
pool-1-thread-1获取了锁 写入 value=-1280947014
pool-1-thread-1释放了锁
pool-1-thread-1获取了锁 value=-1280947014
pool-1-thread-1释放了锁
pool-1-thread-1获取了锁 写入 value=1208950056
pool-1-thread-1释放了锁
pool-1-thread-1获取了锁 value=1208950056
pool-1-thread-1释放了锁
pool-1-thread-1获取了锁 写入 value=1089508899
pool-1-thread-1释放了锁
pool-1-thread-1获取了锁 写入 value=760397757
pool-1-thread-1释放了锁
pool-1-thread-2获取了锁 value=760397757
pool-1-thread-2释放了锁
pool-1-thread-2获取了锁 value=760397757
pool-1-thread-1获取了锁 value=760397757
pool-1-thread-2释放了锁
pool-1-thread-1释放了锁

   可以发现写锁是独占锁,读锁是共享锁,那么读锁是不是无限共享呢?实际上不是的,最大同时可以背65534个共享。

public class ReentrantTest {

	private Lock lock = new ReentrantReadWriteLock().readLock();
	static long count = 0;

	/**
	 * 
	 * @param args
	 * @author zhangwei
	 */
	public static void main(String[] args) {
		ReentrantTest test = new ReentrantTest();
		for (;;) {

			if (test.lock.tryLock()) {

				System.out.println(count++);

			}
		}
	}

}

 

65530
65531
65532
65533
65534
Exception in thread "main" java.lang.Error: Maximum lock count exceeded
	at java.util.concurrent.locks.ReentrantReadWriteLock$Sync.tryReadLock(ReentrantReadWriteLock.java:588)
	at java.util.concurrent.locks.ReentrantReadWriteLock$ReadLock.tryLock(ReentrantReadWriteLock.java:803)
	at org.demo.core.lock.ReentrantTest.main(ReentrantTest.java:34)

  锁降级  写线程获取写入锁后可以获取读取锁,然后释放写入锁,这样就从写入锁变成了读取锁,从而实现锁降级的特性。

 

public class DegradeReentrantReadWriteLockTest2 {

	// 公平锁
	private ReentrantReadWriteLock readWriteLock = new ReentrantReadWriteLock();

	// 写锁
	private Lock writeLock = readWriteLock.writeLock();

	// 读锁
	private Lock readLock = readWriteLock.readLock();

	// 资源
	private Resource resource = new Resource();

	public static void main(String[] args) {
		final DegradeReentrantReadWriteLockTest2 test = new DegradeReentrantReadWriteLockTest2();
		ExecutorService service = Executors.newCachedThreadPool();
		for (int i = 0; i < 10; i++) {
			service.submit(new Runnable() {

				@Override
				public void run() {
					Random random = ThreadLocalRandom.current();

					test.writeAndRead(random.nextInt());
				}
			});

			service.submit(new Runnable() {

				@Override
				public void run() {
					test.read();
				}
			});
		}
		service.shutdown();
	}

	private void writeAndRead(final int value) {
		// 如果可以获取锁
		try {
			try {
				// 写锁锁定
				writeLock.lock();
				// 执行业务逻辑
				System.out.println(Thread.currentThread().getName() + "获取了写锁 写入 value=" + value);

				resource.setValue(String.valueOf(value));

			} finally {
				System.out.println(Thread.currentThread().getName() + "写锁降级为读锁");
				// 读锁锁定
				readLock.lock();
				// 释放写锁
				writeLock.unlock();
			}
			System.out.println(resource.getValue());
		} finally {
			// 释放读锁
			System.out.println(Thread.currentThread().getName() + "释放了读锁");
			readLock.unlock();
		}
	}

	private void read() {
		if (readLock.tryLock()) {
			try {

				System.out.println(Thread.currentThread().getName() + "获取了读锁 value=" + resource.getValue());
			} finally {
				System.out.println(Thread.currentThread().getName() + "释放了读锁");
				readLock.unlock();
			}
		}
	}

}

 

 公平锁的实现:公平性是指最先试图获取锁的线程一定可以保证最先获取

          如果当前线程之前还有线程在等待,获取锁失败!通过队列排序保证获取锁的公平性。

    /**
     * Sync object for fair locks
     */
    static final class FairSync extends Sync {
        private static final long serialVersionUID = -3000897897090466540L;

        final void lock() {
            acquire(1);
        }

        /**
         * Fair version of tryAcquire.  Don't grant access unless
         * recursive call or no waiters or is first.
         */
        protected final boolean tryAcquire(int acquires) {
            final Thread current = Thread.currentThread();
            int c = getState();
            if (c == 0) {
                if (!hasQueuedPredecessors() &&
                    compareAndSetState(0, acquires)) {
                    setExclusiveOwnerThread(current);
                    return true;
                }
            }
            else if (current == getExclusiveOwnerThread()) {
                int nextc = c + acquires;
                if (nextc < 0)
                    throw new Error("Maximum lock count exceeded");
                setState(nextc);
                return true;
            }
            return false;
        }
    }

 

 

 

 

 

 

  

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