Java读写锁基本使用

    ReadWriteLock使得你可以同时具有多个读取者，只要他们都不试图写入即可。如果写锁被其他任务持有，那么任何读取者均不能访问，直至这个写锁被释放为止。
    ReadWriteLock能否提高性能是不确定的，取决于数据读取与修改频率相比较的结果，读取和写入操作的时间，竞争的线程数以及是否是多处理器等等。
    下面展示了ReadWriteLock最基本的用法。

package concurrency;

//: concurrency/ReaderWriterList.java
import java.util.concurrent.*;
import java.util.concurrent.locks.*;
import java.util.*;

public class ReaderWriterList<T> {
	private ArrayList<T> lockedList;
	// Make the ordering fair:
	private ReentrantReadWriteLock lock = new ReentrantReadWriteLock(true);

	public ReaderWriterList(int size, T initialValue) {
		lockedList = new ArrayList<T>(Collections.nCopies(size, initialValue));
	}

	public T set(int index, T element) {
		Lock wlock = lock.writeLock();
		wlock.lock();
		try {
			return lockedList.set(index, element);
		} finally {
			wlock.unlock();
		}
	}

	public T get(int index) {
		Lock rlock = lock.readLock();
		rlock.lock();
		try {
			// Show that multiple readers
			// may acquire the read lock:
			if (lock.getReadLockCount() > 1)
				System.out.println(lock.getReadLockCount());
			return lockedList.get(index);
		} finally {
			rlock.unlock();
		}
	}

	public static void main(String[] args) throws Exception {
		new ReaderWriterListTest(30, 1);
	}
}

class ReaderWriterListTest {
	ExecutorService exec = Executors.newCachedThreadPool();
	private final static int SIZE = 100;
	private static Random rand = new Random(47);
	private ReaderWriterList<Integer> list = new ReaderWriterList<Integer>(
			SIZE, 0);

	private class Writer implements Runnable {
		public void run() {
			try {
				for (int i = 0; i < 20; i++) { // 2 second test
					list.set(i, rand.nextInt());
					TimeUnit.MILLISECONDS.sleep(100);
				}
			} catch (InterruptedException e) {
				// Acceptable way to exit
			}
			System.out.println("Writer finished, shutting down");
			exec.shutdownNow();
		}
	}

	private class Reader implements Runnable {
		public void run() {
			try {
				while (!Thread.interrupted()) {
					for (int i = 0; i < SIZE; i++) {
						list.get(i);
						TimeUnit.MILLISECONDS.sleep(1);
					}
				}
			} catch (InterruptedException e) {
				// Acceptable way to exit
			}
		}
	}

	public ReaderWriterListTest(int readers, int writers) {
		for (int i = 0; i < readers; i++)
			exec.execute(new Reader());
		for (int i = 0; i < writers; i++)
			exec.execute(new Writer());
	}
} /* (Execute to see output) */// :~

程序输出结果如下：
24
25
22
19
17
16
...
...//省略
7
6
5
4
3
Writer finished, shutting down
set()方法获取写锁，get方法获取读锁，并且检查是否有多个线程获取了读锁，如果是就打印这种读取者的数量到控制台，以证明可以有多个读取者获得读锁。