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.javaimport 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。