读写锁

读写所ReadWriteLock

JDK1.5提供读写分离锁，读写分离锁可以有效的减少锁竞争，以提升系统性能。比如：线程A1、A2、A3进行读操作，B1、B2、B3进行写操作，如果是重入锁或者内部锁，所有的读之间，读写之间和写写之间都是串行操作，如B1进行读取时，B2、B3需要等待锁。由于读操作并不对数据完整性造成破坏，因此这种等待是没有意义的。所以就有了读写锁。
读写所允许多个线程同时读，但是写写操作和读写操作已让需要相互等待和持有锁的。

读写锁的约束情况：

import java.util.Random;import java.util.concurrent.locks.Lock;import java.util.concurrent.locks.ReentrantLock;import java.util.concurrent.locks.ReentrantReadWriteLock;public class ReadWriteDemo {    private static Lock reenlock = new  ReentrantLock();    private static ReentrantReadWriteLock readWriteLock = new ReentrantReadWriteLock();    private static Lock readLock = readWriteLock.readLock();    private static Lock writeLock = readWriteLock.writeLock();    private int value;    public Object handleRead(Lock lock) throws InterruptedException{        try {            lock.lock();            Thread.sleep(1000);            return value;        }finally{            lock.unlock();        }    }    public void handleWrite(Lock lock,int i) throws InterruptedException{        try {            lock.lock();            Thread.sleep(1000);            value = i;        }finally{            lock.unlock();        }    }    public static void main(String args[]) throws InterruptedException {        final ReadWriteDemo demo = new ReadWriteDemo();        Runnable readRunnable = new Runnable(){            public void run() {                try {                    //demo.handleRead(readLock);                    demo.handleRead(reenlock);                } catch (InterruptedException e) {                    e.printStackTrace();                }            }        };        Runnable writeRunnable = new Runnable(){            public void run() {                try {                    //demo.handleWrite(writeLock,new Random().nextInt());                    demo.handleWrite(reenlock,new Random().nextInt());                } catch (InterruptedException e) {                    e.printStackTrace();                }            }        };        //System.out.println(System.currentTimeMillis());        for(int i=0;i<18;i++){            Thread t = new Thread(readRunnable);            t.start();            //t.join();        }        for(int i=0;i<2;i++){            Thread t = new Thread(writeRunnable);            t.start();            //t.join();        }        //System.out.println(System.currentTimeMillis());    }}