读写锁–ReadWriteLock接口及其实现类ReentrantReadWriteLock

ReentrantReadWriteLock中定义了2个内部类, ReentrantReadWriteLock.ReadLock和ReentrantReadWriteLock.WriteLock, 分别用来代表读取锁和写入锁. ReentrantReadWriteLock对象提供了readLock()和writeLock()方法, 用于获取读取锁和写入锁. 

• 读取锁允许多个reader线程同时持有, 而写入锁最多只能有一个writter线程持有.
• 读写锁的使用场合: 读取共享数据的频率远大于修改共享数据的频率. 在上述场合下, 使用读写锁控制共享资源的访问, 可以提高并发性能.
• 如果一个线程已经持有了写入锁, 则可以再持有读写锁. 相反, 如果一个线程已经持有了读取锁, 则在释放该读取锁之前, 不能再持有写入锁.
• 可以调用写入锁的newCondition()方法获取与该写入锁绑定的Condition对象, 此时与普通的互斥锁并没有什么区别. 但是调用读取锁的newCondition()方法将抛出异常. 

使用读写锁的一个例子:

public class ReadWriteLockTest {
    private static ReadWriteLock lock = new ReentrantReadWriteLock();
    private static Person person = new Person("David Beckham", true);

    public static void main(String[] args) {
        new Thread() {
            public void run() {
                while(true) {
                    try {
                        lock.readLock().lock();
                        System.out.print("name = " + person.getName());
                        System.out.println(", isMan = " + person.isMan());
                    } finally {
                        lock.readLock().unlock();
                    }
                }
            };
        }.start();
        new Thread() {
            public void run() {
                boolean state = true;
                while(true) {
                    try {
                        lock.writeLock().lock();
                        if (state) {
                            person.setName("Lady GaGa");
                            person.setMan(false);
                            state = false;
                        } else {
                            person.setName("David Beckham");
                            person.setMan(true);
                            state = true;
                        }
                        
                    } finally {
                        lock.writeLock().unlock();
                    }
                }
            };
        }.start();
    }
}

class Person {
    private String name;
    private boolean isMan;

    public Person(String name, boolean isMan) {
        this.name = name;
        this.isMan = isMan;
    }

    public String getName() {
        return name;
    }

    public void setName(String name) {
        this.name = name;
    }

    public boolean isMan() {
        return isMan;
    }

    public void setMan(boolean isMan) {
        this.isMan = isMan;
    }
}

不管程序运行多久, 也不会有人妖的出现.

转自 http://coolxing.iteye.com/blog/1236909