读写锁ReadWriteLock和缓存实例
12/12/2015来源：Java教程人气：6580
读写锁：多个读锁不互斥，读锁与写锁互斥，写锁与写锁互斥。即：读的时候不允许写，写的时候不允许读，可以同时读。
synchronized关键字和普通的Lock构造的锁，会造成读与读之间的互斥，因此读写锁可提高性能。
例子1：三个线程同时对一个共享数据进行读写。
import java.util.Random;
import java.util.concurrent.locks.ReadWriteLock;
import java.util.concurrent.locks.ReentrantReadWriteLock;
public class ReadWriteLockTest {
public static void main(String[] args) {
final Queue queue = new Queue();
for (int i = 0; i < 3; i++) {
new Thread() {
public void run() {
while (true) {
queue.get();
}
}
}.start();
new Thread() {
public void run() {
while (true) {
queue.put( new Random().nextInt(10000));
}
}
}.start();
}
}
}
class Queue {
ReadWriteLock rwl = new ReentrantReadWriteLock();
public void get() {
rwl.readLock().lock();
try {
System. out.println(Thread.currentThread().getName() + " be ready to read data!");
Thread. sleep((long) (Math. random() * 1000));
System. out.println(Thread.currentThread().getName() + " have read data :" + data);
} catch (InterruptedException e) {
e.printStackTrace();
} finally {
rwl.readLock().unlock();
}
}
public void put(Object data) {
rwl.writeLock().lock();
try {
System. out.println(Thread.currentThread().getName() + " be ready to write data!");
Thread. sleep((long) (Math. random() * 1000));
this.data = data;
System. out.println(Thread.currentThread().getName() + " have write data: " + data);
} catch (InterruptedException e) {
e.printStackTrace();
} finally {
rwl.writeLock().unlock();
}
}
}
例子2：缓存实例
import java.util.HashMap;
import java.util.Map;
import java.util.concurrent.locks.ReadWriteLock;
import java.util.concurrent.locks.ReentrantReadWriteLock;
public class CacheDemo {
private static Map<String, Object> cache = new HashMap<String, Object>();
private ReadWriteLock rwl = new ReentrantReadWriteLock();
public Object getData(String key) {
// 当线程开始读时，首先开始加上读锁
rwl.readLock().lock();
Object value = null;
try {
value = cache.get(key);
// 判断是否存在值
if (value == null) {
// 在开始写之前，首先要释放读锁，否则写锁无法拿到
rwl.readLock().unlock();
// 获取写锁开始写数据
rwl.writeLock().lock();
try {
// 再次判断该值是否为空，因为如果两个写线程都阻塞在这里，
// 当一个线程被唤醒后value的值为null则进行数据加载，当另外一个线程也被唤醒如果不判断就会执行两次写
if (value == null) {
cache.put(key, value);
}
} finally {
rwl.writeLock().unlock(); // 释放写锁
}
rwl.readLock().lock(); // 写完之后降级为读锁
}
} finally {
rwl.readLock().unlock(); // 释放读锁
}
return value;
}
}
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