多线程基础总结八--ReentrantReadWriteLock
2010-01-22 00:00:00 来源:WEB开发网说到ReentrantReadWriteLock,首先要做的是与ReentrantLock划清界限。它和后者都是单独的实现,彼此之间没有继承或实现的关系。然后就是总结这个锁机制的特性了:
(a).重入方面其内部的WriteLock可以获取ReadLock,但是反过来ReadLock想要获得WriteLock则永远都不要想。
(b).WriteLock可以降级为ReadLock,顺序是:先获得WriteLock再获得ReadLock,然后释放WriteLock,这时候线程将保持Readlock的持有。反过来ReadLock想要升级为WriteLock则不可能,为什么?参看(a),呵呵.
(c).ReadLock可以被多个线程持有并且在作用时排斥任何的WriteLock,而WriteLock则是完全的互斥。这一特性最为重要,因为对于高读取频率而相对较低写入的数据结构,使用此类锁同步机制则可以提高并发量。
(d).不管是ReadLock还是WriteLock都支持Interrupt,语义与ReentrantLock一致。
(e).WriteLock支持Condition并且与ReentrantLock语义一致,而ReadLock则不能使用Condition,否则抛出UnsupportedOperationException异常。
以上就是比较重要的,或者衡量是否使用ReentrantReadWriteLock的基础了。下面还是写个小例子说明部分内容:
Java代码
import java.util.HashMap;
import java.util.Map;
import java.util.concurrent.locks.Lock;
import java.util.concurrent.locks.ReentrantReadWriteLock;
/**
* @author: yanxuxin
* @date: 2010-1-7
*/
public class ReentrantReadWriteLockSample {
public static void main(String[] args) {
testReadLock();
// testWriteLock();
}
public static void testReadLock() {
final ReadWriteLockSampleSupport support = new ReadWriteLockSampleSupport();
support.initCache();
Runnable runnable = new Runnable() {
public void run() {
support.get("test");
}
};
new Thread(runnable).start();
new Thread(runnable).start();
new Thread(new Runnable() {
public void run() {
support.put("test", "test");
}
}).start();
}
public static void testWriteLock() {
final ReadWriteLockSampleSupport support = new ReadWriteLockSampleSupport();
support.initCache();
new Thread(new Runnable() {
public void run() {
support.put("key1", "value1");
}
}).start();
new Thread(new Runnable() {
public void run() {
support.put("key2", "value2");
}
}).start();
new Thread(new Runnable() {
public void run() {
support.get("key1");
}
}).start();
}
}
class ReadWriteLockSampleSupport {
private final ReentrantReadWriteLock lock = new ReentrantReadWriteLock();
private final Lock readLock = lock.readLock();
private final Lock writeLock = lock.writeLock();
private volatile boolean completed;
private Map<String,String> cache;
public void initCache() {
readLock.lock();
if(!completed) {
// Must release read lock before acquiring write lock
readLock.unlock(); // (1)
writeLock.lock(); // (2)
if(!completed) {
cache = new HashMap<String,String>(32);
completed = true;
}
// Downgrade by acquiring read lock before releasing write lock
readLock.lock(); // (3)
writeLock.unlock(); // (4) Unlock write, still hold read
}
System.out.println("empty? " + cache.isEmpty());
readLock.unlock();
}
public String get(String key) {
readLock.lock();
System.out.println(Thread.currentThread().getName() + " read.");
startTheCountdown();
try{
return cache.get(key);
}
finally{
readLock.unlock();
}
}
public String put(String key, String value) {
writeLock.lock();
System.out.println(Thread.currentThread().getName() + " write.");
startTheCountdown();
try{
return cache.put(key, value);
}
finally {
writeLock.unlock();
}
}
/**
* A simple countdown,it will stop after about 5s.
*/
public void startTheCountdown() {
long currentTime = System.currentTimeMillis();
for(;;) {
long diff = System.currentTimeMillis() - currentTime;
if(diff > 5000) {
break;
}
}
}
}
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