内部锁（对象监视器）使用Object类的wait(), notify(), notifyAll()方法来进行线程之间的协作通信；Lock锁引入Condition来支持线程之间通信协作，Condition提供await(), signal(), signalAll()方法实现与内部锁同样的等待与唤醒功能，但与内部锁不同的是一个Lock可以绑定多个Condition，以满足不同条件下唤醒不同线程的功能。
最典型的线程协作例子就是生产者与消费者。如果使用内部锁控制线程通信，所有线程，不管生产者还是消费者，都被同一个对象监视，当新生产的对象放入消费队列后，生产者会唤醒所有的线程，包括其它生产者，这时队列可能已经满了，所以被唤醒的生产者只好继续回头进入等待队列，一直轮询直到唤醒一个消费者才会进行消费，然后继续。因为Lock支持绑定多个Condition，所以如果使用Lock实现，可以为当队列有对象可消费创建一个Condition，当队列有空位创建一个Condition，这样，生产者和消费者可以相互明确告知，而不是像内部锁广播的方式而缺乏明确的目标对象。
简而言之，内部锁的notify()与notifyAll()会唤醒任何一个线程，不管线程的等待条件是否已经满足；Condition则提供了针对不同类型线程定制唤醒条件的实现，减少无谓的唤醒。
另外，经过测试，在多核情况下，一个线程还可能被一些其它的条件唤醒，所以测试结果并不一定会满意，有待细究。
内部锁与Lock方式实现的生产者与消费者代码案例：
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import java.util.LinkedList;
import java.util.Queue;
import java.util.concurrent.TimeUnit;
import java.util.concurrent.locks.Condition;
import java.util.concurrent.locks.Lock;
import java.util.concurrent.locks.ReentrantLock;
public class TestLockCondition {
private static final int MAX_SIZE = 3;
private static final int PRODUCER_NUM = 5;
private static final int CONSUMER_NUM = 1;
private Queue<Object> queue = new LinkedList<Object>();
private volatile boolean isWorkingTime = true;
private volatile boolean isUsingLock = true;
private Lock lock = new ReentrantLock();
private Condition full = lock.newCondition();
private Condition empty = lock.newCondition();
private void produceWithLock() throws InterruptedException{
lock.lock();
try{
while(queue.size() >= MAX_SIZE){
System.out.println(Thread.currentThread().getName() + " go wait.");
full.await();
System.out.println(Thread.currentThread().getName() + " was awaken.");
}
queue.offer(new Object());
System.out.println(Thread.currentThread().getName() + " produce one object.");
empty.signalAll();
//empty.signal();
}
finally{
lock.unlock();
}
}
private void consumeWithLock() throws InterruptedException{
lock.lock();
try{
while(queue.isEmpty()){
System.out.println(Thread.currentThread().getName() + " go wait.");
empty.await();
System.out.println(Thread.currentThread().getName() + " was awaken.");
}
queue.poll();
System.out.println(Thread.currentThread().getName() + " consume one object.");
full.signalAll();
//full.signal();
}
finally{
lock.unlock();
}
}
private synchronized void produce() throws InterruptedException{
while(queue.size() >= MAX_SIZE){
System.out.println(Thread.currentThread().getName() + " go wait.");
wait();
System.out.println(Thread.currentThread().getName() + " was awaken.");
}
queue.offer(new Object());
System.out.println(Thread.currentThread().getName() + " produce one object.");
//notifyAll();
notify();
}
private synchronized void consume() throws InterruptedException{
while(queue.isEmpty()){
System.out.println(Thread.currentThread().getName() + " go wait.");
wait();
System.out.println(Thread.currentThread().getName() + " was awaken.");
}
queue.poll();
System.out.println(Thread.currentThread().getName() + " consume one object.");
//notifyAll();
notify();
}
public static void main(String[] args) throws InterruptedException {
TestLockCondition testLockCondition = new TestLockCondition();
Thread[] producers = new Thread[PRODUCER_NUM];
for(int i=0; i<PRODUCER_NUM; i++){
producers[i] = new Thread(testLockCondition.new Producer());
producers[i].start();
}
Thread[] consumers = new Thread[CONSUMER_NUM];
for(int i=0; i<CONSUMER_NUM; i++){
consumers[i] = new Thread(testLockCondition.new Consumer());
consumers[i].start();
}
TimeUnit.SECONDS.sleep(10);
for(int i=0; i<PRODUCER_NUM; i++){
System.out.println(producers[i].getName() + " : " + producers[i].getState());
producers[i].interrupt();
}
for(int i=0; i<CONSUMER_NUM; i++){
System.out.println(consumers[i].getName() + " : " + consumers[i].getState());
consumers[i].interrupt();
}
testLockCondition.isWorkingTime = false;
}
private class Consumer implements Runnable{
@Override
public void run() {
try {
while(isWorkingTime){
if(isUsingLock){
//System.out.println(Thread.currentThread().getName() + " is using Lock.");
consumeWithLock();
}
else{
consume();
}
}
} catch (InterruptedException e) {
e.printStackTrace();
}
}
}
private class Producer implements Runnable{
@Override
public void run() {
try {
while(isWorkingTime){
if(isUsingLock){
//System.out.println(Thread.currentThread().getName() + " is using Lock.");
produceWithLock();
}
else{
produce();
}
}
} catch (InterruptedException e) {
e.printStackTrace();
}
}
}
} |
本文转自sarchitect 51CTO博客，原文链接http://blog.51cto.com/stevex/1300223，如需转载请自行联系原作者