下面的代码在某处发现后,立马发现存在问题。
public class BlockingQ {
private Object notEmpty = new Object();
private Object notFull = new Object();
private Queue<Object> linkedList = new LinkedList<Object>();
private int maxLength = 10;
public Object take() throws InterruptedException{
synchronized(notEmpty){
if(linkedList.size() == 0){
notEmpty.wait();
}
synchronized(notFull){
if(linkedList.size() == maxLength){
notFull.notifyAll();
}
return linkedList.poll();
}
}
}
public void offer(Object object) throws InterruptedException{
synchronized(notEmpty){
if(linkedList.size() == 0){
notEmpty.notifyAll();
}
synchronized(notFull){
if(linkedList.size() == maxLength){
notFull.wait();
}
linkedList.add(object);
}
}
}
}
最简单的情况就是一个生产者,两个消费者,
if(linkedList.size() == 0){
notEmpty.wait();
}
上面的if语句在第二线程重新获取锁后,进来的时候不会再次判断是否有消费元素,会直接返回null。
public Object take() throws InterruptedException{
synchronized(notEmpty){
while(linkedList.size() == 0){
notEmpty.wait();
}
synchronized(notFull){
if(linkedList.size() == maxLength){
notFull.notifyAll();
}
return linkedList.poll();
}
}
}
修正代码如上面所示。
整个代码主要用作示例,所以代码上存在很冗余的点,下面是代码优化后的版本:
public class SimpleBlockingQ {
private Queue<Object> linkedList = new LinkedList<Object>();
private int maxLength = 10;
public synchronized Object take() throws InterruptedException{
while(linkedList.size() == 0){
wait();
}
notifyAll();
return linkedList.poll();
}
public synchronized void offer(Object object) throws InterruptedException{
notifyAll();
if(linkedList.size() == maxLength){
wait();
}
linkedList.add(object);
}
}
从性能上优化,可以参考《一种简单无锁队列的实现》和《无锁同步栈实现》。
下面是测试代码:
import java.util.concurrent.atomic.AtomicInteger;
/**
* @author 天水
* @date 2013-4-12 下午03:44:15
*/
public class BlockingQTest {
public static AtomicInteger index = new AtomicInteger(0);
public static void main(String[] args){
int tCount = 10; // thread count
final BlockingQ BQ = new BlockingQ();
final SimpleBlockingQ SBQ = new SimpleBlockingQ();
// provider
Runnable pr = new Runnable(){
@Override
public void run() {
while(true){
try {
Thread.sleep(100);
int tindex = index.getAndIncrement();
//BQ.offer(tindex);
//System.out.println("BQ offer: " + tindex);
SBQ.offer(tindex);
System.out.println("SBQ offer: " + tindex);
Thread.sleep(100);
} catch (InterruptedException e) {
e.printStackTrace();
}
}
}
};
// consumer
Runnable cr = new Runnable(){
@Override
public void run() {
while(true){
try {
Thread.sleep(100);
//System.out.println("BQ take: " + BQ.take());
System.out.println("SBQ take: " + SBQ.take());
Thread.sleep(100);
} catch (InterruptedException e) {
e.printStackTrace();
}
}
}
};
for(int i=0; i<tCount; i++){
new Thread(cr).start();
}
for(int i=0; i<tCount; i++){
new Thread(pr).start();
}
}
}