JAVA多线程---Condition使用---线程通信

线程之间除了同步互斥，还要考虑通信。在Java5之前我们的通信方式为：wait 和 notify。那么Condition的优势是支持多路等待，就是我可以定义多个Condition，每个condition控制线程的一条执行通路。传统方式只能是一路等待。我们可以先分析下Java5 Api中的缓冲队列的实现：
假定有一个绑定的缓冲区，它支持 put 和 take 方法。如果试图在空的缓冲区上执行take 操作，则在某一个项变得可用之前，线程将一直阻塞；如果试图在满的缓冲区上执行put 操作，则在有空间变得可用之前，线程将一直阻塞。我们喜欢在单独的等待 set 中保存put 线程和take 线程，这样就可以在缓冲区中的项或空间变得可用时利用最佳规划，一次只通知一个线程。可以使用两个

Condition

实例来做到这一点。

Java代码

class BoundedBuffer {
final Lock lock = new ReentrantLock();//实例化一个锁对象
final Condition notFull = lock.newCondition(); //实例化两个condition
final Condition notEmpty = lock.newCondition();

final Object[] items = new Object[100];//初始化一个长度为100的队列
int putptr, takeptr, count;

public void put(Object x) throws InterruptedException {
lock.lock();//获取锁
try {
while (count == items.length)
notFull.await();//当计数器count等于队列的长度时，不能在插入，因此等待
items[putptr] = x; //将对象放入putptr索引处
if (++putptr == items.length) putptr = 0;//当索引长度等于队列长度时，将putptr置为0
//原因是，不能越界插入
++count;//没放入一个对象就将计数器加1
notEmpty.signal();//一旦插入就唤醒取数据线程
} finally {
lock.unlock();//最后释放锁
}
}

public Object take() throws InterruptedException {
lock.lock();//获取锁
try {
while (count == 0) //如果计数器等于0那么等待
notEmpty.await();
Object x = items[takeptr]; //取得takeptr索引处对象
if (++takeptr == items.length) takeptr = 0;//当takeptr达到队列长度时，从零开始取
--count;//每取一个讲计数器减1
notFull.signal();//枚取走一个就唤醒存线程
return x;
} finally {
lock.unlock();//释放锁
}
}
}

class BoundedBuffer {
final Lock lock = new ReentrantLock();//实例化一个锁对象
final Condition notFull  = lock.newCondition(); //实例化两个condition
final Condition notEmpty = lock.newCondition();

final Object[] items = new Object[100];//初始化一个长度为100的队列
int putptr, takeptr, count;

public void put(Object x) throws InterruptedException {
lock.lock();//获取锁
try {
while (count == items.length)
notFull.await();//当计数器count等于队列的长度时，不能在插入，因此等待
items[putptr] = x; //将对象放入putptr索引处
if (++putptr == items.length) putptr = 0;//当索引长度等于队列长度时，将putptr置为0
//原因是，不能越界插入
++count;//没放入一个对象就将计数器加1
notEmpty.signal();//一旦插入就唤醒取数据线程
} finally {
lock.unlock();//最后释放锁
}
}

public Object take() throws InterruptedException {
lock.lock();//获取锁
try {
while (count == 0) //如果计数器等于0那么等待
notEmpty.await();
Object x = items[takeptr]; //取得takeptr索引处对象
if (++takeptr == items.length) takeptr = 0;//当takeptr达到队列长度时，从零开始取
--count;//每取一个讲计数器减1
notFull.signal();//枚取走一个就唤醒存线程
return x;
} finally {
lock.unlock();//释放锁
}
}
}

下面还有一个例子：

启动main,sub2,sub3三个线程，sub2运行完后sub3运行，sub3运行完成后main运行，main运行完成后sub2运行，如此循环往复50次。实现代码如下：

Java代码

import java.util.concurrent.locks.Condition;
import java.util.concurrent.locks.Lock;
import java.util.concurrent.locks.ReentrantLock;

public class ConditionCommunication {

/**
* @param args
*/
public static void main(String[] args) {

final Business business = new Business();

new Thread(new Runnable(){

public void run() {
for(int i=0; i<50; i++){
business.sub2(i);
}
}

}).start();

new Thread(new Runnable(){

public void run() {
for(int i=0; i<50; i++){
business.sub3(i);
}
}

}).start();

new Thread(new Runnable(){

public void run() {
for(int i=0; i<50; i++){
business.main(i);
}
}

}).start();

}

static class Business{

private int shouldSub = 1;

private Lock lock = new ReentrantLock();

Condition condition1 = lock.newCondition();

Condition condition2 = lock.newCondition();

Condition condition3 = lock.newCondition();

public void sub2(int i){
try{
lock.lock();
while(shouldSub != 2){
try {
// this.wait();
condition2.await();
} catch (Exception e) {
e.printStackTrace();
}
}
for(int j=1; j<=10; j++){
System.out.println("sub2 thread sequence is " + j + " loop of " + i);
}
shouldSub = 3;
// this.notify();
condition3.signal();
}finally{
lock.unlock();
}
}

public void sub3(int i){
try{
lock.lock();
while(shouldSub != 3){
try {
// this.wait();
condition3.await();
} catch (Exception e) {
e.printStackTrace();
}
}
for(int j=1; j<=20; j++){
System.out.println("sub3 thread sequence is " + j + " loop of " + i);
}
shouldSub = 1;
// this.notify();
condition1.signal();
}finally{
lock.unlock();
}
}

public void main(int i){
try{
lock.lock();
while(shouldSub != 1){
try {
// this.wait();
condition1.await();
} catch (Exception e) {
e.printStackTrace();
}
}
for(int j=1; j<=100; j++){
System.out.println("main thread sequence is " + j + " loop of " + i);
}
shouldSub = 2;
// this.notify();
condition2.signal();
}finally{
lock.unlock();
}
}
}

}