Java并发库（十三）：Condition、几个线程有顺序地一个搞一会儿

深切怀念传智播客张孝祥老师，特将其代表作——Java并发库视频研读两遍，受益颇丰，记以后阅

13.java5条件阻塞Condition的应用

       Condition的功能类似在传统线程技术中的Object.wait()和Object.natify()的功能，传统线程技术实现的互斥只能一个线程单独干，不能说这个线程干完了通知另一个线程来干，Condition就是解决这个问题的，实现线程间的通信。比如CPU让小弟做事，小弟说我先歇着并通知大哥，大哥就开始做事。

public interface Condition

Condition 将 Object 监视器方法（wait、notify和notifyAll）分解成截然不同的对象，以便通过将这些对象与任意Lock
实现组合使用，为每个对象提供多个等待 set（wait-set）。其中，Lock 替代了 synchronized 方法和语句的使用，Condition 替代了 Object 监视器方法的使用。

Condition 实例实质上被绑定到一个锁上。要为特定
Lock 实例获得 Condition 实例，请使用其
newCondition()方法。

使用方法：

Lock lock = new ReentrantLock();

Condition condition = lock.newCondition();

this.wait()àcondition.await()

this.notify()àcondition.signal()

注意：判断条件时用while防止虚假唤醒，等待在那里，唤醒后再进行判断，确认符合要求后再执行任务。

public class ConditionCommunication {

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

final Business business = new Business();
new Thread(
new Runnable() {

@Override
public void run() {

for(int i=1;i<=50;i++){
business.sub(i);
}

}
}
).start();

for(int i=1;i<=50;i++){
business.main(i);
}

}

static class Business {
Lock lock = new ReentrantLock();
Condition condition = lock.newCondition();
private boolean bShouldSub = true;
public  void sub(int i){
lock.lock();
try{
while(!bShouldSub){
try {
condition.await();
} catch (Exception e) {
// TODO Auto-generated catch block
e.printStackTrace();
}
}
for(int j=1;j<=10;j++){
System.out.println("sub thread sequence of " + j + ",loop of " + i);
}
bShouldSub = false;
condition.signal();
}finally{
lock.unlock();
}
}

public  void main(int i){
lock.lock();
try{
while(bShouldSub){
try {
condition.await();
} catch (Exception e) {
// TODO Auto-generated catch block
e.printStackTrace();
}
}
for(int j=1;j<=100;j++){
System.out.println("main thread sequence of " + j + ",loop of " + i);
}
bShouldSub = true;
condition.signal();
}finally{
lock.unlock();
}
}

}
}

作为一个示例，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。

 class BoundedBuffer {阻塞队列 满了不能放，空了不能取

  final Lock lock = new ReentrantLock();

  final Condition notFull  =lock.newCondition();

  final Condition notEmpty = lock.newCondition();

 

   final Object[] items = new Object[100];

  int putptr, takeptr, count;

 

  public void put(Object x) throws InterruptedException {

    lock.lock();

    try {

      while (count == items.length)

        notFull.await();

      items[putptr] = x;

       if (++putptr == items.length) putptr = 0;

      ++count;

      notEmpty.signal();

    } finally {

      lock.unlock();

    }

   }

 

  public Object take() throws InterruptedException {

    lock.lock();

    try {

      while (count == 0)

        notEmpty.await();

      Object x = items[takeptr];

      if (++takeptr == items.length) takeptr = 0;

      --count;

      notFull.signal();

      return x;

    } finally {

      lock.unlock();

    }

   }

 }

三个线程一人搞一会儿

public class ThreeConditionCommunication {

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

final Business business = new Business();
new Thread(
new Runnable() {

@Override
public void run() {

for(int i=1;i<=50;i++){
business.sub2(i);
}

}
}
).start();

new Thread(
new Runnable() {

@Override
public void run() {

for(int i=1;i<=50;i++){
business.sub3(i);
}

}
}
).start();

for(int i=1;i<=50;i++){
business.main(i);
}

}

static class Business {
Lock lock = new ReentrantLock();
Condition condition1 = lock.newCondition();
Condition condition2 = lock.newCondition();
Condition condition3 = lock.newCondition();
private int shouldSub = 1;
public void sub2(int i){
lock.lock();
try{
while(shouldSub != 2){
try {
condition2.await();
} catch (Exception e) {
// TODO Auto-generated catch block
e.printStackTrace();
}
}
for(int j=1;j<=10;j++){
System.out.println("sub2 thread sequence of " + j + ",loop of " + i);
}
shouldSub = 3;
condition3.signal();
}finally{
lock.unlock();
}
}

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

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

}
}