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(); } } } }