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歡迎跳轉到本文的原文鏈接：https://honeypps.com/java/java-multi-thread-of-condition/

? ? 內置條件隊列存在一些缺陷。每個內置鎖都只能有一個相關聯的條件隊列，因而在像BounderBuffer這種類中，多個線程可能在同一個條件隊列上等待不同的條件謂詞，并且在最常見的加鎖模式下公開條件隊列對象。這些因素都使得無法滿足在使用notifyAll時所有等待線程為統一類型的需求。如果想編寫一個帶有多個條件謂詞的并發對象，或者想獲得除了條件隊列可見性之外的更多控制權，就可以使用顯示的Lock和Condition而不是內置鎖和條件隊列，這是一種更靈活的選擇。

? ? 一個Condition和一個Lock關聯在一起，就想一個條件隊列和一個內置鎖相關聯一樣。要創建一個Condition，可以在相關聯的Lock上調用Lock.newCondition方法。正如Lock比內置加鎖提供了更為豐富的功能，Condition同樣比內置條件隊列提供了更豐富的功能：在每個鎖上可存在多個等待、條件等待可以是可中斷的或者不可中斷的、基于時限的等待，以及公平的或非公平的隊列操作。

? ? 與內置條件隊列不同的是，對于每個Lock，可以有任意數量的Condition對象。Condition對象繼承了相關的Lock對象的公平性，對于公平的鎖，線程會依照FIFO順序從Condition.await中釋放。

Condition接口：

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public interface Condition{void await() throws InterruptedException;boolean await(long time, TimeUnit unit) throws InterruptedException;long awaitNanos(long nanosTimeout) throws InterruptedException;void awaitUniterruptibly();boolean awaitUntil(Date deadline) throws InterruptedException;void signal();void signalAll(); }

? ? 注意：在Condition對象中，與wait,notify和notifyAll方法對于的分別是await,signal,signalAll。但是，Condition對Object進行了擴展，因而它也包含wait和notify方法。一定要確保使用的版本——await和signal.
官方使用方法如下：

class BoundedBuffer {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();}} }

? ? 或者通過一個實際的例子來解釋Condition的用法：
? ? 我們要打印1-9這0個數字，由A線程先打印1-3，然后由B線程打印4-6，然后再由A線程打印7-9.我們采用Condition來演示解決方法：

package com.cooperation;import java.util.concurrent.locks.Condition; import java.util.concurrent.locks.Lock; import java.util.concurrent.locks.ReentrantLock;public class TestCondition {private static int value = 1;private Lock lock = new ReentrantLock();private Condition Condition456 = lock.newCondition();private Condition Condition789 = lock.newCondition();class ThreadA implements Runnable{@Overridepublic void run(){try{lock.lock();System.out.println("首先輸出1-3");while(value<=3){System.out.println(value++);}Condition456.signal();}finally{lock.unlock();}try{lock.lock();while(value<=6){Condition789.await();}System.out.println("輸出7-9");while(value<=9){System.out.println(value++);}}catch (InterruptedException e){e.printStackTrace();}finally{lock.unlock();}}}class ThreadB implements Runnable{@Overridepublic void run(){try{lock.lock();while(value<=3){Condition456.await();}}catch (InterruptedException e){e.printStackTrace();}finally{lock.unlock();}try{lock.lock();System.out.println("輸出4-6");while(value<=6){System.out.println(value++);}Condition789.signal();}finally{lock.unlock();}}}public static void main(String[] args){TestCondition test = new TestCondition();Thread threadA = new Thread(test.new ThreadA());Thread threadB = new Thread(test.new ThreadB());threadA.start();threadB.start();}}

輸出結果：

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首先輸出1-3 1 2 3 輸出4-6 4 5 6 輸出7-9 7 8 9

? ? 如果需要采用Object方法的wait,notify,notifyAll方法實現這個實例可以參考：http://outofmemory.cn/java/java.util.concurrent/thread-sync-with-object-wait-notify-notifyAll
? ? 同樣可以擴張，ThreadA打印123，ThreadB打印456，再讓ThreadA打印789，最后然ThreadB打印10 11 12.如下：

package com.cooperation;import java.util.concurrent.locks.Condition; import java.util.concurrent.locks.Lock; import java.util.concurrent.locks.ReentrantLock;public class TestCondition {private static int value = 1;private Lock lock = new ReentrantLock();private Condition Condition456 = lock.newCondition();private Condition Condition789 = lock.newCondition();private Condition Condition101112 = lock.newCondition();class ThreadA implements Runnable{@Overridepublic void run(){try{lock.lock();System.out.println("首先輸出1-3");while(value<=3){System.out.println(value++);}Condition456.signal();}finally{lock.unlock();}try{lock.lock();while(value<=6){Condition789.await();}System.out.println("輸出7-9");while(value<=9){System.out.println(value++);}Condition101112.signal();}catch (InterruptedException e){e.printStackTrace();}finally{lock.unlock();}}}class ThreadB implements Runnable{@Overridepublic void run(){try{lock.lock();while(value<=3){Condition456.await();}}catch (InterruptedException e){e.printStackTrace();}finally{lock.unlock();}try{lock.lock();System.out.println("輸出4-6");while(value<=6){System.out.println(value++);}Condition789.signal();}finally{lock.unlock();}try{lock.lock();while(value<=9){Condition101112.await();}}catch (InterruptedException e){e.printStackTrace();}finally{lock.unlock();}try{lock.lock();System.out.println("輸出10-12");while(value<=12){System.out.println(value++);}}finally{lock.unlock();}}}public static void main(String[] args){TestCondition test = new TestCondition();Thread threadA = new Thread(test.new ThreadA());Thread threadB = new Thread(test.new ThreadB());threadA.start();threadB.start();} }

輸出結果：

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首先輸出1-3 1 2 3 輸出4-6 4 5 6 輸出7-9 7 8 9 輸出10-12 10 11 12

歡迎跳轉到本文的原文鏈接：https://honeypps.com/java/java-multi-thread-of-condition/

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歡迎支持筆者新作：《深入理解Kafka:核心設計與實踐原理》和《RabbitMQ實戰指南》，同時歡迎關注筆者的微信公眾號：朱小廝的博客。

超強干貨來襲 云風專訪：近40年碼齡，通宵達旦的技術人生

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