最近面試遇到的一道題，需要三個(gè)線(xiàn)程交替打印0-100，當(dāng)時(shí)對(duì)多線(xiàn)程并不是很熟悉因此沒(méi)怎么寫(xiě)出來(lái)，網(wǎng)上搜了之后得到現(xiàn)

synchronized + wait/notifyAll

實(shí)現(xiàn)思路：判斷當(dāng)前打印數(shù)字和線(xiàn)程數(shù)的取余，不等于當(dāng)前線(xiàn)程則處于等待狀態(tài)。循環(huán)結(jié)束喚醒所有等待線(xiàn)程。

public class PrintExample {//創(chuàng)建一個(gè)公共鎖對(duì)象private static final Object Lock = new Object();//執(zhí)行線(xiàn)程數(shù)private static final int THREAD_COUNT = 3;//打印數(shù)字的起始點(diǎn)private static volatile int START = 0;//打印數(shù)字的結(jié)束點(diǎn)private static final int END = 100;private static class Print implements Runnable{private final int index;public Print(int index){this.index = index;}@Overridepublic void run() {while(START<END){synchronized (Lock){//START和線(xiàn)程數(shù)進(jìn)行取余，如果不等于當(dāng)前線(xiàn)程的則等待while(START % THREAD_COUNT != index){try{Lock.wait();}catch (Exception e){e.printStackTrace();}}//否則進(jìn)行輸出if(START<=END){System.out.println("Thread" + (index+1) +  ",打印結(jié)果：" + START);}START++;//喚醒等待線(xiàn)程Lock.notifyAll();}}}public static void main(String[] args) {for(int i = 0; i < THREAD_COUNT; i++){new Thread(new Print(i)).start();}}}
}

ReetrantLock + await/signalAll

實(shí)現(xiàn)思路：實(shí)現(xiàn)方式和synchronized + wait/notifyAll兒乎完全一樣。我們只需要4步：
1.synchronized 替換為ReentrantLock

2.根據(jù)鎖對(duì)象創(chuàng)建一個(gè)Condition對(duì)象

3.wait替換成await

4.notifyAll 替換為 signalAll
?

public class PrintExample {//創(chuàng)建一個(gè)公共鎖對(duì)象private static final ReentrantLock Lock = new ReentrantLock();//根據(jù)鎖對(duì)象創(chuàng)建一個(gè)Condition對(duì)象private static final Condition CONDITION = Lock.newCondition();//執(zhí)行線(xiàn)程數(shù)private static final int THREAD_COUNT = 3;//打印數(shù)字的起始點(diǎn)private static volatile int START = 0;//打印數(shù)字的結(jié)束點(diǎn)private static final int END = 100;private static class Print implements Runnable{private final int index;public Print(int index){this.index = index;}@Overridepublic void run() {while(START<END){Lock.lock();try {//START和線(xiàn)程數(shù)進(jìn)行取余，如果不等于當(dāng)前線(xiàn)程的則等待while(START % THREAD_COUNT != index){try{CONDITION.await();}catch (Exception e){e.printStackTrace();}}//否則進(jìn)行輸出if(START<=END){System.out.println("Thread" + (index+1) +  ",打印結(jié)果：" + START);}START++;//喚醒等待線(xiàn)程CONDITION.signalAll();}catch (Exception e){e.printStackTrace();}finally {Lock.unlock();}}}public static void main(String[] args) {for(int i = 0; i < THREAD_COUNT; i++){new Thread(new Print(i)).start();}}}
}

ReetrantLock + await/signal

因?yàn)镃ondition相對(duì)wait/notify方式，可以喚醒指定線(xiàn)程。那我們就完全不用每次都喚醒全部線(xiàn)程，僅需要喚醒下一次需要執(zhí)行的線(xiàn)程就可以了。
相比較 ReentrantLock + await/signalAll 改進(jìn)方法：
1.去除公共的Condition對(duì)象，替換為L(zhǎng)ist<Condition> conditions；
2.調(diào)用"下一個(gè)線(xiàn)程的"Condition對(duì)象的signal方法喚醒下一個(gè)線(xiàn)程；

public class PrintExample {//創(chuàng)建一個(gè)公共鎖對(duì)象private static final ReentrantLock Lock = new ReentrantLock();//根據(jù)鎖對(duì)象創(chuàng)建一個(gè)Condition對(duì)象//private static final Condition CONDITION = Lock.newCondition();//執(zhí)行線(xiàn)程數(shù)private static final int THREAD_COUNT = 3;//打印數(shù)字的起始點(diǎn)private static volatile int START = 0;//打印數(shù)字的結(jié)束點(diǎn)private static final int END = 100;private static class Print implements Runnable{private final int index;private final List<Condition> conditions;public Print(int index,List<Condition> conditions){this.index = index;this.conditions = conditions;}//只喚醒下一個(gè)線(xiàn)程private void signalNext(){int nextIndex = (index + 1) % THREAD_COUNT;conditions.get(nextIndex).signal();}@Overridepublic void run() {while(START<END){Lock.lock();try {//START和線(xiàn)程數(shù)進(jìn)行取余，如果不等于當(dāng)前線(xiàn)程的則等待while(START % THREAD_COUNT != index){try{conditions.get(index).await();}catch (Exception e){e.printStackTrace();}}//否則進(jìn)行輸出if(START<=END){System.out.println("Thread" + (index+1) +  ",打印結(jié)果：" + START);}START++;//喚醒等待線(xiàn)程signalNext();}catch (Exception e){e.printStackTrace();}finally {Lock.unlock();}}}public static void main(String[] args) {List<Condition> conditionList = new ArrayList<>();conditionList.add(Lock.newCondition());conditionList.add(Lock.newCondition());conditionList.add(Lock.newCondition());for(int i = 0; i < THREAD_COUNT; i++){new Thread(new Print(i,conditionList)).start();}}}
}

此處使用?List<Condition> conditions讓每個(gè)線(xiàn)程都擁有屬于自己的condition，這樣可以單獨(dú)喚醒和等待。

Condition是什么

概念：

condition可以理解為條件隊(duì)列。當(dāng)一個(gè)線(xiàn)程在調(diào)用了其await方法以后，直到線(xiàn)程等待的某個(gè)條件為真的時(shí)候才會(huì)被喚醒。Condition必須要配合鎖一起使用，因?yàn)閷?duì)共享狀態(tài)變量的訪(fǎng)問(wèn)發(fā)生在多線(xiàn)程環(huán)境下。一個(gè)Condition的實(shí)例必須與一個(gè)Lock綁定，因此Condition一般都是作為L(zhǎng)ock的內(nèi)部實(shí)現(xiàn)

方法：

Condition依賴(lài)于Lock接口

方法	解釋
lock.newCondition()	生成一個(gè)Condition
await()	對(duì)應(yīng)Object的wait()；使線(xiàn)程等待
signal()	對(duì)應(yīng)Object的notify()；喚醒線(xiàn)程

注意：調(diào)用Condition的await()和signal()方法，都必須在lock.lock()和lock.unlock()之間使用

在生產(chǎn)者和消費(fèi)者中Condition的執(zhí)行方式：

• 當(dāng)在線(xiàn)程Consumer中調(diào)用await方法后，線(xiàn)程Consumer將釋放鎖，并且將自己沉睡，等待喚醒。
• 這時(shí)等到線(xiàn)程Producer獲取到鎖后，開(kāi)始執(zhí)行任務(wù)，完畢后，調(diào)用Condition的signalall方法，喚醒線(xiàn)程Consumer，線(xiàn)程Consumer恢復(fù)執(zhí)行。

以上說(shuō)明Condition是一個(gè)多線(xiàn)程間協(xié)調(diào)通信的工具類(lèi)，使得某個(gè)或某些線(xiàn)程一起等待某個(gè)條件（Condition）,只有當(dāng)該條件具備( signal 或者 signalAll方法被帶調(diào)用)時(shí) ，這些等待線(xiàn)程才會(huì)被喚醒，從而重新?tīng)?zhēng)奪鎖

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