notify() 和 notifyAll() 有什么區(qū)別？

先解釋兩個(gè)概念。 等待池：假設(shè)一個(gè)線程A調(diào)用了某個(gè)對(duì)象的wait()方法，線程A就會(huì)釋放該對(duì)象的鎖后，進(jìn)入到了該對(duì)象的等待池，等待池中的線程不會(huì)去競(jìng)爭(zhēng)該對(duì)象的鎖。 鎖池：只有獲取了對(duì)象的鎖，線程才能執(zhí)行對(duì)象的 synchronized 代碼，對(duì)象的鎖每次只有一個(gè)線程可以獲得，其他線程只能在鎖池中等待然后再來(lái)說(shuō)notify和notifyAll的區(qū)別 如果線程調(diào)用了對(duì)象的 wait()方法，那么線程便會(huì)處于該對(duì)象的等待池中，等待池中的線程不會(huì)去競(jìng)爭(zhēng)該對(duì)象的鎖。 當(dāng)有線程調(diào)用了對(duì)象的 notifyAll()方法（喚醒所有 wait 線程）或 notify()方法（只隨機(jī)喚醒一個(gè) wait 線程），被喚醒的的線程便會(huì)進(jìn)入該對(duì)象的鎖池中，鎖池中的線程會(huì)去競(jìng)爭(zhēng)該對(duì)象鎖。也就是說(shuō)，調(diào)用了notify后只要一個(gè)線程會(huì)由等待池進(jìn)入鎖池，而notifyAll會(huì)將該對(duì)象等待池內(nèi)的所有線程移動(dòng)到鎖池中，等待鎖競(jìng)爭(zhēng) 優(yōu)先級(jí)高的線程競(jìng)爭(zhēng)到對(duì)象鎖的概率大，假若某線程沒(méi)有競(jìng)爭(zhēng)到該對(duì)象鎖，它還會(huì)留在鎖池中，唯有線程再次調(diào)用 wait()方法，它才會(huì)重新回到等待池中。而競(jìng)爭(zhēng)到對(duì)象鎖的線程則繼續(xù)往下執(zhí)行，直到執(zhí)行完了 synchronized 代碼塊，它會(huì)釋放掉該對(duì)象鎖，這時(shí)鎖池中的線程會(huì)繼續(xù)競(jìng)爭(zhēng)該對(duì)象鎖。

綜上，所謂喚醒線程，另一種解釋可以說(shuō)是將線程由等待池移動(dòng)到鎖池，notifyAll調(diào)用后，會(huì)將全部線程由等待池移到鎖池，然后參與鎖的競(jìng)爭(zhēng)，競(jìng)爭(zhēng)成功則繼續(xù)執(zhí)行，如果不成功則留在鎖池等待鎖被釋放后再次參與競(jìng)爭(zhēng)。而notify只會(huì)喚醒一個(gè)線程。

有了這些理論基礎(chǔ)，后面的notify可能會(huì)導(dǎo)致死鎖，而notifyAll則不會(huì)的例子也就好解釋了

測(cè)試代碼

public class TestNotifyNotifyAll { private static Object obj = new Object(); public static void main(String[] args) { //測(cè)試 RunnableImplA wait() Thread t1 = new Thread(new RunnableImplA(obj)); Thread t2 = new Thread(new RunnableImplA(obj)); t1.start(); t2.start(); //RunnableImplB notify() Thread t3 = new Thread(new RunnableImplB(obj)); t3.start(); // //RunnableImplC notifyAll()// Thread t4 = new Thread(new RunnableImplC(obj));// t4.start(); } } class RunnableImplA implements Runnable { private Object obj; public RunnableImplA(Object obj) { this.obj = obj; } public void run() { System.out.println('run on RunnableImplA'); synchronized (obj) { System.out.println('obj to wait on RunnableImplA'); try { obj.wait(); } catch (InterruptedException e) { e.printStackTrace(); } System.out.println('obj continue to run on RunnableImplA'); } }} class RunnableImplB implements Runnable { private Object obj; public RunnableImplB(Object obj) { this.obj = obj; } public void run() { System.out.println('run on RunnableImplB'); System.out.println('睡眠3秒...'); try { Thread.sleep(3000); } catch (InterruptedException e) { e.printStackTrace(); } synchronized (obj) { System.out.println('notify obj on RunnableImplB'); obj.notify(); } }} class RunnableImplC implements Runnable { private Object obj; public RunnableImplC(Object obj) { this.obj = obj; } public void run() { System.out.println('run on RunnableImplC'); System.out.println('睡眠3秒...'); try { Thread.sleep(3000); } catch (InterruptedException e) { e.printStackTrace(); } synchronized (obj) { System.out.println('notifyAll obj on RunnableImplC'); obj.notifyAll(); } }}

結(jié)果：僅調(diào)用一次 obj.notify()，線程 t1 或 t2 中的一個(gè)始終在等待被喚醒，程序不終止

run on RunnableImplAobj to wait on RunnableImplArun on RunnableImplAobj to wait on RunnableImplArun on RunnableImplB睡眠3秒...notify obj on RunnableImplBobj continue to run on RunnableImplA

把 t3 注掉，啟動(dòng) t4 線程。調(diào)用 obj.notifyAll() 方法

public class TestNotifyNotifyAll { private static Object obj = new Object(); public static void main(String[] args) { //測(cè)試 RunnableImplA wait() Thread t1 = new Thread(new RunnableImplA(obj)); Thread t2 = new Thread(new RunnableImplA(obj)); t1.start(); t2.start(); // //RunnableImplB notify()// Thread t3 = new Thread(new RunnableImplB(obj));// t3.start(); //RunnableImplC notifyAll() Thread t4 = new Thread(new RunnableImplC(obj)); t4.start(); } }

結(jié)果：t1、t2線程均可以執(zhí)行完畢

run on RunnableImplAobj to wait on RunnableImplArun on RunnableImplAobj to wait on RunnableImplArun on RunnableImplC睡眠3秒...notifyAll obj on RunnableImplCobj continue to run on RunnableImplAobj continue to run on RunnableImplA

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