問(wèn)題描述

package test;import java.text.DateFormat;import java.text.SimpleDateFormat;import java.util.Date;import java.util.concurrent.TimeUnit;/** * Created by rhwayfun on 16-4-3. */public class ThreadTest { private static DateFormat format = new SimpleDateFormat('HH:mm:ss'); public synchronized void tryOther(ThreadTest other) throws InterruptedException {System.out.println(Thread.currentThread().getName() + ' enter tryOther method at ' + format.format(new Date())); System.out.println(Thread.currentThread().getName() + ' tryOther method is about to invoke other method at ' + format.format(new Date()));other.other(); } public synchronized void other() throws InterruptedException {System.out.println(Thread.currentThread().getName() + ' enter other method atatatatat ' + format.format(new Date())); } public static void main(String[] args) throws InterruptedException {final ThreadTest d1 = new ThreadTest();final ThreadTest d2 = new ThreadTest();Thread t1 = new Thread(new Runnable() { public void run() {try { d1.tryOther(d2);} catch (InterruptedException e) { e.printStackTrace();} }}, 'threadA');Thread t2 = new Thread(new Runnable() { public void run() {try { d2.tryOther(d1);} catch (InterruptedException e) { e.printStackTrace();} }}, 'threadB');t1.start();//讓threadA先運(yùn)行一秒TimeUnit.SECONDS.sleep(1);t2.start(); }}

如上，隨便找的產(chǎn)生死鎖的代碼，問(wèn)題：TimeUnit.SECONDS.sleep(1);加上這行后，不存在死鎖問(wèn)題。sleep并不釋放鎖，為何這邊死鎖情況會(huì)消失。輸出結(jié)果為：threadA enter tryOther method at 15:37:39threadA tryOther method is about to invoke other method at 15:37:39threadA enter other method atatatatat 15:37:39threadB enter tryOther method at 15:37:40threadB tryOther method is about to invoke other method at 15:37:40threadB enter other method atatatatat 15:37:40

注掉這行，正常死鎖。輸出結(jié)果為：threadB enter tryOther method at 15:37:10threadA enter tryOther method at 15:37:10threadB tryOther method is about to invoke other method at 15:37:10threadA tryOther method is about to invoke other method at 15:37:10

問(wèn)題解答

線程A拿到tryOther鎖但是他還要得到other的鎖線程B拿到tryOther的鎖但是他還要拿到other的鎖 有可能A剛剛釋放鎖B也剛剛釋放tryOther的鎖.此時(shí)但是他們同時(shí)都想要獲取other的鎖 此時(shí)誰(shuí)也不讓誰(shuí) 發(fā)生死鎖解決方法讓倆個(gè)線程不要同時(shí)去搶第二把鎖.讓A停一會(huì)但是如果你把時(shí)間調(diào)成納秒級(jí)別 多次嘗試也會(huì)發(fā)生死鎖不建議這樣預(yù)防死鎖.如果并發(fā)量高的情況下.

雙方在爭(zhēng)同一把鎖，不會(huì)死鎖啊

自己突然知道為什么了，懶得刪帖子了。寫下我的看法，如果有錯(cuò)，歡迎指正，輕噴在沒(méi)有sleep時(shí)，a線程啟動(dòng)，完成tryOther方法，釋放鎖并去執(zhí)行other方法，此時(shí)b獲得鎖執(zhí)行tryOther方法，此時(shí)a在other方法中所需資源被b線程鎖住，b在執(zhí)行完tryOther后需要獲得a資源，由此產(chǎn)生死鎖。

加上sleep后。在a執(zhí)行tryOther方法釋放鎖，此時(shí)b線程并沒(méi)有執(zhí)行，此時(shí)順利獲得other鎖。2s后b線程執(zhí)行，無(wú)死鎖環(huán)境。