java - 多線程死鎖測試
問題描述
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先運行一秒TimeUnit.SECONDS.sleep(1);t2.start(); }}
如上,隨便找的產生死鎖的代碼,問題:TimeUnit.SECONDS.sleep(1);加上這行后,不存在死鎖問題。sleep并不釋放鎖,為何這邊死鎖情況會消失。輸出結果為: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
問題解答
回答1: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.此時但是他們同時都想要獲取other的鎖 此時誰也不讓誰 發(fā)生死鎖解決方法讓倆個線程不要同時去搶第二把鎖.讓A停一會但是如果你把時間調成納秒級別 多次嘗試也會發(fā)生死鎖不建議這樣預防死鎖.如果并發(fā)量高的情況下.
回答2:雙方在爭同一把鎖,不會死鎖啊
回答3: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,歡迎指正,輕噴在沒有sleep時,a線程啟動,完成tryOther方法,釋放鎖并去執(zhí)行other方法,此時b獲得鎖執(zhí)行tryOther方法,此時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,由此產生死鎖。
加上sleep后。在a執(zhí)行tryOther方法釋放鎖,此時b線程并沒有執(zhí)行,此時順利獲得other鎖。2s后b線程執(zhí)行,無死鎖環(huán)境。
相關文章:
1. javascript - 原生canvas中如何獲取到觸摸事件的canvas內坐標?2. showpassword里的this 是什么意思?代表哪個元素3. mysql scripts提示 /usr/bin/perl: bad interpreter4. css3 - css怎么實現圖片環(huán)繞的效果5. html - vue項目中用到了elementUI問題6. 對mysql某個字段監(jiān)控的功能7. java - spring boot @Autowired8. JavaScript事件9. css3 - border-bottom 的長度可否超過盒子的寬度呢?實現如下圖效果。(我的書下面的線)10. mysql優(yōu)化 - mysql EXPLAIN之后怎么看結果進行優(yōu)化 ?
