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

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 多次嘗試也會發生死鎖不建議這樣預防死鎖.如果并發量高的情況下.

雙方在爭同一把鎖，不會死鎖啊

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，歡迎指正，輕噴在沒有sleep時，a線程啟動，完成tryOther方法，釋放鎖并去執行other方法，此時b獲得鎖執行tryOther方法，此時a在other方法中所需資源被b線程鎖住，b在執行完tryOther后需要獲得a資源，由此產生死鎖。

加上sleep后。在a執行tryOther方法釋放鎖，此時b線程并沒有執行，此時順利獲得other鎖。2s后b線程執行，無死鎖環境。