Java基礎(chǔ)：一個100%會發(fā)生死鎖的程序

?多線程是Java工程師進階所必須掌握的一項技能，也是面試中繞不過的一個環(huán)節(jié)，而死鎖又是多線程同步失敗的經(jīng)典案例，對于復(fù)雜的系統(tǒng)，死鎖是很難通過代碼層面來做靜態(tài)檢測和排查的，所以有的面試官會從反向出發(fā)，讓你手寫一個死鎖程序。
????先來看一個網(wǎng)絡(luò)上常見的死鎖程序（可能存在問題）：

public?class?DeadLockTest?{????private?static?Object?lock1?=?new?Object();????private?static?Object?lock2?=?new?Object();????public?static?void?main(String[]?args)?{????????new?Thread(()?->?{
????????????synchronized?(lock1)?{
????????????????System.out.println("thread1?acquired?lock1");????????????????try?{
????????????????????Thread.sleep(1000);
????????????????}?catch?(InterruptedException?e)?{
????????????????????e.printStackTrace();
????????????????}
????????????????System.out.println("thread1?try?to?acquire?lock2");
????????????????synchronized?(lock2)?{
????????????????????System.out.println("thread1?acquired?lock2");
????????????????}
????????????}
????????},?"t1").start();????????new?Thread(()?->?{
????????????synchronized?(lock2)?{
????????????????System.out.println("thread2?acquired?lock2");????????????????try?{
????????????????????Thread.sleep(1000);
????????????????}?catch?(InterruptedException?e)?{
????????????????????e.printStackTrace();
????????????????}
????????????????System.out.println("thread2?try?to?acquire?lock1");
????????????????synchronized?(lock1)?{
????????????????????System.out.println("thread2?acquired?lock1");
????????????????}
????????????}
????????},?"t2").start();????????
????????//?檢測死鎖
????????checkDeadLock();
????????System.out.println("main?thread?end");
????}????public?static?void?checkDeadLock()?{
????????ThreadMXBean?mxBean?=?ManagementFactory.getThreadMXBean();
????????ScheduledExecutorService?scheduled?=?Executors.newScheduledThreadPool(1);????????//?初始等待5秒，每隔10秒檢測一次
????????scheduled.scheduleAtFixedRate(()->{????????????long[]?threadIds?=?mxBean.findDeadlockedThreads();????????????if?(threadIds?!=?null)?{
????????????????System.out.println("檢測到死鎖線程：");
????????????????ThreadInfo[]?threadInfos?=?mxBean.getThreadInfo(threadIds);????????????????for?(ThreadInfo?info?:?threadInfos)?{
????????????????????System.out.println(info.getThreadId()?+?"："?+?info.getThreadName());
????????????????}
????????????}
????????},?5L,?10L,?TimeUnit.SECONDS);
????}
}

????上面這段程序在99.99%的情況下都會發(fā)生死鎖，但是從理論的角度來講，死鎖并不是100%會發(fā)生的，比如：線程t1先啟動并獲取了鎖lock1，在休眠的這1s的過程中，JVM并未發(fā)生線程調(diào)度（實際上基本不可能），t2未得到執(zhí)行也未獲取到鎖lock2，這時候t1休眠結(jié)束繼續(xù)執(zhí)行并獲取了鎖lock2，那么這種情況下就不會發(fā)生死鎖了。
????如何寫一個100%會發(fā)生死鎖的程序呢？直接上代碼：

public?class?DeadLockTest?{????private?static?Object?lock1?=?new?Object();????private?static?Object?lock2?=?new?Object();????//?這里的flag需要用volatile修飾，以保證線程間的可見性
????private?static?volatile?boolean?flag1?=?false;????private?static?volatile?boolean?flag2?=?false;????public?static?void?main(String[]?args)?{????????new?Thread(()?->?{
????????????synchronized?(lock1)?{
????????????????flag1?=?true;
????????????????System.out.println("thread1?acquired?lock1");????????????????while?(!flag2)?{????????????????????//?無限循環(huán)，等待thread2獲取到lock2后再繼續(xù)往下執(zhí)行（相比使用Thread.sleep(1000)在理論上是100%會出現(xiàn)死鎖）
????????????????????Thread.yield();
????????????????}
????????????????System.out.println("thread1?try?to?acquire?lock2");
????????????????synchronized?(lock2)?{
????????????????????System.out.println("thread1?acquired?lock2");
????????????????}
????????????}
????????},?"t1").start();????????new?Thread(()?->?{
????????????synchronized?(lock2)?{
????????????????flag2?=?true;
????????????????System.out.println("thread2?acquired?lock2");????????????????while?(!flag1)?{
????????????????????Thread.yield();
????????????????}
????????????????System.out.println("thread2?try?to?acquire?lock1");
????????????????synchronized?(lock1)?{
????????????????????System.out.println("thread2?acquired?lock1");
????????????????}
????????????}
????????},?"t2").start();????????//?檢測死鎖
????????checkDeadLock();
????????System.out.println("main?thread?end");
????}????public?static?void?checkDeadLock()?{
????????ThreadMXBean?mxBean?=?ManagementFactory.getThreadMXBean();
????????ScheduledExecutorService?scheduled?=?Executors.newScheduledThreadPool(1);????????//?初始等待5秒，每隔10秒檢測一次
????????scheduled.scheduleAtFixedRate(()?->?{????????????long[]?threadIds?=?mxBean.findDeadlockedThreads();????????????if?(threadIds?!=?null)?{
????????????????System.out.println("檢測到死鎖線程：");
????????????????ThreadInfo[]?threadInfos?=?mxBean.getThreadInfo(threadIds);????????????????for?(ThreadInfo?info?:?threadInfos)?{
????????????????????System.out.println(info.getThreadId()?+?"："?+?info.getThreadName());
????????????????}
????????????}
????????},?5L,?10L,?TimeUnit.SECONDS);
????}
}