多线程是Java工程师进阶所必须掌握的一项技能,也是面试中绕不过的一个环节,而死锁又是多线程同步失败的经典案例,对于复杂的系统,死锁是很难通过代码层面来做静态检测和排查的,所以有的面试官会从反向出发,让你手写一个死锁程序。
先来看一个网络上常见的死锁程序(可能存在问题):
创新互联建站主营峨眉山网站建设的网络公司,主营网站建设方案,成都app软件开发公司,峨眉山h5微信小程序开发搭建,峨眉山网站营销推广欢迎峨眉山等地区企业咨询
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(); // 检测死锁 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秒检测一次 scheduled.scheduleAtFixedRate(()->{ long[] threadIds = mxBean.findDeadlockedThreads(); if (threadIds != null) { System.out.println("检测到死锁线程:"); ThreadInfo[] threadInfos = mxBean.getThreadInfo(threadIds); for (ThreadInfo info : threadInfos) { System.out.println(info.getThreadId() + ":" + info.getThreadName()); } } }, 5L, 10L, TimeUnit.SECONDS); } }
上面这段程序在99.99%的情况下都会发生死锁,但是从理论的角度来讲,死锁并不是100%会发生的,比如:线程t1先启动并获取了锁lock1,在休眠的这1s的过程中,JVM并未发生线程调度(实际上基本不可能),t2未得到执行也未获取到锁lock2,这时候t1休眠结束继续执行并获取了锁lock2,那么这种情况下就不会发生死锁了。
如何写一个100%会发生死锁的程序呢?直接上代码:
public class DeadLockTest { private static Object lock1 = new Object(); private static Object lock2 = new Object(); // 这里的flag需要用volatile修饰,以保证线程间的可见性 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) { // 无限循环,等待thread2获取到lock2后再继续往下执行(相比使用Thread.sleep(1000)在理论上是100%会出现死锁) 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(); // 检测死锁 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秒检测一次 scheduled.scheduleAtFixedRate(() -> { long[] threadIds = mxBean.findDeadlockedThreads(); if (threadIds != null) { System.out.println("检测到死锁线程:"); ThreadInfo[] threadInfos = mxBean.getThreadInfo(threadIds); for (ThreadInfo info : threadInfos) { System.out.println(info.getThreadId() + ":" + info.getThreadName()); } } }, 5L, 10L, TimeUnit.SECONDS); } }