死锁是指两个或两个以上的进程在执行过程中，由于竞争资源或者由于彼此通信而造成的一种阻塞的现象，若无外力作用，它们都将无法推进下去。此时称系统处于死锁状态或系统产生了死锁，这些永远在互相等待的进程称为死锁进程。
Java线程中我们构造两个相互等待对方释放资源的线程就构成了死锁。
代码：
public class DeadLockSimulation {
private static String A = "a";
private static String B = "b";
public static void main(String[] args) {
new DeadLockSimulation().deadLock();
}
private void deadLock() {
Thread t1 = new Thread(new Runnable() {
@Override
public void run() {
synchronized (A) {
try {
Thread.sleep(1000);
} catch (InterruptedException e) {
e.printStackTrace();
}
synchronized (B) {
System.out.println("t1");
}
}
}
});
Thread t2 = new Thread(new Runnable() {
@Override
public void run() {
synchronized (B) {
synchronized (A) {
System.out.println("t2");
}
}
}
});
t1.start();
t2.start();
}
}
运行上边的代码产生死锁。JDK自带的工具有jconsole、jvisualvm、jstack等。
jvisualvm
运行jvisualvm如下：
发现提示检测到死锁。
jstack
查看进程号：
jps // 查看Java任务进程号
2784 Jps
8672 Main
18068 Launcher
12520 KotlinCompileDaemon
16776 DeadLockSimulation
9576
16668 RemoteMavenServer
运行jstack。
jstack [进程号] // 查看当前进程堆栈
Found one Java-level deadlock:
=============================
"Thread-0":
waiting to lock monitor 0x0000012f11b53e80 (object 0x0000000741d763e8, a java.lang.String),
which is held by "Thread-1"
"Thread-1":
waiting to lock monitor 0x0000012f11b51e80 (object 0x0000000741d76400, a java.lang.String),
which is held by "Thread-0"
Java stack information for the threads listed above:
===================================================
"Thread-0":
at com.lin.juc.mydesign.DeadLockSimulation$1.run(DeadLockSimulation.java:28)
- waiting to lock <0x0000000741d763e8> (a java.lang.String)
- locked <0x0000000741d76400> (a java.lang.String)
at java.lang.Thread.run([email protected]/Thread.java:834)
"Thread-1":
at com.lin.juc.mydesign.DeadLockSimulation$2.run(DeadLockSimulation.java:39)
- waiting to lock <0x0000000741d76400> (a java.lang.String)
- locked <0x0000000741d763e8> (a java.lang.String)
at java.lang.Thread.run([email protected]/Thread.java:834)
Found 1 deadlock.
MXBean
MXBean是JDK自带的用于扫描程序是否存在死锁包， 但是扫描的过程中存在性能损耗。
代码：
public class ScanDeadLock {
public void scanDeadLock() {
ThreadMXBean mxBean = ManagementFactory.getThreadMXBean();
Runnable runnable = () -> {
long[] ids = mxBean.findDeadlockedThreads();
System.out.println("扫描死锁...");
if (ids != null) {
ThreadInfo[] threadInfos = mxBean.getThreadInfo(ids);
for (ThreadInfo info : threadInfos) {
System.out.println("info = " + info);
}
}
};
ExecutorService executorService = Executors.newScheduledThreadPool(4);
executorService.execute(runnable);
}
}
死锁预防
- 避免一个线程同时获取多个锁。
- 避免一个线程在锁内同时占用多个资源，尽量保证每个锁只占用一个资源。
- 尝试使用定时锁，使用lock.tryLock（timeout）来替代使用内部锁机制。
- 对于数据库锁，加锁和解锁必须在一个数据库连接里，否则会出现解锁失败的情况。