1. 死锁

 死锁指的是两个或以上的线程在执行过程中，因争夺资源而造成的一种互相等待的现象。

// 死锁代码
public class DeadLockDemo {
    public static void main(String[] args) {
        final Object objectA = new Object();
        final Object objectB = new Object();

        new Thread(()->{
            synchronized (objectA) {
                System.out.println("A get LockA and try to get LockB");
                try {
                    TimeUnit.SECONDS.sleep(3);
                } catch (InterruptedException e) {
                    throw new RuntimeException(e);
                }
                synchronized (objectB) {
                    System.out.println("A get LockB");
                }
            }
        }, "A").start();

        new Thread(()->{
            synchronized (objectB) {
                System.out.println("B get LockB and try to get LockA");
                try {
                    TimeUnit.SECONDS.sleep(3);
                } catch (InterruptedException e) {
                    throw new RuntimeException(e);
                }
                synchronized (objectA) {
                    System.out.println("B get LockA");
                }
            }
        }, "B").start();
    }
}

排查死锁代码：jstack pid