java 多线程死锁

死锁案例：

package com.test;

public class DealThread implements Runnable {
    
    public String username;
    public Object lock1 = new Object();
    public Object lock2 = new Object();
    
    public void setFlag(String username) {
        this.username = username;
    }
    
    @Override
    public void run() {
        if (username.equals("a")) {
            synchronized (lock1) {
                try {
                    System.out.println("username = " + username);
                    Thread.sleep(3000);
                } catch (Exception e) {
                    e.printStackTrace();
                }
                synchronized (lock2) {
                    System.out.println("按 lock1 -> lock2 代码顺序执行了");
                }
            }
        }
        if (username.equals("b")) {
            synchronized (lock2) {
                try {
                    System.out.println("username = " + username);
                    Thread.sleep(3000);
                } catch (Exception e) {
                    e.printStackTrace();
                }
                synchronized (lock1) {
                    System.out.println("按 lock2 -> lock1 代码顺序执行了");
                }
            }
        }
    }

}

package com.test;

public class Run {
    
    public static void main(String[] args) {
        
        try {
            DealThread d1 = new DealThread();
            d1.setFlag("a");
            Thread thread1 = new Thread(d1);
            thread1.start();
            Thread.sleep(100);
            d1.setFlag("b");
            Thread thread2 = new Thread(d1);
            thread2.start();
        } catch (Exception e) {
            e.printStackTrace();
        }
    }
}

运行结果：
username = a
username = b

死锁是程序设计的Bug，在设计程序时就要避免双方互相持有对方锁的情况。本案例使用synchronized嵌套的代码结构来实现死锁，其实不适用嵌套的代码结构也会出现死锁。 只要互相等待对方释放锁就有可能出现死锁。