What are the situations that cause deadlock in Java?

Four situations

1. Cross lock causes program deadlock: For example, thread A holds the lock of R1 and waits for the lock of R2, and thread B holds the lock of R2 and waits for the lock of R1. Lock

2. Insufficient memory: For example, there are two threads T1 and T2. T1 has obtained 10MB of memory, and T2 has obtained 15MB of memory. Both T1 and T2 need to obtain 30MB of memory to work, but the remaining available memory is 10MB. , so that both threads are waiting for each other to release memory resources

3. Question-and-answer data exchange: The server opens a certain port and waits for the client to access it. After the client sends the request, the server fails due to some The reason is that the client request is missed, causing the client to wait for the server to respond, while the server is waiting for the client to send a request

4. Deadlock caused by an infinite loop: It is relatively common. You cannot see the deadlock using tools such as jstack, but The program does not work and the CPU usage is high. This kind of deadlock is also called system suspended animation, which is difficult to troubleshoot and reproduce

Instance

public class Main {
    private final Object MUTEX_READ = new Object();
    private final Object MUTEX_WRITE = new Object();
 
    public void read(){
        synchronized (MUTEX_READ){
            synchronized (MUTEX_WRITE){
            }
        }
    }
 
    public void write(){
        synchronized (MUTEX_WRITE){
            synchronized (MUTEX_READ){
            }
        }
    }
 
    public static void main(String[] args) throws InterruptedException {
        Main m = new Main();
        new Thread(()->{
            while (true){
                m.read();
            }
        }).start();
        new Thread(()->{
            while (true){
                m.write();
            }
        }).start();
    }
}

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