How to deal with Java deadlock problem

1. Introduction to Deadlock

Deadlock refers to a situation in a Java program where multiple threads wait for each other due to competition for resources, resulting in the inability to continue executing the process. The occurrence of deadlock prevents the threads involved from continuing to execute, causing the entire program to stall.

2. The conditions for Java deadlock can be summarized into the following four:

• Mutual Exclusion Conditions (Mutual Exclusion): Resources are in the same Time can only be occupied by one thread. When a thread has occupied a resource, other threads cannot access the resource until the resource is released by the occupying thread.

• Hold and Wait (Hold and Wait): While holding at least one resource, a thread attempts to request resources occupied by other threads. This will cause the thread to still hold the resource it already owns while waiting for other resources.

• Non-preemption condition (No Preemption): The resources occupied by the thread cannot be preempted by other threads. Only when a thread actively releases the resource can other threads acquire the resource.

• Circular Wait (Circular Wait): There is a group of threads T1, T2,..., Tn, among which T1 waits for the resources occupied by T2, and T2 waits for T3 The occupied resources,...,Tn waits for the resources occupied by T1, forming a cyclic waiting relationship.

3. Causes of deadlock

• Resource competition between threads: When multiple threads access shared resources at the same time, resource competition may occur. , resulting in deadlock.

• Circular waiting: There is a cyclic waiting relationship between threads for resources, causing each thread to wait for other threads to release resources.

• Inconsistent order: When threads request resources, if they do not request them in a fixed order, it is easy to cause deadlock.

4. Strategies to avoid deadlock

• Request resources in a fixed order: Ensure that all threads request resources in the same order. This reduces the possibility of deadlock.

• Avoid circular waiting: Ensure that there is no circular waiting relationship between threads for resources.

• Use lock timeout settings: In Java, you can use the tryLock() method to set the lock timeout so that the lock can be automatically released after the timeout and reduce the possibility of deadlock. occur.

5. Code Example

The following is a Java deadlock example:

public class DeadlockDemo {
    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("Thread 1: Holding lock 1");
                try {
                    Thread.sleep(100);
                } catch (InterruptedException e) {
                    e.printStackTrace();
                }
                System.out.println("Thread 1: Waiting for lock 2");
                synchronized (lock2) {
                    System.out.println("Thread 1: Holding lock 1 & 2");
                }
            }
        }).start();

        new Thread(() -> {
            synchronized (lock2) {
                System.out.println("Thread 2: Holding lock 2");
                try {
                    Thread.sleep(100);
                } catch (InterruptedException e) {
                    e.printStackTrace();
                }
                System.out.println("Thread 2: Waiting for lock 1");
                synchronized (lock1) {
                    System.out.println("Thread 2: Holding lock 1 & 2");
                }
            }
        }).start();
    }
}

In the above example, thread 1 and thread 2 are locked respectively lock1 and lock2. However, when trying to obtain the resources locked by the other party, a deadlock occurs because both parties are waiting for the other party to release the resources.

6. Diagnosing deadlocks

Java provides some tools and methods to detect and analyze deadlock problems.

• Use the jstack tool: jstack is a command line tool for Java that can be used to analyze thread stack information. When a deadlock occurs in a program, you can view the thread status through jstack to determine which threads have deadlocked.

• Using ThreadMXBean: ThreadMXBean is part of the Java Management Extensions (JMX) and can be used to detect deadlocks. Here is a simple example:

import java.lang.management.ManagementFactory;
import java.lang.management.ThreadInfo;
import java.lang.management.ThreadMXBean;

public class DeadlockDetector {
    public static void main(String[] args) {
        ThreadMXBean threadMXBean = ManagementFactory.getThreadMXBean();
        long[] deadlockedThreads = threadMXBean.findDeadlockedThreads();

        if (deadlockedThreads != null) {
            ThreadInfo[] threadInfos = threadMXBean.getThreadInfo(deadlockedThreads);
            for (ThreadInfo threadInfo : threadInfos) {
                System.out.println("Deadlocked thread: " + threadInfo.getThreadId() + " - " + threadInfo.getThreadName());
            }
        } else {
            System.out.println("No deadlocked threads found.");
        }
    }
}

The above is the detailed content of How to deal with Java deadlock problem. For more information, please follow other related articles on the PHP Chinese website!