How to find deadlocked threads?

　In Java, a deadlock is a situation in which two or more threads are blocked indefinitely, waiting for resources held by each other, so that the program cannot continue to execute. Deadlocks are usually caused by threads cyclically waiting for resources. To find deadlocked threads, you can do the following:

　　1. Thread dump (Thread Dump)

　　Through the thread dump, you can view the status and stack information of all current threads, so as to identify whether there is a deadlock. A thread dump can be obtained with the following steps:

　　a. While the program is running, use the console or command line to enter the following command:

jstack <PID>

　　Among them, PID is the process ID of the running Java program.

　　b. The jstack command generates a thread dump of the current Java process and outputs it to the console or to a log file.

　　c. Find the thread state in the dump. If there are threads waiting for resources in a loop, it is likely to be a deadlock thread.

　　2. Use tool analysis

　　Java provides various tools to analyze threads and deadlocks. One of the commonly used tools is VisualVM, which can monitor the running status of Java applications and can detect deadlocks. Here are the steps to use VisualVM to detect deadlocks:

　　a. Start VisualVM and connect to the running Java application.

　　b. In the left navigation bar of VisualVM, find the "Threads" tab, and you can see all the currently running threads.

　　c. Check the thread status and stack information to find out whether any threads are in the BLOCKED state and wait for each other's lock at the same time.

　　d. If multiple threads are found to be in the BLOCKED state and they are waiting for each other's locks, then a deadlock is likely to have occurred.

　　3. Use ThreadMXBean

　　Java provides the ThreadMXBean class to detect deadlocks. ThreadMXBean allows obtaining information about threads at runtime, including deadlock information. Deadlock can be detected by the following code snippet:

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[] threadIds = threadMXBean.findDeadlockedThreads();

        if (threadIds != null) {
            ThreadInfo[] threadInfos = threadMXBean.getThreadInfo(threadIds);
            System.out.println("Detected Deadlock Threads:");
            for (ThreadInfo threadInfo : threadInfos) {
                System.out.println(threadInfo.getThreadName());
            }
        } else {
            System.out.println("No Deadlock Detected.");
        }
    }
}

　　Running the above code will output the thread name (if any) where the deadlock occurred.

　　Note that deadlocks are a complex concurrency problem that may sometimes not be easy to detect and resolve. Therefore, when writing multithreaded applications, it is important to design and use locks carefully to minimize deadlock situations.