How to Fix: Java Concurrency Error: Deadlock Avoidance

How to solve: Java concurrency error: deadlock avoidance

Introduction:

In Java program development, multi-thread concurrency is essential . However, concurrent programming also brings some problems, one of the most common and potentially serious problems is deadlock. Deadlock refers to a situation where two or more threads hold the resources required by each other, but cannot continue execution because the other party does not release the resources. This article will explore how to resolve deadlock issues in concurrency errors in Java and provide some code examples.

1. Understand the causes of deadlock:

Before solving the deadlock problem, you first need to understand the cause of the deadlock. Deadlock usually occurs when multiple threads compete for multiple resources at the same time. A deadlock occurs when two or more threads wait for each other to release a required resource. The following is a simple example code:

class Resource {
    private String name;

    public Resource(String name) {
        this.name = name;
    }

    public synchronized void doSomething() {
        System.out.println(name + " is doing something.");
    }

    public synchronized void doAnotherthing(Resource otherResource) {
        System.out.println(name + " is doing anotherthing.");
        otherResource.doSomething();
    }
}

public class DeadlockExample {
    public static void main(String[] args) {
        Resource resource1 = new Resource("Resource1");
        Resource resource2 = new Resource("Resource2");

        Thread t1 = new Thread(() -> {
            resource1.doAnotherthing(resource2);
        });

        Thread t2 = new Thread(() -> {
            resource2.doAnotherthing(resource1);
        });

        t1.start();
        t2.start();
    }
}

In the above example, there are two resources resource1 and resource2. Two threads t1 and t2 are created in the main method, and the doAnotherthing method of the resource is called respectively. In the t1 thread, it calls the doAnotherthing method of resource1 and passes in resource2 as a parameter. In the t2 thread, it calls the doAnotherthing method of resource2 and passes in resource1 as a parameter.

As the two threads wait for each other to release the required resources, a deadlock occurs. Of course, this is just a simple example, and actual scenarios may include more resources and threads.

2. Solve the deadlock problem:

1. Prevent deadlock:

To prevent deadlock, you first need to understand the cause of deadlock. In the example code above, the deadlock is caused by threads acquiring resources in an inconsistent order. Therefore, we can prevent deadlocks by specifying the order in which threads acquire resources. Modify the sample code as follows:

public class DeadlockExample {
    public static void main(String[] args) {
        Resource resource1 = new Resource("Resource1");
        Resource resource2 = new Resource("Resource2");

        Thread t1 = new Thread(() -> {
            synchronized (resource1) {
                System.out.println("Thread 1 acquired resource 1.");
                synchronized (resource2) {
                    System.out.println("Thread 1 acquired resource 2.");
                }
            }
        });

        Thread t2 = new Thread(() -> {
            synchronized (resource1) {
                System.out.println("Thread 2 acquired resource 1.");
                synchronized (resource2) {
                    System.out.println("Thread 2 acquired resource 2.");
                }
            }
        });

        t1.start();
        t2.start();
    }
}

By stipulating the order in which resources are obtained, it is ensured that each other will not wait for the resources required by the other party, thereby avoiding the occurrence of deadlock.

2. Deadlock detection and recovery:

In addition to preventing deadlocks, deadlock problems can also be solved through deadlock detection and recovery. Java provides the ThreadMXBean interface for monitoring and managing the status of threads. The following is a sample code:

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

public class DeadlockExample {
    public static void main(String[] args) {
        Resource resource1 = new Resource("Resource1");
        Resource resource2 = new Resource("Resource2");

        Thread t1 = new Thread(() -> {
            synchronized (resource1) {
                System.out.println("Thread 1 acquired resource 1.");

                try {
                    Thread.sleep(1000);
                } catch (InterruptedException e) {
                    e.printStackTrace();
                }

                synchronized (resource2) {
                    System.out.println("Thread 1 acquired resource 2.");
                }
            }
        });

        Thread t2 = new Thread(() -> {
            synchronized (resource2) {
                System.out.println("Thread 2 acquired resource 2.");

                try {
                    Thread.sleep(1000);
                } catch (InterruptedException e) {
                    e.printStackTrace();
                }

                synchronized (resource1) {
                    System.out.println("Thread 2 acquired resource 1.");
                }
            }
        });

        t1.start();
        t2.start();

        ThreadMXBean threadMxBean = ManagementFactory.getThreadMXBean();
        long[] deadlockedThreadIds = threadMxBean.findDeadlockedThreads();

        if (deadlockedThreadIds != null) {
            ThreadInfo[] threadInfos = threadMxBean.getThreadInfo(deadlockedThreadIds);
            for (ThreadInfo threadInfo : threadInfos) {
                System.out.println(threadInfo.getThreadName() + " is deadlocked.");
                // 恢复死锁线程的执行，或者进行其他操作
            }
        }
    }
}

In the above sample code, we find the thread where the deadlock occurred through the findDeadlockedThreads method of ThreadMXBean, and handle it accordingly. . You can resume the execution of the deadlocked thread, or perform other operations.

Conclusion:

Deadlock is one of the common problems in multi-threaded concurrent programming. If not solved, it may cause the program to crash or be unable to continue execution. This article introduces two methods to solve the deadlock problem, namely deadlock prevention and deadlock detection and recovery. Of course, these are just some basic solutions, and more complex strategies may be needed to solve the deadlock problem in actual applications. Developers should pay attention to avoiding deadlocks when writing multi-threaded concurrent programs and handle them appropriately to ensure the stability and reliability of the program.

Reference materials:

1. [Java concurrent programming: in-depth understanding of synchronized](https://www.jianshu.com/p/6d293a1a412c)
2. [Java Analysis and solution of thread deadlock problem](https://blog.csdn.net/coslay/article/details/78387673)

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