How to fix: Java Concurrency Error: Thread deadlock

How to solve: Java concurrency error: Thread deadlock

Introduction:
In concurrent programming, thread deadlock is a very common problem. When multiple threads are competing for resources, deadlock may occur if the threads wait for each other to release resources. This article will introduce the concept of thread deadlock, its causes, and how to solve this problem.

1. The concept of thread deadlock
   When multiple threads wait for each other to release resources, all threads are unable to continue executing, forming a thread deadlock. The occurrence of thread deadlock is usually caused by the following four conditions being true at the same time:
2. Mutual exclusion condition: a resource is only allowed to be accessed by one thread at the same time.
3. Request and hold conditions: While a thread occupies resources, it also requests resources occupied by other threads.
4. No deprivation condition: The resource can only be released by the thread that owns it, and other threads cannot deprive it.
5. Loop waiting condition: There is a thread waiting sequence in which each thread is waiting for the next thread to release resources.
6. Causes of thread deadlock
   The causes of thread deadlock are usually the following:
7. Resource contention: multiple threads compete for the same resource at the same time without a suitable scheduling strategy , leading to a deadlock.
8. Lock order deadlock: Threads acquire locks in different orders, causing each other to wait for each other to release the lock.
9. Threads wait for each other: Thread A waits for thread B to release the lock, and thread B waits for thread A to release the lock, resulting in a deadlock.
10. Methods to solve thread deadlock
    In order to solve the thread deadlock problem, we can consider the following methods:

3.1 Avoid circular waiting
Circular waiting is a thread One of the main causes of deadlocks. In order to avoid circular waiting, you can use a resource sorting algorithm, which requires threads to acquire locks in a certain order and release locks in the same order. This eliminates the possibility of waiting in a loop.

3.2 Unify the locking order
A common situation in thread deadlock is that different threads acquire locks in different orders, resulting in waiting for each other. To solve this problem, we can stipulate that all threads must acquire locks in the same order. This can avoid the occurrence of lock sequence deadlock.

3.3 Using the lock timeout mechanism
In multi-threaded programming, the lock timeout mechanism can be used to avoid thread deadlock. When a thread attempts to acquire a lock for more than a certain time limit and fails to acquire the lock, it can choose to give up acquiring the lock and try other processing methods.

The following is a sample code that uses the lock's timeout mechanism to avoid thread deadlock:

public class DeadlockExample {
    private static Object lock1 = new Object();
    private static Object lock2 = new Object();

    public static void main(String[] args) {
        new Thread(() -> {
            synchronized (lock1) {
                try {
                    Thread.sleep(1000);
                } catch (InterruptedException e) {
                    e.printStackTrace();
                }
                synchronized (lock2) {
                    System.out.println("Thread 1");
                }
            }
        }).start();

        new Thread(() -> {
            synchronized (lock2) {
                try {
                    Thread.sleep(1000);
                } catch (InterruptedException e) {
                    e.printStackTrace();
                }
                synchronized (lock1) {
                    System.out.println("Thread 2");
                }
            }
        }).start();

        // 设置超时时间为2秒
        CompletableFuture<Object> future = CompletableFuture.supplyAsync(() -> {
            while (true) {
                if (Thread.holdsLock(lock1) && Thread.holdsLock(lock2)) {
                    return true;
                }
            }
        }).orTimeout(2000, TimeUnit.MILLISECONDS);
        try {
            future.get();
        } catch (TimeoutException e) {
            System.out.println("Deadlock detected!");
            // 执行适当的处理逻辑
        } catch (InterruptedException | ExecutionException e) {
            e.printStackTrace();
        }
    }
}

4. Summary
   Thread deadlock is one of the common problems in concurrent programming. To solve the thread deadlock problem, we can avoid loop waiting, unify the locking sequence, and use the lock timeout mechanism. Through appropriate strategies and technical means, problems caused by thread deadlock can be effectively avoided and the robustness and performance of concurrent programs can be improved.

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