出现死锁情况并建议我如何修复，我需要一般建议

Question

我学习 java 多线程。出现了死锁情况，建议我如何解决，我需要一般建议。我明白死锁概念是什么意思，但它可以解决一个问题。这是代码：

java 效用 class:

// in multithreading. 
class Util 
{ 
    // Util class to sleep a thread 
    static void sleep(long millis) 
    { 
        try
        { 
            Thread.sleep(millis); 
        } 
        catch (InterruptedException e) 
        { 
            e.printStackTrace(); 
        } 
    } 
}

java 共享 class:

// This class is shared by both threads 
class Shared 
{ 
    // first synchronized method 
    synchronized void test1(Shared s2) 
    { 
        System.out.println("test1-begin"); 
        Util.sleep(1000); 

        // taking object lock of s2 enters 
        // into test2 method 
        s2.test2(this); 
        System.out.println("test1-end"); 
    } 

    // second synchronized method 
    synchronized void test2(Shared s1) 
    { 
        System.out.println("test2-begin"); 
        Util.sleep(1000); 

        // taking object lock of s1 enters 
        // into test1 method 
        s1.test1(this); 
        System.out.println("test2-end"); 
    } 
}

java 线程 1 class:

class Thread1 extends Thread 
{ 
    private Shared s1; 
    private Shared s2; 

    // constructor to initialize fields 
    public Thread1(Shared s1, Shared s2) 
    { 
        this.s1 = s1; 
        this.s2 = s2; 
    } 

    // run method to start a thread 
    @Override
    public void run() 
    { 
        // taking object lock of s1 enters 
        // into test1 method 
        s1.test1(s2); 
    } 
}

java 线程 2 class:

class Thread2 extends Thread 
{ 
    private Shared s1; 
    private Shared s2; 

    // constructor to initialize fields 
    public Thread2(Shared s1, Shared s2) 
    { 
        this.s1 = s1; 
        this.s2 = s2; 
    } 

    // run method to start a thread 
    @Override
    public void run() 
    { 
        // taking object lock of s2 
        // enters into test2 method 
        s2.test2(s1); 
    } 
}

最终 GFC class 主要方法：

public class GFG 
{ 
    public static void main(String[] args) 
    { 
        // creating one object 
        Shared s1 = new Shared(); 

        // creating second object 
        Shared s2 = new Shared(); 

        // creating first thread and starting it 
        Thread1 t1 = new Thread1(s1, s2); 
        t1.start(); 

        // creating second thread and starting it 
        Thread2 t2 = new Thread2(s1, s2); 
        t2.start(); 

        // sleeping main thread 
        Util.sleep(2000); 
    } 
}

Answer 1

您提供的代码确实会导致死锁。最初，t1 获得 s1 的内在锁，t2 获得 s2 的锁。然后 t1 尝试获取 s2 锁，同时仍然持有 s1 锁。由于s2锁被t2持有，t1会阻塞等待锁被释放。类似的情况发生在 t2 中：它尝试获取 s1 锁，同时仍然持有 s2 锁。由于 s1 锁由 t1 持有，因此 t2 也会阻塞。这会导致死锁。

要解决这个问题，两个线程应该只使用一个锁来同步它们的工作。一个可能的解决方案是：

class Shared
{
    private static final Object LOCK = new Object();

    // first synchronized method
    void test1(Shared s2)
    {
        synchronized (LOCK) {
            System.out.println("test1-begin");
            Util.sleep(1000);

            // taking object lock of s2 enters
            // into test2 method
            s2.test2(this);
            System.out.println("test1-end");
        }
    }

    // second synchronized method
    void test2(Shared s1)
    {
        synchronized (LOCK) {
            System.out.println("test2-begin");
            Util.sleep(1000);

            // taking object lock of s1 enters
            // into test1 method
            s1.test1(this);
            System.out.println("test2-end");
        }
    }
}

这将修复死锁。但是，考虑到目前的实现，会导致间接递归造成的死循环： s1.test1(s2) -> s2.test2(s1) -> s1.test1(s2) -> s2.test2(s1) -> ...

出现死锁情况并建议我如何修复，我需要一般建议

there are caused deadlock condition and advise me how to fix and i need general advises

java

multithreading

deadlock