Thread Deadlocks

Thread deadlocks occur when two or more threads are blocked forever, waiting for each other to release the resources they hold. It happens when threads acquire resources in different orders, resulting in a cyclical dependency between the threads.

Causes of Thread Deadlocks

There are four necessary conditions for thread deadlocks:

1. Mutual Exclusion: Threads must request exclusive control of the resources they need and cannot be shared.
2. Hold and Wait: Threads must hold at least one resource while waiting for another resource.
3. No Preemption: Resources cannot be forcibly taken away from a thread; they can only be released voluntarily.
4. Circular Wait: There is a circular chain of two or more threads, where each thread is waiting for a resource held by the next thread.

Example of Thread Deadlock

Consider the following code snippet:

1// Create two resources
2Resource resource1 = new Resource();
3Resource resource2 = new Resource();
4
5// Create two threads
6Thread thread1 = new Thread(() -> {
7  synchronized (resource1) {
8    System.out.println("Thread 1 acquired resource 1");
9    try {
10      Thread.sleep(1000);
11    } catch (InterruptedException e) {
12      e.printStackTrace();
13    }
14    synchronized (resource2) {
15      System.out.println("Thread 1 acquired resource 2");
16    }
17  }
18});
19
20Thread thread2 = new Thread(() -> {
21  synchronized (resource2) {
22    System.out.println("Thread 2 acquired resource 2");
23    synchronized (resource1) {
24      System.out.println("Thread 2 acquired resource 1");
25    }
26  }
27});
28
29// Start the threads
30thread1.start();
31thread2.start();

In this example, thread1 acquires resource1 and then waits for resource2, while thread2 acquires resource2 and then waits for resource1. This creates a circular dependency between the two threads, leading to a deadlock.

Preventing Thread Deadlocks

To prevent deadlocks, you can follow the following guidelines:

• Avoid holding multiple locks: If possible, try to use only one lock at a time to avoid circular dependencies.
• Lock resources in a consistent order: If multiple locks are needed, make sure to acquire them in a consistent order across all threads.
• Use timeout and tryLock(): Use timouts and non-blocking lock acquisition to handle exceptional cases.
• Avoid nested locks: Be cautious when acquiring locks within locks, as it can increase the chances of deadlocks.

By following these guidelines and designing your code carefully, you can minimize the chances of thread deadlocks and ensure the smooth execution of concurrent programs.

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}

class Main {

  public static void main(String[] args) {

    // Create two resources

    Resource resource1 = new Resource();

    Resource resource2 = new Resource();

​

    // Create two threads

    Thread thread1 = 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 thread2 = new Thread(() -> {

      synchronized (resource2) {

        System.out.println("Thread 2 acquired resource 2");

        synchronized (resource1) {

          System.out.println("Thread 2 acquired resource 1");

        }

      }

    });