The key to avoiding Java deadlocks is one of the four necessary conditions for breaking the deadlock. 1. Avoid the "request and hold" state. You can retry by applying for all resources at once or releasing existing resources, and ensure that threads access multiple locks in the same order; 2. Introduce the hierarchical order of locks, assign numbers to each lock and require threads to lock in the numbering order to prevent loop waiting; 3. Use the ReentrantLock.tryLock() method to cooperate with the timeout mechanism to try to acquire the lock within a specified time, and release the existing lock to avoid blocking if it fails; 4. Use jstack, VisualVM and other tools to regularly detect potential deadlocks to assist in troubleshooting and monitoring the use of locks.

The deadlock problem in Java often causes headaches for developers, especially in multi-threaded environments. The key to avoiding deadlocks is to understand its formation conditions and adopt corresponding strategies . A common deadlock scenario is when multiple threads are waiting for each other's resources held by each other, causing the program to get stuck. Let’s start from several practical perspectives and talk about how to avoid this pit when writing code.

1. Avoid "request and hold" status

This is one of the four necessary conditions for deadlock: a thread does not release the resources it already holds while waiting for other resources. This situation is most likely to cause deadlocks.

The solution is simple: apply for all required resources at one time , or if you cannot obtain all resources, first release the existing resources and try again.

For example, suppose you have two locks lockA and lockB , and the two threads lock in different orders will easily lead to problems:

 // Thread 1
synchronized(lockA) {
    synchronized(lockB) { ... }
}

// Thread 2
synchronized(lockB) {
    synchronized(lockA) { ... }
}

In this case, threads may hold one of the locks on each other and then get stuck waiting for the other lock. so:

• All threads try to access multiple resources in the same order.
• If you have to take multiple locks at the same time, you can consider using ReentrantLock.tryLock() method and set the timeout time.

2. The hierarchical order of the introduction of locks

Assign each lock a "level" or "sequential number" to it, requiring the thread to only lock in the order of the number from small to large, so that there will be no loop waiting.

for example:

• Lock A number is 1
• Lock B number is 2

Then the thread can lock A first and then B, but it cannot be reversed. If a module violates this rule, it can be found through tool inspection or unit testing.

The advantage of this method is that it has clear structure and easy maintenance, and is suitable for resource management design in medium and large projects.

3. Use tryLock timeout mechanism

Java provides ReentrantLock class, which supports trying to acquire locks and setting timeouts. This method is more flexible than traditional synchronized and can effectively avoid deadlocks.

The sample code is as follows:

 ReentrantLock lockA = new ReentrantLock();
ReentrantLock lockB = new ReentrantLock();

if (lockA.tryLock(1, TimeUnit.SECONDS)) {
    try {
        if (lockB.tryLock(1, TimeUnit.SECONDS)) {
            try {
                // Perform an action} finally {
                lockB.unlock();
            }
        }
    } finally {
        lockA.unlock();
    }
}

This code means:

• Try to acquire the first lock first and wait for a second at most.
• After success, try the second lock, and set the maximum waiting time.
• If any of these fails, the acquired lock is automatically released, thus avoiding deadlocks.

Some points to pay attention to when using tryLock :

• Setting the appropriate timeout time is too short, which may fail frequently, and too long will lose its meaning.
• After the operation is completed, remember to unlock, it is best to put it in the finally block.

4. Regular testing and tool assistance

Even if a lot of preventive measures are taken, some situations may be missed sometimes. At this time, you can use some tools to help troubleshoot potential deadlock risks.

JDK comes with some tools, such as:

• jstack : Print thread stack information, and you can see which threads hold which locks and which locks are waiting for.
• VisualVM or JConsole : The graphical interface can view the thread status and can even directly prompt deadlock.

In addition, some monitoring logic can be added during the development stage, such as the order of acquisition and release of record locks, and periodic analysis can be performed.

In general, Java deadlock is not a mysterious problem. As long as you pay attention to the order of resource application, use the lock mechanism reasonably, and check it with tool, you can greatly reduce the chance of occurrence.
Basically that's it.

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