Multiple threads compete for resources and wait for each other, causing the program to be unable to continue execution.

In a Java project, when multiple threads wait for each other due to resource competition, the program may not be able to continue executing and a deadlock may occur. The following will introduce in detail the concept of deadlock, the causes of deadlock, and how to identify, prevent and solve deadlock problems.

1. The concept of deadlock

Deadlock refers to a situation where two or more threads are waiting for each other to release the resources they occupy, causing them to be unable to continue execution. If a deadlock occurs, the program will fall into an infinite waiting state and be unable to complete the task normally.

The necessary conditions for deadlock to occur include:

1. Mutual exclusion: Resources can only be occupied by one thread at the same time.

2. Possession and waiting: While maintaining resources, a thread enters the waiting state because it needs other resources.

3. Non-preemptible: Resources allocated to one thread cannot be preempted by other threads.

4. Circular waiting: There is a thread chain, and each thread is waiting for the resources held by the next thread.

2. Causes of deadlock

In Java projects, deadlocks can be caused by the following reasons:

1. Thread synchronization problem: When multiple threads share resources, if these resources are not accessed correctly and synchronized, deadlock may occur. For example, thread A holds resource X but needs resource Y, while thread B holds resource Y but needs resource X.

2. Thread waiting problem: When a thread waits for other threads to release resources, if the waiting conditions are incorrect or the waiting time is too long, it may lead to deadlock. For example, thread A is waiting for thread B to release resource X, and thread B is also waiting for thread A to release resource Y.

3. Thread scheduling problem: The thread scheduling mechanism of the operating system may cause deadlock. For example, when thread A occupies resource X and thread B occupies resource Y, if the operating system allocates the CPU time slice to thread A and thread A waits for thread B to release resource Y, a deadlock may occur.

3. Identify deadlocks

Identifying deadlock is the first step in solving the deadlock problem. The following are some commonly used deadlock identification methods:

1. Observe program behavior: By observing the behavior of the program when it is running, such as the program cannot continue to execute, the thread is blocked, etc., you can initially determine whether there is a deadlock problem.

2. Stack analysis: Use tools to analyze the stack information of each thread to check whether there is a loop waiting situation.

3. Deadlock detection tools: Use specialized deadlock detection tools, such as JConsole, VisualVM, etc., to help detect potential deadlock problems.

4. Prevent deadlock

In order to prevent deadlock problems from occurring, the following measures can be taken:

1. Avoid nested locks: Try to avoid requesting another lock while holding one lock to reduce the possibility of deadlock.

2. Unify the resource application sequence: Specify the order in which threads apply for resources, so that all threads apply for resources in the same order, which can reduce the occurrence of deadlocks.

3. Timeout waiting: When a thread applies for resources, you can set a timeout waiting mechanism. If the resource is not obtained within the specified time, the obtained resource will be released and retry or perform other operations.

4. Deadlock detection and recovery: You can use deadlock detection algorithms to detect the occurrence of deadlock and try to recover the system by depriving certain threads of resources.

5. Solve deadlock

If a deadlock has occurred, you can take the following methods to solve it:

1. Restart the program: The simplest solution is to restart the program, but this is only a temporary measure and does not really solve the deadlock problem.

2. Forcefully terminate threads: If you can determine which threads caused the deadlock, you can choose to forcefully terminate these threads and release the resources they occupy.

3. Deprive resources: By depriving certain threads of resources, the loop caused by the deadlock is destroyed, thereby lifting the deadlock.

4. Optimize resource allocation strategy: Redesign and implement resource allocation strategy to reduce the probability of deadlock.

In a Java project, deadlock may occur when multiple threads wait for each other due to resource competition. By identifying, preventing, and resolving deadlock problems, program stability and reliability can be improved. To identify deadlocks, you can observe program behavior, perform stack analysis, or use deadlock detection tools. In order to prevent deadlocks, you can avoid nested locks, unify the resource application sequence, set up a timeout waiting mechanism, and use deadlock detection algorithms. If a deadlock has occurred, you can choose to restart the program, forcefully terminate the thread, deprive resources, or optimize the resource allocation strategy to solve the problem. Through reasonable concurrency control and resource management, the probability of deadlock can be reduced and the reliability and performance of the system can be improved.