Deadlock refers to the phenomenon that multiple threads cannot continue to execute because they are waiting for each other's resources. Its generation requires four conditions: 1. Mutual exclusion, resources cannot be shared; 2. Hold and wait, threads do not release the occupied resources while waiting for other resources; 3. It cannot be preempted, resources can only be actively released by the holding thread; 4. Loop waiting, thread chains are waiting for each other. To detect deadlocks, you can view the "DEADLOCK" prompt in the thread stack through the jstack command, or use IDE tools, VisualVM and other visual tools to analyze. Methods to avoid deadlocks include: 1. Unify the locking order to break loop waiting; 2. Set a timeout mechanism, such as using tryLock(); 3. Reduce the granularity and range of locks; 4. Use concurrent tool classes such as ReentrantLock. In addition, we need to pay attention to issues such as live locks and resource hunger, and reasonably design thread collaboration mechanisms to ensure the stability and robustness of multi-threaded programs.

In Java multithreaded programming, deadlock is a common but easily overlooked problem. It usually occurs when multiple threads are waiting for each other's resources held by each other, causing the program to be stuck and unable to continue execution. The key to avoiding deadlocks is to understand the conditions in which they occur and adopt reasonable strategies to prevent them.

What is a deadlock?

The generation of deadlocks requires meeting four necessary conditions: mutual exclusion, holding and waiting, non-preemptive and circular waiting. In other words, as long as these four conditions are valid at the same time, a deadlock may occur.

• Mutual Exclusion : Resources cannot be shared and can only be used by one thread at a time.
• Hold and wait : The thread does not release the already held resources while waiting for other resources.
• Not preemptive : Resources can only be released actively by the threads that hold them and cannot be forced to be deprived.
• Loop waiting : There is a thread chain, each thread is waiting for the resources held by the next thread.

If your program has "no threads move" phenomenon, it is likely that it has encountered a deadlock.

How to detect deadlocks?

The most direct way is to assist detection with tool:

• Use the jstack command to analyze thread stack information. After running your Java application, open the terminal and enter jstack <pid></pid> to view the "DEADLOCK" prompt in the output.
• IDE built-in tools or visual tools such as VisualVM can also help you intuitively discover dependencies between threads.
• The thread does not respond for a long time, or some operations have not been completed for a long time, which may also be a deadlock signal.

It is recommended to do thread inspections regularly during the development stage, rather than wait until it is launched before troubleshooting.

How to avoid deadlocks?

To completely avoid deadlocks, you can start by breaking one of the four conditions mentioned above. Here are some commonly used practice methods:

• Unified locking order : Ensure that all threads request resources in the same order. For example, always acquire the A lock first and then the B lock, so that there will be no loop waiting.
• Set the timeout mechanism : Use tryLock() method instead of synchronized . If the lock cannot be acquired within the specified time, the current operation will be abandoned and the existing resources will be released.
• Reduce the granularity and range of locks : Only locks are added where synchronization is really needed, and try to shorten the lock holding time.
• Use the tool classes provided by the concurrent package : For example, ReentrantLock supports trying to acquire locks, timeout control and other functions, which is more flexible than native synchronized .

For example, deadlocks are prone to occur when two threads acquire two locks in different orders. But if the lock must be acquired in a fixed order, this problem can be effectively avoided.

Things to note outside deadlock

In addition to deadlocks, you should also pay attention to similar issues, such as live locks (threads constantly retry but always fail) and resource hunger (some threads do not get execution opportunities for a long time). Although these situations do not completely block the program like deadlock, they can also affect the stability and performance of the system.

Rationally designing thread collaboration mechanisms to avoid over-reliance on locks, while keeping the code simple and clear, is the basis for building robust multi-threaded applications.

Basically that's it.

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