Thread deadlock and its solution

Four necessary conditions for thread deadlock

If the following four conditions are true in a system , Then it can cause a deadlock ：

1. Mutual exclusivity ： The possession of resources by threads is exclusive , A resource can only be occupied by one thread , Until released .
2. Request and hold conditions ： When a thread blocks a resource that is requested to be occupied , Don't release the acquired resources .
3. Not to deprive ： Before a thread releases resources , Other threads cannot be deprived of occupation .
4. Loop waiting for ： When you have a life and death lock , Thread goes into a dead loop , Permanent blocking .

resolvent ：

1. Mutual exclusivity , You can use ThreadLocal
2. Request and hold conditions , Kick one out （ End one ）
3. Not to deprive ,trylcok, Add a timeout , Unable to seize and release resources

Distributed lock , It can be used DB,redis Of nx Implement distributed locks

DB Distributed lock
1、 Generate... According to the business field transaction_id, And thread safe creation of lock resources
2、 according to transaction_id To apply for the lock
3、 Release the lock

Threads within a process can be based on obj To synchronize .obj In this case, it can be understood as a lock object . If the thread wants to enter synchronized In code block , Must hold first obj A lock on an object . This kind of lock is JAVA The built-in lock inside , The creation process is thread safe . Then with the help of DB, How to ensure that the process of creating a lock is thread safe ？ You can use DB Medium UNIQUE
KEY characteristic , Once there's a repeat key, because UNIQUE
KEY Uniqueness , Will throw an exception . stay JAVA Inside , yes SQLIntegrityConstraintViolationException abnormal .