Several common locks in Java

1. Spin lock

As the name suggests, the spin lock will wait for a certain period of time (spin), and nothing will happen during this period. If you don't do it, you just have to wait for the resource to be released. The advantage is that there is no efficiency loss in switching between kernel mode and user mode. However, if it cannot access the resource, it will continue to occupy the CPU resource, so it will cycle for a period of time and then enter the blocking state.

2. Heavyweight locks

synchronized is the implementation mechanism of heavyweight locks. Processes that cannot grab resources will enter a blocking state.

3. Biased lock

As the name suggests, it will bias the first process to access the resource. If only one process executes the synchronization code block, then the last one will be Bias lock, if other threads seize resources, it will be upgraded to a lightweight lock.

4. Lightweight lock

After the biased lock is upgraded, it becomes a lightweight lock. The lock can only be upgraded but not downgraded. Other processes in the lightweight lock will enter the self-selection state. If the self-selection fails, the heavyweight lock will be upgraded.

5. Fair and unfair locks

Mainly refers to whether threads get the lock in a first-come, first-served manner. Synchronized is unfair, while ReentrantLock defaults to unfair and can be set to fair. Lock.

6. Pessimistic lock

Always assume the worst case scenario. Every time you get the data, you think others will modify it, so every time you get the data It will be locked at all times, so that if someone else wants to get the data, it will be blocked until it gets the lock (the shared resource is only used by one thread at a time, other threads are blocked, and the resource is transferred to other threads after use).

Traditional relational databases use many such lock mechanisms, such as row locks, table locks, read locks, write locks, etc., which are all locked before operations. Exclusive locks such as synchronized and ReentrantLock in Java are the implementation of the pessimistic lock idea.

7. Optimistic locking

Always assume the best situation. Every time you go to get the data, you think that others will not modify it, so you will not lock it. However, when updating, it will be judged whether others have updated the data during this period. This can be implemented using the version number mechanism (in the database) and the CAS algorithm.

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