Brief introduction of optimistic Lock and pessimistic Lock in Java

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1. Category 1: optimistic lock and pessimistic lock

A) pessimistic locks: locks are added because they think that other threads will interfere with their own thread operations

i. Specific forms: synchronized keyword and lock implementation class

B) optimistic lock: think that no other thread will affect its own thread operation, so do not lock

i. Specific form: the incremental operation of the atomic class of java

ii. Principle: using cas algorithm

C) Cas algorithm: exchange and comparison

i. Three numbers are involved: the memory value V that needs to be read and written, the value A to be compared, and the new value B to be written

ii. Specific operation: if you want V, update V with B, otherwise nothing will be done.

iii. Can use ABA problem: cas algorithm needs to determine whether the memory value V has changed, if a value becomes b value and then changes back to a value, cas algorithm will be unable to judge, resulting in an error. Solve the above problem: add the version number before the variable to change aba into 1a2b3c

iv. Long cycle time and high overhead because spin consumes cpu

v. Only one atomic operation of a shared variable is guaranteed.

2. Category 2

A) reentrant locks: locks that support reentry, exclusive locks

3. Classification 3

A) read-write locks: a pair of locks, read locks, write locks, allowing multiple threads to access at the same time

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