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It is assumed that data generally does not produce concurrency conflicts , So when the data is submitted for update , Will formally determine whether the data is generated and Send conflict detection , If concurrency conflicts are found , Returns the user's error message , Let the user decide how to do it .

（2） Explanation of pessimistic lock

Always assume the worst , Every time I go to get the data, I think others will modify it , So every time I get the data, I lock it , So people who want to take this data will block it until it gets the lock .

（3） Examples of two strategies

When the thread conflict is not serious , Optimistic locking strategy can be adopted to avoid multiple lock adding and lock releasing operations
When thread conflicts are serious , Pessimistic locking can be used to prevent threads from frequently accessing shared data CPU Idling problem

（4） Optimistic lock detects data conflicts

Optimistic locking does not really block threads , The implementation of optimistic locking can introduce “ Version number ” To solve the problem

The thread 2 Operation strategy of ：
● Direct error , Threads 2 sign out , If you don't write to the main memory
● Read and write the latest value from main memory 50 And version number 2, And then execute it again -20 operation =30, Then write it back to the main memory
The core of the above illustration is , Whether the thread can successfully refresh the value of main memory depends on
Version number in working memory == The version number in the main memory can be updated , After the update is successful , Synchronously refresh your version number and the version number of main memory , That is, the update is successful

2、 Read-write lock

（1） Introduction to read-write lock

Between threads , There is no thread safety problem between data readers , But the writers of the data need to communicate with each other and with the readers To be mutually exclusive . If the same lock is used in both scenarios , There will be great performance loss . Therefore, read-write locks are generated .
Read-write lock （readers-writer lock）, Look at English, as the name suggests , When performing the lock operation, it is necessary to indicate the read-write intent additionally , Readers are not mutually exclusive , The writer requires mutual exclusion from anyone .
A thread's access to data , There are two main operations :
● Reading data and Writing data .
● Both threads just read one data , There is no thread safety issue at this time . Read directly and concurrently .
● Both threads have to write a data , There are thread safety issues , Only one thread can obtain a write lock , Other threads are blocked
● One thread reads and another thread writes , There are also thread safety issues , When the write thread ends, the read thread can continue
Read / write locks are mostly used for threads that are basically reading data , Write less data

（2）java Read write lock in

Read write lock is to distinguish between read operation and write operation .
Java The standard library provides ReentrantReadWriteLock class , Realize the read-write lock . ●ReentrantReadWriteLock.ReadLock Class represents a read lock . This object provides lock / unlock methods Lock and unlock .
●ReentrantReadWriteLock.WriteLock Class represents a write lock . This object also provides lock / unlock Method to lock and unlock .
among , Between read lock and read lock , Not mutually exclusive . Between write lock and write lock , Mutually exclusive . Between read lock and write lock , Mutually exclusive .

3、 Heavyweight locks and lightweight locks

（1） Heavyweight lock

Need the support of operating system and hardware , The thread obtains the heavyweight lock and enters the blocking state ( Dependent operating system , Frequently switch between user mode and kernel mode , Very expensive ).
If you go to the bank to handle business , User mode is to handle business by yourself outside the window , The kernel mode requires staff to assist in handling business in the window , When a business needs to switch frequently between user mode and kernel mode ( That is, they keep running around between the external window and the staff , It will be very time-consuming )

（2） Lightweight lock

Try to perform operations in user mode , Threads don't block , No state switching
Let's say when we deal with business in the bank , The current business can be processed in an external window , There is no need to run back and forth , You can save a lot of time , Save money .

4、 spinlocks

The common implementation of lightweight lock is spin lock
As before , The thread enters the blocking state after the lock grabbing failure , give up CPU, It will take a long time to be scheduled again . But actually , In most cases , Although the current lock grab failed , But it won't be long , The lock will be released . There's no need to give up CPU. This Spin lock can be used to deal with such problems .
while ( Grab the lock (lock) ==  Failure ) {}
If the lock acquisition fails , Try to get the lock again now , Infinite loop , Until we get the lock . Failed to acquire lock for the first time , The second attempt will come in a very short time . Once the lock is released by another thread , You can get the lock at the first time .

5、 Fair and non-fair locks

Fair lock : comply with " first come , first served ". B Than C First come . When A After releasing the lock , B Before C Get lock .
Not fair lock : Non compliance " first come , first served ". B and C It's possible to get the lock .