Read write lock, shared lock, exclusive lock

Optimism lock ： Judge when modifying

Pessimistic locking ： Lock before operation

Table locks : A deadlock will not occur , The probability of lock conflict is high , Low concurrency .

Row lock : A deadlock occurs , Low probability of lock conflict , High concurrency .

Read the lock ： Shared lock , There's a deadlock

Write lock ： An exclusive lock , There's a deadlock

Read-write lock ： A resource can be accessed by multiple read threads , Or it can be accessed by a write thread , But there can't be read and write threads at the same time , Reading and writing are mutually exclusive

First of all unlocked

Multithreading grabs resources The disorderly

second Add lock

Use synchronized and ReentrantLock

It's all exclusive , Only one operation at a time

Third Read-write lock

ReentrantReadWriteLock

Read Can be Shared , Lifting performance , Read by multiple people at the same time

shortcoming ：

（1） Cause lock hunger , Read all the time , No write operation

（2） Reading time , Can't write , Only after reading , To write , Write operation can read

Lock down ： Demote write lock to read lock , Read lock cannot be upgraded to write lock

jdk8 explain ：

Write lock Downgrade Read the lock

Get write lock ——》 Get read lock ——》 Release the write lock ——》 Release read lock

Add modify delete —— Write

Inquire about —— read

Lock demotion example

package juc;

import java.util.concurrent.locks.ReentrantReadWriteLock;

public class Demo1 {

    public static void main(String[] args) {

        ReentrantReadWriteLock rwLock = new ReentrantReadWriteLock();

        ReentrantReadWriteLock.ReadLock readLock = rwLock.readLock();// Read the lock

        ReentrantReadWriteLock.WriteLock writeLock = rwLock.writeLock();// Write lock

        // Lock down

        //1 Get write lock

        writeLock.lock();

        System.out.println("Linsa");

        //2 Get read lock

        readLock.lock();

        System.out.println("read");

        //3 Release the write lock

        writeLock.unlock();

        //4 Release read lock

        readLock.unlock();

    }

}