悲观锁和乐观锁

每次操作的时候都在上锁解锁，能解决并发中的各种问题，不支持并发操作，效率低

每次操作都加上版本号，操作提交的时候会比较数据库版本和目前的版本号是否一致，不一致就会提交失败

表锁和行锁

表锁是对整张表进行上锁，其他人则不可以再操作整张表

行锁是对某一行进行上锁，别人还可以操作其他的行，行锁会发生死锁

读锁和写锁

读锁也叫共享锁会发生死锁问题

写锁也叫独占锁会发生死锁问题

先来看例子，不加读写锁

import java.util.HashMap;
import java.util.Map;
import java.util.concurrent.TimeUnit;

public class Test {
    public static void main(String[] args) {
        MyCache myCache = new MyCache();
        //写操作
        new Thread(()->{
            for (int i = 1; i <= 5; i++) {
                myCache.put(i,i);
            }
        }, "0").start();
        //读操作
        new Thread(()->{
            for (int i = 1; i <= 5; i++) {
                myCache.get(i);
            }
        },"1").start();

    }
}

class MyCache{
    private volatile Map<Integer, Integer> map = new HashMap<>(5);

    public void put(Integer key, Integer value){
        System.out.println(Thread.currentThread().getName()+"正在写操作"+key);
        try {
            TimeUnit.MICROSECONDS.sleep(300);
        } catch (InterruptedException e) {
            throw new RuntimeException(e);
        }
        map.put(key,value);
        System.out.println(Thread.currentThread().getName()+"写完了"+key);
    }
    public Integer get(Integer key){
        System.out.println(Thread.currentThread().getName()+"正在读操作"+key);
        try {
            Thread.sleep(300);
        } catch (InterruptedException e) {
            throw new RuntimeException(e);
        }
        Integer i = map.get(key);
        System.out.println(Thread.currentThread().getName()+"读完了"+i);
        return i;
    }

}

添加读写锁之后

import java.util.HashMap;
import java.util.Map;
import java.util.concurrent.TimeUnit;
import java.util.concurrent.locks.ReadWriteLock;
import java.util.concurrent.locks.ReentrantReadWriteLock;

public class Test {
    public static void main(String[] args) {
        MyCache myCache = new MyCache();
        //写操作
        new Thread(()->{
            for (int i = 1; i <= 5; i++) {
                myCache.put(i,i);
            }
        }, "0").start();
        //读操作
        new Thread(()->{
            for (int i = 1; i <= 5; i++) {
                myCache.get(i);
            }
        },"1").start();

    }
}

class MyCache{
    private volatile Map<Integer, Integer> map = new HashMap<>(5);

    //创建读写锁
    private ReadWriteLock rwLock = new ReentrantReadWriteLock();
    public void put(Integer key, Integer value){
        //添加写锁
        rwLock.writeLock().lock();
        try {
            System.out.println(Thread.currentThread().getName()+"正在写操作"+key);
            TimeUnit.MICROSECONDS.sleep(300);
            map.put(key,value);
            System.out.println(Thread.currentThread().getName()+"写完了"+key);
        } catch (InterruptedException e) {
            throw new RuntimeException(e);
        }finally {
            //释放写锁
            rwLock.writeLock().unlock();
        }
    }
    public Integer get(Integer key){
        //添加读锁
        rwLock.readLock().lock();
        Integer i = null;
        try {
            System.out.println(Thread.currentThread().getName()+"正在读操作"+key);
            Thread.sleep(300);
            i = map.get(key);
            System.out.println(Thread.currentThread().getName()+"读完了"+i);
        } catch (InterruptedException e) {
            throw new RuntimeException(e);
        }finally {
            //释放读锁
            rwLock.readLock().unlock();
        }
        return i;
    }

}