多线程（四） AQS底层原理分析

public class ReentrantDemo{
 public synchronized void demo(){
 System.out.println("begin:demo");
 demo2();
 }
 public void demo2(){
 System.out.println("begin:demo1");
 synchronized (this){
 }
 }
 public static void main(String[] args) {
 ReentrantDemo rd=new ReentrantDemo();
 new Thread(rd::demo).start();
 } }

public class AtomicDemo {
    private static int count=0;
    static Lock lock=new ReentrantLock();
    public static void inc(){
        lock.lock();
        try {
            Thread.sleep(1);
        } catch (InterruptedException e) {
            e.printStackTrace();
        }
        count++;
        lock.unlock();
    }
    public static void main(String[] args) throws
            InterruptedException {
        for(int i=0;i<1000;i++){
            new Thread(()->{AtomicDemo.inc();}).start();;
        }
        Thread.sleep(3000);
        System.out.println("result:"+count);
    }
}

package com.lf.threaddemo;

import java.util.HashMap;
import java.util.Map;
import java.util.concurrent.locks.Lock;
import java.util.concurrent.locks.ReentrantReadWriteLock;

public class LockDemo {
    static Map<String, Object> cacheMap = new HashMap<>();
    static ReentrantReadWriteLock rwl = new
            ReentrantReadWriteLock();
    static Lock read = rwl.readLock();
    static Lock write = rwl.writeLock();

    public static final Object get(String key) {
        System.out.println("开始读取数据");
        read.lock(); //读锁
        try {
            return cacheMap.get(key);
        } finally {
            read.unlock();
        }
    }

    public static final Object put(String key, Object value) {
        write.lock();
        System.out.println("开始写数据");
        try {
            return cacheMap.put(key, value);
        } finally {
            write.unlock();
        }
    }
}

final StampedLock sl = new StampedLock();
  
// 获取/释放悲观读锁示意代码
long stamp = sl.readLock();
try {
  //省略业务相关代码
} finally {
  sl.unlockRead(stamp);
}

// 获取/释放写锁示意代码
long stamp = sl.writeLock();
try {
  //省略业务相关代码
} finally {
  sl.unlockWrite(stamp);
}

final StampedLock lock
  = new StampedLock();
Thread T1 = new Thread(()->{
  // 获取写锁
  lock.writeLock();
  // 永远阻塞在此处，不释放写锁
  LockSupport.park();
});
T1.start();
// 保证T1获取写锁
Thread.sleep(100);
Thread T2 = new Thread(()->
  //阻塞在悲观读锁
  lock.readLock()
);
T2.start();
// 保证T2阻塞在读锁
Thread.sleep(100);
//中断线程T2
//会导致线程T2所在CPU飙升
T2.interrupt();
T2.join();

final StampedLock sl =   new StampedLock();
// 乐观读
long stamp =   sl.tryOptimisticRead();
// 读入方法局部变量
......
// 校验stamp
if (!sl.validate(stamp)){ 
 // 升级为悲观读锁  
stamp = sl.readLock(); 
    try {   
    // 读入方法局部变量   
     ..... 
     } finally { 
   //释放悲观读锁
       sl.unlockRead(stamp); 
 }
}
//使用方法局部变量执行业务操作
......

long stamp = sl.writeLock();
try {
  // 写共享变量
  ......
} finally {
  sl.unlockWrite(stamp);
}