Java Atomic类及线程同步新机制原理解析

Z时代
2024-01-10
分类：综合

一、为什么要使用Atomic类？

看一下下面这个小程序，模拟计数，创建10个线程，共同访问这个int count = 0 ；每个线程给count往上加10000，这个时候你需要加锁，如果不加锁会出现线程安全问题，但是使用AtomicInteger之后就不用再做加锁的操作了，因为AtomicInteger内部使用了CAS操作，直接无锁往上递增，有人会问问什么会出现无锁操作，答案只有一个：那就是快呗；

下面是AtomicInteger的使用方法：


package com.example.demo.threaddemo.juc_008;
import java.util.ArrayList;
import java.util.List;
import java.util.concurrent.atomic.AtomicInteger;
/**
 * @author D-L
 * @Classname T01_AtomicInteger
 * @Version 1.0
 * @Description 使用AtomicInteger类代替synchronized
 * @Date 2020/7/22
 */
public class T01_AtomicInteger {
//  int count = 0;
   AtomicInteger count = new AtomicInteger(0);
  public /**synchronized*/ void m(){
    for (int i = 0; i < 10000; i++) {
//      count++;
      count.incrementAndGet();
    }
  }
  public static void main(String[] args) {
    T01_AtomicInteger t = new T01_AtomicInteger();
    List<Thread> threads = new ArrayList<>();
    for (int i = 0; i < 10; i++) {
      threads.add(new Thread(t::m ,"Thread" + i));
    }
    threads.forEach(o -> o.start());
    threads.forEach(o ->{
      try {
        o.join();
      } catch (InterruptedException e) {
        e.printStackTrace();
      }
    });
    /* for (int i = 0; i < 10; i++) {
      new Thread(t::m ,"Thread"+i).start();
    }
    try {
      TimeUnit.SECONDS.sleep(3);
    } catch (InterruptedException e) {
      e.printStackTrace();
    }*/
    System.out.println(t.count);
  }
}

二、Atomic类，synchronized、LongAdder的效率验证及分析

模拟多个线程对一个数进行递增，多线程对一个共享变量进行递增的方法大概有三种；验证一下它们的效率，这里做一些粗糙的测试，基本已经能说明问题，具体情况还要根据实际情况：

第一种：使用long count = 0; 加锁来实现；

第二种：使用AtomicLong类来实现；

第三种：使用LongAdder实现；


package com.example.demo.threaddemo.juc_008;
import java.util.concurrent.atomic.AtomicLong;
import java.util.concurrent.atomic.LongAdder;
/**
 * @author D-L
 * @Classname T02_AtomicVsSyncVsLongAdder
 * @Version 1.0
 * @Description 测试Atomic类 synchronized LongAdder效率
 * @Date 2020/7/22
 */
public class T02_AtomicVsSyncVsLongAdder {
  static AtomicLong count1 = new AtomicLong(0L);
  static Long count2 = 0L;
  static LongAdder count3 = new LongAdder();
  public static void main(String[] args) throws InterruptedException {
    Thread [] threads = new Thread[1000];
    /*-----------------------------------Atomic类-----------------------------------*/
    for (int i = 0; i < threads.length; i++) {
      threads[i] = new Thread(() ->{
        for (int j = 0; j < 100000; j++) {
          count1.incrementAndGet();
        }
      });
    }
    long start = System.currentTimeMillis();
    for (Thread t : threads) t.start();
    for (Thread t : threads) t.join();
    long end = System.currentTimeMillis();
    System.out.println("Atomic:" + count1.get() +"-----time:" +(end - start));
    /*----------------------------------synchronized---------------------------------*/
    Object lock = new Object();
    for (int i = 0; i < threads.length; i++) {
      threads[i] = new Thread(new Runnable() {
        @Override
        public void run() {
          for (int j = 0; j < 100000; j++) {
            synchronized (lock) {
              count2++;
            }
          }
        }
      });
    }
    long start2 = System.currentTimeMillis();
    for (Thread t : threads) t.start();
    for (Thread t : threads) t.join();
    long end2 = System.currentTimeMillis();
    System.out.println("synchronized:" + count1.get() +"-----time:" +(end2 - start2));
    /*-------------------------------------LongAdder----------------------------------*/
    for (int i = 0; i < threads.length; i++) {
      threads[i] = new Thread(() ->{
        for (int j = 0; j < 100000; j++) {
          count3.increment();
        }
      });
    }
    long start3 = System.currentTimeMillis();
    for (Thread t : threads) t.start();
    for (Thread t : threads) t.join();
    long end3 = System.currentTimeMillis();
    System.out.println("LongAdder:" + count1.get() +"-----time:" +(end3 - start3));
  }
}
 /*----------------------------------运行结果---------------------------------*/
Atomic:100000000-----time:2096synchronized:100000000-----time:5765LongAdder:100000000-----time:515