Java多线程CAS操作原理代码实例解析

package com.designmodal.design.juc01;

import java.util.ArrayList;
import java.util.List;
import java.util.concurrent.atomic.AtomicInteger;

/**
 * @author D-L
 * @Classname T03_AtomicInteger
 * @Version 1.0
 * @Description 使用 AtomicInteger 类解决常见的 多线程count++
 *        其内部使用了CAS操作来保证原子性 但是不能保证多个方法连续调用都是原子性
 * @Date 2020/7/21 0:35
 */
public class T03_AtomicInteger {
  //使用AtomicInteger类
  AtomicInteger count = new AtomicInteger(0);

  public void m(){
    for (int i = 0; i < 10000; i++) {
      //等同于 在 count++ 上加锁
      count.incrementAndGet();
    }
  }

  public static void main(String[] args) {
    T03_AtomicInteger t = new T03_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();
      }
    });

    System.out.println(t.count);
  }
}

 AtomicInteger count = new AtomicInteger(0);
 public void add(){
  count.incrementAndGet();
 }

/**
   * Atomically increments by one the current value.
   *
   * @return the updated value
   */
  public final int incrementAndGet() {
    return unsafe.getAndAddInt(this, valueOffset, 1) + 1;
  }

public native int getIntVolatile(Object var1, long var2);
public final native boolean compareAndSwapInt(Object var1, long var2, int var4, int var5);


 public final int getAndAddInt(Object var1, long var2, int var4) {
    int var5;
    do {
      var5 = this.getIntVolatile(var1, var2);
    } while(!this.compareAndSwapInt(var1, var2, var5, var5 + var4));

    return var5;
  }

/**
   * CAS操作 ：Unsafe类中的本地方法 由于Java语言无法访问操作系统底层信息（比如：底层硬件设备等），
   * 这时候就需要借助C C++语言来完成了
   * @param var1 对象
   * @param var2 偏移量
   * @param var4 期望值
   * @param var5 新值
   * @return 修改成功返回true 失败返回false
   */
  public final native boolean compareAndSwapObject(Object var1, long var2, Object var4, Object var5);

  public final native boolean compareAndSwapInt(Object var1, long var2, int var4, int var5);

  public final native boolean compareAndSwapLong(Object var1, long var2, long var4, long var6);