并发编程原理与实战（三十一）从原子类到CPU原子指令，CAS实现全链路拆解

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已于 2025-09-21 21:30:55 修改 · 1.1k 阅读

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于 2025-09-21 19:37:49 首次发布

在前面学习原子类的核心API过程中，CAS方法compareAndSet(int expect, int update)是每个原子类的核心方法，本文来深入这一核心算法底层的实现原理。

CAS底层原理（JDK1.8）

以AtomicInteger为例，我们分析下compareAndSet(int expect, int update)的源码。

/**
 * Atomically sets the value to the given updated value
 * if the current value {@code ==} the expected value.
 *
 * @param expect the expected value
 * @param update the new value
 * @return {@code true} if successful. False return indicates that
 * the actual value was not equal to the expected value.
 */
public final boolean compareAndSet(int expect, int update) {
   
   
    return unsafe.compareAndSwapInt(this, valueOffset, expect, update);
}

该方法传入两个参数，当原子对象的当前值等于传入的期望值expect的时候，原子对象的值被设置为传入的更新值update，更新成功返回true，更新失败返回false，表示原子对象的当前值不等于传入的期望值expect。

其内部调用unsafe变量的compareAndSwapInt(this, valueOffset, expect, update)方法，那么问题来了？什么是unsafe，参数valueOffset又是什么？

认识Unsafe

我们发现，每个原子类里面都Unsafe类型的变量unsafe和long类型的变量valueOffset。

public class AtomicInteger extends Number implements java.io.Serializable {
   
   
    private static final long serialVersionUID = 6214790243416807050L;

    // setup to use Unsafe.compareAndSwapInt for updates
    private static final Unsafe unsafe = Unsafe.getUnsafe();
    private static final long valueOffset;
    static {
   
   
        try {
   
   
            valueOffset = unsafe.objectFieldOffset
                (AtomicInteger.class.getDeclaredField("value"));
        } catch (Exception ex) {
   
    throw new Error(ex); }
    }    
    ...
}

public class AtomicBoolean implements java.io.Serializable {
   
   
    private static final long serialVersionUID = 4654671469794556979L;
    // setup to use Unsafe.compareAndSwapInt for updates
    private static final Unsafe unsafe = Unsafe.getUnsafe();
    private static final long valueOffset;
    ...
}

public class AtomicLong extends Number implements java.io.Serializable {
   
   
    private static final long serialVersionUID = 1927816293512124184L;

    // setup to use Unsafe.compareAndSwapLong for updates
    private static final Unsafe unsafe = Unsafe.getUnsafe();
    private static final long valueOffset;
    ...