前言

由于并发包中很多数据结构的底层实现都是依赖于CAS,CAS也就是用Unsafe实现. CAS操作有3个操作数, 内存位置, 预期原值, 和 新值, 如果内存位置的值与预期原值相等,则在该内存位置填写新的值, 否则不修改原值.

本文源码: 源码下载

例子

先通过一个简单的例子了解一下Unsafe的操作.

package com.usafe;

import sun.misc.Unsafe;
import java.lang.reflect.Field;

public class TestField {
    private int age;
    private String name;
    private static int value;

    public static void main(String[] args) throws Exception {
        Field theUnsafe = Unsafe.class.getDeclaredField("theUnsafe");
        theUnsafe.setAccessible(true);
        Unsafe unsafe = (Unsafe) theUnsafe.get(null);

        long setoff = unsafe.objectFieldOffset(TestField.class.getDeclaredField("age"));
        System.out.println("setoff:" + setoff);

        TestField test1 = new TestField();
        test1.age = 10;
        TestField test2 = new TestField();
        test2.age = 10;

        unsafe.compareAndSwapInt(test1,  setoff, 10, 11);
        unsafe.compareAndSwapInt(test2,  setoff, 10, 12);
        System.out.println("test1 age:" + test1.age);
        System.out.println("test2 age:" + test2.age);
    }
}

输出如下, 从输出结果可以看到通过objectFieldOffset方法可以得到该成员变量在类TestField中的偏移量, 在方法compareAndSwapInt中第一个变量是类对象, 和第二个变量是偏移量即可得到该变量在内存中真正的位置, 然后进行CAS操作.

setoff:12
test1 age:11
test2 age:12

Unsafe方法

以下只是Unsafe的一些方法,基本上看到方法名字就可以知道大概什么作用,可以自己尝试一下, 由于本文是为其他文章做铺垫, 因此就不对其源码(C++实现)进行分析.

    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);
    public native Object getObjectVolatile(Object var1, long var2);
    public native void putObjectVolatile(Object var1, long var2, Object var4);
    public native int getIntVolatile(Object var1, long var2);
    public native void putIntVolatile(Object var1, long var2, int var4);
    public native boolean getBooleanVolatile(Object var1, long var2);
    public native void putBooleanVolatile(Object var1, long var2, boolean var4);
    public native byte getByteVolatile(Object var1, long var2);
    public native void putByteVolatile(Object var1, long var2, byte var4);
    public native short getShortVolatile(Object var1, long var2);
    public native void putShortVolatile(Object var1, long var2, short var4);
    public native char getCharVolatile(Object var1, long var2);
    public native void putCharVolatile(Object var1, long var2, char var4);
    public native long getLongVolatile(Object var1, long var2);
    public native void putLongVolatile(Object var1, long var2, long var4);
    public native float getFloatVolatile(Object var1, long var2);
    public native void putFloatVolatile(Object var1, long var2, float var4);
    public native double getDoubleVolatile(Object var1, long var2);
    public native void putDoubleVolatile(Object var1, long var2, double var4);
    public native void putOrderedObject(Object var1, long var2, Object var4);
    public native void putOrderedInt(Object var1, long var2, int var4);
    public native void putOrderedLong(Object var1, long var2, long var4);
    public native void unpark(Object var1);
    public native void park(boolean var1, long var2);