java Unsafe类的compareAndSwap方法

compareAndSwap是个原子方法,原理是cas.就是说如果他是xx,那么就改为xxx.
这个是高效,而且是原子的,不用加锁.
也不用但是其他值改了而产生误操作,应为会先判断当前值,符合期望才去改变.

直接上代码吧

package com.huangyunbin;
import java.lang.reflect.Field;
import sun.misc.Unsafe;


public class UnsafeTest {

    private static Unsafe unsafe;

    static {
        try {
            //通过反射获取rt.jar下的Unsafe类
            Field field = Unsafe.class.getDeclaredField("theUnsafe");
            field.setAccessible(true);
            unsafe = (Unsafe) field.get(null);
        } catch (Exception e) {
            System.out.println("Get Unsafe instance occur error"+ e);
        }
    }




    public static void main(String[] args) throws Exception
    {
        Class clazz = Target.class;
        Field[] fields = clazz.getDeclaredFields();
        System.out.println("fieldName:fieldOffset");
        for (Field f : fields) {
            // 获取属性偏移量，可以通过这个偏移量给属性设置
            System.out.println(f.getName() + ":" + unsafe.objectFieldOffset(f));
        }
        Target target = new Target();
        Field intFiled  =  clazz.getDeclaredField("intParam")  ;
        int a=(Integer)intFiled.get(target ) ;
        System.out.println("原始值是:"+a);
        //intParam的字段偏移是12 原始值是3 我们要改为10
        System.out.println(unsafe.compareAndSwapInt(target, 12, 3, 10));
        int b=(Integer)intFiled.get(target) ;
        System.out.println("改变之后的值是:"+b);

        //这个时候已经改为10了,所以会返回false
        System.out.println(unsafe.compareAndSwapInt(target, 12, 3, 10));

        System.out.println(unsafe.compareAndSwapObject(target, 24, null, "5"));
    }
}


 class Target {
     int intParam=3;
     long longParam;
     String strParam;
     String strParam2;
}

结果是:
fieldName:fieldOffset
intParam:12
longParam:16
strParam:24
strParam2:28
原始值是:3
true
改变之后的值是:10
false
true

compareAndSwapInt是通过反射根据字段偏移去修改对象的.
可以看到int是4个字节的偏移量,long是4个字节的偏移量,string是4个字节的偏移量
注意 Unsafe的对象不能直接new,要通过反射去获取.

评论

1 楼 mxdxm 2015-10-09