多线程 Unsafe类的使用

标签：and   ESS   类加载器   ctf   dba   rac   swa   修改   var   

Unsafe类提供了原子性操作CAS

package com.example.demo.utils;

import com.alibaba.druid.pool.DruidDataSource;
import sun.misc.Unsafe;

import java.lang.reflect.Field;
import java.util.concurrent.*;

public class Test1 {

    static Unsafe unsafe;  //sun.misc.Unsafe
    static long stateOffset;
    private volatile long state = 0;

    public Test1() {
    }

    static {
        try {
            //因为Unsafe的类加载和项目中的类加载器bootstrap不一样，Unsafe的getUnsafe()判断了类加载器不同会报错，所以需要使用反射获取unsafe实例
            Field field = Unsafe.class.getDeclaredField("theUnsafe");
            field.setAccessible(true);
            unsafe = (Unsafe) field.get(null);
            //获取state变量在test1中的内存偏移量
            stateOffset = unsafe.objectFieldOffset(Test1.class.getDeclaredField("state"));
        } catch (Exception e) {
            System.out.println(e.getLocalizedMessage());
            e.printStackTrace();
        }
    }

    public static void main(String args[]) {
        Test1 test1 = new Test1();
        System.out.println("first:"+stateOffset);
        System.out.println(test1.state);
        long state = test1.state;//此时state为0
        //非阻塞算法CAS操作，在系统层面把该操作锁住，禁止中途被其他线程修改。
        // 操作失败代表中途有人截胡，把state的值改了
        //这样就可以知道数据被改动，可以预防产生脏数据
        Boolean sucess = unsafe.compareAndSwapLong(test1, stateOffset, state,1);
        System.out.println(sucess);

        System.out.println("2rd:"+stateOffset);
        System.out.println(test1.state);



    }
}

Unsafe的部分其他方法

public native long staticFieldOffset(Field var1);

public native long objectFieldOffset(Field var1);

public native Object staticFieldBase(Field var1);

public native boolean shouldBeInitialized(Class<?> var1);

public native void ensureClassInitialized(Class<?> var1);

public native int arrayBaseOffset(Class<?> var1);

public native int arrayIndexScale(Class<?> var1);

public native long getLongVolatile(Object var1, long var2);

public native void putLongVolatile(Object var1, long var2, long 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);

多线程 Unsafe类的使用

标签：and   ESS   类加载器   ctf   dba   rac   swa   修改   var   

原文地址：https://www.cnblogs.com/shihx/p/12114258.html