Java Unsafe 常用API 例子。

Posted 2020-10-23 guojunwei

 

1 线程挂起恢复

2 CAS操作

2 直接内存操作（非JVM内存）

3 实例化对象

4 直接操作对象属性

5 直接操作数组

示例代码如下

 

public class Player {

    private String name;
    
    private int age;

    public String getName() {
        return name;
    }

    public void setName(String name) {
        this.name = name;
    }

    public int getAge() {
        return age;
    }

    public void setAge(int age) {
        this.age = age;
    }
    
}

 

import java.lang.reflect.Field;

import com.alibaba.fastjson.JSON;
import com.guo.util.Util;

import sun.misc.Unsafe;


/**
 * unsafe 操作数组
 * @author GuoJunWei
 *
 */
public class Test1 {
    
    public static Unsafe unsafe = Util.getUnsafe();

    public static void main(String[] args) throws Exception {
        //function1();
        //function2();
        //function3();
        //function4();
        //function5();
        function6();
    }
    
    /**
     * park():方法返回条件
     * 1 当前现在调用过 unpark 方法 (多次调用 按照一次计算)
     * 2 当前线程被中断
     * 3 当park 为 false:时间块到了 单位纳秒
     * 4 当park 为 true:时间是绝对时间（1970）年 到期 单位毫秒
     */
    public static void function6() {
        System.out.println("Start");
        long time = System.currentTimeMillis()+3000l;
        unsafe.park(true,time);
        System.out.println("end");
    }
    
    /**
     * CAS操作
     * @throws Exception
     */
    public static void function5() throws Exception {
        
        Player player = (Player) unsafe.allocateInstance(Player.class);
        
        Field age = player.getClass().getDeclaredField("age");
        
        long addressAge = unsafe.objectFieldOffset(age);
        
        unsafe.compareAndSwapInt(player, addressAge, 0, 100);
        
        System.out.println(player.getAge());
        
    }
    
    
    /**
     * 直接分配内存地址：内存管理
     */
    public static void function4() {
        //分配100字节内存  返回初始地址
        long address = unsafe.allocateMemory(100);
        //往分配的内存地址写入值
        unsafe.putInt(address, 55);
        //获取值
        System.out.println(unsafe.getInt(address));
        
        //分配100字节内存  返回初始地址
        long address1 = unsafe.allocateMemory(100);
        
        //copy 内存值
        unsafe.copyMemory(address, address1, 4);
        
        System.out.println(unsafe.getInt(address1));
        
        //释放内存
        unsafe.freeMemory(address);
        unsafe.freeMemory(address1);
        
    }
    
    
    /**
     * 操作对象属性值
     * @throws Exception
     */
    public static void function3() throws Exception {
        
        Player player = (Player) unsafe.allocateInstance(Player.class);
        
        Field fieldName = player.getClass().getDeclaredField("name");
        
        Field fieldAge = player.getClass().getDeclaredField("age");
        
        long fileNameaddres = unsafe.objectFieldOffset(fieldName);
        
        long fileAgeaddres = unsafe.objectFieldOffset(fieldAge);
        
        unsafe.putObjectVolatile(player, fileNameaddres, "wangWu");
        
        unsafe.putInt(player,fileAgeaddres, 100);
        
        System.out.println(player.getAge()+"  "+player.getName());
        
    }
    
    
    /**
     * 实例化对象
     * @throws InstantiationException
     */
    public static void function2() throws InstantiationException {
        
        Player player = (Player) unsafe.allocateInstance(Player.class);
        
        player.setAge(100);
        
        player.setName("zhangShan");
        
        System.out.println(player.getAge()+"  "+player.getName());
    }
    
    /**
     * 对数组的操作
     */
    public static void function1() {
        int[] num = new int[7];
        
        //数组的起始地址
        long adress = unsafe.arrayBaseOffset(int[].class);
        //block 大小
        long index = unsafe.arrayIndexScale(int[].class);
        
        unsafe.putInt(num, adress,1);
        unsafe.putInt(num, adress+index, 2);
        unsafe.putInt(num, adress+index+index, 3);
        unsafe.putInt(num, adress+index+index+index, 4);
        
        System.out.println(JSON.toJSONString(num));
        
    }
}