Unsafe 簡介

Unsafe 是sun.misc包中的一個類,可以通過內存偏移量操作類變量/成員變量

Unsafe 用途

AQS(AbstractQueuedSynchronizer) 常用作實現輕量級鎖,它里面有一個雙向鏈表,用于封裝未搶到鎖的線程 ,其中有用到Unsafe的compareAndSwapObject修改鏈表

Unsafe 簡單使用示例

package com.xh.kssfwjg.idgenerator;

import sun.misc.Unsafe;

import java.lang.reflect.Field;

/**
 * TODO
 *
 * @auther xh
 * @date 12/26/18 6:11 PM
 */
public class UnSaveTest {
    private static final Unsafe unsafe = getUnsafe();
    private static final long objectNameOffset;
    private static final long staticNameOffset;

    private static String staticName = "qq";
    private String objectName = "123";

    static {
        try {
            objectNameOffset = unsafe.objectFieldOffset
                    (UnSaveTest.class.getDeclaredField("objectName"));
            staticNameOffset = unsafe.staticFieldOffset(UnSaveTest.class.getDeclaredField("staticName"));
        } catch (NoSuchFieldException e) {
            throw new Error(e);
        }
    }

    public static void main(String[] args) {
        UnSaveTest unSaveTest = new UnSaveTest();
        // unsafe.compareAndSwapObject(unSaveTest, testOffset, "123", "456");//CAS
        unsafe.putObject(unSaveTest, objectNameOffset, "haha");//直接修改
        unsafe.putObject(UnSaveTest.class, staticNameOffset, "hehe");//直接修改
        System.out.println(unSaveTest.objectName);
        System.out.println(UnSaveTest.staticName);
    }


    private static Unsafe getUnsafe() {
        Field singleoneInstanceField = null;
        try {
            singleoneInstanceField = Unsafe.class.getDeclaredField("theUnsafe");
            singleoneInstanceField.setAccessible(true);
            return (Unsafe) singleoneInstanceField.get(null);
        } catch (Exception e) {
            e.printStackTrace();
        }
        return null;
    }
}

output:

haha
hehe

ps:

如果變量使用final修飾,如:

private static final String staticName = "qq";
private final String objectName = "123";

是不能修改的.

另外附上反射的示例

        try {
            Class clazz = Class.forName("com.xh.kssfwjg.idgenerator.UnSaveTest");
            Object o = clazz.newInstance();
            Field object_field_name = clazz.getDeclaredField("objectName");
            object_field_name.setAccessible(true);
            object_field_name.set(o, "ooo");

            Field static_field_name = clazz.getDeclaredField("staticName");
            static_field_name.setAccessible(true);
            static_field_name.set(clazz, "ppp");

            System.out.println(UnSaveTest.staticName);
            System.out.println(((UnSaveTest) o).objectName);

        } catch (Exception e) {
            e.printStackTrace();
        }

結果一樣

總結

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