java引用如果是成员变量则引用本身不保存在栈上的汇编级调试证明

た入场券 2023-06-04 15:56 5阅读 0赞

很久很久没有更新博客了，因为发生太多太多猝不及防的事情，再加上自己本身也特别忙，这里补上一直想发的自己觉得很有意义的一次探索过程。

就是很多java开发人员都曾被误导的一个点——“如果一个变量是引用，则它会被存放在栈上”，其实这个证明早在2017.5月就完成了，时间有点久远，细节也许稍有遗忘。

这里先说下结论：一个变量是否会被存放在栈上，在于这个变量是不是写在一个方法里面，而不在于这个变量本身是否是个引用。

下面是示例代码：

public class Main {
    	public static void main(String[] args) {
            Test test = new Test();
            System.out.println("test->" + test);
            System.out.println("t2->" + test.t2);
            test.fn(1,2);
            System.out.println("i->" + test.t2.test2_age);
    	}
    }
    
    public class Test {
        int test_age = 5;
        Test2 t2 = new Test2();
        
        public int fn(int a, int b){
        	return a+b;
        }
    }
    
    class Test2 {
    	int test2_age = 6;
    }

调试工具参考R大写的一篇文章[https://rednaxelafx.iteye.com/blog/1847971][https_rednaxelafx.iteye.com_blog_1847971]。我这边用文章中的命令打不开，所以写了个程序来间接打开，代码如下：

import sun.jvm.hotspot.HSDB;
    
    
    public class OpenHSDB {
    
    	public static void main(String[] args) {
    		HSDB.main(args);
    	}
    }

然后经常用到的HSDB命令如下：

查看堆：
    universe
    
    从堆中搜索对象：
    scanoops 0x0f200000 0x15200000 Test2
    结果
    0x0f2d7c50 Test2
    0x0f2d7c68 Test2
    0x0f2d84e8 Test2
    
    查看具体对象内容：
    inspect 0x0f2d7c50
    
    查找反向引用：
    revptrs 0x0f2d7c50
    
    查看在main线程中的TLAB中：
    whatis 0x0f2d6d50

再利用jdb命令来单步调试最开始的java程序，具体的过程截图按顺序如下：

![790694-20190816113934795-368337831.png][]

![790694-20190816113948606-350261787.png][]

![790694-20190816113956987-1935316963.png][]

![790694-20190816114005005-795238163.png][]

![790694-20190816114012637-1324129716.png][]

![790694-20190816114020728-2096628795.png][]

![790694-20190816114027147-280199012.png][]

![790694-20190816114034700-1441653183.png][]

![790694-20190816114041173-1766176952.png][]

![790694-20190816114047357-1622138806.png][]

![790694-20190816114054780-1109304915.png][]

![790694-20190816114100912-536837746.png][]

![790694-20190816114406145-851497423.png][]

![790694-20190816114422864-1564066108.png][]

![790694-20190816114431313-1731181880.png][]

![790694-20190816114440176-1023746693.png][]

![790694-20190816114451130-224036699.png][]

![790694-20190816114618397-1035544850.png][]

![790694-20190816114631336-617739961.png][]

具体的描述见每张截图上的描述，[虚拟机字节码指令表地址][Link 1]。

完结。

转载于:https://www.cnblogs.com/joey-hua/p/11363006.html

[https_rednaxelafx.iteye.com_blog_1847971]: https://rednaxelafx.iteye.com/blog/1847971
[790694-20190816113934795-368337831.png]: /images/20230601/584568fbd29b438aa492f28ca7d0eb26.png
[790694-20190816113948606-350261787.png]: /images/20230601/a2176aa186c64df18f9fb31071cff3f9.png
[790694-20190816113956987-1935316963.png]: /images/20230601/aa9c86d961374fd09d1c04b074e3a97c.png
[790694-20190816114005005-795238163.png]: /images/20230601/bc4a5c4b66614a4c974555e252f1b2af.png
[790694-20190816114012637-1324129716.png]: /images/20230601/0368ca43aacd4022882e88b170337fb6.png
[790694-20190816114020728-2096628795.png]: /images/20230601/b6f4b7cccb2a4922959a1ed112251601.png
[790694-20190816114027147-280199012.png]: /images/20230601/4208ac7a77a74222a0703998d8b44e29.png
[790694-20190816114034700-1441653183.png]: /images/20230601/64035270a188499293f571693e1b70b5.png
[790694-20190816114041173-1766176952.png]: /images/20230601/4631b4c728324befa533e11880fd3415.png
[790694-20190816114047357-1622138806.png]: /images/20230601/ef66dd27fc5944e683071278bc3429c3.png
[790694-20190816114054780-1109304915.png]: /images/20230601/e752c2f0730e4ebd880a6e9181c316cd.png
[790694-20190816114100912-536837746.png]: /images/20230601/6554a99a359c47a680457c330af7a279.png
[790694-20190816114406145-851497423.png]: /images/20230601/e7ccd608febf4d31ba074798ed2d0ffb.png
[790694-20190816114422864-1564066108.png]: /images/20230601/c42b32d2a9ca4d0c8b83361746c6aa1f.png
[790694-20190816114431313-1731181880.png]: /images/20230601/89d3397021cd4d5791b83ece95e8116a.png
[790694-20190816114440176-1023746693.png]: /images/20230601/d8eb3889831e4fd99b7cd64168cd7f6b.png
[790694-20190816114451130-224036699.png]: /images/20230601/836aa2c4780f4ef0a5638e6094a9b990.png
[790694-20190816114618397-1035544850.png]: /images/20230601/9033397deaa644ca92ca15a5ab7e502f.png
[790694-20190816114631336-617739961.png]: /images/20230601/71bb2982795e48edae4f682d41f0fb5b.png
[Link 1]: https://github.com/huaxiaozhou/android-protection/blob/master/%E9%99%84%E5%BD%95B%20%E8%99%9A%E6%8B%9F%E6%9C%BA%E5%AD%97%E8%8A%82%E7%A0%81%E6%8C%87%E4%BB%A4%E8%A1%A8.pdf