关于 Java 字节码的请教

This topic created in 2245 days ago, the information mentioned may be changed or developed.

例如，我有这么一份代码

package lock;

public class ByteCodeTest {
    private volatile String name="view";
    public void work(){
        name+="hello";
    }
}

编译后，通过字节码工具查看

// class version 56.0 (56)
// access flags 0x21
public class lock/ByteCodeTest {

  // compiled from: ByteCodeTest.java
  // access flags 0x19
  public final static INNERCLASS java/lang/invoke/MethodHandles$Lookup java/lang/invoke/MethodHandles Lookup

  // access flags 0x42
  private volatile Ljava/lang/String; name

  // access flags 0x1
  public <init>()V
   L0
    LINENUMBER 3 L0
    ALOAD 0
    INVOKESPECIAL java/lang/Object.<init> ()V
   L1
    LINENUMBER 4 L1
    ALOAD 0
    LDC "view"
    PUTFIELD lock/ByteCodeTest.name : Ljava/lang/String;
    RETURN
   L2
    LOCALVARIABLE this Llock/ByteCodeTest; L0 L2 0
    MAXSTACK = 2
    MAXLOCALS = 1

  // access flags 0x1
  public work()V
   L0
    LINENUMBER 6 L0
    ALOAD 0
    DUP
    GETFIELD lock/ByteCodeTest.name : Ljava/lang/String;
    INVOKEDYNAMIC makeConcatWithConstants(Ljava/lang/String;)Ljava/lang/String; [
      // handle kind 0x6 : INVOKESTATIC
      java/lang/invoke/StringConcatFactory.makeConcatWithConstants(Ljava/lang/invoke/MethodHandles$Lookup;Ljava/lang/String;Ljava/lang/invoke/MethodType;Ljava/lang/String;[Ljava/lang/Object;)Ljava/lang/invoke/CallSite;
      // arguments:
      "\u0001hello"
    ]
    PUTFIELD lock/ByteCodeTest.name : Ljava/lang/String;
   L1
    LINENUMBER 7 L1
    RETURN
   L2
    LOCALVARIABLE this Llock/ByteCodeTest; L0 L2 0
    MAXSTACK = 2
    MAXLOCALS = 1
}

想请教下如何阅读上述指令语法
或是 google 什么关键字可以搜到相关内容

Supplement 1 · Apr 22, 2020

这几天翻阅了书籍有些头绪，特来回帖，如果错误望指出

代码

package core.object_create;
public class ObjectTest {
    private String name = "v2";
    public void add() {
        int i10 = -1;
        int i0 = 5;
        int i1 = 6;
        int i2 = 127;
        int i3 = 150;
        int sum = i10 + i0 + i1 + i2 + i3;
    }
}

字节码

public class core/object_create/ObjectTest {
  private Ljava/lang/String; name

  // access flags 0x1
  public <init>()V
   L0
    LINENUMBER 3 L0
    ALOAD 0 //将指定的引用类型本地变量推送至栈顶
    INVOKESPECIAL java/lang/Object.<init> ()V //调用超类构造方法，实例初始化方法，私有方法
   L1
    LINENUMBER 4 L1
    ALOAD 0
    LDC "v2" //将int, float或String型常量值从常量池中推送至栈顶
    PUTFIELD core/object_create/ObjectTest.name : Ljava/lang/String; //为指定的类的实例域赋值
    RETURN //从当前方法返回void
   L2
    LOCALVARIABLE this Lcore/object_create/ObjectTest; L0 L2 0
    MAXSTACK = 2 //代表了操作数栈（Operand Stacks）深度的最大值
    MAXLOCALS = 1 //代表了局部变量表所需的存储空间
  // access flags 0x1
  public add()V
   L0 //表示一个局部变量
    LINENUMBER 6 L0 //标识变量对应行
    ICONST_M1 //将int型-1推送至栈顶
    ISTORE 1 //将栈顶int型数值存入指定本地变量
   L1
    LINENUMBER 7 L1
    ICONST_5 //将int型5推送至栈顶
    ISTORE 2
   L2
    LINENUMBER 8 L2
    BIPUSH 6  //将单字节的常量值(-128~127)推送至栈顶
    ISTORE 3
   L3
    LINENUMBER 9 L3
    BIPUSH 127
    ISTORE 4
   L4
    LINENUMBER 10 L4
    SIPUSH 150
    ISTORE 5
   L5
    LINENUMBER 11 L5
    ILOAD 1 //将指定的int型本地变量推送至栈顶
    ILOAD 2 
    IADD  //将栈顶两int型数值相加并将结果压入栈顶
    ILOAD 3
    IADD
    ILOAD 4
    IADD
    ILOAD 5
    IADD
    ISTORE 6
   L6
    LINENUMBER 12 L6
    RETURN
   L7
    LOCALVARIABLE this Lcore/object_create/ObjectTest; L0 L7 0
    LOCALVARIABLE i10 I L1 L7 1
    LOCALVARIABLE i0 I L2 L7 2
    LOCALVARIABLE i1 I L3 L7 3
    LOCALVARIABLE i2 I L4 L7 4
    LOCALVARIABLE i3 I L5 L7 5
    LOCALVARIABLE sum I L6 L7 6
    MAXSTACK = 2
    MAXLOCALS = 7
}

11 replies • 2020-04-14 13:23:23 +08:00

anerinck

Apr 14, 2020

我觉得直接看字节码的十六进制比阅读反编译要清楚。。如果是要了解指令的话不如去搜索一下 jvm 的指令集一类的？

misaka19000

Apr 14, 2020

《深入理解 Java 虚拟机》第 6 章

CRUD

Apr 14, 2020

Java 虚拟机指令集： https://docs.oracle.com/javase/specs/jvms/se8/html/jvms-6.html

guxingke

Apr 14, 2020

https://github.com/guxingke/mini-jvm

java 实现的解释字节码的虚拟机可以了解一下

xyooyx

Apr 14, 2020

@anerinck
@misaka19000
@CRUD
@guxingke
太棒了,感谢各位😄

maokabc

Apr 14, 2020 via Android

java 虚拟机规范，或者可以看 jasmin 的语法，感觉比这个更好理解，还能修改了再编译回 class

choiwanxy

Apr 14, 2020

首先你要分得清哪里是什么，比如哪里是常量池，哪里是无参构架函数，哪里是方法对应的字节码。找到你要看的方法，看 jvm 指令，一个个去查是什么意思。

itning

Apr 14, 2020 via Android

不要脸一下
https://blog.itning.top/post/2020/04/10/jvm%E7%B3%BB%E5%88%972-class%E6%96%87%E4%BB%B6%E6%A0%BC%E5%BC%8F/

xyooyx

Apr 14, 2020

@maokabc
@choiwanxy
@itning
有认真去看，感谢哈😄

CFM880

Apr 14, 2020

jclasslib idea 插件，常量池，方法区，点击指令可以跳转到#3 楼说的网页中的指定指令

CFM880

Apr 14, 2020

https://docs.oracle.com/javase/specs/jvms/se8/html/jvms-4.html
这一章介绍了 ClassFile Structure，先整体了解字节码文件结构，结构嵌套结构，后面再到方法区的指令，比较容易一些，直接上指令集，可以按照字节码文件结构规定的字节来分析一下
ClassFile {
u4 magic; #u4 4 个字节魔数
u2 minor_version; #2 个字节小版本号
u2 major_version;
u2 constant_pool_count;
cp_info constant_pool[constant_pool_count-1];
u2 access_flags;
u2 this_class;
u2 super_class;
u2 interfaces_count;
u2 interfaces[interfaces_count];
u2 fields_count;
field_info fields[fields_count];
u2 methods_count;
method_info methods[methods_count];
u2 attributes_count;
attribute_info attributes[attributes_count];
}