概述
纸上得来终觉浅，绝知此事要躬行，今天就动手实现一个jdk自带的工具javap
- 阅读本文可以带你完整的自己实现javap的全部功能
- 本文只提到了核心代码，完整代码git地址请访问：https://github.com/lzc-alioo/itstack-demo-jvm
- 项目访问入口类是com.lzc.wuxin.Jad1，com.lzc.wuxin.Jad2，将分别代表javap，javap -v的实现逻辑，如果你在运行过程中有任何问题欢迎留言，看到后一定为你解答，同时如果你发现代码的缺陷，也欢迎提交issue或者直接提交pr。
- 本文的创作是建立在github中一个已经开源的项目基础上进行的，源项目地址为：itstack-demo-jvm （https://github.com/fuzhengwei/itstack-demo-jvm）
准备工作
用来测试的java文件 HelloWorld.java
package com.lzc.wuxin.demo;
public class HelloWorld {
private final static byte abc = -14;
private final static Byte abc2 = Byte.valueOf((byte) -15);
private static int lzc;
public static void main(String[] args) {
int lzc2 = 2;
int lzc3 = 3;
int lzc4 = lzc2 + lzc3;
}
}
javap VS jad1
javap效果
javap com.lzc.wuxin.demo.HelloWorld
Compiled from "HelloWorld.java"
public class com.lzc.wuxin.demo.HelloWorld {
public com.lzc.wuxin.demo.HelloWorld();
public static void main(java.lang.String[]);
static {};
}
jad1效果
Compiled from "HelloWorld.java" (反编译工具作者：Lzc)
public class com.lzc.wuxin.demo.HelloWorld {
private static final byte abc = -14;
private static final Byte abc2;
private static int lzc;
public HelloWorld();
public static void main(String[]);
static {};
}
jad1相应的源码
public static void deCompile(Cmd cmd) {
Classpath classpath = new Classpath(cmd.jre, cmd.classpath);
ClassLoader classLoader = new ClassLoader(classpath);
String className = cmd.getMainClass().replace(".", "/");
String simpleClassName = className.substring(className.lastIndexOf("/") + 1);
System.out.println("Compiled from \"" + simpleClassName + ".java\" (反编译工具作者：Lzc)");
Class clazz = classLoader.loadClass(className);
StringBuilder buf = new StringBuilder();
buf.append(clazz.deprecated ? "@Deprecated " : "").append(AccessFlagsUtil.getAccessFlagsStr(clazz.accessFlags)).append(className).append(" {\n");
for (Field memberInfo : clazz.fields) {
Object val = ValueUtil.getValue(memberInfo, clazz);
buf.append(" ").append(memberInfo.deprecated ? "@Deprecated " : "").append(AccessFlagsUtil.getAccessFlagsStr(memberInfo.accessFlags)).append(FieldDescriptorUtil.getDescriptorStr(memberInfo.descriptor())).append(memberInfo.name()).append(Optional.ofNullable(val).map(tmp -> " = " + tmp).orElse("")).append(";\n");
}
buf.append("\n");
for (Method memberInfo : clazz.methods) {
//方法参数类型 方法返回值类型
MethodInfo methodInfo = MethodDescriptorUtil.getMethodInfo(memberInfo);
buf.append(" ").append(memberInfo.deprecated ? "@Deprecated " : "").append(AccessFlagsUtil.getAccessFlagsStr(memberInfo.accessFlags)).append(methodInfo.getValueType()).append(methodInfo.getMethodName()).append(methodInfo.getParamType()).append(";\n");
}
buf.append("}");
System.out.println(buf.toString());
}
javap -v VS jad2
javap -v效果
javap -v com.lzc.wuxin.demo.HelloWorld
Classfile /Users/mac/work/gitstudy/itstack-demo-jvm/itstack-lzc/target/test-classes/com/lzc/wuxin/demo/HelloWorld.class
Last modified 2022-4-17; size 711 bytes
MD5 checksum 17e695fb34c8c10f716ac6e92c8609f1
Compiled from "HelloWorld.java"
public class com.lzc.wuxin.demo.HelloWorld
minor version: 0
major version: 52
flags: ACC_PUBLIC, ACC_SUPER
Constant pool:
#1 = Methodref #5.#31 // java/lang/Object."<init>":()V
#2 = Methodref #32.#33 // java/lang/Byte.valueOf:(B)Ljava/lang/Byte;
#3 = Fieldref #4.#34 // com/lzc/wuxin/demo/HelloWorld.abc2:Ljava/lang/Byte;
#4 = Class #35 // com/lzc/wuxin/demo/HelloWorld
#5 = Class #36 // java/lang/Object
#6 = Utf8 abc
#7 = Utf8 B
#8 = Utf8 ConstantValue
#9 = Integer -14
#10 = Utf8 abc2
#11 = Utf8 Ljava/lang/Byte;
#12 = Utf8 lzc
#13 = Utf8 I
#14 = Utf8 <init>
#15 = Utf8 ()V
#16 = Utf8 Code
#17 = Utf8 LineNumberTable
#18 = Utf8 LocalVariableTable
#19 = Utf8 this
#20 = Utf8 Lcom/lzc/wuxin/demo/HelloWorld;
#21 = Utf8 main
#22 = Utf8 ([Ljava/lang/String;)V
#23 = Utf8 args
#24 = Utf8 [Ljava/lang/String;
#25 = Utf8 lzc2
#26 = Utf8 lzc3
#27 = Utf8 lzc4
#28 = Utf8 <clinit>
#29 = Utf8 SourceFile
#30 = Utf8 HelloWorld.java
#31 = NameAndType #14:#15 // "<init>":()V
#32 = Class #37 // java/lang/Byte
#33 = NameAndType #38:#39 // valueOf:(B)Ljava/lang/Byte;
#34 = NameAndType #10:#11 // abc2:Ljava/lang/Byte;
#35 = Utf8 com/lzc/wuxin/demo/HelloWorld
#36 = Utf8 java/lang/Object
#37 = Utf8 java/lang/Byte
#38 = Utf8 valueOf
#39 = Utf8 (B)Ljava/lang/Byte;
{
public com.lzc.wuxin.demo.HelloWorld();
descriptor: ()V
flags: ACC_PUBLIC
Code:
stack=1, locals=1, args_size=1
0: aload_0
1: invokespecial #1 // Method java/lang/Object."<init>":()V
4: return
LineNumberTable:
line 3: 0
LocalVariableTable:
Start Length Slot Name Signature
0 5 0 this Lcom/lzc/wuxin/demo/HelloWorld;
public static void main(java.lang.String[]);
descriptor: ([Ljava/lang/String;)V
flags: ACC_PUBLIC, ACC_STATIC
Code:
stack=2, locals=4, args_size=1
0: iconst_2
1: istore_1
2: iconst_3
3: istore_2
4: iload_1
5: iload_2
6: iadd
7: istore_3
8: return
LineNumberTable:
line 10: 0
line 11: 2
line 12: 4
line 13: 8
LocalVariableTable:
Start Length Slot Name Signature
0 9 0 args [Ljava/lang/String;
2 7 1 lzc2 I
4 5 2 lzc3 I
8 1 3 lzc4 I
static {};
descriptor: ()V
flags: ACC_STATIC
Code:
stack=1, locals=0, args_size=0
0: bipush -15
2: invokestatic #2 // Method java/lang/Byte.valueOf:(B)Ljava/lang/Byte;
5: putstatic #3 // Field abc2:Ljava/lang/Byte;
8: return
LineNumberTable:
line 6: 0
}
SourceFile: "HelloWorld.java"
jad2效果
Classfile : /Users/mac/work/gitstudy/itstack-demo-jvm/itstack-lzc/target/test-classes/com/lzc/wuxin/demo/HelloWorld.class
Last modified : 2022-04-17; size 711 bytes
MD5 checksum : 17e695fb34c8c10f716ac6e92c8609f1
Compiled from : HelloWorld.java" (反编译工具作者：Lzc)
public class com.lzc.wuxin.demo.HelloWorld
minor version: 0
major version: 52
flags : public class
Constant pool:
#0 = null
#1 = MethodRef #0.#0 // java/lang/Object.<init>:()V
#2 = MethodRef #0.#0 // java/lang/Byte.valueOf:(B)Ljava/lang/Byte;
#3 = FieldRef #0.#0 // com/lzc/wuxin/demo/HelloWorld.abc2:Ljava/lang/Byte;
#4 = ClassRef #0 // com/lzc/wuxin/demo/HelloWorld
#5 = ClassRef #0 // java/lang/Object
#6 = Utf8 abc
#7 = Utf8 B
#8 = Utf8 ConstantValue
#9 = Integer -14
#10 = Utf8 abc2
#11 = Utf8 Ljava/lang/Byte;
#12 = Utf8 lzc
#13 = Utf8 I
#14 = Utf8 <init>
#15 = Utf8 ()V
#16 = Utf8 Code
#17 = Utf8 LineNumberTable
#18 = Utf8 LocalVariableTable
#19 = Utf8 this
#20 = Utf8 Lcom/lzc/wuxin/demo/HelloWorld;
#21 = Utf8 main
#22 = Utf8 ([Ljava/lang/String;)V
#23 = Utf8 args
#24 = Utf8 [Ljava/lang/String;
#25 = Utf8 lzc2
#26 = Utf8 lzc3
#27 = Utf8 lzc4
#28 = Utf8 <clinit>
#29 = Utf8 SourceFile
#30 = Utf8 HelloWorld.java
#31 = NameAndType #14:#15 // <init>:()V
#32 = ClassRef #0 // java/lang/Byte
#33 = NameAndType #38:#39 // valueOf:(B)Ljava/lang/Byte;
#34 = NameAndType #10:#11 // abc2:Ljava/lang/Byte;
#35 = Utf8 com/lzc/wuxin/demo/HelloWorld
#36 = Utf8 java/lang/Object
#37 = Utf8 java/lang/Byte
#38 = Utf8 valueOf
#39 = Utf8 (B)Ljava/lang/Byte;
{
private static final byte abc = -14;
private static final Byte abc2;
private static int lzc;
public HelloWorld();
descriptor:()V
flags: public class
Code:
stack=1, locals=1, args_size=1
aload_0
invoke_special
nop
aconst_null
return
LineNumberTable:
line 3: 0
LocalVariableTable:
Start Length Slot Name Signature
0 5 0 this Lcom/lzc/wuxin/demo/HelloWorld;
public static void main(String[]);
descriptor:([Ljava/lang/String;)V
flags: public class
Code:
stack=2, locals=4, args_size=1
iconst_2
istore_1
iconst_3
istore_2
iload_1
iload_2
iadd
istore_3
return
LineNumberTable:
line 10: 0
line 11: 2
line 12: 4
line 13: 8
LocalVariableTable:
Start Length Slot Name Signature
0 9 0 args [Ljava/lang/String;
2 7 1 lzc2 I
4 5 2 lzc3 I
8 1 3 lzc4 I
static {};
descriptor:()V
flags: public class
Code:
stack=1, locals=0, args_size=0
bipush
unkown
invoke_static
nop
iconst_m1
put_static
nop
iconst_0
return
LineNumberTable:
line 6: 0
LocalVariableTable:
Start Length Slot Name Signature
}
jad2相应的源码
public static void deCompile(Cmd cmd) {
String className = cmd.getMainClass().replace(".", "/");
String simpleClassName = className.substring(className.lastIndexOf("/") + 1);
ClassInfo classInfo = null;
try {
classInfo = new Classpath(cmd.jre, cmd.classpath).readClass2ClassInfo(className);
} catch (Exception e) {
e.printStackTrace();
return;
}
System.out.println(StringUtils.rightPad("Classfile", 16) + ": " + classInfo.classFromPath);
System.out.println(StringUtils.rightPad(" Last modified ", 16) + ": " + DateTimeUtil.toDateTimeString(classInfo.lastModified, "yyyy-MM-dd") + "; size " + classInfo.length + " bytes");
System.out.println(StringUtils.rightPad(" MD5 checksum ", 16) + ": " + classInfo.md5);
System.out.println(StringUtils.rightPad(" Compiled from ", 16) + ": " + simpleClassName + ".java\" (反编译工具作者：Lzc)");
Classpath classpath = new Classpath(cmd.jre, cmd.classpath);
ClassLoader classLoader = new ClassLoader(classpath);
Class clazz = classLoader.loadClass(className);
StringBuilder buf = new StringBuilder();
buf.append(clazz.deprecated ? "@Deprecated " : "").append(AccessFlagsUtil.getAccessFlagsStr(clazz.accessFlags)).append(cmd.getMainClass()).append("\n");
buf.append(" minor version: ").append(classInfo.classFile.minorVersion()).append("\n");
buf.append(" major version: ").append(classInfo.classFile.majorVersion()).append("\n");
buf.append(" flags : ").append(AccessFlagsUtil.getAccessFlagsStr(clazz.accessFlags)).append("\n");
System.out.print(buf.toString());
buf.delete(0, buf.length());
buf.append("Constant pool:\n");
Object[] constants = clazz.constantPool().constants;
for (int i = 0; i < constants.length; i++) {
// #1 = Methodref #5.#34 // java/lang/Object."<init>":()V
buf.append(StringUtils.leftPad("#" + i, 4)).append(" = ");
buf.append(ConstantPoolUtil.readableConstant(constants[i]));
buf.append("\n");
}
System.out.print(buf.toString());
buf.delete(0, buf.length());
buf.append("{\n");
for (Field memberInfo : clazz.fields) {
Object val = ValueUtil.getValue(memberInfo, clazz);
buf.append(" ").append(memberInfo.deprecated ? "@Deprecated " : "").append(AccessFlagsUtil.getAccessFlagsStr(memberInfo.accessFlags)).append(FieldDescriptorUtil.getDescriptorStr(memberInfo.descriptor())).append(memberInfo.name()).append(Optional.ofNullable(val).map(tmp -> " = " + tmp).orElse("")).append(";\n");
}
buf.append("\n");
for (Method method : clazz.methods) {
//方法参数类型 方法返回值类型
MethodInfo methodInfo = MethodDescriptorUtil.getMethodInfo(method);
buf.append(" ").append(AccessFlagsUtil.getAccessFlagsStr(method.accessFlags)).append(methodInfo.getValueType()).append(methodInfo.getMethodName()).append(methodInfo.getParamType()).append(";\n");
buf.append(" descriptor:").append(method.descriptor).append("\n");
buf.append(" flags: ").append(AccessFlagsUtil.getAccessFlagsStr(clazz.accessFlags)).append("\n");
buf.append(" Code: ").append("\n");
buf.append(" stack=").append(method.maxStack).append(", locals=").append(method.maxLocals).append(", args_size=").append(method.argSlotCount()).append("\n");
for (byte opcode : method.code) {
Instruction inst = Factory.newInstruction(opcode);
buf.append(" ").append(InstructionUtil.readableInstruction(inst).toLowerCase()).append("\n");
}
buf.append(" LineNumberTable:").append("\n");
for (LineNumberTableAttribute.LineNumberTableEntry lineNumber : method.lineNumberTableAttribute.lineNumberTable) {
buf.append(" line ").append(lineNumber.lineNumber).append(": ").append(lineNumber.startPC).append("\n");
}
buf.append(" LocalVariableTable:").append("\n");
buf.append(" Start Length Slot Name Signature").append("\n");
if(method.localVariableTableAttribute!=null && method.localVariableTableAttribute.localVariableTable!=null){
for (LocalVariableTableAttribute.LocalVariableTableEntry localVar : method.localVariableTableAttribute.localVariableTable) {
buf.append(StringUtils.leftPad(localVar.startPC + "", 13))
.append(StringUtils.leftPad(localVar.length + "", 8))
.append(StringUtils.leftPad(localVar.idx + "", 6))
.append(StringUtils.leftPad(clazz.runTimeConstantPool.getConstants(localVar.nameIdx) + "", 6)).append(" ")
.append(StringUtils.rightPad(clazz.runTimeConstantPool.getConstants(localVar.descriptorIdx)+"" , 14))
.append("\n");
}
}
}
buf.append("}");
System.out.println(buf.toString());
}
