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This article introduces the knowledge of "how to decompile java bytecode files by javap". In the operation of actual cases, many people will encounter such a dilemma, so let the editor lead you to learn how to deal with these situations. I hope you can read it carefully and be able to achieve something!

Overview:

Javap is a Java class file decomposer, which can decompile or view the bytecode generated by the java compiler to analyze the execution logic inside the code.

Syntax:

Compile the java file into a class file: javac Test.java (Test.java is the name of the java file) to generate the corresponding .class file Test.class

Perform javap operation: javap command line class file name (without .class suffix)

For example: javap-c Test

Command Lin

-help outputs help information for javap.

-l output line and local variable scale.

-b ensures backward compatibility with JDK 1.1 javap.

-public displays only public classes and members.

-protected displays only protected and public classes and members.

-package displays only the package, protected, and public classes and members. This is the default setting.

-private displays all classes and members.

-J [flag] passes the flag directly to the runtime system.

-s outputs the internal type signature.

-c outputs the unparsed code of the methods in the class, that is, the instructions that make up the Java bytecode.

-verbose output stack size, number of locals and args of each method, and compiled version of the class file

-classpath [path] specifies the path that javap uses to find the class. If this option is set, it overrides the default value or CLASSPATH environment variable. Directories are separated by colons.

-bootclasspath [path] specifies the path used to load the bootstrap class. By default, bootstrap classes are classes that implement the core Java platform and are located in jrelibt.jar and jrelibi18n.jar.

-extdirs [dirs] overrides the location of the search for installation method extensions. The default location for the extension is jrelibext.

Example:

A Demo.java file (stored on the desktop)

Class Demo {public static void main (String [] args) {Integer aInteger = 56; int aInt=0; int aInt2 = 123; String s = "helloworld"; String aString = new String ("IamString");}}

Disassembly result

Compiled from "Demo.java" class Demo {Demo (); Code: 0: aload_0 1: invokespecial # 1 / / Method java/lang/Object. "": () V 4: return public static void main (java.lang.String []); Code: 0: bipush 56 2: invokestatic # 2 / / Method java/lang/Integer.valueOf: (I) Ljava/lang/Integer 5: astore_1 6: iconst_0 7: istore_2 8: bipush 123 10: istore_3 11: ldc # 3 / / String helloworld 13: astore 4 15: new # 4 / / class java/lang/String 18: dup 19: ldc # 5 / / String IamString 21: invokespecial # 6 / / Method java/lang/String. "(Ljava/lang/String ) V 24: astore 5 26: return}

Integer stack instructions for JVM bytecodes (iconst, bipush, sipush, ldc)

Common decompilation tools:

1. Jd-gui: it's best to use it, but sometimes the result of decompilation will be omitted.

2. DJ Java Decompiler: it is less easy to use than jd-gui, but the decompiled result is more accurate.

3. Jclasslib bytecode viewer: a tool for converting bytecode into JVM instructions.

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