Ability to decompile a Java class file is quite helpful for any Java
developer who wants to look into the source of any open source or propriety library
used in project. Though I always prefer to attach source in Eclipse of most
common libraries like JDK but it’s not always possible with increasing number
of dependencies. Java decompiler (a program which can decompile Java class files to produce
source files) is very helpful on such situation. By using Java decompiler you
can easily check out source of any .class file.
Thanks to Eclipse IDE and increasing number of free plug-in available for Java
developers, You can have powerful Java decompile in your armory. Earlier I
used to used JadEclipse an Eclipse plugin which works
well with JAD decompiler but knowing about JAD decompiler is not supporting
Java 1.5 source, I was in hunt of another suitable and useful Eclipse plugin which
can decompile .class file. My search ends with JD-Eclipse , this is
another free Eclipse plugin for non commercial and personal use which helps you
to get source from class files. JD-Eclipse is easy to
install and its site has detailed step by step guide on how to install JD-Eclipse plug-in. If
you are already familiar with Eclipse plugin installation than it just a cake
walk.
How to
decompile Class file in Eclipse IDE
Once you have JD-Eclipse installedand running, you probably need to restart Eclipse once you installed
plug-in. You are ready to decompile Java class file from Eclipse. In order to
decompile class file, just open any of your Java project and go to Maven
dependencies of libraries to see the jar files included in your
project. Just expand any jar file for which you don't have source attached in
Eclipse and click on .class file. Now you can see the source code for
that file in Java editor, simple and elegant isn't it. Though I highly
recommend to attach source code for JAR in Eclipse, at least for frequently used libraries like JDK or Spring as quality of
decompiled code and original code is different. Original code gives better
visibility and also shows comment while decompiled code is just bare minimum
version.
How to decompile class file in Java – javap
command example
Even with powerful Eclipse IDE and plugin, we may some time needs to work
on command prompt especially
while working in Linux development servers and its not convenient to switch
back and fourth for quick look on .class file or
get some information from compiled form of source. Thanks to javap command
you can decompile class file on the fly in command prompt. javap is
standard binary which comes with JDK installation and resides in JAVA_HOME/bin directory.
javap is similar to javac (java compiler) and work directly with .class
file. In order to use javap command you must have JAVA_HOME in your system path.
you can verify this by typing "javap" in
command prompt and if it doesn't complain and instead show some output like
below than you are good to go. If it doesn't recognize the command
means you need to set path, check how to set path in java more detailed steps.
abc@localhost:~/java javap
No classes were specified on the command line. Try
-help.
Now let's see what javap command offers us. We have a simple Java class with one field and
one method.
abc@localhost:~/java cat Hello.java
public class
Hello{
private String
version="1.2";
public static void
main(String args[]){
System.out.println("Running
Hello");
}
}
abc@localhost:~/java javap Hello
Compiled from "Hello.java"
public class
Hello extends java.lang.Object{
public Hello();
public static void
main(java.lang.String[]);
}
So it looks javap only provides information related to method
in .class file. It also state the constructor even default
constructor added by Java compiler. javap can
provide more information depending upon its command line option like -private will show
all private members. here is an
example of running javap command with -private option:
abc@localhost:~/java javap -private
Hello
Compiled from "Hello.java"
public class
Hello extends java.lang.Object{
private java.lang.String version;
public Hello();
public static void
main(java.lang.String[]);
}
How to
see bytecode from .class file
javap command can also show bytecodes form compiled class files. Just run
javap with -c option and it will print byte codes of class file, as shown in below example:
abc@localhost:~/java javap -c Hello
Compiled from "Hello.java"
public class
Hello extends java.lang.Object{
public Hello();
Code:
0: aload_0
1: invokespecial
#1; //Method java/lang/Object."
4: aload_0
5: ldc #2; //String 1.2
7: putfield
#3; //Field
version:Ljava/lang/String;
10: return
public static
void main(java.lang.String[]);
Code:
0: getstatic
#4;
//Field
java/lang/System.out:Ljava/io/PrintStream;
3: ldc #5; //String Running Hello
5: invokevirtual
#6; //Method
java/io/PrintStream.println:(Ljava/lang/String;)V
8: return
}
That's all on how to decompile class files in Java and Eclipse IDE.
JD-Eclipse is an easy to use eclipse plugin and has detailed
installation steps documented. if you are running on JDK lower than Java 5 than
You can still use famous JAD decompiler and JADEclipse plug-in. Apart
from these are many more which I haven't try. Just go to Eclipse market place
and you will see lot of those.
Relate Eclipse shortcuts and tutorials from Javarevisited Blog
Source:http://javarevisited.blogspot.com/2013/01/how-to-decompile-class-file-in-java-eclipse-javap-example.html
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