How can I open Java .class files in a human-readable way?

Question

I'm trying to figure out what a Java applet's class file is doing under the hood. Opening it up with Notepad or Textpad just shows a bunch of gobbledy-gook.

Is there any way to wrangle it back into a somewhat-readable format so I can try to figure out what it's doing?

Environment == Windows w/ VS 2008 installed.

Can you clarify this question? My immediate thought was that you wanted to read the actual bytecode, but it seems that you actually want to decompile someone else's code. — Tim Frey, Sep 16 '08 at 20:39
@icco, = means assignment, == means equality. He is saying that (Environment == Windows w/ VS2008 installed) is true, not giving an order to set Environment to Windows w/ VS2008 installed. Man that was offtopic and nerdy. — Blorgbeard, Sep 16 '08 at 20:47
For the record == doesn't declare equality; it tests equality. You can write true == false, and it's about as meaningful as writing Environment == Windows w/ VS 2008 installed, i.e. not meaningful at all. — bwerks, Mar 21 '13 at 06:44

DarenW · Accepted Answer · 2013-10-19T02:20:22.193

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jd-gui is the best decompiler at the moment. it can handle newer features in Java, as compared to the getting-dusty JAD.

edited Oct 19 '13 at 02:20

answered Sep 16 '08 at 20:11

DarenW

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How up to date is this answer in 2020? – The Coding Wombat Mar 24 '20 at 20:36
As long as the link goes somewhere, I suppose it's good. I wouldn't know if there's now a better Java decompiler, as I haven't used Java since long ago. – DarenW Mar 30 '20 at 20:20
@DarenW the jd-gui seems to be an in secure place is it actually trustworthy? – MauricioTL Aug 18 '23 at 23:43

score 86 · Answer 2 · answered Sep 16 '08 at 20:08

If you don't mind reading bytecode, javap should work fine. It's part of the standard JDK installation.

Usage: javap <options> <classes>...

where options include:
   -c                        Disassemble the code
   -classpath <pathlist>     Specify where to find user class files
   -extdirs <dirs>           Override location of installed extensions
   -help                     Print this usage message
   -J<flag>                  Pass <flag> directly to the runtime system
   -l                        Print line number and local variable tables
   -public                   Show only public classes and members
   -protected                Show protected/public classes and members
   -package                  Show package/protected/public classes
                             and members (default)
   -private                  Show all classes and members
   -s                        Print internal type signatures
   -bootclasspath <pathlist> Override location of class files loaded
                             by the bootstrap class loader
   -verbose                  Print stack size, number of locals and args for methods
                             If verifying, print reasons for failure

score 22 · Answer 3 · answered Feb 16 '18 at 15:51

22

As pointed out by @MichaelMyers, use

javap -c <name of java class file>

to get the JVM assembly code. You may also redirect the output to a text file for better visibility.

javap -c <name of java class file> > decompiled.txt

answered Feb 16 '18 at 15:51

Nilashish C

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score 7 · Answer 4 · answered Sep 16 '08 at 20:05

7

You want a java decompiler, you can use the command line tool javap to do this. Also, Java Decompiler HOW-TO describes how you can decompile a class file.

answered Sep 16 '08 at 20:05

Drew Frezell

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score 3 · Answer 5 · answered Feb 23 '18 at 11:50

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you can also use the online java decompilers available. For e.g. http://www.javadecompilers.com

answered Feb 23 '18 at 11:50

Prachi Sharma

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Rasmus Faber · Answer 6 · 2018-10-25T13:07:15.090

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Using Jad to decompile it is probably your best option. Unless the code has been obfuscated, it will produce an okay result.

edited Oct 25 '18 at 13:07

answered Sep 16 '08 at 20:03

Rasmus Faber

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score 2 · Answer 7 · answered Sep 27 '13 at 10:05

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jd-gui "http://code.google.com/p/innlab/downloads/detail?name=jd-gui-0.3.3.windows.zip&can=2&q=" is the best and user friendly option for decompiling .class file....

answered Sep 27 '13 at 10:05

Karthikeyan Subramanian

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score 2 · Answer 8 · answered Dec 23 '08 at 17:56

cpuguru, if your applet has been compiled with javac 1.3 (or less), your best option is to use Jad.

Unfortunately, the last JDK supported by JAD 1.5.8 (Apr 14, 2001) is JDK 1.3.

If your applet has been compiled with a more recent compiler, you could try JD-GUI : this decompiler is under development, nevertheless, it generates correct Java sources, most of time, for classes compiled with the JDKs 1.4, 1.5 or 1.6.

DarenW, thank you for your post. JD-GUI is not the best decompiler yet ... but I'm working on :)

score 2 · Answer 9 · answered Sep 16 '08 at 20:05

what you are looking for is a java de-compiler. I recommend JAD http://www.kpdus.com/jad.html It's free for non commercial use and gets the job done.

Note: this isn't going to make the code exactly the same as what was written. i.e. you're going to lose comments and possibly variable names, so it's going to be a little bit harder than just reading normal source code. If the developer is really secretive they will have obfuscated their code as well, making it even harder to read.

score 1 · Answer 10 · answered Sep 16 '08 at 20:03

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That's compiled code, you'll need to use a decompiler like JAD: http://www.kpdus.com/jad.html

answered Sep 16 '08 at 20:03

cynicalman

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score 1 · Answer 11 · answered Sep 16 '08 at 20:12

You need to use a decompiler. Others have suggested JAD, there are other options, JAD is the best.

I'll echo the comments that you may lose a bit compared to the original source code. It is going to look especially funny if the code used generics, due to erasure.

score 1 · Answer 12 · answered Sep 16 '08 at 20:35

1

JAD and/or JADclipse Eclipse plugin, for sure.

answered Sep 16 '08 at 20:35

John Gardner

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arturh · Answer 13 · 2009-05-13T10:54:10.920

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If the class file you want to look into is open source, you should not decompile it, but instead attach the source files directly into your IDE. that way, you can just view the code of some library class as if it were your own

edited May 13 '09 at 10:54

answered Sep 17 '08 at 15:29

arturh

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score 0 · Answer 14 · answered Jan 18 '17 at 13:54

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CFR - another java decompiler is a great decompiler for modern Java written i Java 6.

answered Jan 18 '17 at 13:54

tidbeck

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score 0 · Answer 15 · answered Sep 16 '08 at 20:07

As suggested you can use JAD to decompile it and view the files. To make it easier to read you can use the JADclipse plugin for eclipse to integrate JAD directly to eclipse or use DJ Java Decompiler which is much easier to use than command line JAD

score 0 · Answer 16 · answered Sep 16 '08 at 20:09

JAD is an excellent option if you want readable Java code as a result. If you really want to dig into the internals of the .class file format though, you're going to want javap. It's bundled with the JDK and allows you to "decompile" the hexadecimal bytecode into readable ASCII. The language it produces is still bytecode (not anything like Java), but it's fairly readable and extremely instructive.

Also, if you really want to, you can open up any .class file in a hex editor and read the bytecode directly. The result is identical to using javap.

score 0 · Answer 17 · answered Sep 16 '08 at 20:45

There is no need to decompile Applet.class. The public Java API classes sourcecode comes with the JDK (if you choose to install it), and is better readable than decompiled bytecode. You can find compressed in src.zip (located in your JDK installation folder).

How can I open Java .class files in a human-readable way?

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