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Hello! Today I'm happy to announce the release of a project that me and my friends have been working on over the course of the last year, Quiltflower! Originally intended just for use with the QuiltMC toolchain with Minecraft, Quiltflower quickly expanded to be a general purpose java decompiler aiming to create code that is as accurate and clean as possible. If the name sounds familiar it's because Quiltflower is a fork of Fernflower, the (in)famous decompiler that was developed by Stiver, maintained by Jetbrains, and became the default decompiler in Intellij IDEA. Fernflower also quickly found its way into many other tools. After many frustrations with it myself with its decompiled code structuring and quality I decided to do something about it, and here we are! Over the past year, Quiltflower has added support for features such as modern string concatenation, a code formatter, sealed classes, pattern matching, switch expressions, try-with-resources, and more. Quiltflower also focuses on the code quality of the decompiled output, and takes readability very seriously. We'd greatly appreciate it if you'd give it a try, with our Intellij Plugin, as a standalone jar, or on our maven. While it has come a long way it's still a work in progress, and feedback can be reported on our issue tracker.
Here's a comparison of Fernflower and Quiltflower's output.
I'd also like to thank the MinecraftForge Team for creating ForgeFlower, the fork that QuiltFlower was based on, and Lee Benfield for creating CFR and it's truly incredible test suite.
I have a couple of .jar games on my disk. I want to decompile the bytecode back to the readable .java format.
What are my options? Google wasn't very helpful, throwing old sites at me with broken download links.
I just want to learn - I played the game lots of times so I know what it does, now I want to know how it does it.
EDIT: Thanks to everyone who replied, the post stays so that others can use. I decided to use jd-gui, but it's good to know many Java IDEs have plugins to do this too (I use Netbeans and I couldn't find a decompiler plugin for it)
Update February 2016:
www.javadecompilers.com lists JAD as being:
the most popular Java decompiler, but primarily of this age only. Written in C++, so very fast.
Outdated, unsupported and does not decompile correctly Java 5 and later
So your mileage may vary with recent jdk (7, 8).
The same site list other tools.
And javadecompiler, as noted by Salvador Valencia in the comments (Sept 2017), offers a SaaS where you upload the .class file to the cloud and it returns you the decompiled code.
Original answer: Oct. 2008
- The final release of JSR 176, defining the major features of J2SE 5.0 (Java SE 5), has been published on September 30, 2004.
- The lastest Java version supported by JAD, the famous Java decompiler written by Mr. Pavel Kouznetsov, is JDK 1.3.
- Most of the Java decompilers downloadable today from the Internet, such as “DJ Java Decompiler” or “Cavaj Java Decompiler”, are powered by JAD: they can not display Java 5 sources.
Java Decompiler (Yet another Fast Java decompiler) has:
- Explicit support for decompiling and analyzing Java 5+ “.class” files.
- A nice GUI:
It works with compilers from JDK 1.1.8 up to JDK 1.7.0, and others (Jikes, JRockit, etc.).
It features an online live demo version that is actually fully functional! You can just drop a jar file on the page and see the decompiled source code without installing anything.
There are a few decompilers out there... A quick search yields:
- Procyon: open-source (Apache 2) and actively developed
- Krakatau: open-source (GPLv3) and actively developed
- CFR: open-source (MIT) and actively developed
- JAD
- DJ Java Decompiler
- Mocha
And many more.
These produce Java code. Java comes with something that lets you see JVM byte code (javap).
2023: splashout suggests in the comments the Vineflower/vineflower decompiler (releases), renaming from Quiltflower to Vineflower.
java -jar vineflower.jar -dgs=1 c:\Temp\binary\library.jar c:\Temp\souce
2022 update: QuiltMC/quiltflower is the latest most advanced Java decompiler:
Quiltflower is a modern, general purpose decompiler focused on improving code quality, speed, and usability.
Quiltflower is a fork of Fernflower and Forgeflower.
Changes include:
- New language features (Try with resources, switch expressions, pattern matching, and more)
- Better control flow generation (loops, try-catch, and switch, etc.)
- More configurability
- Better error messages
- Javadoc application
- Multithreading
- Optimization
- Many other miscellaneous features and fixes
Originally intended just for use with the QuiltMC toolchain with Minecraft, Quiltflower quickly expanded to be a general purpose java decompiler aiming to create code that is as accurate and clean as possible.
If the name sounds familiar it's because Quiltflower is a fork of Fernflower, the (in)famous decompiler that was developed by Stiver, maintained by Jetbrains, and became the default decompiler in Intellij IDEA.
Fernflower also quickly found its way into many other tools.Over the past year, Quiltflower has added support for features such as modern string concatenation, a code formatter, sealed classes, pattern matching, switch expressions, try-with-resources, and more. Quiltflower also focuses on the code quality of the decompiled output, and takes readability very seriously.
See output examples.
Runs nice with jbang
https://github.com/QuiltMC/quiltflower/releases/download/1.8.1/quiltflower-1.8.1.jar
Or:
java -jar quiltflower.jar -dgs=1 c:\Temp\binary\library.jar c:\Temp\binary\Boot.class c:\Temp\source\
2009: JavaDecompiler can do a good job with a jar: since 0.2.5, All files, in JAR files, are displayed.
See also the question "How do I “decompile” Java class files?".
The JD-Eclipse doesn't seem to have changed since late 2009 though (see Changes).
So its integration with latest Eclipse (3.8, 4.2+) might be problematic.
JD-Core is actively maintained.
Both are the result of the fantastic work of (SO user) Emmanuel Dupuy.
2018: A more modern option, mentioned in the comments by David Kennedy Araujo:
JetBrains/intellij-community/plugins/java-decompiler/engine
Fernflower is the first actually working analytical decompiler for Java and probably for a high-level programming language in general.
java -jar fernflower.jar [-<option>=<value>]* [<source>]+ <destination> java -jar fernflower.jar -hes=0 -hdc=0 c:\Temp\binary\ -e=c:\Java\rt.jar c:\Temp\source\
See also How to decompile to java files intellij idea for a command working with recent IntelliJ IDEA.
2022 update: Florian Wendelborn suggests in the comments
this one works well: jdec.app from Leonardo Santos.
First of all, it's worth remembering that all Java archive files (.jar/.war/etc...) are all basically just fancy.zip files, with a few added manifests and metadata.
Second, to tackle this problem I personally use several tools which handle this problem on all levels:
- Jad + Jadclipse while working in IDE for decompiling
.classfiles - WinRAR, my favorite compression tool natively supports Java archives (again, see first paragraph).
- Beyond Compare, my favorite diff tool, when configured correctly can do on-the-fly comparisons between any archive file, including
jars. Well worth a try.
The advantage of all the aforementioned, is that I do not need to hold any other external tool which clutters my work environment. Everything I will ever need from one of those files can be handled inside my IDE or diffed with other files natively.