How to Decompile a Whole Jar File

How to decompile a whole Jar file?

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.

How to decompile a jar into .java files from command prompt

The JD-CMD GitHub project claims to be able to do so. However, most people I know use JD-GUI.

JD-Core is a library that reconstructs Java source code from one or more “.class” files. JD-Core may be used to recover lost source code and explore the source of Java runtime libraries. New features of Java 5, such as annotations, generics or type “enum”, are supported. JD-GUI and JD-Eclipse include JD-Core library.

I should include that when a source file is compiled, things like the variable assignment and the names of those variables are changed. Similarly, syntactic sugar is changed into what it actually is (such as x += i turns into x = x + i).

How can I decompile many jars at once?

Download JAD Decompiler.

Unjar all your jar files (using the command jar xvf) to some directory. Let's call this ${unjar.dir}.
Create a directory for JAD to write out the decompiled sources. Let's call this ${target.dir}.
Execute the following command:

    jad -o -r -sjava -d${target.dir} ${unjar.dir}/**/*.class

options are:

-o       - overwrite output files without confirmation
-r       - restore package directory structure
-s <ext> - output file extension (default: .jad)

Can jar files be decompiled to get original source?

Unless you explicitly packaged your source code in the JAR file, there's no way to get back the original source. You could try using a decompiler (like this) but that will hardly give you nice, readable code.

The best solution would be to stop using your computer right now. Then use a recovery tool to recover your deleted files. The more recently it was deleted, the higher the chance you'll get it back. There are lots of tools that can be used to do this (just Google it).

Java decompiler, open jar excecutable file in eclipse and see source code

Unzip the .jar file. (A jar file is really a zip file), and then open the .class files in eclipse.

How to Decompile a Whole Jar File