How to decompile to java files intellij idea

As of August 2017 and IntelliJ V2017.2, the accepted answer does not seem to be entirely accurate anymore: there is no fernflower.jar to use.

The jar file is called java-decompiler.jar and does not include a main manifest... Instead you can use the following command (from a Mac install):

java -cp "/Applications/IntelliJ IDEA.app/Contents/plugins/java-decompiler/lib/java-decompiler.jar" org.jetbrains.java.decompiler.main.decompiler.ConsoleDecompiler

(you will get the wrong Usage command, but it does work).

1. Open an existing project or create a new one.

2. Go to Project structure settings > Libraries. Add the jar you want to decompile in libraries by clicking the + symbol.

3. Go to the Project tool window shown on the left. Search for jar name that you added in the previous step. Navigate to the desired class or package.

You can see the decompiled java files for that jar.

Follow instructions for IntelliJ JD plugin. Or see an excerpt from the instructions below.

java -jar fernflower.jar [<source>]+ <destination>

+ means 1 or more times
<source>: file or directory with files to be decompiled. Directories are recursively scanned. Allowed file extensions are class, zip and jar.
<destination>: destination directory

Example:

java -jar fernflower.jar -hdc=0 -dgs=1 -rsy=1 -lit=1 c:\Temp\binary\ -e=c:\Java\rt.jar c:\Temp\source\

Be aware that if you pass it a ".jar" file for the source, it will create another ".jar" file in the destination, however, within the new ".jar" file, the files will be .java instead of .class files (it doesn't explode the jar).

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UPDATE

People ask me: How do I get the fernflower.jar?

If you have any IntelliJ product installed, chances are that you already have the Fernflower decompiler on your computer. IntelliJ IDEA comes with Java Bytecode Decompiler plugin (bundled) which is a modern extension of Fernflower.

1. Locate the file in ${IntelliJ_INSTALL_DIR}\plugins\java-decompiler\lib\java-decompiler.jar (example: C:\Program Files\JetBrains\IntelliJ IDEA 2018\plugins\java-decompiler\lib).
2. Copy it somewhere and rename to fernflower.jar (optional).

3. This JAR is not executable, so we can't run it using java -jar. However something like this works:

   java -cp fernflower.jar org.jetbrains.java.decompiler.main.decompiler.ConsoleDecompiler [<source>]+ <destination>
   

   org.jetbrains.java.decompiler.main.decompiler.ConsoleDecompiler is the class that contains the main method to run the decompiler.

   Example:

   mkdir output_src
   java -cp fernflower.jar org.jetbrains.java.decompiler.main.decompiler.ConsoleDecompiler -hdc=0 -dgs=1 -rsy=1 -lit=1 ./input.jar ./output_src
   

If you don't have IntelliJ products installed, either download it now (available on jetbrains.com) or make your own decompiler executable from sources (available on Github).