Skip to content
master
Switch branches/tags
Code

CFR - Another Java Decompiler \o/

This is the public repository for the CFR decompiler, main site hosted at benf.org/other/cfr

CFR will decompile modern Java features - including much of Java 9, 12 & 14, but is written entirely in Java 6, so will work anywhere! (FAQ) - It'll even make a decent go of turning class files from other JVM languages back into java!

To use, simply run the specific version jar, with the class name(s) you want to decompile (either as a path to a class file, or as a fully qualified classname on your classpath). (--help to list arguments).

Alternately, to decompile an entire jar, simply provide the jar path, and if you want to emit files (which you probably do!) add --outputdir /tmp/putithere.

Getting CFR

The main site for CFR is benf.org/other/cfr, where releases are available with a bunch of rambling musings from the author.

Since 0.145, Binaries are published on github along with release tags.

You can also download CFR from your favourite maven repo, though releases are published a few days late usually, to allow for release regret.

Issues

If you have an issue, please DO NOT include copyright materials. I will have to delete your issue.

Building CFR

Dead easy!

Just ensure you have Maven installed. Then mvn compile in the root directory of this project will get you what you need.

Note: If you encounter a maven-compiler-plugin...: Compilation failure error while trying to compile the project then your JAVA_HOME environment variable is probably pointing to a JDK version that doesn't support 6 for the source or target compile options. Fix this by pointing JAVA_HOME to a JDK version that still supports compiling to Java 1.6 such as JDK 11. Also note, the version of Java on your JAVA may need to be greater than 1.6 if you are using Maven version >=3.3.1 which requires Java 1.7. The best solution is to use JDK 8, 9, 10 or 11 for both your PATH and JAVA_HOME.

The main class is org.benf.cfr.reader.Main, so once you've built, you can test it out (from target/classes)

java org.benf.cfr.reader.Main java.lang.Object

to get CFR to decompile java.lang.Object.

Decompilation tests

As part of the Maven build automatic decompilation tests are performed. They verify that the current decompiled output of CFR matches the expected previous output. The test data (Java class and JAR files) are part of a separate Git repository; it is therefore necessary to clone this repository with git clone --recurse-submodules. The expected output and CFR test configuration is however part of this repository to allow altering it without having to modify the corresponding test data. The test data is in the decompilation-test/test-data directory, and the respective expected data and custom configuration is in the decompilation-test/test-data-expected-output directory (with a similar directory structure, see Expected data structure below).

The decompilation tests are also performed by the GitHub workflow, and in case of test failures the unified diff is available in a workflow artifact called "decompilation-test-failures-diff".

The expected output is not the gold standard, it merely describes the currently expected output. There is nothing wrong with adjusting the expected output, if the changes to the decompilation results are reasonable.

The test class is org.benf.cfr.test.DecompilationTest. It can be modified to adjust the test directories, or to ignore certain class files or JARs. Additionally it is possible to directly execute the tests there from the IDE. This usually gives better output than what is shown by Maven, and allows using the built-in IDE functionality for showing differences between the expected and the actual data.

Options file

The decompilation process can be customized by adding an options file. Each line of it specifies a CFR option, with key and value separated by a space. Empty lines and lines starting with # are ignored and can be used for comments.

Example:

# Enable identifier renaming
renameillegalidents true

See Expected data structure below for how to name the file and where to place it.

Expected data structure

Class files

For class files the expected data and custom configuration is in the same respective location under test-data-expected-output, with the file names being based on the class file name.

For example, for the class file test-data/classes/subdir/MyClass.class the following files can be used:

  • test-data-expected-output/classes/subdir/
    • MyClass.expected.java
      Contains the expected decompiled Java output, optionally with decompilation notes.
    • MyClass.options
      An optional options file customizing decompilation.
    • MyClass.expected.summary
      Contains the expected summary reported by the CFR API. Can be omitted when no summary is produced.
    • MyClass.expected.exceptions
      Contains the expected exceptions reported by the CFR API. Can be omitted when no exception is reported.

JAR files

For JAR files the expected data and custom configuration is inside a directory with the name of the JAR file in the respective location under test-data-expected-output. Expected Java output files include the package name in their file name, for example mypackage.MyClass.java. The options file and the expected summary and exceptions file use the "file name" _. For multi-release JARs the directory contains subdirectories for version specific classes. The directory name has the form java-<version>.

For example, for the multi-release JAR file test-data/jars/subdir/MyJar.jar the following files can be used:

  • test-data-expected-output/jars/subdir/MyJar/
    • mypackage.MyClass.java
      Contains the expected decompiled Java output for the class mypackage.MyClass, optionally with decompilation notes.
    • java-11/mypackage.MyClass.java
      Contains the expected decompiled Java output for a class file specific to Java 11 and higher (for multi-release JARs).
    • _.options
      An optional options file customizing decompilation.
    • _.expected.summary
      Contains the expected summary reported by the CFR API. Can be omitted when no summary is produced.
    • _.expected.exceptions
      Contains the expected exceptions reported by the CFR API. Can be omitted when no exception is reported.

Decompilation note comments

The expected Java output files support comments representing decompilation notes. They are ignored during comparison with the actual Java output and can for example be used to indicate incorrect or improvable CFR output. There are two kinds of decompilation notes:

  • Line: Start with //# (optionally prefixed with whitespace)
  • Inline: /*# ... #*/

Line decompilation notes should be used sparingly, especially in large files, because they shift line numbers for the diff files (due to being removed during comparison), which can be confusing.

Example:

//# Line decompilation note
public class MyClass {
    public static void main(String[] stringArray/*# Inline decompilation note #*/) {
        ...
    }
}

Updating / creating expected data

When adding a lot of new classes or JAR files for the decompilation tests or when a change to CFR affects the output for a lot of classes or JAR files, manually creating or updating the expected output can be rather cumbersome. For these cases the following system properties exist which help with this. They can be set with -D<system-property> when running tests. However, when these system properties are set, the respective tests will still fail (but the expected data is updated) to prevent accidentally using them for regular test execution.

  • cfr.decompilation-test.create-expected
    Generates all missing expected test data based on the current CFR output.
  • cfr.decompilation-test.update-expected
    Updates the expected test data to match the actual data produced by CFR. Note that this does not work for expected Java output using decompilation notes because those comments would get lost. The affected tests have to be updated manually.