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A Database of Real Faults and an Experimental Infrastructure to Enable Controlled Experiments in Software Engineering Research


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Defects4J -- version 2.0.1 .github/workflows/ci.yml

Defects4J is a collection of reproducible bugs and a supporting infrastructure with the goal of advancing software engineering research.

Contents of Defects4J

The projects

Defects4J contains 835 bugs (plus 29 deprecated bugs) from the following open-source projects:

Identifier Project name Number of active bugs Active bug ids Deprecated bug ids (*)
Chart jfreechart 26 1-26 None
Cli commons-cli 39 1-5,7-40 6
Closure closure-compiler 174 1-62,64-92,94-176 63,93
Codec commons-codec 18 1-18 None
Collections commons-collections 4 25-28 1-24
Compress commons-compress 47 1-47 None
Csv commons-csv 16 1-16 None
Gson gson 18 1-18 None
JacksonCore jackson-core 26 1-26 None
JacksonDatabind jackson-databind 112 1-112 None
JacksonXml jackson-dataformat-xml 6 1-6 None
Jsoup jsoup 93 1-93 None
JxPath commons-jxpath 22 1-22 None
Lang commons-lang 64 1,3-65 2
Math commons-math 106 1-106 None
Mockito mockito 38 1-38 None
Time joda-time 26 1-20,22-27 21

* Due to behavioral changes introduced under Java 8, some bugs are no longer reproducible. Hence, Defects4J distinguishes between active and deprecated bugs:

  • Active bugs can be accessed through active-bugs.csv.

  • Deprecated bugs are removed from active-bugs.csv, but their metadata is retained in the project directory.

  • Deprecated bugs can be accessed through deprecated-bugs.csv, which also details when and why a bug was deprecated.

We do not re-enumerate active bugs because publications using Defects4J artifacts usually refer to bugs by their specific bug id.

The bugs

Each bug has the following properties:

  • Issue filed in the corresponding issue tracker, and issue tracker identifier mentioned in the fixing commit message.
  • Fixed in a single commit.
  • Minimized: the Defects4J maintainers manually pruned out irrelevant changes in the commit (e.g., refactorings or feature additions).
  • Fixed by modifying the source code (as opposed to configuration files, documentation, or test files).
  • A triggering test exists that failed before the fix and passes after the fix -- the test failure is not random or dependent on test execution order.

The (b)uggy and (f)ixed program revisions are labelled with <id>b and <id>f, respectively (<id> is an integer).

Setting up Defects4J


  • Java 1.8
  • Git >= 1.9
  • SVN >= 1.8
  • Perl >= 5.0.12

Defects4J version 1.x and 0.x required Java 1.7.

Java version

All bugs have been reproduced and triggering tests verified, using the latest version of Java 1.8. Using a different version of Java might result in unexpected failing tests on a fixed program version.


Defects4J generates and executes tests in the timezone America/Los_Angeles. If you are using the bugs outside of the Defects4J framework, set the TZ environment variable to America/Los_Angeles and export it.

Perl dependencies

All required Perl modules are listed in cpanfile. On many Unix platforms, these required Perl modules are installed by default. If this is not the case, see instructions below for how to install them.

Steps to set up Defects4J

  1. Clone Defects4J:

    • git clone
  2. Initialize Defects4J (download the project repositories and external libraries, which are not included in the git repository for size purposes and to avoid redundancies): If you do not have cpanm installed, use cpan or a cpan wrapper to install the perl modules listed in cpanfile.

    • cd defects4j
    • cpanm --installdeps .
    • ./
  3. Add Defects4J's executables to your PATH:

    • export PATH=$PATH:"path2defects4j"/framework/bin ("path2defects4j" points to the directory to which you cloned Defects4J; it looks like "/user/yourComputerUserName/desktop/defects4j".)
  4. Check installation:

    • defects4j info -p Lang

On some platforms such as Windows, you might need to use perl "fullpath"\defects4j where these instructions say to use defects4j.

Using Defects4J

Example commands

  1. Get information for a specific project (commons lang):

    • defects4j info -p Lang
  2. Get information for a specific bug (commons lang, bug 1):

    • defects4j info -p Lang -b 1
  3. Checkout a buggy source code version (commons lang, bug 1, buggy version):

    • defects4j checkout -p Lang -v 1b -w /tmp/lang_1_buggy
  4. Change to the working directory, compile sources and tests, and run tests:

    • cd /tmp/lang_1_buggy
    • defects4j compile
    • defects4j test
  5. Some Defects4J commands take the project id as a command-line argument (possibly along with other arguments). Examples include info, checkout, and query. Note that info and query report infromation that is derived from the Defects4J metadata and do not require access to project files that are in the project's VCS.

    Other commands require a working directory, either set explicitly (-w command-line argument) or implicitly (executed from within a working directory). Examples include any command that requires access to files under version control, including source code and build files. This includes all commands that build or test the code (compile, test, coverage, mutation) and commands that return version-specific information (export).

  6. The scripts in framework/test/ are examples of how to use Defects4J, which you might find useful as inspiration when you are writing your own scripts that use Defects4J.

Command-line interface: defects4j command

Use framework/bin/defects4j to execute any of the following commands:

Command Description
info View configuration of a specific project or summary of a specific bug
env Print the environment of defects4j executions
checkout Checkout a buggy or a fixed project version
compile Compile sources and developer-written tests of a buggy or a fixed project version
test Run a single test method or a test suite on a buggy or a fixed project version
mutation Run mutation analysis on a buggy or a fixed project version
coverage Run code coverage analysis on a buggy or a fixed project version
monitor.test Monitor the class loader during the execution of a single test or a test suite
bids Print the list of active or deprecated bug IDs for a specific project
pids Print a list of available project IDs
export Export version-specific properties such as classpaths, directories, or lists of tests
query Query the metadata to generate a CSV file of requested information for a specific project

Export version-specific properties

Use defects4j export -p <property_name> [-o output_file] in the working directory to export a version-specific property:

Property Description
classes.modified Classes modified by the bug fix
classes.relevant Classes loaded by the JVM when executing all triggering tests
cp.compile Classpath to compile and run the project
cp.test Classpath to compile and run the developer-written tests
dir.src.classes Source directory of classes (relative to working directory)
dir.bin.classes Target directory of classes (relative to working directory)
dir.src.tests Source directory of tests (relative to working directory)
dir.bin.tests Target directory of test classes (relative to working directory)
tests.all List of all developer-written test classes
tests.relevant List of relevant tests classes (a test class is relevant if, when executed, the JVM loads at least one of the modified classes)
tests.trigger List of test methods that trigger (expose) the bug

Export project-specific metadata

Use defects4j query -p <pid> -q <field_list> [-o <output_file>] [-D|-A] to generate a CSV file containing a set of requested metadata for each bug in a specific project.

By default, defects4j query returns a list of active bug IDs for a project. To request specific metadata, the -q flag should be provided with a comma-separated list of variables from the list below. For example, defects4j query -p Chart -q ",report.url" will provide the a list of all active bug IDs, along with the bug report ID and bug report URL for each.

Property Description Assigned bug IDs (included in all results) Assigned project ID Original project name Location of the Defects4J build file for the project
project.vcs Version control system used by the project
project.repository Location of the project repository
project.bugs.csv Location of the CSV containing information on that bug Commit hashes for the buggy version of each bug Commit hashes for the fixed version of each bug Date of the buggy commit for each bug Date of the fixed commit for each bug Bug report ID from the version tracker for each bug
report.url Bug report URL from the version tracker for each bug
classes.modified Classes modified by the bug fix
classes.relevant.src Source classes loaded by the JVM when executing all triggering tests
classes.relevant.test Test classes loaded by the JVM when executing all triggering tests
tests.relevant List of relevant tests classes (a test class is relevant if, when executed, the JVM loads at least one of the modified classes)
tests.trigger List of test methods that trigger (expose) the bug, separated by semicolons (;)
tests.trigger.cause List of test methods that trigger (expose) the bug, along with the exception thrown. Each list element has the form "methodName --> exceptionClass[: message]", and list elements are separated by semicolons (;)
deprecated.version (for deprecated bugs only) Version of Defects4J where a bug was deprecated
deprecated.reason (for deprecated bugs only) Reason for deprecation

By default, defects4j query returns information on active bugs. The [-D] flag returns information only on deprecated bugs, while the [-A] flag returns information for all active and deprecated bugs.

To determine the methods that are changed between the buggy and fixed version of the code:

  1. Add this line to your user-level git attributes file: *.java diff=java

  2. Run git diff --no-index, for example git diff --no-index /tmp/lang_1_buggy /tmp/lang_1_fixed. In the output, every line starting with "@" gives the method name of a changed method.

Test execution framework

The test execution framework for generated test suites (framework/bin) provides the following scripts:

Script Description
defects4j Main script, described above
gen_tests Generate test suites using EvoSuite or Randoop
run_bug_detection Determine the real fault detection rate
run_mutation Determine the mutation score
run_coverage Determine code coverage ratios (statement and branch coverage)

Mining and contributing additional bugs to Defects4J

We welcome your contributions to Defects4J! The bug-mining README details the bug-mining process.

Additional resources

Scripts built on Defects4J

Fault localization (FL)

Automated program repair (APR)


  • "Defects4J: A Database of Existing Faults to Enable Controlled Testing Studies for Java Programs" René Just, Darioush Jalali, and Michael D. Ernst, ISSTA 2014 [download].

  • "Are Mutants a Valid Substitute for Real Faults in Software Testing?" René Just, Darioush Jalali, Laura Inozemtseva, Michael D. Ernst, Reid Holmes, and Gordon Fraser, FSE 2014 [download].

  • "Challenges in Using Search-Based Test Generation to Identify Real Faults in Mockito" Gregory Gay, SSBSE 2016 [download].

  • "Detecting Real Faults in the Gson Library Through Search-Based Unit Test Generation" Gregory Gay, SSBSE 2018 [download].

  • "Defects4J as a Challenge Case for the Search-Based Software Engineering Community" Gregory Gay and René Just, SSBSE 2020 [download].

More publications

Implementation details

Documentation for any script or module is available as HTML documentation.

The directory structure of Defects4J is as follows:

   |--- project_repos:     The version control repositories of the provided projects.
   |--- major:             The Major mutation framework.
   |--- framework:         Libraries and executables of the core, test execution,
       |                   and bug-mining frameworks.
       |--- bin:           Command line interface to Defects4J.
       |--- bug-mining:    Bug-mining framework.
       |--- core:          The modules of the core framework.
       |--- lib:           Libraries used in the core framework.
       |--- util:          Util scripts used by Defects4J.
       |--- projects:      Project-specific resource files.
       |--- test:          Scripts to test the framework.

Versioning information

Defects4J uses a semantic versioning scheme (major.minor.patch):

Change major minor patch
Addition/Deletion of bugs X
New/upgraded internal or external tools X
Fixes and documentation changes X


MIT License, see license.txt for more information.


A Database of Real Faults and an Experimental Infrastructure to Enable Controlled Experiments in Software Engineering Research