How to submit a bug report

If you received an error message, please include it and any exceptions.

We commonly need to know what platform you are on:

  • JDK/JRE version (i.e., java -version)
  • Operating system (i.e., uname -a)

How to contribute

We definitely welcome patches and contributions to OpenCensus! Here are some guidelines and information about how to do so.

Before getting started

In order to protect both you and ourselves, you will need to sign the Contributor License Agreement.

Eclipse and IntelliJ style configurations are commonly useful. For IntelliJ 14, copy the style to ~/.IdeaIC14/config/codestyles/, start IntelliJ, go to File > Settings > Code Style, and set the Scheme to GoogleStyle.


We follow the Google Java Style Guide. Our build automatically will provide warnings for simple style issues.

Run the following command to format all files. This formatter uses google-java-format:

OS X or Linux

./gradlew goJF


gradlew.bat goJF

We also follow these project-specific guidelines:


  • All public classes and their public and protected methods MUST have javadoc. It MUST be complete (all params documented etc.) Everything else (package-protected classes, private) MAY have javadoc, at the code writer's whim. It does not have to be complete, and reviewers are not allowed to require or disallow it.
  • Each API element should have a @since tag specifying the minor version when it was released (or the next minor version).
  • There MUST be NO javadoc errors.
  • See section 7.3.1 in the guide for exceptions to the Javadoc requirement.
  • Reviewers may request documentation for any element that doesn't require Javadoc, though the style of documentation is up to the author.
  • Try to do the least amount of change when modifying existing documentation. Don't change the style unless you have a good reason.


  • Use AutoValue, when possible, for any new value classes. Remember to add package-private constructors to all AutoValue classes to prevent classes in other packages from extending them.

Building opencensus-java

Continuous integration builds the project, runs the tests, and runs multiple types of static analysis.

Run the following commands to build, run tests and most static analysis, and check formatting:

OS X or Linux

./gradlew clean assemble check verGJF


gradlew.bat clean assemble check verGJF

Use these commands to run Checker Framework null analysis:

OS X or Linux

./gradlew clean assemble -PcheckerFramework


gradlew.bat clean assemble -PcheckerFramework

Checker Framework null analysis

OpenCensus uses the Checker Framework to prevent NullPointerExceptions. Since the project uses Java 6, and Java 6 doesn't allow annotations on types, all Checker Framework type annotations must be put in comments. Putting all Checker Framework annotations and imports in comments also avoids a dependency on the Checker Framework library.

OpenCensus uses org.checkerframework.checker.nullness.qual.Nullable for all nullable annotations on types, since javax.annotation.Nullable cannot be applied to types. However, it uses javax.annotation.Nullable in API method signatures whenever possible, so that the annotations can be uncommented and be included in .class files and Javadocs.

Checkstyle import control

This project uses Checkstyle to specify the allowed dependencies between packages, using its ImportControl feature ( buildscripts/import-control.xml specifies the allowed imports and contains some guidelines on OpenCensus' inter-package dependencies. An error messsage such as Disallowed import - edu.umd.cs.findbugs.annotations.SuppressFBWarnings. [ImportControl] could mean that import-control.xml needs to be updated.


Invoke all benchmarks on a sub-project

$ ./gradlew clean :opencensus-impl-core:jmh

Invoke on a single benchmark class

./gradlew -PjmhIncludeSingleClass=BinaryFormatImplBenchmark clean :opencensus-impl-core:jmh

Debug compilation errors

When you make incompatible changes in the Benchmarks classes you may get compilation errors which are related to the old code not being compatible with the new code. Some of the reasons are:

  • Any plugin cannot delete the generated code (jmh generates code) because if the user configured the directory as the same as source code the plugin will delete users source code.
  • After you run jmh, a gradle daemon will stay alive which may cache the generated code in memory and use use that generated code even if the files were changed. This is an issue for classes generated with auto-value.

Run this commands to clean the Gradle's cache:

./gradlew --stop
rm -fr .gradle/
rm -fr benchmarks/build

Proposing changes

Create a Pull Request with your changes. Please add any user-visible changes to The continuous integration build will run the tests and static analysis. It will also check that the pull request branch has no merge commits. When the changes are accepted, they will be merged or cherry-picked by an OpenCensus core developer.