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Gradle Checker Framework Plugin

This plugin configures JavaCompile tasks to use the Checker Framework for pluggable type-checking.

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Add the following to your build.gradle file:

plugins {
    // Checker Framework pluggable type-checking
    id 'org.checkerframework' version '0.6.12'
}

apply plugin: 'org.checkerframework'

The org.checkerframework plugin modifies existing Java compilation tasks. You should apply it after whatever plugins introduce your Java compilation tasks (usually the java or java-library plugin for non-Android builds).

Configuration

Configuring which checkers to use

The checkerFramework.checkers property lists which checkers will be run.

For example, using Groovy syntax in a build.gradle file:

checkerFramework {
  checkers = [
    'org.checkerframework.checker.nullness.NullnessChecker',
    'org.checkerframework.checker.units.UnitsChecker'
  ]
}

The same example, using Kotlin syntax in a build.gradle.kts file:

// In Kotlin, you need to import CheckerFrameworkExtension explicitly:
import org.checkerframework.gradle.plugin.CheckerFrameworkExtension

configure<CheckerFrameworkExtension> {
    checkers = listOf(
        "org.checkerframework.checker.nullness.NullnessChecker",
        "org.checkerframework.checker.units.UnitsChecker"
    )
}

For a list of checkers, see the Checker Framework Manual.

Providing checker-specific options to the compiler

You can set the checkerFramework.extraJavacArgs property in order to pass additional options to the compiler when running a typechecker.

For example, to use a stub file:

checkerFramework {
  extraJavacArgs = [
    '-Werror',
    '-Astubs=/path/to/my/stub/file.astub'
  ]
}

Configuring third-party checkers

To use a third-party typechecker (i.e. one that is not distributed with the Checker Framework), add a dependency to the checkerFramework dependency configuration.

For example, to use the Glacier immutability checker:

dependencies {
  ...
  checkerFramework 'edu.cmu.cs.glacier:glacier:0.1'
}

You should also use a checkerFramework dependency for anything needed by a checker you are running. For example, if you are using the Subtyping Checker with custom type qualifiers, you should add a checkerFramework dependency referring to the definitions of the custom qualifiers.

Specifying a Checker Framework version

Version 0.6.12 of this plugin uses Checker Framework version 3.22.1 by default. Anytime you upgrade to a newer version of this plugin, it might use a different version of the Checker Framework.

You can use a Checker Framework version that is different than this plugin's default. For example, if you want to use Checker Framework version 3.4.0, then you should add the following text to build.gradle, after apply plugin: 'org.checkerframework':

dependencies {
  compileOnly 'org.checkerframework:checker-qual:3.4.0'
  testCompileOnly 'org.checkerframework:checker-qual:3.4.0'
  checkerFramework 'org.checkerframework:checker:3.4.0'
}

You can also use a locally-built version of the Checker Framework:

// To use a locally-built Checker Framework, run gradle with "-PcfLocal".
if (project.hasProperty("cfLocal")) {
  def cfHome = String.valueOf(System.getenv("CHECKERFRAMEWORK"))
  dependencies {
    compileOnly files(cfHome + "/checker/dist/checker-qual.jar")
    testCompileOnly files(cfHome + "/checker/dist/checker-qual.jar")
    checkerFramework files(cfHome + "/checker/dist/checker.jar")
  }
}

Incremental compilation

By default, the plugin assumes that all checkers are "isolating incremental annotation processors" according to the Gradle terminology here. This assumption speeds up builds by enabling incremental compilation, but is unsafe: Gradle's documentation warns that annotation processors that use internal Javac APIs may crash, because Gradle wraps some of those APIs. The Checker Framework does use internal Javac APIs, so you might encounter such a crash, which would appear as a ClassCastException referencing some internal Javac class. If you encounter such a crash, you can disable incremental compilation in your build using the following code in your checkerFramework configuration block:

  checkerFramework {
    incrementalize = false
  }

Per-Task Configuration

You can also use a checkerFramework block to configure individual tasks. This can be useful for skipping the Checker Framework on generated code:

tasks.withType(JavaCompile).configureEach {
  // Don't run the checker on generated code.
  if (name.equals("compileMainGeneratedDataTemplateJava")
      || name.equals("compileMainGeneratedRestJava")) {
    checkerFramework {
      skipCheckerFramework = true
    }
  }
}

Currently, the only supported option is skipCheckerFramework.

Other options

• You can disable the Checker Framework temporarily (e.g. when testing something unrelated) either in your build file or from the command line. In your build file:

  checkerFramework {
    skipCheckerFramework = true
  }

  From the command line, add -PskipCheckerFramework to your gradle invocation. This property can also take an argument: anything other than false results in the Checker Framework being skipped.

• By default, the plugin applies the selected checkers to all JavaCompile targets, including test targets such as testCompileJava.

  Here is how to prevent checkers from being applied to test targets:

  checkerFramework {
    excludeTests = true
  }

  The check for test targets is entirely syntactic: this option will not apply the checkers to any task whose name includes "test", ignoring case.

• If you encounter errors of the form zip file name too long when configuring your Gradle project, you can use the following code to skip this plugin's version check, which reads the manifest file of the version of the Checker Framework you are actually using:

  checkerFramework {
    skipVersionCheck = true
  }

Multi-project builds

In most projects with subprojects, the top-level project is not a Java project. You should not apply the plugin to such a non-Java project. Instead, move all Checker Framework configuration (the checkerFramework block and any dependencies) into a subprojects block, and do not apply the plugin to the top-level project. For example:

plugins {
  id 'org.checkerframework' version '0.6.12' apply false
}

subprojects { subproject ->
  apply plugin: 'org.checkerframework'

  checkerFramework {
    checkers = ['org.checkerframework.checker.index.IndexChecker']
  }
  dependencies {
    checkerFramework 'org.checkerframework:checker:3.22.1'
    implementation 'org.checkerframework:checker-qual:3.22.1'
  }
}

If the top-level project is a Java project that you wish to typecheck, follow these instructions but replace the 'subprojects' block with the 'allprojects' block.

Alternately, apply the plugin in the build.gradle in each subproject as if it were a stand-alone project. You must do this if you require different configuration for different subprojects (for instance, if you want to run different checkers).

Incompatibility with Error Prone 2.3.4 and earlier

Error Prone uses the Checker Framework's dataflow analysis library. Unfortunately, Error Prone version 2.3.4 and earlier uses an old version of the library, so you cannot use both Error Prone and the current Checker Framework (because each one depends on a different version of the library).

You can resolve this by:

• upgrading to Error Prone version 2.4.0 or later, or
• using a switch that causes your build to use either Error Prone or the Checker Framework, but not both.

Here is an example of the latter approach:

plugins {
  id "net.ltgt.errorprone" version "1.1.1" apply false
  // To do Checker Framework pluggable type-checking (and disable Error Prone), run:
  // ./gradlew compileJava -PuseCheckerFramework=true
  id 'org.checkerframework' version '0.6.12' apply false
}

if (!project.hasProperty("useCheckerFramework")) {
    ext.useCheckerFramework = "false"
}
if ("true".equals(project.ext.useCheckerFramework)) {
  apply plugin: 'org.checkerframework'
} else {
  apply plugin: 'net.ltgt.errorprone'
}

def errorProneVersion = "2.3.4"
def checkerFrameworkVersion = "3.22.1

dependencies {
  if ("true".equals(project.ext.useCheckerFramework)) {
    checkerFramework 'org.checkerframework:checker:' + checkerFrameworkVersion
    checkerFramework 'org.checkerframework:checker-qual:' + checkerFrameworkVersion
  } else {
    errorprone group: 'com.google.errorprone', name: 'error_prone_core', version: errorProneVersion
  }
}

if ("true".equals(project.ext.useCheckerFramework)) {
  checkerFramework {
    checkers = [
      'org.checkerframework.checker.interning.InterningChecker',
      'org.checkerframework.checker.signature.SignatureChecker'
    ]
  }
} else {
  // Configuration for the Error Prone linter.
  tasks.withType(JavaCompile).each { t ->
    if (!t.name.equals("compileTestInputJava") && !t.name.startsWith("checkTypes")) {
      t.toolChain ErrorProneToolChain.create(project)
      t.options.compilerArgs += [
        '-Xep:StringSplitter:OFF',
        '-Xep:ReferenceEquality:OFF' // use Interning Checker instead
      ]
    }
  }
}

Java 9+ compatibility

When running the plugin on a Java 9+ project that uses modules, you may need to add annotations to the module path. First add requires org.checkerframework.checker.qual; to your module-info.java. The Checker Framework inserts inferred annotations into bytecode even if none appear in source code, so you must do this even if you write no annotations in your code.

Then, add this line to the checkerFramework block to add the checker-qual.jar artifact (which only contains annotations) to the module path:

checkerFramework {
  extraJavacArgs = [
    '--module-path', compileOnly.asPath
  ]
}

Lombok compatibility

This plugin automatically interacts with the Lombok Gradle Plugin to delombok your source code before it is passed to the Checker Framework for typechecking. This plugin does not support any other use of Lombok.

By default, Lombok suppresses all warnings in the code it generates. If you want to typecheck the code that Lombok generates, use the suppressLombokWarnings configuration key:

checkerFramework {
  suppressLombokWarnings = false
}

Note that doing so will cause all tools (including Javac itself) to begin issuing warnings in the code that Lombok generates.

Using a locally-built plugin

You can build the plugin locally rather than downloading it from Maven Central.

To build the plugin from source, run ./gradlew build.

If you want to use a locally-built version of the plugin, you can publish the plugin to your local Maven repository by running ./gradlew publishToMavenLocal. Then, add the following to the settings.gradle file in the Gradle project that you want to use the plugin:

pluginManagement {
    repositories {
        mavenLocal()
        gradlePluginPortal()
    }
}

JDK 8 vs JDK 9+ implementation details

The plugin attempts to automatically configure the Checker Framework on both Java 8 and Java 9+ JVMs, following the best practices in the Checker Framework manual. In particular:

• If both the JVM and target versions are 8, it applies the Java 8 annotated JDK.
• If the JVM version is 9+ and the target version is 8 (and the Checker Framework version is >= 2.11.0), use the Error Prone javac compiler.
• If the JVM version is 9+, use the --add-opens option to javac.

Credits

This project started as a fork of an abandoned plugin built by jaredsburrows.

License

Copyright (C) 2017 Jared Burrows, 2018-2020 Martin Kellogg

Licensed under the Apache License, Version 2.0 (the "License");
you may not use this file except in compliance with the License.
You may obtain a copy of the License at

   http://www.apache.org/licenses/LICENSE-2.0

Unless required by applicable law or agreed to in writing, software
distributed under the License is distributed on an "AS IS" BASIS,
WITHOUT WARRANTIES OR CONDITIONS OF ANY KIND, either express or implied.
See the License for the specific language governing permissions and
limitations under the License.