Chapter 54. The Scala Plugin

Table of Contents

54.1. Usage
54.2. Tasks
54.3. Project layout
54.4. Dependency management
54.5. Automatic configuration of scalaClasspath
54.6. Convention properties
54.7. Source set properties
54.8. Compiling in external process
54.9. Incremental compilation
54.10. Eclipse Integration
54.11. IntelliJ IDEA Integration

The Scala plugin extends the Java plugin to add support for Scala projects. It can deal with Scala code, mixed Scala and Java code, and even pure Java code (although we don't necessarily recommend to use it for the latter). The plugin supports joint compilation, which allows you to freely mix and match Scala and Java code, with dependencies in both directions. For example, a Scala class can extend a Java class that in turn extends a Scala class. This makes it possible to use the best language for the job, and to rewrite any class in the other language if needed.

54.1. Usage

To use the Scala plugin, include the following in your build script:

Example 54.1. Using the Scala plugin

build.gradle

apply plugin: 'scala'

54.2. Tasks

The Scala plugin adds the following tasks to the project.

Table 54.1. Scala plugin - tasks

Task name Depends on Type Description
compileScala compileJava ScalaCompile Compiles production Scala source files.
compileTestScala compileTestJava ScalaCompile Compiles test Scala source files.
compileSourceSetScala compileSourceSetJava ScalaCompile Compiles the given source set's Scala source files.
scaladoc - ScalaDoc Generates API documentation for the production Scala source files.

The Scala plugin adds the following dependencies to tasks added by the Java plugin.

Table 54.2. Scala plugin - additional task dependencies

Task nameDepends on
classes compileScala
testClasses compileTestScala
sourceSetClasses compileSourceSetScala

Figure 54.1. Scala plugin - tasks

Scala plugin - tasks

54.3. Project layout

The Scala plugin assumes the project layout shown below. All the Scala source directories can contain Scala and Java code. The Java source directories may only contain Java source code. None of these directories need to exist or have anything in them; the Scala plugin will simply compile whatever it finds.

Table 54.3. Scala plugin - project layout

Directory Meaning
src/main/java Production Java source
src/main/resources Production resources
src/main/scala Production Scala sources. May also contain Java sources for joint compilation.
src/test/java Test Java source
src/test/resources Test resources
src/test/scala Test Scala sources. May also contain Java sources for joint compilation.
src/sourceSet/java Java source for the given source set
src/sourceSet/resources Resources for the given source set
src/sourceSet/scala Scala sources for the given source set. May also contain Java sources for joint compilation.

54.3.1. Changing the project layout

Just like the Java plugin, the Scala plugin allows you to configure custom locations for Scala production and test sources.

Example 54.2. Custom Scala source layout

build.gradle

sourceSets {
    main {
        scala {
            srcDirs = ['src/scala']
        }
    }
    test {
        scala {
            srcDirs = ['test/scala']
        }
    }
}

54.4. Dependency management

Scala projects need to declare a scala-library dependency. This dependency will then be used on compile and runtime class paths. It will also be used to get hold of the Scala compiler and Scaladoc tool, respectively. [28]

If Scala is used for production code, the scala-library dependency should be added to the compile configuration:

Example 54.3. Declaring a Scala dependency for production code

build.gradle

repositories {
    mavenCentral()
}

dependencies {
    compile 'org.scala-lang:scala-library:2.11.1'
}

If Scala is only used for test code, the scala-library dependency should be added to the testCompile configuration:

Example 54.4. Declaring a Scala dependency for test code

build.gradle

dependencies {
    testCompile "org.scala-lang:scala-library:2.11.1"
}

54.5. Automatic configuration of scalaClasspath

The ScalaCompile and ScalaDoc tasks consume Scala code in two ways: on their classpath, and on their scalaClasspath. The former is used to locate classes referenced by the source code, and will typically contain scala-library along with other libraries. The latter is used to load and execute the Scala compiler and Scaladoc tool, respectively, and should only contain the scala-compiler library and its dependencies.

Unless a task's scalaClasspath is configured explicitly, the Scala (base) plugin will try to infer it from the task's classpath. This is done as follows:

  • If a scala-library Jar is found on classpath, and the project has at least one repository declared, a corresponding scala-compiler repository dependency will be added to scalaClasspath.
  • Otherwise, execution of the task will fail with a message saying that scalaClasspath could not be inferred.

54.6. Convention properties

The Scala plugin does not add any convention properties to the project.

54.7. Source set properties

The Scala plugin adds the following convention properties to each source set in the project. You can use these properties in your build script as though they were properties of the source set object.

Table 54.4. Scala plugin - source set properties

Property name Type Default value Description
scala SourceDirectorySet (read-only) Not null The Scala source files of this source set. Contains all .scala and .java files found in the Scala source directories, and excludes all other types of files.
scala.srcDirs Set<File>. Can set using anything described in Section 18.5, “Specifying a set of input files”. [projectDir/src/name/scala] The source directories containing the Scala source files of this source set. May also contain Java source files for joint compilation.
allScala FileTree (read-only) Not null All Scala source files of this source set. Contains only the .scala files found in the Scala source directories.

These convention properties are provided by a convention object of type ScalaSourceSet.

The Scala plugin also modifies some source set properties:

Table 54.5. Scala plugin - source set properties

Property name Change
allJava Adds all .java files found in the Scala source directories.
allSource Adds all source files found in the Scala source directories.

54.8. Compiling in external process

Scala compilation takes place in an external process.

Memory settings for the external process default to the defaults of the JVM. To adjust memory settings, configure the scalaCompileOptions.forkOptions property as needed:

Example 54.5. Adjusting memory settings

build.gradle

tasks.withType(ScalaCompile) {
    configure(scalaCompileOptions.forkOptions) {
        memoryMaximumSize = '1g'
        jvmArgs = ['-XX:MaxPermSize=512m']
    }
}


54.9. Incremental compilation

By compiling only classes whose source code has changed since the previous compilation, and classes affected by these changes, incremental compilation can significantly reduce Scala compilation time. It is particularly effective when frequently compiling small code increments, as is often done at development time.

The Scala plugin defaults to incremental compilation by integrating with Zinc, a standalone version of sbt's incremental Scala compiler. If you want to disable the incremental compilation, set force = true in your build file:

Example 54.6. Forcing all code to be compiled

build.gradle

tasks.withType(ScalaCompile) {
    scalaCompileOptions.with {
        force = true
    }
}


Note: This will only cause all classes to be recompiled if at least one input source file has changed. If there are no changes to the source files, the compileScala task will still be considered UP-TO-DATE as usual.

The Scala plugin adds a configuration named zinc to resolve the Zinc library and its dependencies. Gradle will have a default version of the Zinc library, but if you want to override the Zinc version that Gradle uses, add an explicit dependency like “com.typesafe.zinc:zinc:0.3.6”. Gradle will support version 0.3.0 of Zinc and above, although due to a regression in the Zinc library, versions 0.3.2 through 0.3.5.2 cannot be used. Regardless of which Zinc version is used, Zinc will always use the Scala compiler found on the scalaTools configuration.

The Zinc-based Scala Compiler supports joint compilation of Java and Scala code. By default, all Java and Scala code under src/main/scala will participate in joint compilation. Even Java code will be compiled incrementally.

Incremental compilation requires dependency analysis of the source code. The results of this analysis are stored in the file designated by scalaCompileOptions.incrementalOptions.analysisFile (which has a sensible default). In a multi-project build, analysis files are passed on to downstream ScalaCompile tasks to enable incremental compilation across project boundaries. For ScalaCompile tasks added by the Scala plugin, no configuration is necessary to make this work. For other ScalaCompile tasks that you might add, the property scalaCompileOptions.incrementalOptions.publishedCode needs to be configured to point to the classes folder or Jar archive by which the code is passed on to compile class paths of downstream ScalaCompile tasks. Note that if publishedCode is not set correctly, downstream tasks may not recompile code affected by upstream changes, leading to incorrect compilation results.

Note that Zinc's Nailgun based daemon mode is not supported. Instead, we plan to enhance Gradle's own compiler daemon to stay alive across Gradle invocations, reusing the same Scala compiler. This is expected to yield another significant speedup for Scala compilation.

54.10. Eclipse Integration

When the Eclipse plugin encounters a Scala project, it adds additional configuration to make the project work with Scala IDE out of the box. Specifically, the plugin adds a Scala nature and dependency container.

54.11. IntelliJ IDEA Integration

When the IDEA plugin encounters a Scala project, it adds additional configuration to make the project work with IDEA out of the box. Specifically, the plugin adds a Scala SDK (IntelliJ IDEA 14+) and a Scala compiler library that matches the Scala version on the project's class path. The Scala plugin is backwards compatible with earlier versions of IntelliJ IDEA and it is possible to add a Scala facet instead of the default Scala SDK by configuring targetVersion on IdeaModel.

Example 54.7. Explicitly specify a target IntelliJ IDEA version

build.gradle

idea {
    targetVersion = "13"
}