# Compiling Groovy

version 6.0.0-beta-1

## 1. groovyc, the Groovy compiler

`groovyc` is the Groovy compiler command line tool. It allows you to compile Groovy sources into bytecode. It plays the same role as `javac` in the Java world. The easiest way to compile a Groovy script or class is to run the following command:

    groovyc MyClass.groovy

This will produce a `MyClass.class` file (as well as other .class files depending on the contents of the source). `groovyc` supports a number of command line switches:

Short version | Long version | Description | Example
--- | --- | --- | ---
-cp | -classpath, --classpath | Specify the compilation classpath. Must be the first argument. | groovyc -cp lib/dep.jar MyClass.groovy
| --sourcepath | Directory where to find source files. Not used anymore. Specifying this parameter will have no effect. |
| --temp | Temporary directory for the compiler |
| --encoding | Encoding of the source files | groovyc --encoding utf-8 script.groovy
| --help | Displays help for the command line groovyc tool | groovyc --help
-d |  | Specify where to place generated class files. | groovyc -d target Person.groovy
| --check | Check sources for errors without generating class files (stops compilation after static analysis) | groovyc --check Person.groovy
-v | --version | Displays the compiler version | groovyc -v
-e | --exception | Displays the stack trace in case of compilation error | groovyc -e script.groovy
-w | --warningLevel | Sets how many warnings are reported after a successful compile: `0` (none), `1` (likely errors, the default), `2` (possible errors) or `3` (all). See Compiler warnings. | groovyc -w 2 MyClass.groovy
-j | --jointCompilation* | Enables joint compilation | groovyc -j A.groovy B.java
-b | --basescript | Base class name for scripts (must derive from Script) |
| --configscript | Advanced compiler configuration script | groovyc --configscript config/config.groovy src/Person.groovy
-Jproperty=value |  | Properties to be passed to `javac` if joint compilation is enabled | groovyc -j -Jtarget=1.6 -Jsource=1.6 A.groovy B.java
-Fflag |  | Flags to be passed to `javac` if joint compilation is enabled | groovyc -j -Fnowarn A.groovy B.java
-pa | --parameters | Generates metadata for reflection on method parameter names. Requires Java 8+. | groovyc --parameters Person.groovy
-pr | --enable-preview | Enable preview Java features (jdk12+ only). | groovy --enable-preview Person.groovy
@argfile |  | Read options and source files from specified file. | groovyc @conf/args

Notes: * for a full description of joint compilation, see the joint compilation section.

## 2. Compiler warnings

While compiling, Groovy may report warnings: diagnostics that don’t stop compilation but flag code that is suspicious or unlikely to behave as intended — for example, an unreachable `main` method or a property that can’t override a `final` accessor. On a successful compile, `groovyc` prints any collected warnings to `stderr`, using the same format (message, source line and caret) as compilation errors, followed by a summary count.

Every warning carries an importance, and the `-w` (`--warningLevel`) option controls how many are reported. Only warnings whose importance is at or below the configured level are kept, so a lower level is quieter:

Level | Name | Reports
--- | --- | ---
`0` | none | No warnings at all.
`1` | likely errors | (default) Diagnostics that very probably indicate a mistake — for example an unreachable `main` method, a property that can’t override a `final` accessor, or a duplicate constructor generated from default argument values or `@TupleConstructor`/`@MapConstructor`.
`2` | possible errors | The above, plus diagnostics that often have benign causes — for example a field hidden by a dynamic property under `@TypeChecked`, or problems found while scanning global AST transformation descriptors.
`3` | paranoia | The above, plus any remaining lower-priority warnings.

For example, to also see the level 2 warnings:

    groovyc -w 2 MyClass.groovy

| The warning level can also be set programmatically via `CompilerConfiguration#setWarningLevel`, or for the `groovy` command through the `groovy.warnings` system property. Displaying the collected warnings on a successful compile is done by the `groovyc` command line; other embeddings (for instance, code driving a `CompilationUnit` directly) can instead read them from the unit’s `ErrorCollector`.

## 3. Ant task

See the groovyc Ant task documentation. It allows the Groovy compiler to be invoked from Apache Ant.

## 4. Gant

Gant is a tool for scripting Ant tasks using Groovy instead of XML to specify the logic. As such, it has exactly the same features as the Groovyc Ant task.

## 5. Gradle

Gradle is a build tool that allows you to leverage the flexibility of Ant, while keeping the simplicity of convention over configuration that tools like Maven offer. Builds are specified using a Groovy DSL, which offers great flexibility and succinctness.

## 6. Maven integration

There are several approaches to compiling Groovy code in your Maven projects. GMavenPlus is the most flexible and feature rich, but like most Groovy compiler tools, it can have difficulties with joint Java-Groovy projects (for the same reason GMaven and Gradle can have issues). The Groovy-Eclipse compiler plugin for Maven sidesteps the joint compilation issues. Read this for a deeper discussion of the benefits and disadvantages of the two approaches.

A third approach is to use Maven’s Ant plugin to compile a groovy project. Note that the Ant plugin is bound to the compile and test-compile phases of the build in the example below. It will be invoked during these phases and the contained tasks will be carried out which runs the Groovy compiler over the source and test directories. The resulting Java classes will coexist with and be treated like any standard Java classes compiled from Java source and will appear no different to the JRE, or the JUnit runtime.

    <project xmlns="http://maven.apache.org/POM/4.0.0" xmlns:xsi="http://www.w3.org/2001/XMLSchema-instance"
        xsi:schemaLocation="http://maven.apache.org/POM/4.0.0 http://maven.apache.org/maven-v4_0_0.xsd">
        <modelVersion>4.0.0</modelVersion>
        <groupId>com.mycomp.MyGroovy</groupId>
        <artifactId>MyGroovy</artifactId>
        <packaging>jar</packaging>
        <version>1.0-SNAPSHOT</version>
        <name>Maven Example building a Groovy project</name>
        <dependencies>
            <dependency>
                <groupId>junit</groupId>
                <artifactId>junit</artifactId>
                <version>3.8.1</version>
                <scope>test</scope>
            </dependency>
            <dependency>
                <groupId>org.codehaus.groovy</groupId>
                <artifactId>groovy-all</artifactId>
                <version>2.5.0</version>
                <type>pom</type> <!-- required JUST since Groovy 2.5.0 -->
            </dependency>
        </dependencies>
        <build>
            <plugins>
                <plugin>
                    <artifactId>maven-antrun-plugin</artifactId>
                    <executions>
                        <execution>
                            <id>compile</id>
                            <phase>compile</phase>
                            <configuration>
                                <tasks>
                                    <mkdir dir="${basedir}/src/main/groovy"/>
                                    <taskdef name="groovyc"
                                        classname="org.codehaus.groovy.ant.Groovyc">
                                        <classpath refid="maven.compile.classpath"/>
                                    </taskdef>
                                    <mkdir dir="${project.build.outputDirectory}"/>
                                    <groovyc destdir="${project.build.outputDirectory}"
                                        srcdir="${basedir}/src/main/groovy/" listfiles="true">
                                        <classpath refid="maven.compile.classpath"/>
                                    </groovyc>
                                </tasks>
                            </configuration>
                            <goals>
                                <goal>run</goal>
                            </goals>
                        </execution>
                        <execution>
                            <id>test-compile</id>
                            <phase>test-compile</phase>
                            <configuration>
                                <tasks>
                                    <mkdir dir="${basedir}/src/test/groovy"/>
                                    <taskdef name="groovyc"
                                        classname="org.codehaus.groovy.ant.Groovyc">
                                        <classpath refid="maven.test.classpath"/>
                                    </taskdef>
                                    <mkdir dir="${project.build.testOutputDirectory}"/>
                                    <groovyc destdir="${project.build.testOutputDirectory}"
                                        srcdir="${basedir}/src/test/groovy/" listfiles="true">
                                        <classpath refid="maven.test.classpath"/>
                                    </groovyc>
                                </tasks>
                            </configuration>
                            <goals>
                                <goal>run</goal>
                            </goals>
                        </execution>
                    </executions>
                </plugin>
            </plugins>
        </build>
    </project>

This assumes you have a Maven project setup with `groovy` subfolders as peers to the java src and test subfolders. You can use the `java`/`jar` archetype to set this up then rename the java folders to groovy or keep the java folders and just create groovy peer folders. There exists, also a groovy plugin which has not been tested or used in production. After defining the build section as in the above example, you can invoke the typical Maven build phases normally. For example, `mvn test` will execute the test phase, compiling Groovy source and Groovy test source and finally executing the unit tests. If you run `mvn jar` it will execute the jar phase bundling up all of your compiled production classes into a jar after all the unit tests pass. For more detail on Maven build phases consult the Maven2 documentation.

### 6.1. GMaven and GMavenPlus

#### 6.1.1. GMaven

GMaven is the original Maven plugin for Groovy, supporting both compiling and scripting Groovy.

Important:

You should be aware that GMaven is not supported anymore and can have difficulties with joint compilation. GMavenPlus can be a good replacement, but if you are having problems with joint compilation, you might consider the Groovy Eclipse maven plugin.

#### 6.1.2. GMavenPlus

GMavenPlus is a rewrite of GMaven and is in active development. It supports most of the features of GMaven (a couple notable exceptions being mojo Javadoc tags and support for older Groovy versions). Its joint compilation uses stubs (which means it has the same potential issues as GMaven and Gradle). The main advantages over its predecessor are that it supports recent Groovy versions, InvokeDynamic, Groovy on Android, GroovyDoc, and configuration scripts.

#### 6.1.3. GMaven 2

Unlike the name might seem to suggest, GMaven 2 is not aimed at replacing GMaven. In fact, it removes the non-scripting features of the GMaven plugin. It has not yet had any release and appears to be inactive currently.

### 6.2. The Groovy Eclipse Maven plugin

Groovy-Eclipse provides a compiler plugin for Maven. Using the compiler plugin, it is possible to compile your maven projects using the Groovy-Eclipse compiler. One feature unavailable elsewhere is stubless joint compilation.

## 7. Joint compilation

Joint compilation means that the Groovy compiler will parse the Groovy source files, create stubs for all of them, invoke the Java compiler to compile the stubs along with Java sources, and then continue compilation in the normal Groovy compiler way. This allows mixing of Java and Groovy files without constraint.

Joint compilation can be enabled using the `-j` flag with the command-line compiler, or using a nested tag and all the attributes and further nested tags as required for the Ant task.

It is important to know that if you don’t enable joint compilation and try to compile Java source files with the Groovy compiler, the Java source files will be compiled as if they were Groovy sources. In some situations, this might work since most of the Java syntax is compatible with Groovy, but there are a number of places where semantics could be different.

## 8. Android support

It is possible to write an Android application in Groovy. However this requires a special version of the compiler, meaning that you cannot use the regular groovyc tool to target Android bytecode. In particular, Groovy provides specific JAR files for Android, which have a classifier of `grooid`. In order to make things easier, a Gradle plugin adds support for the Groovy language in the Android Gradle toolchain.

The plugin can be applied like this:

    buildscript {
        repositories {
            mavenCentral()
        }
        dependencies {
            classpath 'com.android.tools.build:gradle:2.1.2'
            classpath 'org.codehaus.groovy:groovy-android-gradle-plugin:1.0.0'
        }
    }

    apply plugin: 'groovyx.android'

Then you will need to add a dependency on the `grooid` version of the Groovy compiler:

    dependencies {
        compile 'org.codehaus.groovy:groovy:2.4.7:grooid'
    }

Note that if a Groovy jar does not provide a `grooid` classifier alternative, then it means that the jar is directly compatible with Android. In that case, you can add the dependency directly like this:

    dependencies {
        compile 'org.codehaus.groovy:groovy:2.4.7:grooid'       // requires the grooid classifier
        compile ('org.codehaus.groovy:groovy-json:2.4.7') {     // no grooid version available
            transitive = false                                  // so do not depend on non-grooid version
        }
    }

Note that the `transitive=false` parameter for `groovy-json` will let Gradle download the JSON support jar without adding a dependency onto the normal jar of Groovy.

Please make sure to go to the plugin homepage in order to find the latest documentation and version.