Xtext 2.2 finally brings Maven support for Xtend

I cannot tell how often I was asked since introduction of Xtend2 how to compile them in a Maven build. This was just not possible until now due to the problem that to load and compile an Xtend class it is necessary to compile all of the Java classes that the Xtend class on before, and Java classes might depend on Xtend classes to be translated in order to be compilable.

Xtext 2.2.0 was just released yesterday, and for me the most important new feature is direct support through the xtend-maven-plugin plugin. I could not hesitate to test this feature, and created a small example.

Scenario

I have set up a small Maven project with 2 Java classes and 1 Xtend class that depend on each other.

To translate XtendClass1 to Java code and compile it requires that JavaClass1 is compiled before, and to compile JavaClass2 it is necessary that XtendClass is translated to Java code before.

Maven POM Configuration

I have decided to not use Maven Tycho for this example, and use the typical structure of an Eclipse project with usage of Xtend, i.e. sources in folder /src (instead /src/main/java) and xtend-gen to generate the Java code for Xtend classes to.

Repositories

Since shortly Xtext artifacts are available on maven.eclipse.org. The new Maven plugin is available on http://build.eclipse.org/common/xtend/maven/

	<repositories>
		<repository>
			<id>maven.eclipse.org</id>
			<url>http://maven.eclipse.org/nexus/content/groups/public/</url>
		</repository>
		<repository>
			<id>xtend</id>
			<url>http://build.eclipse.org/common/xtend/maven/</url>
		</repository>
	</repositories>
	<pluginRepositories>
		<pluginRepository>
			<id>xtend</id>
			<url>http://build.eclipse.org/common/xtend/maven/</url>
		</pluginRepository>
		<pluginRepository>
			<id>fornax</id>
			<url>http://fornax-platform.org/nexus/content/groups/public/</url>
		</pluginRepository>
	</pluginRepositories>

Dependencies

This small project setup requires just a mimimal set of dependencies to be configured:

	<dependencies>
		<dependency>
			<groupId>org.eclipse.xtend2</groupId>
			<artifactId>org.eclipse.xtend2.lib</artifactId>
			<version>2.2.0</version>
		</dependency>
		<dependency>
			<groupId>org.eclipse.xtext</groupId>
			<artifactId>org.eclipse.xtext.xtend2.lib</artifactId>
			<version>2.2.0.v201112061305</version>
		</dependency>
		<dependency>
			<groupId>com.google.inject</groupId>
			<artifactId>com.google.inject</artifactId>
			<version>2.0.0.v201105231817</version>
		</dependency>
	</dependencies>

Source folders

The main source folder src/ can be configured with the build/sourceDirectory setting, for xtend-gen we need the support of the build-helper-maven-plugin.

	<build>
		<sourceDirectory>src</sourceDirectory>
		<plugins>
			<plugin>
				<groupId>org.codehaus.mojo</groupId>
				<artifactId>build-helper-maven-plugin</artifactId>
				<version>1.7</version>
				<executions>
					<execution>
						<id>add-source</id>
						<phase>generate-sources</phase>
						<goals>
							<goal>add-source</goal>
						</goals>
						<configuration>
							<sources>
								<source>xtend-gen</source>
							</sources>
						</configuration>
					</execution>
				</executions>
			</plugin>
			...

The xtend-gen folder must be emptied when executing mvn clean, this requires some additional configuration of the maven-clean-plugin.

			<plugin>
				<artifactId>maven-clean-plugin</artifactId>
				<version>2.4.1</version>
				<configuration>
					<filesets>
						<fileset>
							<directory>xtend-gen</directory>
							<includes>
								<include>**</include>
							</includes>
						</fileset>
					</filesets>
				</configuration>
			</plugin>

xtend-maven-plugin

Now let’s finally come to the Xtend plugin. The plugin is configured as follows:

			<plugin>
				<groupId>org.eclipse.xtend2</groupId>
				<artifactId>xtend-maven-plugin</artifactId>
				<version>2.2.2</version>
				<executions>
					<execution>
						<goals>
							<goal>compile</goal>
							<goal>testCompile</goal>
						</goals>
						<configuration>
							<outputDirectory>xtend-gen</outputDirectory>
						</configuration>
					</execution>
				</executions>
			</plugin>

The outputDirectory is optional, but leaving it out the sources are generated to src/main/xtend-gen, and we want to generate to xtend-gen in the project root.

The plugin does not define a lifecycle mapping for M2E, which leads to an error marker “Plugin execution not covered by lifecycle configuration”. This is a well known issue when using M2E, and requires some additional work on the plugin. I have opened Bug#366118 for this.

Sample Project

The project described above is shared on Github: https://github.com/kthoms/xtext-experimental

In Eclipse the project looks like this after import (assuming the M2E plugin installed):

Build Execution

The classes in the project can now be built using mvn clean install. This will produce the following output:

[INFO] Scanning for projects...
[INFO]                                                                         
[INFO] ------------------------------------------------------------------------
[INFO] Building xtend-maven-classic 2.2.0-SNAPSHOT
[INFO] ------------------------------------------------------------------------
[INFO] 
[INFO] --- maven-clean-plugin:2.4.1:clean (default-clean) @ xtend-maven-classic ---
[INFO] Deleting /Users/thoms/git/xtext-experimental/maven/xtend.maven/target
[INFO] Deleting /Users/thoms/git/xtext-experimental/maven/xtend.maven/xtend-gen (includes = [**], excludes = [])
[INFO] 
[INFO] --- build-helper-maven-plugin:1.7:add-source (add-source) @ xtend-maven-classic ---
[INFO] Source directory: /Users/thoms/git/xtext-experimental/maven/xtend.maven/xtend-gen added.
[INFO] 
[INFO] --- xtend-maven-plugin:2.2.0:compile (default) @ xtend-maven-classic ---
[WARNING] 
WARNING: 	XtendClass1.xtend - /Users/thoms/git/xtext-experimental/maven/xtend.maven/src/mypackage/XtendClass1.xtend
7: The value of the field XtendClass1.cls is not used
[INFO] Compiling 1 source file to xtend-gen
[INFO] 
[INFO] --- maven-resources-plugin:2.4.3:resources (default-resources) @ xtend-maven-classic ---
[WARNING] Using platform encoding (MacRoman actually) to copy filtered resources, i.e. build is platform dependent!
[INFO] skip non existing resourceDirectory /Users/thoms/git/xtext-experimental/maven/xtend.maven/src/main/resources
[INFO] 
[INFO] --- maven-compiler-plugin:2.3.2:compile (default-compile) @ xtend-maven-classic ---
[WARNING] File encoding has not been set, using platform encoding MacRoman, i.e. build is platform dependent!
[INFO] Compiling 3 source files to /Users/thoms/git/xtext-experimental/maven/xtend.maven/target/classes
[INFO] ------------------------------------------------------------------------
[INFO] BUILD SUCCESS
[INFO] ------------------------------------------------------------------------
[INFO] Total time: 3.923s
[INFO] Finished at: Thu Dec 08 23:01:42 CET 2011
[INFO] Final Memory: 20M/81M
[INFO] ------------------------------------------------------------------------

From the output it can be seen that at the end 3 Java classes are compiled, after the xtend-maven-plugin created the Java code for the Xtend class.

Enabling debug output with -X reveals some more insight, how the plugin works:

[DEBUG] Configuring mojo 'org.eclipse.xtend2:xtend-maven-plugin:2.2.0:compile' with basic configurator -->
[DEBUG]   (f) outputDirectory = xtend-gen
[DEBUG]   (f) project = MavenProject: org.eclipse.xtext.example:xtend-maven-classic:2.2.0-SNAPSHOT @ /Users/thoms/git/xtext-experimental/maven/xtend.maven/pom.xml
[DEBUG]   (f) tempDirectory = /Users/thoms/git/xtext-experimental/maven/xtend.maven/target/xtend
[DEBUG] -- end configuration --
[DEBUG] load xtend file 'file:/Users/thoms/git/xtext-experimental/maven/xtend.maven/src/mypackage/XtendClass1.xtend'
[DEBUG] Parsing took: 42 ms
...
[DEBUG] create java stub 'mypackage/XtendClass1.java'
[DEBUG] invoke batch compiler with '-cp /Users/thoms/git/xtext-experimental/maven/xtend.maven/src:/Users/thoms/.m2/repository/org/eclipse/xtend2/org.eclipse.xtend2.lib/2.2.0/org.eclipse.xtend2.lib-2.2.0.jar:/Users/thoms/.m2/repository/com/google/guava/guava/10.0.1/guava-10.0.1.jar:/Users/thoms/.m2/repository/com/google/code/findbugs/jsr305/1.3.9/jsr305-1.3.9.jar:/Users/thoms/.m2/repository/org/eclipse/xtext/org.eclipse.xtext.xtend2.lib/2.2.0.v201112061305/org.eclipse.xtext.xtend2.lib-2.2.0.v201112061305.jar:/Users/thoms/.m2/repository/org/eclipse/xtext/org.eclipse.xtext.xbase.lib/2.2.0.v201112061305/org.eclipse.xtext.xbase.lib-2.2.0.v201112061305.jar:/Users/thoms/.m2/repository/com/google/inject/com.google.inject/2.0.0.v201105231817/com.google.inject-2.0.0.v201105231817.jar -d /Users/thoms/git/xtext-experimental/maven/xtend.maven/target/xtend/classes -1.5 -proceedOnError /Users/thoms/git/xtext-experimental/maven/xtend.maven/src /Users/thoms/git/xtext-experimental/maven/xtend.maven/target/xtend/stubs'
[DEBUG] classpath used for Xtend compilation : [file:/Users/thoms/git/xtext-experimental/maven/xtend.maven/src/, file:/Users/thoms/.m2/repository/org/eclipse/xtend2/org.eclipse.xtend2.lib/2.2.0/org.eclipse.xtend2.lib-2.2.0.jar, file:/Users/thoms/.m2/repository/com/google/guava/guava/10.0.1/guava-10.0.1.jar, file:/Users/thoms/.m2/repository/com/google/code/findbugs/jsr305/1.3.9/jsr305-1.3.9.jar, file:/Users/thoms/.m2/repository/org/eclipse/xtext/org.eclipse.xtext.xtend2.lib/2.2.0.v201112061305/org.eclipse.xtext.xtend2.lib-2.2.0.v201112061305.jar, file:/Users/thoms/.m2/repository/org/eclipse/xtext/org.eclipse.xtext.xbase.lib/2.2.0.v201112061305/org.eclipse.xtext.xbase.lib-2.2.0.v201112061305.jar, file:/Users/thoms/.m2/repository/com/google/inject/com.google.inject/2.0.0.v201105231817/com.google.inject-2.0.0.v201105231817.jar, file:/Users/thoms/git/xtext-experimental/maven/xtend.maven/src/, file:/Users/thoms/git/xtext-experimental/maven/xtend.maven/target/xtend/classes/]

The plugin invokes the Xtend Batch compiler through a spawned JVM. The batch compiler is realized in class org.eclipse.xtext.xtend2.compiler.batch.Xtend2BatchCompiler from the newly added plugin org.eclipse.xtext.xtend2.standalone.

After the build XtendClass1.java is generated as expected to xtend-gen. When opening the target folder it can be seen that the plugin produces a Java stub class to target/stubs for the Xtend class. Finally, everything is compiled and the project is clean 🙂

Limitation

The plugin only takes one source folder into account, namely the one configured by build/sourceDirectory. This is a problem when you have multiple source folders, which is quite typical for Xtext projects, namely src and src-gen. This is reported as , which is fixed in the meantime. I have deployed a patched version as unofficial release 2.2.2.

Conclusion

I deeply desired this plugin and this finally allows that the xtend-gen folder does not need to be checked in. The sample used here was just simple. Next would be a real life project were I want to apply the plugin, most likely in Project Spray. This project uses Xtend based code generation heavily and Maven Tycho for the build. The project has not been upgraded to Xtext 2.2.0 (of course, it was just released), and I guess we have to upgrade to Xtext 2.2.0 before we can use the plugin. But if this allows us to finally remove the xtend-gen folders from the repository this alone is worth upgrading.

Nice work, Xtext team!