Integrate Protocol Buffers into Maven2 build

Question

I'm experimenting with Protocol Buffers in an existing, fairly vanilla Maven 2 project. Currently, I invoke a shell script every time I need to update my generated sources. This is obviously a hassle, as I would like the sources to be generated automatically before each build. Hopefully without resorting to shameful hackery.

So, my question is two-fold:

1. Long shot: is there a "Protocol Buffers plugin" for Maven 2 that can achieve the above in an automagic way? There's a branch on Google Code whose author appears to have taken a shot at implementing such a plugin. Unfortunately, it hasn't passed code review or been merged into protobuf trunk. The status of that plugin is thus unknown.

2. Probably more realistic: lacking an actual plugin, how else might I go about invoking protoc from my Maven 2 build? I suppose I may be able to wire up my existing shell script into an antrun invocation or something similar.

Personal experiences are most appreciated.

Answer 1

I think that using antrun to invoke non-Maven steps is the generally accepted solution.

You could also try the maven-exec-plugin.

Answer 2

You'll find some information about the plugin available in the Protocol Buffers repository in the Protocol Buffers Compiler Maven Plug-In thread on the Protocol Buffers discussion group. My understanding is that it's usable but lacking tests. I'd give it a try.

Or you could just use the antrun plugin (snipet pasted from the thread mentioned above):

<build>
  <plugins>
    <plugin>
      <artifactId>maven-antrun-plugin</artifactId>
      <executions>
        <execution>
          <id>generate-sources</id>
          <phase>generate-sources</phase>
          <configuration>
            <tasks>
              <mkdir dir="target/generated-sources"/>
              <exec executable="protoc">
                <arg value="--java_out=target/generated-sources"/>
                <arg value="src/main/protobuf/test.proto"/>
              </exec>
            </tasks>
            <sourceRoot>target/generated-sources</sourceRoot>
          </configuration>
          <goals>
            <goal>run</goal>
          </goals>
        </execution>
      </executions>
    </plugin>
  </plugins>
</build>

<dependencies>
  <dependency>
    <groupId>com.google.protobuf</groupId>
    <artifactId>protobuf-java</artifactId>
    <version>2.0.3</version>
  </dependency>
</dependencies>

Answer 3

The accepted answer encouraged me to get the Google-provided plugin to work. I merged the branch mentioned in my question into a checkout of 2.2.0 source code, built and installed/deployed the plugin, and was able to use it in my project as follows:

  <build>
    <plugins>
      <plugin>
        <groupId>com.google.protobuf.tools</groupId>
        <artifactId>maven-protoc-plugin</artifactId>
        <version>0.0.1</version>
        <executions>
          <execution>
            <id>generate-sources</id>
            <goals>
              <goal>compile</goal>
            </goals>
            <phase>generate-sources</phase>
            <configuration>
              <protoSourceRoot>${basedir}/src/main/protobuf/</protoSourceRoot>
              <includes>
                <param>**/*.proto</param>
              </includes>
            </configuration>
          </execution>
        </executions>
        <configuration>
          <protocExecutable>/usr/local/bin/protoc</protocExecutable>
        </configuration>
      </plugin>
    </plugins>
  </build>

Note that I changed the plugin's version to 0.0.1 (no -SNAPSHOT) in order to make it go into my non-snapshot thirdparty Nexus repository. YMMV. The takeaway is that this plugin will be usable once it's no longer necessary to jump through hoops in order to get it going.

Answer 4

I just updated the maven plugin to work with 2.2.0 -- the updated pom are attached to the code review bug.

Here are the instructions to build the plugin yourself:

svn co http://protobuf.googlecode.com/svn/branches/maven-plugin/tools/maven-plugin
cd maven-plugin
wget -O pom.xml 'http://protobuf.googlecode.com/issues/attachment?aid=8860476605163151855&name=pom.xml'
mvn install

You can then use the maven config above.

Answer 5

The accepted solution does not scale for multiple proto files. I had to come up with my own:

<build>
    <plugins>
        <plugin>
            <artifactId>maven-antrun-plugin</artifactId>
            <executions>
                <execution>
                    <id>compile-protoc</id>
                    <phase>generate-sources</phase>
                    <configuration>
                        <tasks>
                            <mkdir dir="${generated.sourceDirectory}" />
                            <path id="proto.path">
                                <fileset dir="src/main/proto">
                                    <include name="**/*.proto" />
                                </fileset>
                            </path>
                            <pathconvert pathsep=" " property="proto.files" refid="proto.path" />
                            <exec executable="protoc" failonerror="true">
                                <arg value="--java_out=${generated.sourceDirectory}" />
                                <arg value="-I${project.basedir}/src/main/proto" />
                                <arg line="${proto.files}" />
                            </exec>
                        </tasks>
                    </configuration>
                    <goals>
                        <goal>run</goal>
                    </goals>
                </execution>
            </executions>
        </plugin>
</build>