One of the core motivations for Maven project profiles was to provide for environment-specific configuration settings. In a development environment, you might want to produce bytecode with debug information and you might want to configure your system to use a development database instance. In a production environment you might want to produce a signed JAR and configure the system to use a production database. In this chapter, we defined a number of environments with identifiers like dev and prod. A simpler way to do this would be to define profiles that are activated by environment properties and to use these common environment properties across all of your projects.

For example, if every project had a development profile activated by a property named environment.type having a value of dev, and if those same projects had a production profile activated by a property named environment.type having a value of prod, you could simply pass in the appropriate property value on the command-line to ensure that your builds target the correct environment. You can then use this property to activate profiles defined in a project's pom.xml as follows. Let's take a look at how a project's pom.xml would define a profile activated by environment.type having the value dev.

<project>
  ...
  <profiles>
    <profile>
      <id>development</id>
      <activation>
        <activeByDefault>true</activeByDefault>
        <property>
          <name>environment.type</name>
          <value>dev</value>
        </property>
      </activation>
      <properties>
        <database.driverClassName>com.mysql.jdbc.Driver</database.driverClassName>
        <database.url>
          jdbc:mysql://localhost:3306/app_dev
        </database.url>
        <database.user>development_user</database.user>
        <database.password>development_password</database.password>
      </properties>
    </profile>
    <profile>
      <id>production</id>
      <activation>
        <property>
          <name>environment.type</name>
          <value>prod</value>
        </property>
      </activation>
      <properties>
        <database.driverClassName>com.mysql.jdbc.Driver</database.driverClassName>
        <database.url>jdbc:mysql://master01:3306,slave01:3306/app_prod</database.url>
        <database.user>prod_user</database.user>
      </properties>
    </profile>
  </profiles>
</project>

This project defines some properties like database.url and database.user which might be used to configure another Maven plugin configured in the pom.xml. There are plugins available that can manipulate the database, run SQL, and plugins like the Maven Hibernate3 plugin which can generate annotated model objects for use in persistence frameworks. A few of these plugins, can be configured in a pom.xml using these properties. These properties could also be used to filter resources. If we needed to target the development environment, we would just run the following command:

~/examples/profiles $ mvn install

Because the development profile is active by default, and because there are no other profiles activated, running mvn help:active-profiles will show that the development profile is active. Now, the activeByDefault option will only work if no other profiles are active. If you wanted to be sure that the development profile would be active for a given build, you could explicitly pass in the environment.type variable as follows:

~/examples/profiles $ mvn install -Denvironment.type=dev

Alternatively, if we need to activate the production profile, we could always run Maven with:

~/examples/profiles $ mvn install -Denvironment.type=prod

To test which profiles are active for a given build, use mvn help:active-profiles.

This best practice builds upon the previous section. In Project Profile Activated by setting environment.type to 'dev', the production profile does not contain the database.password property. I've done this on purpose to illustrate the concept of putting secrets in you user-specific settings.xml. If you were developing an application at a large organization which values security, it is likely that the majority of the development group will not know the password to the production database. In an organization that draws a bold line between the development group and the operations group, this will be the norm. Developers may have access to a development and a staging environment, but they might not have (or want to have) access to the production database. There are a number of reasons why this makes sense, particularly if an organization is dealing with extremely sensitive financial, intelligence, or medical information. In this scenario, the production environment build may only be carried out by a lead developer or by a member of the production operations group. When they run this build using the prod environment.type, they will need to define this variable in their settings.xml as follows:

<settings>
  <profiles>
    <profile>
      <activeByDefault>true</activeByDefault>
      <properties>
        <environment.type>prod</environment.type>
        <database.password>m1ss10nimp0ss1bl3</database.password>
      </properties>
    </profile>
  </profiles>
</settings>

This user has defined a default profile which sets the environment.type to prod and which also sets the production password. When the project is executed, the production profile is activated by the environment.type property and the database.password property is populated. This way, you can put all of the production-specific configuration into a project's pom.xml and leave out only the single secret necessary to access the production database.

Let's assume that you have a library or a project that produces platform-specific customizations. Even though Java is platform-neutral, there are times when you might need to write some code that invokes platform-specific native code. Another possibility is that you've written some C code which is compiled by the Maven Native plugin and you want to produce a qualified artifact depending on the build platform. You can set a classifier with the Maven Assembly plugin or with the Maven Jar plugin. The following pom.xml produces a qualified artifact using profiles which are activated by Operating System parameters. For more information about the Maven Assembly plugin, see Chapter 8, Maven Assemblies.

<project>
  ...
  <profiles>
    <profile>
      <id>windows</id>
      <activation>
        <os>
          <family>windows</family>
        </os>
      </activation>
      <build>
        <plugins>
          <plugin>
            <artifactId>maven-jar-plugin</artifactId>
            <configuration>
              <classifier>win</classifier>
            </configuration>
          </plugin>
        </plugins>
      </build>
    </profile>
    <profile>
      <id>linux</id>
      <activation>
        <os>
          <family>unix</family>
        </os>
      </activation>
      <build>
        <plugins>
          <plugin>
            <artifactId>maven-jar-plugin</artifactId>
            <configuration>
              <classifier>linux</classifier>
            </configuration>
          </plugin>
        </plugins>
      </build>
    </profile>
  </profiles>
</project>

If the Operating System is in the Windows family, this pom.xml qualifies the JAR artifact with "-win". If the Operating System is in the Unix family, the artifact is qualified with "-linux". This pom.xml successfully adds the qualifiers to the artifacts, but it is more verbose than it need to be due to the redundant configuration of the Maven Jar plugin in both profiles. This example could be rewritten to use variable substitution to minimize redundancy as follows:

<project>
  ...
  <build>
    <plugins>
      <plugin>
        <artifactId>maven-jar-plugin</artifactId>
        <configuration>
          <classifier>${envClassifier}</classifier>
        </configuration>
      </plugin>
    </plugins>
  </build>
  ...
  <profiles>
    <profile>
      <id>windows</id>
      <activation>
        <os>
          <family>windows</family>
        </os>
      </activation>
      <properties>
        <envClassifier>win</envClassifier>
      </properties>
    </profile>
    <profile>
      <id>linux</id>
      <activation>
        <os>
          <family>unix</family>
        </os>
      </activation>
      <properties>
        <envClassifier>linux</envClassifier>
      </properties>
    </profile>
  </profiles>
</project>

In this pom.xml, each profile doesn't need to include a build element to configure the Jar plugin. Instead, each profile is activated by the Operating System family and sets the envClassifier property to either win or linux. This envClassifier is then referenced in the default pom.xml build element to add a classifier to the project's JAR artifact. The JAR artifact will be named ${finalName}-${envClassifier}.jar and included as a dependency using the following dependency syntax:

<dependency>
      <groupId>com.mycompany</groupId>
      <artifactId>my-project</artifactId>
      <version>1.0</version>
      <classifier>linux</classifier>
    </dependency>