11.1.1. What is Build Portability

A build's "portability" is a measure of how easy it is to take a particular project and build it in different environments. A build which works without any custom configuration or customization of properties files is more portable than a build which requires a great deal of work to build from scratch. The most portable projects tend to be widely used open source projects like Apache Commons of Apache Velocity which ship with Maven builds which require little or no customization. Put simply, the most portable project builds tend to just work, out of the box, and the least portable builds require you to jump through hoops and configure platform specific paths to locate build tools. Before we show you how to achieve build portability, let's survey the different kinds of portability we are talking about.

11.1.1.1. Non-Portable Builds

The lack of portability is exactly what all build tools are made to prevent - however, any tool can be configured to be non-portable (even Maven). A non-portable project is buildable only under a specific set of circumstances and criteria (e.g., your local machine). Unless you are working by yourself and you have no plans on ever deploying your application to another machine, it is best to avoid non-portability entirely. A non-portable build only runs on a single machine, it is a "one-off". Maven is designed to discourage non-portable builds by offering the ability to customize builds using profiles.

When a new developer gets the source for a non-portable project, they will not be able to build the project without rewriting large portions of a build script.

11.1.1.2. Environment Portability

A build exhibits environment portability if it has a mechanism for customizing behavior and configuration when targeting different environments. A project that contains a reference to a test database in a test environment, for example, and a production database in a production environment, is environmentally portable. It is likely that this build has a different set of properties for each environment. When you move to a different environment, one that is not defined and has no profile created for it, the project will not work. Hence, it is only portable between defined environments.

When a new developer gets the source for an environmentally portable project, they will have to run the build within a defined environment or they will have to create a custom environment to successfully build the project.

11.1.1.3. Organizational (In-House) Portability

The center of this level of portability is a project's requirement that only a select few may access internal resources such as source control or an internally-maintained Maven repository. A project at a large corporation may depend on a database available only to in-house developers, or an open source project might require a specific level of credentials to publish a web site and deploy the products of a build to a public repository.

If you attempt to build an in-house project from scratch outside of the in-house network (for example, outside of a corporate firewall), the build will fail. It may fail because certain required custom plugins are unavailable, or project dependencies cannot be found because you don't have the appropriate credentials to retrieve dependencies from a custom remote repository. Such a project is portable only across environments in a single organization.

11.1.1.4. Wide (Universal) Portability

Anyone may download a widely portable project's source, compile, and install it without customizing a build for a specific environment. This is the highest level of portability; anything less requires extra work for those who wish to build your project. This level of portability is especially important for open source projects, which depend on the ability for would-be contributors to easily download and build from source.

Any developer could download the source for a widely portable project.

11.1.2. Selecting an Appropriate Level of Portability

Clearly, you'll want to avoid creating the worst-case scenario: the non-portable build. You may have had the misfortune to work or study at an organization that had critical applications with non-portable builds. In such organizations, you cannot deploy an application without the help of a specific individual on a specific machine. In such an organization, it is also very difficult to introduce new project dependencies or changes without coordinating the change with the single person who maintains such a non-portable build. Non-portable builds tend to grow in highly political environments when one individual or group needs to exert control over how and when a project is built and deployed. "How do we build the system? Oh, we've got to call Jack and ask him to build it for us, no one else deploys to production." That is a dangerous situation which is more common that you would think. If you work for this organization, Maven and Maven profiles provide a way out of this mess.

On the opposite end of the portability spectrum are widely portable builds. Widely portable builds are generally the most difficult build systems to attain. These builds restrict your dependencies to those projects and tools that may be freely distributed and are publicly available. Many commercial software packages might be excluded from the most-portable builds because they cannot be downloaded before you have accepted a certain license. Wide portability also restricts dependencies to those pieces of software that may be distributed as Maven artifacts. For example, if you depend upon Oracle JDBC drivers, your users will have to download and install them manually; this is not widely portable as you will have to distribute a set of environment setup instructions for people interested in building your application. On the other hand, you could use a JDBC driver which is available from the public Maven repositories like MySQL or HSQLDB.

As stated previously, open source projects benefit from having the most widely portable build possible. Widely portable builds reduce the inefficiencies associated with contributing to a project. In an open source project (such as Maven) there are two distinct groups: end-users and developers. When an end-user uses a project like Maven and decides to contribute a patch to Maven, they have to make the transition from using the output of a build to running a build. They have to first become a developer, and if it is difficult to learn how to build a project, this end-user has a disincentive to take the time to contribute to a project. In a widely portable project, an end-user doesn't have to follow a set or arcane build instructions to start becoming a developer, they can download the source, modify the source, build, and submit a contribution without asking someone to help them set up a build environment. When the cost of contributing source back to an open-source project is lower, you'll see an increase in source code contributions, especially casual contributions which can make the difference between a project's success and a project's failure. One side-effect of Maven's adoption across a wide group of open source projects is that it has made it easier for developers to contribute code to various open source projects.

11.2.1. Overriding a Project Object Model

While the previous example showed you how to override the default configuration properties of a single Maven plugin, you still don't know exactly what a Maven profile is allowed to override. The short-answer to that question is that a Maven profile can override almost everything that you would have in a pom.xml. The Maven POM contains an element under project called profiles containing a project's alternate configurations, and under this element are profile elements which define each profile. Each profile must have an id, and other than that, it can contain almost any of the elements one would expect to see under project. The following XML document shows all of the elements, a profile is allowed to override.

<project>
  <profiles>
    <profile>
      <build>
        <defaultGoal>...</defaultGoal>
        <finalName>...</finalName>
        <resources>...</resources>
        <testResources>...</testResources>
        <plugins>...</plugins>
      </build>
      <reporting>...</reporting>
      <modules>...</modules>
      <dependencies>...</dependencies>
      <dependencyManagement>...</dependencyManagement>
      <distributionManagement>...</distributionManagement>
      <repositories>...</repositories>
      <pluginRepositories>...</pluginRepositories>
      <properties>...</properties>
    </profile>
  </profiles>
</project>

A profile can override an element shown with ellipses. A profile can override the final name of a project's artifact in a profile, the dependencies, and the behavior of a project's build via plugin configuration. A profile can also override the configuration of distribution settings depending on the profile; for example, if you need to publish an artifact to a staging server in a staging profile, you would create a staging profile which overrides the distributionManagement element in a profile.

11.3.1. Activation Configuration

Activations can contain one of more selectors including JDK versions, Operating System parameters, files, and properties. A profile is activated when all activation criteria has been satisfied. For example, a profile could list an Operating System family of Windows, and a JDK version of 1.4, this profile will only be activated when the build is executed on a Windows machine running Java 1.4. If the profile is active then all elements override the corresponding project-level elements as if the profile were included with the -P command-line argument. The following example, lists a profile which is activated by a very specific combination of operating system parameters, properties, and a JDK version.

<project>
  ...
  <profiles>
    <profile>
      <id>dev</id>
      <activation>
        <activeByDefault>false</activeByDefault>
        <jdk>1.5</jdk>
        <os>
          <name>Windows XP</name>
          <family>Windows</family>
          <arch>x86</arch>
          <version>5.1.2600</version>
        </os>
        <property>
          <name>mavenVersion</name>
          <value>2.0.5</value>
        </property>
        <file>
          <exists>file2.properties</exists>
          <missing>file1.properties</missing>
        </file>
      </activation>
      ...
    </profile>
  </profiles>
</project>

This previous example defines a very narrow set of activation parameters. Let's examine each activation criterion in detail:

	The activeByDefault element controls whether this profile is considered active by default.
	This profile will only be active for JDK versions that begin with "1.5". This includes "1.5.0_01", "1.5.1".
	This profile targets a very specific version of Windows XP, version 5.1.2600 on a 32-bit platform. If your project uses the native plugin to build a C program, you might find yourself writing projects for specific platforms.
	The property element tells Maven to activate this profile if the property mavenVersion is set to the value 2.0.5. mavenVersion is an implicit property that is available to all Maven builds.
	The file element allows us to activate a profile based on the presence (or absence) of files. The dev profile will be activated if a file named file2.properties exists in the base directory of the project. The dev profile will only be activated if there is no file named file1.properties file in the base directory of the project.

11.3.2. Activation by the Absence of a Property

You can activate a profile based on the value of a property like environment.type. You can activate a development profile if environment.type equals dev, or a production profile if environment.type equals prod. You can also activate a profile in the absence of a property. The following configuration activates a profile is the property environment.type is not present during Maven execution.

<project>
  ...
  <profiles>
    <profile>
      <id>development</id>
      <activation>
        <property>
          <name>!environment.type</name>
        </property>
      </activation>
    </profile>
  </profiles>
</project>

Note the exclamation point prefixing the property name. The exclamation point is often referred to as the "bang" character and signifies "not". This profile is activated when no ${environment.type} property is set.

11.5.1. Global Settings Profiles

Just like settings profiles, you can also define a set of global profiles in ${M2_HOME}/conf/settings.xml. Profiles defined in this configuration file are available across all users using a specific installation of Maven. The ability to define a global settings profile is useful f you are creating a customized distribution of Maven for a specific organization and you want to ensure that every user of Maven has access to a set of build profiles that ensure in-house portability. If you need to add custom plugin repositories or define a custom set of plugins that are only used by your organization, you could distribute a copy of Maven to your users that has these settings "baked in". The configuration of global settings profiles is the same as the configuration of User-specific settings profiles.

11.7.1. Common Environments

One of 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 create a default profile in your settings.xml that always set environment.type to dev on your development machine. This way, each project defines a dev profile activated by the same environment variable. Let's see how this is done, the following settings.xml defines a profile in ~/.m2/settings.xml which sets the environment.type property to dev.

<settings>
  <profiles>
    <profile>
      <activation>
        <activeByDefault>true</activeByDefault>
      </activation>
      <properties>
        <environment.type>dev</environment.type>
      </properties>
    </profile>
  </profiles>
</settings>

This means that every time you run Maven on your machine, this profile will be activated and the property environment.type will have the value dev. 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>
        <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</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. In this example, because we've defined a profile in ~/.m2/settings.xml which sets environment.type to dev, the development profile will always be activated when we run Maven on our development machine. Alternatively, if we wanted to override this default, we could set a property on the command-line. If we need to activate the production profile, we could always run Maven with:

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

Setting a property on the command-line would override the default property set in ~/.m2/settings.xml. We could have just defined a profile with an id of "dev" and invoked it directly with the -P command-line argument, but using this environment.type property allows us to code other project pom.xml files to this standard. Every project in your codebase could have a profile which is activated by the same environment.type property set in every user's ~/.m2/settings.xml. In this way, developers can share common configuration for development without defining this configuration in non-portable settings.xml files.

11.7.2. Protecting Secrets

This best practice builds upon the previous section. In Project Profile Activated by environment.type equalling '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.

11.7.3. Platform Classifiers

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 Operation System parameters. For more information about the Maven Assembly plugin, see Chapter 12, 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>