Properties and Resource Filtering

Throughout this book, you will notice references to properties which can be used in a POM file. Sibling dependencies in a multi-project build can be referenced using the ${project.groupId} and ${project.version} properties and any part of the POM can be referenced by prefixing the variable name with "project.". Environment variables and Java System properties can be referenced, as well as values from your ~/.m2/settings.xml file. What you haven’t seen yet is an enumeration of the possible property values and some discussion about how they can be used to help you create portable builds. This chapter provides such an enumeration.

If you’ve been using property references in your POM, you should also know that Maven has a feature called Resource Filtering which allows you to replace property references in any resource files stored under src/main/resources. By default this feature is disabled to prevent accidental replacement of property references. This feature can be used to target builds toward a specific platform and to externalize important build variables to properties files, POMs, or profiles. This chapter introduces the resource filtering feature and provides a brief discussion of how it can be used to create portable enterprise builds.

When a Maven Project Property is referenced, the property name is referencing a property of the Maven Project Object Model (POM). Specifically, you are referencing a property of the org.apache.maven.model.Model class which is being exposed as the implicit variable project. When you reference a property using this implicit variable, you are using simple dot notation to reference a bean property of the Model object. For example, when you reference ${project.version}, you are really invoking the getVersion() method on the instance of Model that is being exposed as project.

The POM is also represented in the pom.xml document present in all Maven projects. Anything in a Maven POM can be referenced with a property. A complete reference for the POM structure is available at http://maven.apache.org/ref/3.0.3/maven-model/maven.html. The following list shows some common property references from the Maven project.

project.groupId and project.version

Projects in a large, multi-module build often share the same groupId and version identifiers. When you are declaring interdependencies between two modules which share the same groupId and version, it is a good idea to use a property reference for both:

<dependencies>
    <dependency>
        <groupId>${project.groupId}</groupId>
        <artifactId>sibling-project</artifactId>
        <version>${project.version}</version>
    </dependency>
</dependencies>

project.artifactId

A project’s artifactId is often used as the name of a deliverable. For example, in a project with WAR packaging, you will want to generate a WAR file without the version identifiers. To do this, you would reference the project.artifactId in your POM file like this:

<build>
    <finalName>${project.artifactId}</finalName>
</build>

project.name and project.description

The name and project description can often be useful properties to reference from documentation. Instead of having to worry that all of your site documents maintain the same short descriptions, you can just reference these properties.

project.build.*

sourceDirectory, scriptSourceDirectory, and testSourceDirectory provide access to the source directories for the project. outputDirectory and testOutputDirectory provide access to the directories where Maven is going to put bytecode or other build output. directory refers to the directory which contains all of these output directories.

project.baseUri

If you need a valid URI for your project’s base directory, you can use the ${project.baseUri} property. If your project is stored in the directory /tmp/simple, ${project.baseUri} will resolve to file:/private/tmp/simple/.

Other Project Property references

There are hundreds of properties to reference in a POM. A complete reference for the POM structure is available at http://maven.apache.org/ref/3.0.3/maven-model/maven.html.

For a full list of properties available on the Maven Model object, take a look at the JavaDoc for the maven-model project here http://maven.apache.org/ref/3.0.3/maven-model/apidocs/index.html. Once you load this JavaDoc, take a look at the Model class. From this Model class JavaDoc, you should be able to navigate to the POM property you wish to reference. If you needed to reference the output directory of the build, you can use the Maven Model JavaDoc to see that the output directory is referenced via model.getBuild().getOutputDirectory(); this method call would be translated to the Maven property reference ${project.build.outputDirectory}.

For more information about the Maven Model module, the module which defines the structure of the POM, see the Maven Model project page at http://maven.apache.org/ref/3.0.3/maven-model.

You can also reference any properties in the Maven Local Settings file which is usually stored in ~/.m2/settings.xml. This file contains user-specific configuration such as the location of the local repository and any servers, profiles, and mirrors configured by a specific user.

A full reference for the Local Settings file and corresponding properties is available here http://maven.apache.org/ref/3.0.3/maven-settings/settings.html.

Environment variables can be referenced with the env.* prefix. Some interesting environment variables are listed in the following list:

env.PATH

Contains the current PATH in which Maven is running. The PATH contains a list of directories used to locate executable scripts and programs.

env.HOME

(On *nix systems) this variable points to a user’s home directory. Instead of referencing this, you should use the ${user.home}

env.JAVA_HOME

Contains the Java installation directory. This can point to either a Java Development Kit (JDK) installation or a Java Runtime Environment (JRE). Instead of using this, you should consider referencing the ${java.home} property.

env.M2_HOME

Contains the Maven 2 installation directory.

While they are available, you should always use the Java System properties if you have the choice. If you need a user’s home directory use ${user.home} instead of ${env.HOME}. If you do this, you’ll end up with a more portable build that is more likely to adhere to the Write-Once-Run-Anywhere (WORA) promise of the Java platform.

Maven exposes all properties from java.lang.System. Anything you can retrieve from System.getProperty() you can reference in a Maven property. The following table lists available properties:

In addition to the implicit properties provided by the POM, Maven Settings, environment variables, and the Java System properties, you have the ability to define your own arbitrary properties. Properties can be defined in a POM or in a Profile. The properties set in a POM or in a Maven Profile can be referenced just like any other property available throughout Maven. User-defined properties can be referenced in a POM, or they can be used to filter resources via the Maven Resource plugin. Here’s an example of defining some arbitrary properties in a Maven POM.

User-defined Properties in a POM. 

<project>
    ...
    <properties>
        <arbitrary.property.a>This is some text</arbitrary.property.a>
        <hibernate.version>3.3.0.ga</hibernate.version>
    </properties>
    ...
    <dependencies>
        <dependency>
            <groupId>org.hibernate</groupId>
            <artifactId>hibernate</artifactId>
            <version>${hibernate.version}</version>
        </dependency>
    </dependencies>
    ...
</project>

The previous example defines two properties: arbitrary.property.a and hibernate.version. The hibernate.version is referenced in a dependency declaration. Using the period character as a separator in property names is a standard practice throughout Maven POMs and Profiles. The next example shows you how to define a property in a profile from a Maven POM.

User-defined Properties in a Profile in a POM. 

<project>
    ...
    <profiles>
        <profile>
            <id>some-profile</id>
            <properties>
                <arbitrary.property>This is some text</arbitrary.property>
            </properties>
        </profile>
    </profiles>
    ...
</project>

The previous example demonstrates the process of defining a user-defined property in a profile from a Maven POM. For more information about user-defined properties and profiles, see Chapter 5, Build Profiles.

You can use Maven to perform variable replacement on project resources. When resource filtering is activated, Maven will scan resources for property references surrounded by ${ and }. When it finds these references it will replace them with the appropriate value in much the same way the properties defined in the previous section can be referenced from a POM. This feature is especially helpful when you need to parameterize a build with different configuration values depending on the target deployment platform.

Often a .properties file or an XML document in src/main/resources will contain a reference to an external resource such as a database or a network location which needs to be configured differently depending on the target deployment environment. For example, a system which reads data from a database has an XML document which contains the JDBC URL along with credentials for the database. If you need to use a different database in development and a different database in production. You can either use a technology like JNDI to externalize the configuration from the application in an application server, or you can create a build which knows how to replace variables with different values depending on the target platform.

Using Maven resource filtering you can reference Maven properties and then use Maven profiles to define different configuration values for different target deployment environments. To illustrate this feature, assume that you have a project which uses the Spring Framework to configure a BasicDataSource from the Commons DBCP project. Your project may contain a file in src/main/resources named applicationContext.xml which contains the XML listed in Referencing Maven Properties from a Resource.

Referencing Maven Properties from a Resource. 

<beans xmlns="http://www.springframework.org/schema/beans"
       xmlns:xsi="http://www.w3.org/2001/XMLSchema-instance"
       xsi:schemaLocation="http://www.springframework.org/schema/beans
                           http://www.springframework.org/schema/beans/spring-beans-2.5.xsd">

    <bean id="someDao" class="com.example.SomeDao">
        <property name="dataSource" ref="dataSource"/>
    </bean>

    <bean id="dataSource" destroy-method="close"
          class="org.apache.commons.dbcp.BasicDataSource">
        <property name="driverClassName" value="${jdbc.driverClassName}"/>
        <property name="url" value="${jdbc.url}"/>
        <property name="username" value="${jdbc.username}"/>
        <property name="password" value="${jdbc.password}"/>
    </bean>
</beans>

Your program would read this file at runtime, and your build is going to replace the references to properties like jdbc.url and jdbc.username with the values you defined in your pom.xml. Resource filtering is disabled by default to prevent any unintentional resource filtering. To turn on resource filtering, you need to use the resources child element of the build element in a POM. Defining Variables and Activating Resource Filtering shows a POM which defines the variables referenced in Referencing Maven Properties from a Resource and which activates resource filtering for every resource under src/main/resources.

Defining Variables and Activating Resource Filtering. 

<project>
    ...
    <properties>
        <jdbc.driverClassName>
            com.mysql.jdbc.Driver</jdbc.driverClassName>
        <jdbc.url>jdbc:mysql://localhost:3306/development_db</jdbc.url>
        <jdbc.username>dev_user</jdbc.username>
        <jdbc.password>s3cr3tw0rd</jdbc.password>
    </properties>
    ...
    <build>
        <resources>
            <resource>
                <directory>src/main/resources</directory>
                <filtering>true</filtering>
            </resource>
        </resources>
    </build>
    ...
    <profiles>
        <profile>
            <id>production</id>
            <properties>
                <jdbc.driverClassName>oracle.jdbc.driver.OracleDriver</jdbc.driverClassName>
                <jdbc.url>jdbc:oracle:thin:@proddb01:1521:PROD</jdbc.url>
                <jdbc.username>prod_user</jdbc.username>
                <jdbc.password>s00p3rs3cr3t</jdbc.password>
            </properties>
        </profile>
    </profiles>
</project>

The four variables are defined in the properties element, and resource filtering is activated for resources under src/main/resources. Resource filtering is deactivated by default, and to activate it you must explicitly set filtering to true for the resources stored in your project. Filtering is deactivated by default to prevent accidental, unintentional filtering during your build. If you build a project with the resource from Referencing Maven Properties from a Resource and the POM from Defining Variables and Activating Resource Filtering and if you list the contents of the resource in target/classes, you should see that it contains the filtered resource:

$ mvn install
...
$ cat target/classes/applicationContext.xml
...
<bean id="dataSource" destroy-method="close"
      class="org.apache.commons.dbcp.BasicDataSource">
    <property name="driverClassName" value="com.mysql.jdbc.Driver"/>
    <property name="url" value="jdbc:mysql://localhost:3306/development_db"/>
    <property name="username" value="dev_user"/>
    <property name="password" value="s3cr3tw0rd"/>
</bean>
...

The POM in Defining Variables and Activating Resource Filtering also defines a production profile under the profiles/profile element which overrides the default properties with values that would be appropriate for a production environment. In this particular POM, the default values for the database connection are for a local MySQL database installed on a developer’s machine. When the project is built with the production profile activated, Maven will configure the system to connect to a production Oracle database using a different driver class, URL, username, and password. If you build a project with the resource from Referencing Maven Properties from a Resource and the POM from Defining Variables and Activating Resource Filtering with the production profile activated and if you list the contents of the resource in target/classes, you should see that it contains the filtered resource with production values:

$ mvn -Pproduction install
...
$ cat target/classes/applicationContext.xml
...
<bean id="dataSource" destroy-method="close"
      class="org.apache.commons.dbcp.BasicDataSource">
    <property name="driverClassName"
              value="oracle.jdbc.driver.OracleDriver"/>
    <property name="url" value="jdbc:oracle:thin:@proddb01:1521:PROD"/>
    <property name="username" value="prod_user"/>
    <property name="password" value="s00p3rs3cr3t"/>
</bean>
...