Environment support plugin

The Environment plugin is used to have the same environment variables available (1) in maven for build system tasks and (2) while applications are running.

Typically an environment variable is defined as a property in a maven pom file. Here is an example:

<?xml version="1.0" encoding="UTF-8"?>
<project xmlns="http://maven.apache.org/POM/4.0.0"
    xmlns:xsi="http://www.w3.org/2001/XMLSchema-instance"
    xsi:schemaLocation="http://maven.apache.org/POM/4.0.0 http://maven.apache.org/maven-v4_0_0.xsd">
    <modelVersion>4.0.0</modelVersion>

    ...
    
    <properties>
        <rmi.host>localhost</rmi.host>
        <rmi.port>8099</rmi.port>
        
        <db.username>refdb_user</db.username>
        <db.password>my_secret_password</db.password>
        
        ...
        
    </properties>
</project>

To have the (maven) properties rmi.host and rmi.port also available during application runtime we have to share these properties between the 2 "worlds" (maven and the application runtime). In EL4J this is done the Spring way via a properties file. The EL4J env-module expects the file env-placeholder.properties in the classpath.

Maven is able to filter resources which means that placeholders will be replaced by their values. To have the properties rmi.host and rmi.port with exactly the same name during runtime we have to write the env-placeholder.properties file like this:

rmi.host=${rmi.host}
rmi.port=${rmi.port}

You already know the path src/main/resources. Resources placed in this path will be copied 1:1 to the place where the compiled Java sources are (by default target/classes). Now we have a similar path src/main/env. Resources in this path will be copied and filtered i.e. the placeholders will be replaced. The default target path for filtered files is target/env. This path is handled like the src/main/resources path. To have this behavior we must define the following in our parent pom (this is already made in the root pom of EL4J):

<?xml version="1.0" encoding="UTF-8"?>
<project xmlns="http://maven.apache.org/POM/4.0.0"
    xmlns:xsi="http://www.w3.org/2001/XMLSchema-instance"
    xsi:schemaLocation="http://maven.apache.org/POM/4.0.0 http://maven.apache.org/maven-v4_0_0.xsd">
    <modelVersion>4.0.0</modelVersion>

    ...

    <build>
        <resources>
            <resource>
                <directory>src/main/resources</directory>
                <filtering>false</filtering>
            </resource>

            <!-- Used for the environment support of EL4J -->
            <resource>
                <directory>target/env</directory>
                <filtering>false</filtering>
            </resource>
        </resources>
        <testResources>
            <testResource>
                <directory>src/test/resources</directory>
                <filtering>false</filtering>
            </testResource>

            <!-- Used for the environment support of EL4J -->
            <testResource>
                <directory>target/env-test</directory>
                <filtering>false</filtering>
            </testResource>
        </testResources>
    </build>
    
    ...
    
</project>

To complete the example above, we have to place the env-placeholder.properties file in src/main/env.

The filtered env-placeholder.properties file will then look like the following and can be found at target/env.

rmi.host=localhost
rmi.port=8099

If you create an Eclipse project for your non-pom artifact you will have the path target/env as source folder available. Through this you can clean your Eclipse project without loosing the filtered file.

Test scope

The equivalent part for the test scope must be placed in src/test/env and filtered files will be placed by default in target/env-test.

Overriding bean properties

In the same directory as the env-placeholder.properties files you can place another properties file with name env-bean-property.properties. Properties in this file will be used to override Spring bean properties (like Spring's PropertyOverrideConfigurer). Here is an example:

dataSource.username=${db.username}
dataSource.password=${db.password}
dataSource.url=${db.url}

The bean properties username and password of the bean dataSource will then be overridden. BTW the value of db.url is already set when using the corresponding database profile of the EL4J (root) pom (by default it is set to db2 (which is equivalent to derby)).

Global environment resource directory

You can define to use a global resource directory so you do not have to have the src/main/env path in each artifact. But be aware that you have the env-placeholder.properties file only one time in your classpath (otherwise you will get a warning and the wrong properties file may be chosen)!

For detailed usage see the plugin info page.