Question

I need to read and filter a properties file from a location outside my project, say ${user.home}/my.properties. This properties file looks like this:

res.dir=/my/stuff/here
resource.dir=C:/${res.dir}
bin.dir=${resource.dir}/bin
cfg.dir=${resource.dir}/config

I have to do this in both my build and in my application when it runs. This is easy to do in Ant, using the PROPERTY tag. However, there doesn’t seem to be a good way to do it in Maven.

So far I have tried the Maven <property> tag, the Maven <filter> tag and various permutations of other tags. Either my build fails or the unit tests fail, or both.

If I hardcode these properties into the POM, everything works, so I know the problem is just reading the properties.

I looked at the properties-maven-plugin but the plugin no longer seems to be maintained.

Is there a way to do this?

Answer 1

You could simply implement your own maven-plugin that will take care of this for you.

Here is an example with the following structure:

.
 |-- pom.xml
 |-- plugin
 |   `-- pom.xml
 |   `-- src
 |       `-- main
 |           `-- java
 `-- app
     `-- pom.xml
     `-- src
         `-- main
             `-- java

You will need to create a Mojo that takes the properties file as an input and then propagates the properties to the pom.xml of the app. The pom.xml will actually not be updated but just the project data in it.

pom.xml

<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/xsd/maven-4.0.0.xsd">
    <modelVersion>4.0.0</modelVersion>

    <groupId>com.stackoverflow</groupId>
    <artifactId>Q12082277</artifactId>
    <version>1.0-SNAPSHOT</version>
    <packaging>pom</packaging>

    <name>${project.artifactId}-${project.version}</name>

    <modules>
        <module>plugin</module>
        <module>app</module>
    </modules>

</project>

plugin/pom.xml

<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/xsd/maven-4.0.0.xsd">
    <modelVersion>4.0.0</modelVersion>

    <parent>
        <groupId>com.stackoverflow</groupId>
        <artifactId>Q12082277</artifactId>
        <version>1.0-SNAPSHOT</version>
    </parent>

    <artifactId>Q12082277-plugin</artifactId>
    <packaging>maven-plugin</packaging>

    <name>${project.artifactId}-${project.version}</name>

    <dependencies>
        <dependency>
            <groupId>org.apache.maven</groupId>
            <artifactId>maven-plugin-api</artifactId>
            <version>3.0.4</version>
        </dependency>
        <dependency>
            <groupId>org.apache.maven</groupId>
            <artifactId>maven-project</artifactId>
            <version>2.2.1</version>
        </dependency>
    </dependencies>
</project>

plugin/src/main/java/com/stackoverflow/Q12082277/plugin/PropertiesMojo.java

package com.stackoverflow.Q12082277.plugin;

import org.apache.maven.plugin.AbstractMojo;
import org.apache.maven.plugin.MojoExecutionException;
import org.apache.maven.plugin.MojoFailureException;
import org.apache.maven.plugin.logging.Log;
import org.apache.maven.project.MavenProject;

import java.io.File;
import java.io.FileInputStream;
import java.io.FileNotFoundException;
import java.io.IOException;
import java.util.Properties;

/**
 * @author maba, 2012-08-24
 *
 * @goal extract
 */
public class PropertiesMojo extends AbstractMojo {

    private Log log;

    /**
     * The current project representation.
     * @parameter expression="${project}"
     * @required
     * @readonly
     */
    private MavenProject project;

    /**
     * A properties file
     *
     * @parameter expression="${propertiesFile}"
     * @required
     */
    private File propertiesFile;

    @Override
    public void execute() throws MojoExecutionException, MojoFailureException {
        log.info("Executing PropertiesMojo on " + propertiesFile.getAbsolutePath());

        try {
            Properties fileProperties = new Properties();
            fileProperties.load(new FileInputStream(propertiesFile));
            Properties projectProperties = project.getProperties();
            for (Object key : fileProperties.keySet()) {
                projectProperties.setProperty((String)key, (String) fileProperties.get(key));
            }
            project.getProperties().list(System.out);
        } catch (FileNotFoundException e) {
            throw new MojoFailureException("The file " + propertiesFile.getAbsolutePath() + " was not found!", e);
        } catch (IOException e) {
            log.error("");
        }

    }

    @Override
    public void setLog(Log log) {
        this.log = log;
    }
}

You will use this plugin from the following app/pom.xml

<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/xsd/maven-4.0.0.xsd">
    <modelVersion>4.0.0</modelVersion>

    <parent>
        <groupId>com.stackoverflow</groupId>
        <artifactId>Q12082277</artifactId>
        <version>1.0-SNAPSHOT</version>
    </parent>

    <artifactId>Q12082277-app</artifactId>

    <name>${project.artifactId}-${project.version}</name>

    <build>
        <plugins>
            <plugin>
                <groupId>com.stackoverflow</groupId>
                <artifactId>Q12082277-plugin</artifactId>
                <version>1.0-SNAPSHOT</version>
                <executions>
                    <execution>
                        <phase>initialize</phase>
                        <goals>
                            <goal>extract</goal>
                        </goals>
                        <configuration>
                            <propertiesFile>${user.home}/my.properties</propertiesFile>
                        </configuration>
                    </execution>
                </executions>
            </plugin>
            <plugin>
                <groupId>org.codehaus.mojo</groupId>
                <artifactId>exec-maven-plugin</artifactId>
                <version>1.2.1</version>
                <executions>
                    <execution>
                        <goals>
                            <goal>java</goal>
                        </goals>
                    </execution>
                </executions>
                <configuration>
                    <mainClass>com.stackoverflow.Q12082277.App</mainClass>
                </configuration>
            </plugin>
        </plugins>
        <resources>
            <resource>
                <directory>src/main/resources</directory>
                <filtering>true</filtering>
            </resource>
        </resources>
    </build>
</project>

And then you will have to add the following app.properties that will work as a template and take the values that we have just read from file and set them and create a concrete file app.properties that will be reachable from within the jar.

app/src/main/resources/app.properties

res.dir=${res.dir}
resource.dir=${resource.dir}
bin.dir=${bin.dir}
cfg.dir=${cfg.dir}

And finally here is a test application that just loads the app.properties from the classpath and prints the result.

app/src/main/java/com/stackoverflow/Q12082277/App.java

package com.stackoverflow.Q12082277;

import java.io.IOException;
import java.io.InputStream;
import java.util.Properties;

/**
 * @author maba, 2012-08-23
 */
public class App {

    public static void main(String[] args) throws IOException {
        ClassLoader loader = App.class.getClassLoader();
        InputStream in = loader.getResourceAsStream("app.properties");
        Properties properties = new Properties();
        properties.load(in);
        properties.list(System.out);
    }
}

Now you can stand in the top directory and execute

mvn install

Then go down into the app folder and execute

mvn exec:java

And it will print

-- listing properties --
resource.dir=C://my/stuff/here
cfg.dir=C://my/stuff/here/config
bin.dir=C://my/stuff/here/bin
res.dir=/my/stuff/here

Which is exactly what you wanted.