How to share common properties among several maven projects?

I have several projects built by maven, and I want to share some common properties among them - spring version, mysql driver version, svn base url, etc. - so I can update them once and it will be reflected on all projects.

I thought of having a single super pom with all the properties, but if I change one of the problem I need to either increment its version (and to update all the poms inherit from it) or to delete it from all the developers' machines which I don't want to do.

Can specify these parameters externally to the pom? I still want to have the external location definition in a parent pom.

What you can do is to use the Properties Maven plugin. This will let you define your properties in an external file, and the plugin will read this file.

With this configuration :

<build>
    <plugins>
        <plugin>
            <groupId>org.codehaus.mojo</groupId>
            <artifactId>properties-maven-plugin</artifactId>
            <version>1.0-alpha-1</version>
            <executions>
                <execution>
                    <phase>initialize</phase>
                    <goals>
                        <goal>read-project-properties</goal>
                    </goals>
                    <configuration>
                        <files>
                            <file>my-file.properties</file>
                        </files>
                    </configuration>
                </execution>
            </executions>
        </plugin>
    </plugins>
</build>

and if you have, in your properties file the following lines:

spring-version=1.0
mysql-version=4.0.0

then it's the same thing as if you wrote, in your pom.xml, the following lines:

<properties>
    <spring-version>1.0</spring-version>
    <mysql-version>4.0.0</mysql-version>
</properties>

Using this plugin, you will have several benefits:

• Set easily a long list of properties
• Modify the values of these properties without modifying the parent pom.xml.

Note that the original idea I have here is something that I am doing, but that I may have found a much better idea which I've also listed out below. I wanted to keep both ideas here for completeness in case the newer idea does not work.

---

I think you can solve this problem using the parent pom, but you need to have a maven repository and a CI build tool.

I've got several projects which all inherit base properties from a parent POM. We use Java 1.5, so that build property is setup there. Everything is UTF-8. All of the reports that I wish to run, Sonar setup, etc, is inside the parent POM.

Assuming your project is in version control and you've got a CI tool, when you check in, your CI tool can build to POM project and deploy the SNAPSHOT to the maven repos. If your projects point to the SNAPSHOT version of the parent POM, they'll check the repository to validate that they have the latest version...if not they download the latest version. So if you update the parent, all of the other projects will update.

The trick, I suppose is releasing with a SNAPSHOT. I'd say your releases are going to come much less frequently than your changes. So you perform a release of your POM, then update your POMs that inherit from them and check them into version control. Let the devs know that they need to do an update and go from there.

You could just trigger builds there forcing the new POMs into the repository and then have all of the devs pick up the changes automagically upon build.

---

I've removed the LATEST/RELEASE keywords idea because they do not work for parent POMs. They only work for dependencies or plugins. The problem area is in DefaultMavenProjectBuilder. Essentially it has trouble determining which repository to look for the parent to determine what the latest or release version is. Not sure why this is different for dependencies or plugins though.

---

It sounds like these would be less painful than having to update the POMs on every change to the parent POM.

I think the properties-maven-plugin is the right approach long term, but as you responded to that answer it doesn't allow the properties to be inherited. There are some facilities in maven-shared-io that allow you to discover resources on the project classpath. I've included some code below that extends the properties plugin to find properties files in the dependencies of the plugin.

The configuration declares a path to a properties file, because the descriptor project is declared on the plugin configuration, it is accessible to the ClasspathResourceLocatorStrategy. The configuration can be defined in a parent project, and will be inherited by all the child projects (if you do this avoid declaring any files as they won't be discovered, only set the filePaths property).

The configuration below assumes that there is another jar project called name.seller.rich:test-properties-descriptor:0.0.1 that has a file called external.properties packaged into the jar (i.e. it was defined in src/main/resources).

<plugin>
  <groupId>org.codehaus.mojo</groupId>
  <artifactId>properties-ext-maven-plugin</artifactId>
  <version>0.0.1</version>
  <executions>
    <execution>
      <id>read-properties</id>
      <phase>initialize</phase>
      <goals>
        <goal>read-project-properties</goal>
      </goals>
    </execution>
  </executions>                              
  <configuration>
    <filePaths>
      <filePath>external.properties</filePath>
    </filePaths>
  </configuration> 
  <dependencies>
    <!-- declare any jars that host the required properties files here -->
    <dependency>
      <groupId>name.seller.rich</groupId>
      <artifactId>test-properties-descriptor</artifactId>
      <version>0.0.1</version>
    </dependency>
  </dependencies>
</plugin>

The pom for the plugin project looks like this:

<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>
  <groupId>org.codehaus.mojo</groupId>
  <artifactId>properties-ext-maven-plugin</artifactId>
  <packaging>maven-plugin</packaging>
  <version>0.0.1</version>
  <dependencies>
    <dependency>
      <groupId>org.codehaus.mojo</groupId>
      <artifactId>properties-maven-plugin</artifactId>
      <version>1.0-alpha-1</version>
    </dependency>
    <dependency>
      <groupId>org.apache.maven.shared</groupId>
      <artifactId>maven-shared-io</artifactId>
      <version>1.1</version>
    </dependency>
  </dependencies>
</project>

The mojo is a copy of the properties plugin's ReadPropertiesMojo, with an additional "filePaths" property to allow you to define the relative path to the external properties file in the classpath, it makes the files property optional, and adds the readPropertyFiles() and getLocation() methods to locate the files and merge any filePaths into the files array before continuing. I've commented my changes to make them clearer.

package org.codehaus.mojo.xml;

/*
 * Licensed to the Apache Software Foundation (ASF) under one
 * or more contributor license agreements.  See the NOTICE file 
 * distributed with this work for additional information
 * regarding copyright ownership.  The ASF licenses this file
 * to you under the Apache License, Version 2.0 (the
 * "License"); you may not use this file except in compliance
 * with the License.  You may obtain a copy of the License at
 * 
 *   http://www.apache.org/licenses/LICENSE-2.0
 * 
 * Unless required by applicable law or agreed to in writing, 
 * software distributed under the License is distributed on an
 * "AS IS" BASIS, WITHOUT WARRANTIES OR CONDITIONS OF ANY 
 * KIND, either express or implied.  See the License for the 
 * specific language governing permissions and limitations 
 * under the License.
 */

import java.io.File;
import java.io.FileInputStream;
import java.io.IOException;
import java.util.ArrayList;
import java.util.Enumeration;
import java.util.List;
import java.util.Properties;

import org.apache.maven.plugin.AbstractMojo;
import org.apache.maven.plugin.MojoExecutionException;
import org.apache.maven.project.MavenProject;
import org.apache.maven.shared.io.location.ClasspathResourceLocatorStrategy;
import org.apache.maven.shared.io.location.FileLocatorStrategy;
import org.apache.maven.shared.io.location.Location;
import org.apache.maven.shared.io.location.Locator;
import org.apache.maven.shared.io.location.LocatorStrategy;
import org.apache.maven.shared.io.location.URLLocatorStrategy;
import org.codehaus.plexus.util.cli.CommandLineUtils;

/**
 * The read-project-properties goal reads property files and stores the
 * properties as project properties. It serves as an alternate to specifying
 * properties in pom.xml.
 * 
 * @author <a href="mailto:[email protected]">Zarar Siddiqi</a>
 * @author <a href="mailto:[email protected]">Krystian Nowak</a>
 * @version $Id: ReadPropertiesMojo.java 8861 2009-01-21 15:35:38Z pgier $
 * @goal read-project-properties
 */
public class ReadPropertiesMojo extends AbstractMojo {
    /**
     * @parameter default-value="${project}"
     * @required
     * @readonly
     */
    private MavenProject project;

    /**
     * The properties files that will be used when reading properties.
     * RS: made optional to avoid issue for inherited plugins
     * @parameter
     */
    private File[] files;

    //Begin: RS addition
    /**
     * Optional paths to properties files to be used.
     * 
     * @parameter
     */
    private String[] filePaths;
    //End: RS addition

    /**
     * If the plugin should be quiet if any of the files was not found
     * 
     * @parameter default-value="false"
     */
    private boolean quiet;

    public void execute() throws MojoExecutionException {
        //Begin: RS addition
        readPropertyFiles();
        //End: RS addition

        Properties projectProperties = new Properties();
        for (int i = 0; i < files.length; i++) {
            File file = files[i];

            if (file.exists()) {
                try {
                    getLog().debug("Loading property file: " + file);

                    FileInputStream stream = new FileInputStream(file);
                    projectProperties = project.getProperties();

                    try {
                        projectProperties.load(stream);
                    } finally {
                        if (stream != null) {
                            stream.close();
                        }
                    }
                } catch (IOException e) {
                    throw new MojoExecutionException(
                            "Error reading properties file "
                                    + file.getAbsolutePath(), e);
                }
            } else {
                if (quiet) {
                    getLog().warn(
                            "Ignoring missing properties file: "
                                    + file.getAbsolutePath());
                } else {
                    throw new MojoExecutionException(
                            "Properties file not found: "
                                    + file.getAbsolutePath());
                }
            }
        }

        boolean useEnvVariables = false;
        for (Enumeration n = projectProperties.propertyNames(); n
                .hasMoreElements();) {
            String k = (String) n.nextElement();
            String p = (String) projectProperties.get(k);
            if (p.indexOf("${env.") != -1) {
                useEnvVariables = true;
                break;
            }
        }
        Properties environment = null;
        if (useEnvVariables) {
            try {
                environment = CommandLineUtils.getSystemEnvVars();
            } catch (IOException e) {
                throw new MojoExecutionException(
                        "Error getting system envorinment variables: ", e);
            }
        }
        for (Enumeration n = projectProperties.propertyNames(); n
                .hasMoreElements();) {
            String k = (String) n.nextElement();
            projectProperties.setProperty(k, getPropertyValue(k,
                    projectProperties, environment));
        }
    }

    //Begin: RS addition
    /**
     * Obtain the file from the local project or the classpath
     * 
     * @throws MojoExecutionException
     */
    private void readPropertyFiles() throws MojoExecutionException {
        if (filePaths != null && filePaths.length > 0) {
            File[] allFiles;

            int offset = 0;
            if (files != null && files.length != 0) {
                allFiles = new File[files.length + filePaths.length];
                System.arraycopy(files, 0, allFiles, 0, files.length);
                offset = files.length;
            } else {
                allFiles = new File[filePaths.length];
            }

            for (int i = 0; i < filePaths.length; i++) {
                Location location = getLocation(filePaths[i], project);

                try {
                    allFiles[offset + i] = location.getFile();
                } catch (IOException e) {
                    throw new MojoExecutionException(
                            "unable to open properties file", e);
                }
            }

            // replace the original array with the merged results
            files = allFiles;
        } else if (files == null || files.length == 0) {
            throw new MojoExecutionException(
                    "no files or filePaths defined, one or both must be specified");
        }
    }
    //End: RS addition

    /**
     * Retrieves a property value, replacing values like ${token} using the
     * Properties to look them up. Shamelessly adapted from:
     * http://maven.apache.
     * org/plugins/maven-war-plugin/xref/org/apache/maven/plugin
     * /war/PropertyUtils.html
     * 
     * It will leave unresolved properties alone, trying for System properties,
     * and environment variables and implements reparsing (in the case that the
     * value of a property contains a key), and will not loop endlessly on a
     * pair like test = ${test}
     * 
     * @param k
     *            property key
     * @param p
     *            project properties
     * @param environment
     *            environment variables
     * @return resolved property value
     */
    private String getPropertyValue(String k, Properties p,
            Properties environment) {
        String v = p.getProperty(k);
        String ret = "";
        int idx, idx2;

        while ((idx = v.indexOf("${")) >= 0) {
            // append prefix to result
            ret += v.substring(0, idx);

            // strip prefix from original
            v = v.substring(idx + 2);

            idx2 = v.indexOf("}");

            // if no matching } then bail
            if (idx2 < 0) {
                break;
            }

            // strip out the key and resolve it
            // resolve the key/value for the ${statement}
            String nk = v.substring(0, idx2);
            v = v.substring(idx2 + 1);
            String nv = p.getProperty(nk);

            // try global environment
            if (nv == null) {
                nv = System.getProperty(nk);
            }

            // try environment variable
            if (nv == null && nk.startsWith("env.") && environment != null) {
                nv = environment.getProperty(nk.substring(4));
            }

            // if the key cannot be resolved,
            // leave it alone ( and don't parse again )
            // else prefix the original string with the
            // resolved property ( so it can be parsed further )
            // taking recursion into account.
            if (nv == null || nv.equals(nk)) {
                ret += "${" + nk + "}";
            } else {
                v = nv + v;
            }
        }
        return ret + v;
    }

    //Begin: RS addition
    /**
     * Use various strategies to discover the file.
     */
    public Location getLocation(String path, MavenProject project) {
        LocatorStrategy classpathStrategy = new ClasspathResourceLocatorStrategy();

        List strategies = new ArrayList();
        strategies.add(classpathStrategy);
        strategies.add(new FileLocatorStrategy());
        strategies.add(new URLLocatorStrategy());

        List refStrategies = new ArrayList();
        refStrategies.add(classpathStrategy);

        Locator locator = new Locator();

        locator.setStrategies(strategies);

        Location location = locator.resolve(path);
        return location;
    }
    //End: RS addition
}