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August 25, 2009

Grassroots Groovy: Configuration with ConfigSlurper

We can introduce Groovy into our projects at grassroots level. In this post we learn how to use Groovy configuration scripts instead of Java properties files to configure our application. Groovy has the ConfigSlurper class. We use a ConfigSlurper object to read in a Groovy script. This scripts contains a tree of objects with different values. These values can be Java types, which is not possible in a plain properties file. We can even add code to the script file.

The ConfigSlurper is capable of supporting different environments. We pass the environment to the constructor. This way we can set a default value for a property and override it for a specific environment in the same configuration file.

We start with a simple Java application to read in a Groovy configuration script:

package com.mrhaki.sample.config;

import java.util.*;
import groovy.util.*;

public class JavaConfig {
    public static void main(String[] args) throws MalformedURLException {
        // Read in 'config.groovy' for the development environment.
        ConfigObject conf = new ConfigSlurper("development").parse(new File("config.groovy").toURI().toURL());
        // Read in 'mail.groovy'.
        final ConfigObject mailConf = new ConfigSlurper().parse(new File("mail.groovy").toURI().toURL());
        // Merge both configurations into one.
        conf = (ConfigObject) conf.merge(mailConf);
        // Flatten configuration for easy access keys with dotted notation.
        final Map config = conf.flatten();
        assert "1.0".equals(config.get("app.version"));
        assert new Integer(20).equals(config.get("app.setting2"));
        assert new Date().after((Date) config.get(""));
        assert config.get("");
        assert "http://localhost".equals(config.get("server.URL"));
        assert "smtpservername".equals(config.get(""));

It is not difficult to invoke the ConfigSlurper from our Java code. We must make sure the Groovy libraries are in the classpath when we run the application ($GROOVY_HOME/embeddable/groovy-all-<version>.jar). Next we make the configuration script config.groovy:

// We can group settings.
// The following settings are created:
// app.version, app.setting1, app.setting2, app.setting3,,
app {
    version = "1.0"
    // We can write code to set up settings.
    [1, 2, 3].each {
        this."setting${it}" = it * 10
    // We can use Java objects
    date = new Date()
    active = true

server.URL = 'http://default'

// Environment specific settings override settings with the
// same name.
environments {
    development {
        server.URL = 'http://localhost'
    test {
        server.URL = 'http://test:9080'
    integrationtest {
        server.URL = 'http://integrationtest/url'
    production {
        server.URL = 'http://prod/url'

And we make the configuration script mail.groovy which is merged together with config.groovy:

// Classic way of specifying properties without grouping, 
// but we can use Java objects instead of only strings. = "smtpservername"
mail.smtp.ssl = true

We are ready to run our Java application - $ java -ea com.mrhaki.sample.config.JavaConfig - and all works fine. We shouldn't get any assertion errors. In a real world application we can make a class to read in the Groovy configuration files. Other classes use this class to get the values of the settings. All in plain old Java, but we create the settings and their values in a Groovy way.