Though JSON is becoming a popular format for web applications, using JSON files to describe the configuration is a bit cumbersome and, in my opinion, is not as expressive as YAML. Let's see what YAML can do for us to make our life easier.

For sure, let's start with the problem. In order for our application to function properly, we need to feed it following data somehow:

• version and release date
• database connection parameters
• list of supported protocols
• list of users with their passwords

This list of parameters sounds a bit weird, but the purpose is to demonstrate different data types in work: strings, numbers, dates, lists and maps. The Java model consists of two simple classes: Connection

package com.example.yaml;

public final class Connection {
    private String url;
    private int poolSize;
  
    public String getUrl() {
        return url;
    }
 
    public void setUrl(String url) {
        this.url = url;
    }

    public int getPoolSize() {
        return poolSize;
    }

    public void setPoolSize(int poolSize) {
        this.poolSize = poolSize;
    }
 
    @Override
    public String toString() {
        return String.format( "'%s' with pool of %d", getUrl(), getPoolSize() );
    }
}

and Configuration, both are typical Java POJOs, verbose because of property setters and getters (we get used to it, right?).

package com.example.yaml;

import static java.lang.String.format;

import java.util.Date;
import java.util.List;
import java.util.Map;

public final class Configuration { 
    private Date released;
    private String version;
    private Connection connection;
    private List< String > protocols;
    private Map< String, String > users; 
 
    public Date getReleased() {
        return released;
    }
 
    public String getVersion() {
        return version;
    }
 
    public void setReleased(Date released) {
        this.released = released;
    }
 
    public void setVersion(String version) {
        this.version = version;
    }
 
    public Connection getConnection() {
        return connection;
    }
 
    public void setConnection(Connection connection) {
        this.connection = connection;
    }
 
    public List< String > getProtocols() {
        return protocols;
    }

    public void setProtocols(List< String > protocols) {
        this.protocols = protocols;
    }
 
    public Map< String, String > getUsers() {
        return users;
    }
 
    public void setUsers(Map< String, String > users) {
        this.users = users;
    }
 
    @Override
    public String toString() {
        return new StringBuilder()
            .append( format( "Version: %s\n", version ) )
            .append( format( "Released: %s\n", released ) )
            .append( format( "Connecting to database: %s\n", connection ) )
            .append( format( "Supported protocols: %s\n", protocols ) )
            .append( format( "Users: %s\n", users ) )
            .toString();
    }
}

ow, as model is quite clear, let us try to express it as the human being normally does it. Looking back to our list of required configuration, let's try to write it down one by one. 1. version and release date

version: 1.0
released: 2012-11-30

connection:
    url: jdbc:mysql://localhost:3306/db
    poolSize: 5

protocols:
   - http
   - https

users:
    tom: passwd
    bob: passwd

And this is it, our configuration expressed in YAML syntax is completed! The whole file sample.yml looks like this:

version: 1.0
released: 2012-11-30

# Connection parameters
connection:
    url: jdbc:mysql://localhost:3306/db
    poolSize: 5

# Protocols
protocols:
   - http
   - https

# Users
users:
    tom: passwd
    bob: passwd

To make it work in Java, we just need to use the awesome library called snakeyml, respectively the Maven POM file is quite simple:

<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.example</groupid>
    <artifactid>yaml</artifactid>
    <version>0.0.1-SNAPSHOT</version>
    <packaging>jar</packaging>

    <properties>
        <project.build.sourceencoding>UTF-8</project.build.sourceencoding>
    </properties>

    <dependencies>
        <dependency>
            <groupid>org.yaml</groupid>
            <artifactid>snakeyaml</artifactid>
            <version>1.11</version>
        </dependency>
    </dependencies>
 
    <build> 
        <plugins>
            <groupid>org.apache.maven.plugins</groupid>
            <artifactid>maven-compiler-plugin</artifactid>
            <version>2.3.1</version>
            <configuration>
                <source>1.7</source>
                <target>1.7</target>
            </configuration>
        </plugins>
    </build>
</project>

Please notice the usage of Java 1.7, the language extensions and additional libraries simplify a lot of regular tasks as we could see looking into YamlConfigRunner:

package com.example.yaml;

import java.io.IOException;
import java.io.InputStream;
import java.nio.file.Files;
import java.nio.file.Paths;

import org.yaml.snakeyaml.Yaml;

public class YamlConfigRunner {
    public static void main(String[] args) throws IOException {
        if( args.length != 1 ) {
            System.out.println( "Usage: <file.yml>" );
            return;
        }
  
        Yaml yaml = new Yaml();  
        try( InputStream in = Files.newInputStream( Paths.get( args[ 0 ] ) ) ) {
            Configuration config = yaml.loadAs( in, Configuration.class );
            System.out.println( config.toString() );
        }
    }
}

The code snippet here loads the configuration from file (args[ 0 ]), tries to parse it and fill up the Configuration class with meaningful data using JavaBeans conventions, converting to the declared types where possible. Running this class with sample.yml as an argument generates the following output:

Version: 1.0
Released: Thu Nov 29 19:00:00 EST 2012
Connecting to database: 'jdbc:mysql://localhost:3306/db' with pool of 5
Supported protocols: [http, https]
Users: {tom=passwd, bob=passwd}

Totally identical to the values we have configured!