2. Add your first dependency

1. Find the library on Maven Central

Maven downloads libraries from Maven Central, the public repository of Java libraries. Each library has coordinates: a groupId, an artifactId and a version, the same three elements your own pom.xml starts with.

Open https://central.sonatype.com and search for commons-lang3. The page of org.apache.commons:commons-lang3 shows the latest version and, under “Snippets”, the <dependency> element to copy for Maven. This tutorial uses version 3.21.0.

2. Add the dependency

pom.xml has two <dependencies> elements: one inside <dependencyManagement> and one directly under <project>. Add the library to the one directly under <project>, above the JUnit dependencies:

pom.xml

<project ...>
  ...
  <dependencies>
    <dependency>
      <groupId>org.apache.commons</groupId>
      <artifactId>commons-lang3</artifactId>
      <version>3.21.0</version>
    </dependency>
    <dependency>
      <groupId>org.junit.jupiter</groupId>
      <artifactId>junit-jupiter-api</artifactId>
      <scope>test</scope>
    </dependency>
    ...
  </dependencies>
  ...
</project>

A dependency without a <scope> is available to your code, your tests and the program when it runs. The JUnit dependencies have <scope>test</scope>: they are only there for the tests, and do not end up with your program.

3. Use the library

Create a class that builds the greeting. StringUtils.defaultIfBlank from Commons Lang replaces an empty name with world, and StringUtils.capitalize writes the first letter as a capital:

src/main/java/com/mycompany/app/Greeter.java

package com.mycompany.app;

import org.apache.commons.lang3.StringUtils;

public class Greeter {
    public String greet(String name) {
        return "Hello, " + StringUtils.capitalize(StringUtils.defaultIfBlank(name, "world")) + "!";
    }
}

Replace App.java so that it greets the name you give on the command line:

src/main/java/com/mycompany/app/App.java

package com.mycompany.app;

public class App {
    public static void main(String[] args) {
        String name = String.join(" ", args);
        System.out.println(new Greeter().greet(name));
    }
}

4. Build the project

In the my-app directory, run:

mvn package

Maven downloads commons-lang3-3.21.0.jar from Maven Central into your local repository, puts it on the classpath and compiles both classes:

[INFO] --- compiler:3.13.0:compile (default-compile) @ my-app ---
[INFO] Compiling 2 source files with javac [debug release 17] to target/classes
...
[INFO] BUILD SUCCESS

If the compiler reports package org.apache.commons.lang3 does not exist, the dependency is in the wrong <dependencies> element: move it out of <dependencyManagement>.

Commons Lang needs no other libraries. When a library does, its own POM says so, and Maven downloads those libraries as well; How dependencies work explains these transitive dependencies.

5. Run the program with its dependency

Run the program from the JAR as before:

java -cp target/my-app-1.0-SNAPSHOT.jar com.mycompany.app.App

Exception in thread "main" java.lang.NoClassDefFoundError: org/apache/commons/lang3/StringUtils

The JAR contains only your own classes. Maven puts the libraries on the classpath when it compiles and tests, but when you start the program yourself, you have to give them to java. Copy them into target/dependency, leaving out the libraries that are only for tests:

mvn dependency:copy-dependencies -DincludeScope=runtime

Then run the program with the JAR and the libraries on the classpath, and a name:

java -cp "target/my-app-1.0-SNAPSHOT.jar:target/dependency/*" com.mycompany.app.App ada

Hello, Ada!

Without a name, it prints Hello, World!. On Windows, separate the two classpath entries with ; instead of :.

You now have a program that uses a library from Maven Central, and Maven downloads that library for anyone who builds the project.

Continue with a plugin

Starting the program now takes a classpath with two entries. In Add and configure your first plugin you build one JAR that holds your classes and the library, and runs with java -jar.