Develop with Java on Ubuntu¶

This tutorial provides basic guidance on how to use of the Java toolchain for development on Ubuntu. It shows how to create a ‘Hello, world!’ program and explains how to build projects using Gradle or Maven.

For instructions on how to install Java and related tooling, including IDEs, see the dedicated guide on How to set up a development environment for Java on Ubuntu. This article assumes that tooling suggested in that article has been installed.

`javac` vs build systems¶

javac is the actual compiler, but developers usually use build systems to compile, build, and package Java projects. Gradle and Maven are popular tools for building Java projects.

To use javac directly, refer to the example Compiling Java application using javac directly.

Creating a Java project using Maven¶

Setting up and building a new Java project using the Apache Maven tool.

Prerequisites

Java Development Kit; refer to Installing Java Development Kit.
Apache Maven:

To install Maven and the default Java Development Kit from the Ubuntu archive, use:
```
sudo apt install maven
```
Alternatively, download Maven from Apache Maven Project and follow the installation instructions: Installing Apache Maven.

Maven project

Create a new Java project using the archetype:generate Maven sub-command:

mvn archetype:generate -DgroupId=com.yourcompany \
    -DartifactId=helloworld -Dversion=1.0-SNAPSHOT \
    -Dpackage=com.yourcompany.helloworld \
    -DarchetypeGroupId=org.apache.maven.archetypes \
    -DarchetypeArtifactId=maven-archetype-quickstart \
    -DarchetypeVersion=1.4

Press Enter when prompted to confirm your selection.

This creates a new project using Maven Quickstart Archetype.

Maven sets up a basic project structure:

dev@ubuntu:~$

tree

.
└── helloworld
    ├── pom.xml
    └── src
    ├── main
    │   └── java
    │       └── com
    │           └── yourcompany
    │               └── helloworld
    │                   └── App.java
    └── test
        └── java
            └── com
                └── yourcompany
                    └── helloworld
                        └── AppTest.java

That includes a ‘Hello World’ application and a unit test:

src/main/java/com/yourcompany/helloworld/App.java¶

package com.yourcompany.helloworld;

/**
* Hello world!
*
*/
public class App
{
    public static void main( String[] args )
    {
        System.out.println( "Hello World!" );
    }
}

src/test/java/com/yourcompany/helloworld/AppTest.java¶

package com.yourcompany.helloworld;

import static org.junit.Assert.assertTrue;

import org.junit.Test;

/**
* Unit test for simple App.
*/
public class AppTest
{
    /**
    * Rigorous Test :-)
    */
    @Test
    public void shouldAnswerWithTrue()
    {
        assertTrue( true );
    }
}

Change to the project directory:
```
cd helloworld
```
Build and package the application:
```
mvn -Dmaven.compiler.release=8 package
```
Note

Notice the -Dmaven.compiler.release=8 option. The maven-archetype-quickstart archetype generates a project that targets Java 7, which is no longer supported by the Java 21 LTS release. The project target can be changed by updating the maven.compiler.target and maven.compiler.source properties in the pom.xml file.

This builds and runs unit tests.

Run the application:
dev@ubuntu:~$
java -cp target/helloworld-1.0-SNAPSHOT.jar com.yourcompany.helloworld.App
```
Hello World!
```

Creating a Java project using Gradle¶

Setting up and building a new Java project using the Gradle build tool.

Prerequisites

Java Development Kit; refer to Installing Java Development Kit.
Gradle:

Install the Gradle snap:
```
sudo snap install gradle --classic
```
Alternatively, download Gradle from the Gradle Releases page and follow the provided instructions: Installing manually.

Gradle project

Create a Java project using the gradle init command:

mkdir helloworld
cd helloworld
gradle init \
    --type java-application \
    --dsl kotlin \
    --test-framework junit-jupiter \
    --package com.yourcompany.helloworld \
    --project-name helloworld  \
    --no-split-project \
    --no-incubating

Press Enter when prompted for the Java version.

Gradle sets up a basic project structure:

dev@ubuntu:~$

tree

.
└── helloworld-gradle
    ├── app
    │   ├── build.gradle.kts
    │   └── src
    │       ├── main
    │       │   ├── java
    │       │   │   └── com
    │       │   │       └── yourcompany
    │       │   │           └── helloworld
    │       │   │               └── App.java
    │       │   └── resources
    │       └── test
    │           ├── java
    │           │   └── com
    │           │       └── yourcompany
    │           │           └── helloworld
    │           │               └── AppTest.java
    │           └── resources
    ├── gradle
    │   ├── libs.versions.toml
    │   └── wrapper
    │       ├── gradle-wrapper.jar
    │       └── gradle-wrapper.properties
    ├── gradlew
    ├── gradlew.bat
    └── settings.gradle.kts

That includes a ‘Hello World’ application and a unit test:

app/src/main/java/com/yourcompany/helloworld/App.java¶

/*
* This Java source file was generated by the Gradle 'init' task.
*/
package com.yourcompany.helloworld;

public class App {
    public String getGreeting() {
        return "Hello World!";
    }

    public static void main(String[] args) {
        System.out.println(new App().getGreeting());
    }
}

app/src/test/java/com/yourcompany/helloworld/AppTest.java¶

/*
* This Java source file was generated by the Gradle 'init' task.
*/
package com.yourcompany.helloworld;

import org.junit.jupiter.api.Test;
import static org.junit.jupiter.api.Assertions.*;

class AppTest {
    @Test void appHasAGreeting() {
        App classUnderTest = new App();
        assertNotNull(classUnderTest.getGreeting(), "app should have a greeting");
    }
}

Gradle generates gradlew - a script that manages Gradle download and installation for your project. It requires java binary to be present in the path, or JAVA_HOME environment variable set. For example:

export JAVA_HOME=/usr/lib/jvm/java-21-openjdk-amd64

Build and run the project using the generated Gradle Wrapper:

dev@ubuntu:~$

gradle run

gradle run
Calculating task graph as no cached configuration is available for tasks: run

> Task :app:run
Hello World!

BUILD SUCCESSFUL in 424ms
2 actionable tasks: 1 executed, 1 up-to-date
Configuration cache entry stored.

Compiling Java application using `javac` directly¶

Compiling a Java application directly using the javac tool.

Prerequisites

Java Development Kit; refer to Installing Java Development Kit.

Procedure

Create a ‘Hello World’ application in a file named App.java:

public class App {
    public String getGreeting() {
        return "Hello World!";
    }

    public static void main(String[] args) {
        System.out.println(new App().getGreeting());
    }
}

Compile the class file in the out directory:
```
javac App.java -d out
```
Execute the program:
dev@ubuntu:~$
java -cp out/ App
```
Hello World!
```

Running Java application as a shebang script¶

Running a Java application as a script with the java interpreter specified using the ‘shebang’ (#!) interpreter directive.

Prerequisites

Java Development Kit; refer to Installing Java Development Kit.

Procedure

Create a ‘Hello World’ application in a file named App and include the interpreter directive on the first line:

App¶

#!/usr/bin/java
public class App {
    public String getGreeting() {
        return "Hello World!";
    }

    public static void main(String[] args) {
        System.out.println(new App().getGreeting());
    }
}

Note

This file does not have a .java extension.

Make the file executable:
```
chmod +x App
```
Run the application:
dev@ubuntu:~$
./App
```
Hello World!
```

Develop with Java on Ubuntu¶

javac vs build systems¶

Creating a Java project using Maven¶

Creating a Java project using Gradle¶

Compiling Java application using javac directly¶

Running Java application as a shebang script¶

`javac` vs build systems¶

Compiling Java application using `javac` directly¶