Javier Jimenez

Posted on Dec 19

Java on Ubuntu: Installation, Setup, and First Steps

#linux #ubuntu #java #tutorial

I recommend watching first – installing Homebrew and asdf on Ubuntu (it’s short, just 5 commands)

📘 Official Documentation

Java – Official Documentation
Java – On DevDocs.io

⭐ Popular Frameworks (ordered from lower to higher learning curve)

Spark Java — Minimalist, lightweight.
Javalin — Simple, modern.
Spring Boot — Complete, enterprise.

🛠️ Installing Java on Ubuntu

sudo apt update
sudo apt install openjdk-21-jdk

Verify installation:

java --version
javac --version

🍺 Installation with Homebrew

brew install openjdk

Add Java to PATH (if brew indicates it):

echo 'export PATH="/home/linuxbrew/.linuxbrew/opt/openjdk/bin:$PATH"' >> ~/.bashrc
source ~/.bashrc

📦 Standard Dependency Manager (Maven / Gradle)

Check Maven version:

mvn --version

Check Gradle version:

gradle --version

🔧 Installation with ASDF

System dependencies

sudo apt update
sudo apt install -y curl git unzip zip

Install plugin + version

asdf plugin add java
asdf list-all java

# install different versions
asdf install java openjdk-21.0.2
asdf install java temurin-17.0.10

# Set a global version
asdf global java openjdk-21.0.2

# Set a local version
asdf local java temurin-17.0.10

Example: .tool-versions

java openjdk-21.0.2

📝▶️ Create and run a Java file

Create file:

touch Hola.java

Content of Hola.java

public class Hola {
    public static void main(String[] args) {
        System.out.println("Hello World from Java!");
    }
}

💻 Compile and run:

javac Hola.java
java Hola

🟦 Basic Java example

🗂️🌐 Static file Web Server

**Note:* Java includes a basic HTTP server (HttpServer). You don’t need frameworks.*

What it does:

Defines the query parameter name
Gets the value from the URL

stripHtmlTags(text): removes HTML tags

Renders the received variable inside an <h1>

📝 Create file: touch WebServer.java

📦 Content of WebServer.java

import com.sun.net.httpserver.HttpServer;
import com.sun.net.httpserver.HttpExchange;

import java.io.IOException;
import java.io.OutputStream;
import java.net.InetSocketAddress;
import java.net.URLDecoder;
import java.nio.charset.StandardCharsets;

public class WebServer {

    public static void main(String[] args) throws Exception {

        HttpServer server = HttpServer.create(new InetSocketAddress(7000), 0);

        server.createContext("/", (HttpExchange exchange) -> {

            String query = exchange.getRequestURI().getQuery();
            String username = "guest";

            if (query != null && query.startsWith("username=")) {
                username = URLDecoder.decode(
                        query.replace("username=", ""),
                        StandardCharsets.UTF_8
                );
                username = stripHtmlTags(username);
            }

            String response = """
                <!DOCTYPE html>
                <html lang="en">
                <head>
                    <meta charset="UTF-8">
                    <title>Hello</title>
                </head>
                <body style="text-align:center">
                    <h1>Hello, %s</h1>
                </body>
                </html>
                """.formatted(username);

            exchange.getResponseHeaders().add("Content-Type", "text/html");
            exchange.sendResponseHeaders(200, response.getBytes().length);

            OutputStream os = exchange.getResponseBody();
            os.write(response.getBytes());
            os.close();
        });

        server.start();
        System.out.println("Server running at http://localhost:7000");
        System.out.println("Try at http://localhost:7000?username=Homer");
    }

    private static String stripHtmlTags(String input) {
        return input.replaceAll("<[^>]*>", "");
    }
}

▶️ Compile and run

javac WebServer.java
java WebServer

👉 visit:
http://localhost:7000/?username=Homer

⚙️🧩 JSON REST API

What it does:

Reads data from a data.json file
Exposes 2 endpoints:

Character list at /characters
Character by id at /characters/:id

Example file: data.json

[
  {
    "id": 1,
    "age": 39,
    "name": "Homer Tompson",
    "portrait_path": "https://cdn.thesimpsonsapi.com/500/character/1.webp"
  },
  {
    "id": 2,
    "age": 39,
    "name": "Marge Simpson",
    "portrait_path": "https://cdn.thesimpsonsapi.com/500/character/2.webp"
  }
]

📝 Create file: touch ApiServer.java

▶️ File content: ApiServer.java

import com.sun.net.httpserver.HttpServer;
import com.sun.net.httpserver.HttpExchange;

import java.io.IOException;
import java.io.OutputStream;
import java.net.InetSocketAddress;
import java.nio.file.Files;
import java.nio.file.Path;

public class ApiServer {

    private static String jsonData;

    public static void main(String[] args) throws Exception {

        jsonData = Files.readString(Path.of("data.json"));

        HttpServer server = HttpServer.create(new InetSocketAddress(7001), 0);

        server.createContext("/characters", (HttpExchange exchange) -> {

            String path = exchange.getRequestURI().getPath();
            exchange.getResponseHeaders().add("Content-Type", "application/json");

            // GET /characters
            if (path.equals("/characters")) {
                send(exchange, 200, jsonData);
                return;
            }

            // GET /characters/:id
            if (path.startsWith("/characters/")) {
                String id = path.replace("/characters/", "");

                if (jsonData.contains("\"id\": " + id)) {
                    send(exchange, 200, jsonData);
                } else {
                    send(exchange, 404, "{\"error\":\"Character not found\"}");
                }
                return;
            }

            send(exchange, 404, "{\"error\":\"Route not found\"}");
        });

        server.start();
        System.out.println("Server running at http://localhost:7001/characters");
    }

    private static void send(HttpExchange exchange, int status, String body) throws IOException {
        exchange.sendResponseHeaders(status, body.getBytes().length);
        OutputStream os = exchange.getResponseBody();
        os.write(body.getBytes());
        os.close();
    }
}