notes-apache-maven

My notes on Apache Maven

Why Maven?

Let's make a simple Hello World program in Java.

Step 1: Create the HelloWorld.java file.

% vi HelloWorld.java HelloWorld.java

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

Step 2: Compile the HelloWorld.java file, essentially compiling the source file (files with file name extension of .java) to a class file (file name extension of .class).

.java → .class

Source Code → Bytecode

% javac HelloWorld.java

% ls
HelloWorld.class	HelloWorld.java

Step 3: Use Java JVM to execute the Bytecode.

% java HelloWorld
Hello World!

Step 4: Create JAR file.

% jar cf myjar.jar HelloWorld.class

% ls
HelloWorld.class	HelloWorld.java		myjar.jar

Step 5: Add myjar.jar to CLASSPATH and execute the HelloWorld.class from within it.

% java -classpath myjar.jar HelloWorld
Hello World!

Step 5: Make a /tempdirectory and copy myjar.jar into it and then change the present working directory to /temp.

% mkdir temp
% cp myjar.jar ./temp
% cd temp
% ls
myjar.jar

Step 6: Since JAR file is nothing but a zipped file. Let's unzip it.

% unzip myjar.jar
Archive:  myjar.jar
   creating: META-INF/
  inflating: META-INF/MANIFEST.MF
  inflating: HelloWorld.class

% ls
HelloWorld.class	META-INF		myjar.jar

Step 7: Let's checkout the META-INF/MANIFEST.MF file.

% vi META-INF/MANIFEST.MF
Manifest-Version: 1.0
Created-By: 17.0.7 (Eclipse Adoptium)

When we are building Java with Maven; Maven would be doing all of this for us. Building .class files from .java files, making the JAR files, and asking us what we want in the MANIFEST.MF file.

Using 3rd party JARs with Java in the CLI

We are going to use Apache Commons Lang (link) as the 3rd party JAR.

Step 1: Download the Apache Commons Lang JAR (from here). I am using Apache Commons Lang 3.12.0 (Java 8+) with Java 17. You can use any depending on the supported Java verison.

Step 2: Make your directory look like below:

% ls
HelloWorld.java	lib

% ls lib
commons-lang3-3.12.0.jar

Step 3: HelloWorld.java should look like below:

import org.apache.commons.lang3.StringUtils;

public class HelloWorld {
        public static void main(String[] args) {
                System.out.println("Hello World!");
                System.out.println(StringUtils.capitalize("hello apache!"));
        }
}

Step 4: Compile with the 3rd party library. Refer 1

% javac -classpath ./lib/* HelloWorld.java

OR

% javac -cp lib/* HelloWorld.java

Step 5: See what the previous step created.

% ls
HelloWorld.class	HelloWorld.java		lib

Step 6: Run

% java -cp ":./lib/*" HelloWorld
Hello World!
Hello apache!

This is how we have to manually manage the dependencies. If we use Maven, these dependencies will be managed for us. Depency management would be done by Maven.

References:

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[1] javac - http://web.mit.edu/java_v1.1.6/www/tools/javac.html