Have you ever wondered what those jar files are? Why they are used and how to create one? In this blog post, I’ll show you an easy way to make a jar file to be run as an application on your operating system.
Container or Jar files, both refer to the same thing, a file that usually accompanies an application for setting up the framework for the main code to operate. The ability to make jar files is a good skill to master considering that so many java applications are distributed this way. This article will focus on the essentials of how to build jar files.
How to build a jar file
Now that we have a project, let’s create a jar file. This is the file that will be executed by the java command line tool. From the command line, run:
$ mvn package
The above command will create an executable jar file in target/release/myproject.jar . You can run it like so:
$ java -jar target/release/myproject.jar
If you want to build a jar file, you will need to create a basic project with a test class in it. The project should have the following structure:
project/src/test/java/com.example.testpackage/MyTestClass.java
In MyTestClass.java add the following code:
package com.example.testpackage;
public class MyTestClass {
public static void main(String[] args) {
}
}
To build a jar file, you need to create a class that implements the JarBuilder interface. The jar-file class must contain one public constructor with no arguments. This is the constructor that will be invoked by the jar builder when creating the jar file.
The following example shows how a very simple jar file can be created:
import java.util.*;
class SimpleJarBuilder implements JarBuilder {
public SimpleJarBuilder() {}
}
SimpleJarBuilder simpleJar = new SimpleJarBuilder();
In order to build a jar file, you need to create a JAR file group. To do this, you can use the following command:
$ mkdir -p src/main/java && cd src/main/java && touch HelloWorld.java && cd ../..
Now we have created a directory structure for our project. We also created a file called HelloWorld.java in main package and moved back to current directory. This is how we will be creating all the source files for our project. All the source files must be placed in the main package of your project directory structure. The main package name must be same as your project name which is used in Maven’s pom.xml file. In our case it’s hello-world .
Next step is to add dependencies for your project using Maven’s pom file:
JAR files are used for packaging Java code and resources. They are similar to .zip files, but they have an additional layer of structure that allows them to include metadata about their contents. JAR files may contain classes, other resources such as image files or sound clips, and data files such as XML documents.
The most basic way to create a JAR file is by using the jar utility program that comes with the JDK:
$ jar cvf myJar.jar myFile1.txt myFile2.txt
This command creates a new JAR file named myJar.jar in the current directory containing two text files, myFile1.txt and myFile2.txt
