Java Community | Help. Code. Learn.•2y ago

Week 30 — What is required to make a JAR file executable? (Starts on double click)

Question of the Week #30

What is required to make a JAR file executable? (Starts on double click)

7 Replies

Eric McIntyre•2y ago

a main method, and a manifest packaged into the jar which has a property for the Main class

Eric McIntyre•2y ago

usually ides or build tools can generate those manifests for you

Submission from liqui.dev#0000

Eric McIntyre•2y ago

create the jar file. Each jar file have a manifest file, where attributes related to the executable file are. This is the content of my manifest file. $cat manifest.mf Main-class: start.HelloWorldSwing Just describe what the main class is ( the one with the public static void main method ) Once the manifest is ready, the jar executable is invoked. It has many options, here I'm using -c -m -f ( -c to create jar, -m to specify the manifest file , -f = the file should be named) and the folder I want to jar. $jar -cmf manifest.mf hello.jar start This creates the .jar file on the system You can later just double click on that file and it will run as expected.

Submission from _sky#4725

Eric McIntyre•2y ago

JAR files are basically zip files containing class files (Java bytecode). In order for a class to be usable, it needs to be located in the declared package in the JAR file whereas the . character in the package name represents a subdirectory in the JAR file. For example, the class MyClass in the package com.mypackage would need to be located at com/mypackage/MyClass.class in the zip file. A class with a valid main method can be executed using the java command. For example, assume a JAR called myjar.jar with the main class MyClass as outlined before. Then, this main class can be run using the command java --class-path myjar.jar com.mypackage.MyClass. In order to not specify the fully qualified name of the class each time when running it, one needs to create a file called MANIFEST.MF in the META-INF directory of the JAR file. This file would then need to contain a Main-Class entry referencing the class (with a main method) to execute, for example Main-Class: com.mypackage.MyClass. If that entry is present in the JAR, it can be run using the command java -jar myjar.jar. In this case, the JAR can also be run by double-clicking it if Java is correctly registered for opening JAR files.

⭐ Submission from dan1st#0000

Eric McIntyre•2y ago

There are about 2 things you need to make a JAR file executable: 1. A class containing a main() method. Example:

public class Main {

    // The main method must be set exactly like this
    public static void main(String[] args)
    {
        // Making sure it is actually running.
        System.out.println("Hello, World!");
    }
}

public class Main {

    // The main method must be set exactly like this
    public static void main(String[] args)
    {
        // Making sure it is actually running.
        System.out.println("Hello, World!");
    }
}

2. A MANIFEST.MF file inside the JAR. The Manifest file is a text file that contains information about the JAR, like who it was built by, what it was built with, what manifest version, etc, But most importantly, you can add the "Main-Class" field which tells the JRE what class the main() method is in. Example of what it looks like inside a manifest file:

Manifest-Version: 1.0
Created-By: Geuxy
Main-Class: me.geuxy.Main

Manifest-Version: 1.0
Created-By: Geuxy
Main-Class: me.geuxy.Main

⭐ Submission from geuxy#0000

Eric McIntyre•2y ago

For a JAR file to be executable, it must contain at least one class with an executable main method. This class should also be listed as the main class in the manifest file.

Submission from stormofgalaxy#0000

Eric McIntyre•2y ago

When java wants to launch a jar, it looks for a META-INF/MANIFEST.MF file in the jar. If it is found, it is read, and the Main-Class attribute in it is stored. The attribute specifies which class should be considered the "main" class of the jar, containing the static void main(String[]) method. The class is loaded from the jar, and the main method is executed. If the class doesn't exist, or is malformed, or the version is not supported, an error is thrown. example/Hello.class: Compiled from

package example;

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

package example;

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

META-INF/MANIFEST.MF:

Main-Class: example.Hello

Main-Class: example.Hello

Packaging those 2 files in a .zip, then renaming it to .jar will create an executable jar.

⭐ Submission from 0x150#0000

Gaming

Programming

Week 30 — What is required to make a JAR file executable? (Starts on double click)