In the article showing how to run an application bundled in a jar file, I mentioned the simplest statement java -jar YourApp.jar  if your application was included a manifest file. Otherwise, the statement isn’t working. This post, therefore, presents how to add a manifest file to a jar file using either the typical way or Maven.

As far as manifest file introduced, it is normally used to define the following things:

• Define the entry point of the Application, make the Jar executable.
• Add project dependency classpath.

Presumably we have build a class named com.itersdesktop.javatechs.Demo. The following paragraphs will take this class as an illustrated example to ease the reading.

In a typical way

We want to execute the main method in the class Demo  in the package com.itersdesktop.javatechs  when we run the JAR file.

We first create a text file named Manifest.txt  with the following contents:

Main-Class: com.itersdesktop.javatechs.Demo

[tooltip text=”Warning” gravity=”nw”]Warning: The text file must end with a new line or carriage return. The last line will not be parsed properly if it does not end with a new line or carriage return.[/tooltip]

We then create a JAR file named Demo.jar  by entering the following command:

jar cfm Demo.jar Manifest.txt com/itersdektop/javatechs/*.class

This creates the JAR file with a manifest with the following contents:

Manifest-Version: 1.0
Created-By: 1.8.0_102 (Oracle Corporation)
Main-Class: com.itersdesktop.javatechs.Demo

When you run the JAR file with the following command, the main method of Demo  executes:

java -jar Demo.jar

Setting an Entry Point with the JAR Tool

The ‘e’ flag (for ‘entry point’) creates or [highlight]overrides[/highlight] the manifest’s Main-Class  attribute. It can be used while creating or updating a JAR file. Use it to specify the application entry point without editing or creating the manifest file.
For example, this command creates app.jar where the Main-Class  attribute value in the manifest is set to Demo :

jar cfe app.jar Demo Demo.class

You can directly invoke this application by running the following command:

java -jar Demo.jar

If the entry point class name is in a package it may use a ‘.’ (dot) character as the delimiter. For example, if Main.class  is in a package called foo  the entry point can be specified in the following ways:

jar cfe Main.jar foo.Main foo/Main.class

With Maven

This section will help you familiarise with how to use the maven-jar-plugin  to create a manifest file in Maven based Java projects, and package/add it into the final jar file. When you run the command mvn package  to package project into a Jar, the following meta-inf/manifest.mf  file will be generated and added into the your Jar file automatically.

Define maven-jar-plugin  in pom.xml , and configure the manifest file via configuration tag. The basic syntax looks like

<plugin>
<groupId>org.apache.maven.plugins</groupId>
<artifactId>maven-jar-plugin</artifactId>
<version>2.6</version>
<configuration>
<archive>
<manifest>
<mainClass>com.itersdektop.javatechs.Demo</mainClass>
</manifest>
</archive>
</configuration>
</plugin>

[tooltip text=”Notes” gravity=”nw”]Notes: we need to create an executable jar file, so the attribute   is the most important to have in these settings.[/tooltip]

In Intellij IDEA, once clicking package function, we should see the result in the Run windows as shown in the following image.

For your references, you could take a look at the pom.xml content presented at the end of the post.

<?xml version="1.0" encoding="UTF-8"?>
<project xmlns="http://maven.apache.org/POM/4.0.0"
         xmlns:xsi="http://www.w3.org/2001/XMLSchema-instance"
         xsi:schemaLocation="http://maven.apache.org/POM/4.0.0 http://maven.apache.org/xsd/maven-4.0.0.xsd">
    <modelVersion>4.0.0</modelVersion>

    <groupId>com.itersdesktop.javatechs</groupId>
    <artifactId>Java8Features</artifactId>
    <version>1.0-SNAPSHOT</version>
    <build>
        <plugins>
            <plugin>
                <groupId>org.apache.maven.plugins</groupId>
                <artifactId>maven-compiler-plugin</artifactId>
                <version>3.5.1</version>
                <configuration>
                    <source>1.8</source>
                    <target>1.8</target>
                </configuration>
            </plugin>
            <plugin>
                <groupId>org.apache.maven.plugins</groupId>
                <artifactId>maven-jar-plugin</artifactId>
                <version>2.6</version>
                <configuration>
                    <archive>
                        <manifest>
                            <mainClass>com.itersdesktop.javatechs.Java8NewFeatures.LamdasExpressions</mainClass>
                        </manifest>
                    </archive>
                </configuration>
            </plugin>
        </plugins>
    </build>
</project>

Hopefully you find interesting points once getting through my post. All comments are welcomed!

References

1. https://docs.oracle.com/javase/tutorial/deployment/jar/appman.html
2. https://maven.apache.org/plugins/maven-jar-plugin/
3. https://www.mkyong.com/maven/how-to-create-a-manifest-file-with-maven/
4. https://stackoverflow.com/questions/12767886/use-of-the-manifest-mf-file-in-java