You can start a Java program by specifying the class name and the class path to use (containing .class and/or .jar files), or you can specify just a self contained .jar file, which contains all it needs. I wont describe Java Modules ( Java Platform Module System, .jmod -> stronger encapsulation, improved dependency management, better code organization, enhanced security, streamlined JRE, and potentially faster application startup times), or a Java Run Time image (.jrt, which includes only the necessary modules required for a specific application, making it smaller, safer, and capable of starting faster than a full JRE).
Create class files
The documentation for Java says you have to create a manifest so Java knows which classes to use.
I have a simple program hw.java
class HelloWorld {
public static void main(String[] args) {
System.out.println("Hello, World!");
}
}
I compiled it with
/usr/lpp/java/J21.0_64/bin/javac hw.java
This creates .class files for each class in the original source.
- HelloWorld.class
It will start the function “main” within the class.
Create a jar file and specify the entry point
Java needs to know the entry point class of the jar file. The code below creates the jar file, specifying the entry point class name ( -e ..) and includes all of the classes specified (just HelloWorld.class).
The next line runs the jar file.
/usr/lpp/java/J21.0_64/bin/jar -c -f hw.jar -e HelloWorld HelloWorld.class
/usr/lpp/java/J21.0_64/bin/java -jar hw.jar
Under the covers it creates a manifest file META-INF/MANIFEST.MF . It is not in the default code page, so to look at it, I expanded the jar file, used oedit META-INF/. , then used the line command ea to edit it in ASCII.
Manifest-Version: 1.0
Created-By: 21.0.4 (IBM Corporation)
Main-Class: HelloWorld
Create a jar file and specify a manifest.
Java needs to know the entry point class name of the jar file. You can specify it in the manifest. The code below creates the jar file, using the specified manifest file, and includes all of the classes specified (just HelloWorld.class). The manifest is the one in the previous section.
The next line runs the jar file
/usr/lpp/java/J21.0_64/bin/jar -c -f hw.jar-m META*/MAN*.MF HelloWorld.class
/usr/lpp/java/J8.0_64/J21.0_64/bin/java -jar hw.jar
When you create a jar file and do not specify a manifest or entry point
The code below creates the jar file, using the classes specified (just HelloWorld.class), but does not include a manifest.
If you try running the jar file it complains about no manifest.
You can run the jar file in the class path (-cp) and specify the entry class as a parameter
/usr/lpp/java/J21.0_64/bin/jar -c -f hw.jar HelloWorld.class
COLIN:/u/tmp/java: >/usr/lpp/java/J21.0_64/bin/java -jar hw.jar
no main manifest attribute, in hw.jar
/usr/lpp/java/J21.0_64/bin/java -cp hw.jar HelloWorld
It’s not quite that simple…
I was looking at some Jar files, and the manifest does not have a Main-Class entry, and there is no entry point specify on the java command – so there is clearly another way of specifying the entry point.
ColinPaice 