Run class in Jar file - How to run a jar file from command line

Run class in Jar file

In this post, We will learn how to run a Jar file from command prompt with main method and without main method. This is very useful in unix environment to run the jobs in nohup mode. But, now we will be focusing running jar file in windows operating system.

How to run a jar file from command line

If you have a jar file called myJavaJar.jar located in /myJavafolder and you want to use the class called myJavaClass from it, How do you run this java program from command line?

I thought it would be to go into the directory and say java -cp myJavaJar.jar myJavaClass but that isn't working. 

Solution:

The solution for this issue is described below.

Use java -cp myJavaJar.jar com.mypackage.myJavaClass.

If the class is not in a package then simply java -cp myJavaJar.jar myJavaClass.

If you are not within the directory where myJavaJar.jar is located, then you can do:

On Unix or Linux platforms:

java -cp /location_of_jar/myJavaJar.jar com.mypackage.myJavaClass

On Windows:

java -cp c:\location_of_jar\myJavaJar.jar com.mypackage.myJavaClass

Here, flag cp is to set the jar in the java class-path.

How to run a class from Jar which is not the Main-Class in manifest file:

I have a JAR with 4 classes, each one has Main method. I want to be able to run each one of those as per the need. I am trying to run it from command-line on Linux box.

E.g. The name of my JAR is myJavaJar.jar
It has directory structure for the main classes as follows:

com/mycomp/myproj/dir1/MainClass1.class
com/mycomp/myproj/dir2/MainClass2.class
com/mycomp/myproj/dir3/MainClass3.class
com/mycomp/myproj/dir4/MainClass4.class

I know that I can specify one class as main in my Manifest file. But is there any way by which I can specify some argument on command line to run whichever class I wish to run?

I tried this:

jar cfe myJavaJar.jar com.mycomp.myproj.dir2.MainClass2 com/mycomp/myproj/dir2/MainClass2.class /home/myhome/datasource.properties /home/myhome/input.txt

And I got this error:

com/mycomp/myproj/dir2/MainClass2.class : no such file or directory

(In the above command, '/home/myhome/datasource.properties' and '/home/myhome/input.txt' are the command line arguments).

Solution:

The solution for this issue is described below.

You can create your jar without Main-Class in its Manifest file. Then : Just need to pass the complete fully classified name of the class.

java -cp myJavaJar.jar com.mycomp.myproj.dir2.MainClass2 /home/myhome/datasource.properties /home/myhome/input.txt

MANIFEST.MF file:

Each JAR file contains a manifest file that describe the features of the archive. Each JAR file have a manifest file by default. Default manifest file is named as MANIFEST.MF and is present in the META-INF sub-directory of archive. Although the default manifest file contains just two entries, but complex manifest files can have way more. Here, is what a default manifest file looks like 

Default MANIFEST.MF:

Manifest-Version: 1.0
Created-By: 10 (Oracle Corporation)

Below is from official java version jrt-fs.jar from java 11.

Manifest-Version: 1.0
Specification-Title: Java Platform API Specification
Specification-Version: 11
Specification-Vendor: Oracle Corporation
Implementation-Title: Java Runtime Environment
Implementation-Version: 11.0.2
Implementation-Vendor: Oracle Corporation
Created-By: 10 (Oracle Corporation)

We can add our java file to manifest file so that we no need to pass the class name to java command while running it from command line.

Example with custom class in Manifest.mf file :

Manifest-Version: 1.0
Created-By: 10 (Oracle Corporation)
Main-Class: com/mycomp/myproj/dir1/MainClass1

Read more on Working with JAR and Manifest files In Java