Multiple Runnable Classes Inside Jar, How to Run Them

Multiple runnable classes inside JAR, how to run them?

The executable Jar file format only allows you to specify one main class. In order for you to be able to execute different applications, you'll need to either create a "manager" as you suggest, or to use the classpath instead:

java -cp myjar.jar MyClass

However, this approach will ignore the classpath you have configured in the Jar's manifest file.

Use a single external jar in multiple runnable jars

Technically it is possible, but you will need to update source code of all projects which use this single jar file.

Consider JCL Framework

Below, there's sample code from this comment

JarClassLoader jcl = new JarClassLoader();  
jcl.add("myjar.jar"); //Load jar file  
jcl.add(new URL("http://myserver.com/myjar.jar")); //Load jar from a URL  
jcl.add(new FileInputStream("myotherjar.jar")); //Load jar file from stream  
jcl.add("myclassfolder/"); //Load class folder  
jcl.add("myjarlib/"); //Recursively load all jar files in the folder/sub-folder(s)  

JclObjectFactory factory = JclObjectFactory.getInstance();  

//Create object of loaded class  
Object obj = factory.create(jcl,"mypackage.MyClass");

How to make a .jar file with multiple classes?

You can create an executable jar with multiple classes using Maven for packaging your classes. Then set the main class that serves as the application entry point using the MAVEN SHADE PLUGIN.

Read more about maven shade plugin here: [https://maven.apache.org/plugins/maven-shade-plugin/examples/executable-jar.html][1]

Creating a single jar file for multiple jars each with a separate main class

You can only have oneach Main. So I would add an interface, IModule, say. Then each class implements them and you discover them through reflection.

How do I start multiple main programs in a Java executable .jar?

The jar manifest allows you to optionally specify no more than one main class. This is invoked when you execute java with the -jar flag.

java -jar myapp.jar

You may include multiple main classes in a single jar, but each (except the optional 1 above) must be invoked using the -classpath flag and with the fully qualified name of the main class specified.

java -classpath myapp.jar com.mypackage.app.Main01 && \
  java -classpath myapp.jar com.mypackage.app.Main02 && \
  java -classpath myapp.jar com.mypackage.app.Main03

The example above will spawn three separate java VMs, each in their own process. Obviously, this does not meet your requirement for an 'executable jar'.

Alternatively, you may wish to have one main method that starts separate threads, so that there is only one process, but concurrent execution.

Ant is not a suitable choice to help you solve this issue. I suspect you probably want a single main method that spawns multiple threads. Feel free to provide more information on your requirements.

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

You can create your jar without Main-Class in its Manifest file. Then :

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

jar-with-dependencies + Multiple mainClass classes?

Actually, it's perfectly fine for me to use a classpath where the fat jar contains everything:

java -cp myapp-with-dependencies.jar com.myapp.MyApp1
java -cp myapp-with-dependencies.jar com.myapp.MyApp2

---

Related Topics