What happens when there are mutiple main methods in java?

Question

I am currently looking over some java code not written by me. In the java project there are mutiple packages, and I see three main methods, and they are all in different classes. Two of the classes containing the main method are in the same package, and the other main is in a seperate package. The main method alone in the seperate package seems to be called when I execute the code.

The first main method is

public static void main(String[] args) {
      //code written in here seperate package
}

Second main method is

public static void main(String[] args) {
     //code written in here same package as third main
}

Third main method is

public static void main(String[] argv) {
   //code written in here same package as second main
}

All three method contain the same parameter, so it shouldn't be overloaded. They all have code that performs different functionality. How is this possible? How does java know which method to call? I am not getting any errors when executing this code through java jar command in the command line.

You can add as many main classes as you want in your project, but when executing your code you have to choose one of them. If you are running a jar then the jar must specify the Main-Class in a manifest file. — AwaX, Apr 14 '15 at 15:01

score 2 · Accepted Answer · edited May 23 '17 at 12:14

2

Edited

In order to call main in Java, you have to give the class name also. So this way there is no ambiguity. In your case, unzip your jar file with any unzip program, and look for a file called MANIFEST.MF. This file will contain the name of the main class.
Please do not attempt to change the MANIFEST.MF file. Instead, if you need to call another main method, use an alternate command-line, specifting the class that contains the main method you want to use something similar to: java -classpath somejar.jar com.example.Foo
See Can I override and overload static methods in Java? for the answer on overriding and overloading static methods.

Sualeh.

edited May 23 '17 at 12:14

Community

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answered Apr 14 '15 at 14:58

Sualeh Fatehi

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static methods can be overloaded! static methods cannot be overriten! – elias Apr 14 '15 at 15:06
Great! So I open the MANIFEST.MF and it says Main-Class: "class name". If I changed that class name it will then call the main method from another class when I execute the same jar command? – simhuang Apr 14 '15 at 15:25

score 1 · Answer 2 · answered Apr 14 '15 at 14:57

Java doesn't know which method to call.

That is something that the user specifies when starting the JVM. At that point, you select a "target class"; and the JVM will search for a main method in that class. In other words: when you invoke "java" on the command line, you tell which main() method should be executed - by selecting a class name!

It could be that your application is deployed as a JAR archive; and the manifest file within that JAR file can point out that "target" class, too.

score 0 · Answer 3 · answered Apr 14 '15 at 14:58

0

Calling a static method requires a class name during the invocation. Calling the main method is no different. Therefore, it doesn't make a difference that the method in question is the "main" method. There are no restrictions in Java about static methods being named uniquely (or overloaded) if they are in different classes.

answered Apr 14 '15 at 14:58

KyleM

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Nobody called me out on this, but I should add that you can also import static a method so you can callStaticMethod() without specifying the class name. – KyleM Nov 21 '16 at 21:16

score 0 · Answer 4 · answered Apr 14 '15 at 15:04

you can define main class of jar at jar creation time and there you have to set only one class name so no scope of ambiguity and second way is run jar file with command

java -jar jarfilename.jar foo('foo' is class name that has main method)

so there is no scope of ambiguity

score 0 · Answer 5 · answered Apr 14 '15 at 18:05

You have many classes with main method. So, you can run these many classes. To run particular class you have to mention it implicitly:

java -classpath . Foo

where Foo is class you want to use to run main method. When you run java without class name then default class is used to run main method. Default main class is usually defined in the manifest.

What happens when there are mutiple main methods in java?

5 Answers5