executing class without main()

[Poonam Kadu]

Hi everybody
Is " public static void main(String args[])" always necessary to run a java class?
Is there any way in which I can run a class without the main method?

Cheers,
Poonam K.

[Biswajit Paria]

Is there any way in which I can run a class without the main method?

NO. JVM requires main() method to run a class. In other way you can do ...like,if you have a class called Test class having main method, then you can create a thread object of the class that you want to run not having main() method...then call start() and it's run method will be executed..(provided your class should implement Runnable interface).

[Campbell Ritchie]

Terminology: You cannot run a class at all, but you can call its methods.

As Bisjawit Paria says, you must always use a main method to start a Java application, except possibly when using BlueJ.

[Jesper de Jong]

There are tricks to run a program without a main(...) method, but in any normal program you should always have a main(...) method. Here is an example - do not use something like this in any serious program:

public class WithoutMain
{
    static {
        System.out.println("Hello World!");
        System.exit(0);
    }
}

[Jeremy Botha]

Technically, when you run applications inside an application server, your classes are running without a main method.

Of course, the application server itself probably has one.

J

[Ulf Dittmer]

Technically, when you run applications inside an application server, your classes are running without a main method.

Yeah, but that's stretching the meaning of "running" an application a bit, i think. In that sense, an "application" could take many forms (applets, EJB, servlets, RMI, web service, ...), none of which have a main method.

[Peter Chase]

I'm rather surprised no-one has mentioned this yet...

To run a Java program, you need a native program that will instantiate a JVM and tell it to run method(s) in Java class(es). One might call that program a Java launcher. By far the most common one is the "java[.exe]" program that comes with a JRE or JDK.

The main() method is only special when the Java program is run via "java[.exe]" launcher. Another launcher might have entirely different behaviour.

You can obtain, or write for yourself, another Java launcher. You do this via a part of the Java Native Interface (JNI) called the Java Invocation Interface (JII?). A user-written Java launcher would typically be a C or C++ program (other languages are probably possible), which calls native functions in the JII to instantiate a JVM, find some Java classes and execute Java methods.

As an example, I wrote a Java launcher that does some licensing checks before decrypting some Java code. The encrypted Java code won't run with "java[.exe]" command. For obvious reasons, can't tell you more!

See JII documentation.
[ July 03, 2007: Message edited by: Peter Chase ]