Why java (>=7 version) does not support to run program without main method? [closed]

Question

class WithoutMain {   
       static
       {           
            System.out.println("Without main class!!!");
            System.exit(0);            
       }
}

when I am trying to to run above code in java version greater than 7 i am getting below error. The program compiled successfully, but main class was not found. Main class should contain method: public static void main (String[] args).

can someone please guide me why Java does not support to run program without main after java7

Answer 1

AFAIK this change was specific to Java 7. In Java 8 you can do this. You can't do this in Java 7 as it looks for the method without loading the class first which fails. In any case, it has been changed back in Java 8.

public class Main {
    static {
        System.out.println("Without main class!!! with " + System.getProperty("java.version"));
        System.exit(0);
    }
}

prints

Without main class!!! with 1.8.0_66

Note: this will kill the whole program. If you want the program to keep running without a main you can do this

public class Main {
    static {
        // do something which starts threads
        System.out.println("Without main class!!! with " + System.getProperty("java.version"));
        if (true)
            throw new ThreadDeath();
    }
}

This will prevent the error message, but leave background threads running provided there is a non-daemon thread.

Answer 2

the static section

static
       {           
            System.out.println("Without main class!!!");
            System.exit(0);            
       }

will be executed every time the JVM load the class in memory, but if you want to start a java application, you will need a main method, because that it the start point of every java application, if you dont define it, then the JVM will have no idea where to start.

---

you can expand your code and do something like:

class WithoutMain {   
       static
       {           
            System.out.println("Static section!!");

       }

      public static void main(String[] args){
            System.out.println("Main class!!!");
      }   
}

and the output will be the static section first and then the code you define in the main method.