From shell, is there a Java tool to check which compiled .class file(s) have a main method?

In the C++ days, distributing the headers files to use a shared object file was a big deal. People would get one or the other without both, and there was always the chance you'd get mis-matched versions.

Java fixed that with javap which prints the methods (and other major interfaces) of a compiled .class file.

To test if a class file has a main, run

javap SomeFile.class

which will list all public interfaces. Within that list, see if it has the "main entry point"

public static void main(java.lang.String[])

Now to handle this in mass, simply create a Python script that:

1. Locates all the relevant classes.
2. Runs javap on the class.
3. Reads the output for a method that matches (at the beginning, as there can be a variable number of Exceptions at the end "public static void main(java.lang.String[])

And you'll find all entry points.

Keep in mind that sometimes a single library or JAR file has many entry points, some of which are not intended as the primary entry point.

Well simply calling java -cp . <file> will either completely blow out if the class doesn't have a main method or will run the relevant code. Now, if the code fails to run right and errors out you may see it as the same effect as not having a main method.

public class HasMain {
    public static void main(String[] args) {
        System.out.println("Hit main");
    }
}
public class HasDoIt {
    public static void doIt(String[] args) {
        System.out.println("Hit doIt");
    }
}
public class WillBlowUp {
    public static void main(String[] args) {
        System.out.println("Hit blowUp");
        throw new IllegalStateException("oops");
    }
}

Using PowerShell:

PS D:\Development\sandbox> javac HasMain.java
PS D:\Development\sandbox> javac HasDoIt.java
PS D:\Development\sandbox> javac WillBlowUp.java

PS D:\Development\sandbox> java -cp . HasMain                                                                                                                                                                                                                                  
Hit main

PS D:\Development\sandbox> $?                                                                                                                                                                                                                                                  
True

PS D:\Development\sandbox> java -cp . HasDoIt                                                                                                                                                                                                                                  
Error: Main method not found in class HasDoIt, please define the main method as:
   public static void main(String[] args)
or a JavaFX application class must extend javafx.application.Application

PS D:\Development\sandbox> $?                                                                                                                                                                                                                                                  
False

PS D:\Development\sandbox> java -cp . WillBlowUp                                                                                                                                                                                                                               
Hit blowUp
Exception in thread "main" java.lang.IllegalStateException: oops
    at WillBlowUp.main(WillBlowUp.java:4)

PS D:\Development\sandbox> $?                                                                                                                                                                                                                                                  
False

So simply checking return values could be a quick way to test if the class has what you want, albeit any exit(1) type return will throw a false-false