Issue while compiling a hello world class using Javassist

Question

I am using Javassist to write a HelloWorld class with main method. When I compile , I get an error as below. I am not sure what's wrong with String[] args in the main method ?

javassist.CannotCompileException: [source error] syntax error near "ng[] args)"
at javassist.CtNewMethod.make(CtNewMethod.java:78)
at javassist.CtNewMethod.make(CtNewMethod.java:44) 

This is my code

 public void createClass() {
    ClassPool cp = ClassPool.getDefault();
    CtClass ct = cp.makeClass("HelloClass");
    try {
        CtMethod m = CtNewMethod.make("public void sayHello() { System.out.println(\"Hello     World\");}",ct);
        ct.addMethod(m);

       String str="public static void main(String[] args)";
        CtMethod n = CtNewMethod.make(str,ct);
        n.setBody("HelloClass a = new HelloClass();a.sayHello();");
        ct.addMethod(n);
        ct.writeFile();

    } catch (CannotCompileException e) {
        // TODO Auto-generated catch block
        e.printStackTrace();
    } catch (NotFoundException e) {
        // TODO Auto-generated catch block
        e.printStackTrace();
    } catch (IOException e) {
        // TODO Auto-generated catch block
        e.printStackTrace();
    }


public static void main(String[] args) {
     JavaAssistExample inject = new JavaAssistExample();
    inject.createClass();

}

Answer 1

As the javadoc for CtNewMethod states

The source code must include not only the method body but the whole declaration

It must therefore contain the {}, like

String str = "public static void main(String[] args){}";

However, two more things will give you problems.

First, you don't have a default (or a no arg) constructor. Add one

ct.addConstructor(CtNewConstructor.defaultConstructor(ct));

Second, the CtMethod#setBody(..) method completely replaces the method body. So you can't do what you are doing. If you want all of those calls, you'll need to put them together

n.setBody("{HelloClass a = new HelloClass();a.sayHello();}");