Require a default constructor in java?

Question

Is there any way to require that a class have a default (no parameter) constructor, aside from using a reflection check like the following? (the following would work, but it's hacky and reflection is slow)

 boolean valid = false;
 for(Constructor<?> c : TParse.class.getConstructors())
 {
   if(c.getParameterTypes().length == 0) {
      valid = true;
      break; 
   }
 }
 if(!valid)
    throw new MissingDefaultConstructorException(...);

Answer 1

You can build an Annotation processor for that. Annotation Processors are compiler plugins that get run at compile time. Their errors show up as compiler errors, and may even halt the build.

Here is a sample code (I didn't run it though):

@SupportedAnnotationTypes("*")   // needed to run on all classes being compiled
@SupportedSourceVersion(SourceVersion.RELEASE_6)
public class DefaultConstructor extends AbstractProcessor {

    @Override
    public boolean process(Set<? extends TypeElement> annotations,
            RoundEnvironment roundEnv) {

        for (TypeElement type : ElementFilter.typesIn(roundEnv.getRootElements())) {
            if (requiresDefaultConstructor(type))
                checkForDefaultConstructor(type);
        }
        return false;
    }

    private void checkForDefaultConstructor(TypeElement type) {
        for (ExecutableElement cons :
            ElementFilter.constructorsIn(type.getEnclosedElements())) {
            if (cons.getParameters().isEmpty())
                return;
        }

        // Couldn't find any default constructor here
        processingEnv.getMessager().printMessage(
                Diagnostic.Kind.ERROR, "type is missing a default constructor",
                type);
    }

    private boolean requiresDefaultConstructor(TypeElement type) {
        // sample: require any JPA Entity to have a default constructor
        return type.getAnnotation(Entity.class)) != null
               || type.getQualifiedName().toString().contains("POJO");
    }

}

The annotation processor becomes even easier if you introduce an annotation (e.g. RequiresDefaultAnnotation).

Declaring the requirement of having a default qualifier

::I am also assuming that the OP asking for a mechanism that prevents accidental errors for developers, especially written by someone else.::

There has to be a mechanism to declare which classes require a default processor. Hopefully, you already have a criteria for that, whether it is a pattern in the name, pattern in the qualifier, a possible annotation, and/or a base type. In the sample I provided above, you can specify the criteria in the method requiresDefaultConstructor(). Here is a sample of how it can be done:

1. Based on a name pattern. TypeElement provide access to the fully qualified name and package name.

   return type.getQualifiedName().toString().contains("POJO");
   

2. Based on an annotation present on the type declaration. For example, all Java Bean Entity classes should have a non-default constructors

   return type.getAnnotation(Entity.class) != null;
   

3. Based on a abstract class or interface.

   TypeElement basetype = processingEnv.getElements().getTypeElement("com.notnoop.mybase");
   return processingEnv.getTypes().isSubtype(type.asType(), basetype.asType());
   

4. [Recommended Approach]: If you are using the basetype interface, I recommend mixing the annotation approach with the base type interface. You can declare an annotation, e.g. MyPlain, along with the meta annotation: @Inherited. Then you can annotate the base type with that annotation, then all subclasses would inherit the annotation as well. Then your method would just be

   return type.getAnnotation(MyPlain.class) != null;
   

   This is better because it's a bit more configurable, if the pattern is indeed based on type hierarchy, and you own the root class.

As mentioned earlier, just because it is called "annotation processing", it does mean that you have to use annotations! Which approach in the list you want to follow depends on your context. Basically, the point is that whatever logic you would want to configure in your deployment enforcement tools, that logic goes in requiresDefaultConstructor.

Classes the processor will run on

Annotation Processors invocation on any given class depends on SupportedAnnotationTypes. If the SupportedAnnotationTypes meta-annotation specifies a concrete annotation, then the processor will only run on those classes that contain such annotation.

If SupportedAnnotationTypes is "*" though, then the processor will be invoked on all classes, annotated or not! Check out the [Javadoc](http://java.sun.com/javase/6/docs/api/javax/annotation/processing/Processor.html#getSupportedAnnotationTypes()), which states:

Finally, "*" by itself represents the set of all annotation types, including the empty set. Note that a processor should not claim "*" unless it is actually processing all files; claiming unnecessary annotations may cause a performance slowdown in some environments.

Please note how false is returned to ensure that the processor doesn't claim all annotations.

Answer 2

No. The above check can be easier rewritten as:

try {
  MyClass.newInstance();
} catch (InstantiationException E) {
  // no constructor
} catch (IllegalAccessException E) {
  // constructor exists but is not accessible
?

Answer 3

You can employ PMD and Macker in order to guarantee architectural rules. In partilar, Macker provokes compilation errors, breaking your build process when validations fail.

Macker extends some concepts made popular by PMD regarding validations of source code. A good example is when you'd like to guarantee that all classes from a package implements a certain interface.

So, if you are very paranoid (like me!) about verifying all possible architectural rules, Macker is really useful.

http://innig.net/macker/

Note: The website is not great. Colors will hurt your eyes... but the tools is very useful anyway.

Richard Gomes http://www.jquantlib.org/