How to intentionally cause a custom java compiler

Question

I'm about to commit an ugly temporary hack in order to work around a blocking issue while we wait for an external resource to be fixed. Aside from marking it with a big scary comment and a bunch of FIXMEs, I'd love to have the compiler throw an obvious warning message as a reminder so we don't forget to take this out. For example, something like:

[javac] com.foo.Hacky.java:192: warning: FIXME temporary hack to work around library bug, remove me when library is fixed!

Is there a way I can cause an intentional compiler warning with a message of my choosing? Failing that, what's the easiest thing to add to the code to throw an existing warning, with perhaps a message in a string on the offending line so it gets printed in the warning message?

EDIT: Deprecated tags don't seem to be doing anything for me:

/**
 * @deprecated "Temporary hack to work around remote server quirks"
 */
@Deprecated
private void doSomeHackyStuff() { ... }

No compiler or runtime errors in eclipse or from sun javac 1.6 (running from ant script), and it's definitely executing the function.

Answer 1

I wrote a library that does this with annotations: Lightweight Javac @Warning Annotation

Usage is very simple:

// some code...

@Warning("This method should be refactored")
public void someCodeWhichYouNeedAtTheMomentButYouWantToRefactorItLater() {
    // bad stuff going on here...
}

And compiler will throw warning message with your text

Answer 2

One good hack deserves another... I usually generate compiler warnings for the described purpose by introducing an unused variable in the hacky method, thus:

/**
 * @deprecated "Temporary hack to work around remote server quirks"
 */
@Deprecated
private void doSomeHackyStuff() {
    int FIXMEtemporaryHackToWorkAroundLibraryBugRemoveMeWhenLibraryIsFixed;
    ...
}

This unused variable will generate a warning which (depending upon your compiler) will look something like this:

WARNING: The local variable FIXMEtemporaryHackToWorkAroundLibraryBugRemoveMeWhenLibraryIsFixed is never read.

This solution is not as nice as a custom annotation, but it has the advantage that it requires no advance preparation (assuming the compiler is already configured to issue warnings for unused variables). I would suggest that this approach is only suitable for short-lived hacks. For long-lived hacks, I would argue that effort to create a custom annotation would be justified.

Answer 3

how about marking the method or class as @Deprecated? docs here. Note that there is both a @Deprecated and a @deprecated - the capital D version is the annotation and the lowercase d is the javadoc version. The javadoc version allows you to specify an arbitrary string explaining what is going on. But compilers are not required to emit a warning when seeing it (though many do). The annotation should always cause a warning, though i don't think you can add an explanation to it.

UPDATE here is the code I just tested with: Sample.java contains:

public class Sample {
    @Deprecated
    public static void foo() {
         System.out.println("I am a hack");
    }
}

SampleCaller.java contains:

public class SampleCaller{
     public static void main(String [] args) {
         Sample.foo();
     }
}

when i run "javac Sample.java SampleCaller.java" i get the following output:

Note: SampleCaller.java uses or overrides a deprecated API.
Note: Recompile with -Xlint:deprecation for details.

I am using sun's javac 1.6. If you want an honest to goodness warning rather than just a note, use the -Xlint option. Maybe that will percolate up through Ant properly.

Answer 4

I think that a custom annotation, which will be processed by the compiler, is the solution. I frequently write custom annotations to do things at runtime, but I never tried to use them at compilation time. So, I can only give you pointers on the tools you may need :

• Write a custom annotation type. This page explains how to write an annotation.
• Write an annotation processor, that processes your custom annotation to emit a warning. The tool that runs such annotation processors is called APT. You can find an indroduction on this page. I think what you need in the APT API is AnnotationProcessorEnvironment, which will let you emit warnings.
• From Java 6, APT is integrated into javac. That is, you can add an annotation processor in the javac command line. This section of the javac manual will tell you how to call your custom annotation processor.

I don't know if this solution is really practicable. I'll try to implement it myself when I find some time.

Edit

I successfully implemented my solution. And as a bonus, I used java's service provider facility to simplify its use. Actually, my solution is a jar that contains 2 classes : the custom annotation and the annotation processor. To use it, just add this jar in the classpath of your project, and annotate whatever you want ! This is working fine right inside my IDE (NetBeans).

Code of the annotation :

package fr.barjak.hack;

import java.lang.annotation.ElementType;
import java.lang.annotation.Retention;
import java.lang.annotation.RetentionPolicy;
import java.lang.annotation.Target;

@Retention(RetentionPolicy.SOURCE)
@Target({ElementType.ANNOTATION_TYPE, ElementType.CONSTRUCTOR, ElementType.FIELD, ElementType.LOCAL_VARIABLE, ElementType.METHOD, ElementType.PACKAGE, ElementType.PARAMETER, ElementType.TYPE})
public @interface Hack {

}

Code of the processor :

package fr.barjak.hack_processor;

import java.util.Set;
import javax.annotation.processing.AbstractProcessor;
import javax.annotation.processing.ProcessingEnvironment;
import javax.annotation.processing.RoundEnvironment;
import javax.annotation.processing.SupportedAnnotationTypes;
import javax.lang.model.element.Element;
import javax.lang.model.element.TypeElement;
import javax.tools.Diagnostic.Kind;

@SupportedAnnotationTypes("fr.barjak.hack.Hack")
public class Processor extends AbstractProcessor {

    private ProcessingEnvironment env;

    @Override
    public synchronized void init(ProcessingEnvironment pe) {
        this.env = pe;
    }

    @Override
    public boolean process(Set<? extends TypeElement> annotations, RoundEnvironment roundEnv) {
        if (!roundEnv.processingOver()) {
            for (TypeElement te : annotations) {
                final Set< ? extends Element> elts = roundEnv.getElementsAnnotatedWith(te);
                for (Element elt : elts) {
                    env.getMessager().printMessage(Kind.WARNING,
                            String.format("%s : thou shalt not hack %s", roundEnv.getRootElements(), elt),
                            elt);
                }
            }
        }
        return true;
    }

}

To enable the resulting jar as a service provider, add the file META-INF/services/javax.annotation.processing.Processor in the jar. This file is an acsii file that must contain the following text :

fr.barjak.hack_processor.Processor

Answer 5

Some quick and not so dirty approach, may be to use a @SuppressWarnings annotation with a deliberately wrong String argument:

@SuppressWarnings("FIXME: this is a hack and should be fixed.")

This will generate a warning because it is not recognized by the compiler as a specific warning to suppress:

Unsupported @SuppressWarnings("FIXME: this is a hack and should be fixed.")

Answer 6

One technique that I've seen used is to tie this into unit testing (you do unit test, right?). Basically you create a unit test that fails once the external resource fix is achieved. Then you comment that unit test to tell others how to undo your gnarly hack once the issue is resolved.

What's really slick about this approach is that the trigger for undoing your hack is a fix of the core issue itself.