如何故意导致自定义 Java 编译器警告消息？

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.

You should use a tool to compile, like ant ou maven. With it, you should define some tasks at compile time which could produce some logs (like messages or warnings) about your FIXME tags, for example.

And if you want some errors, it is possible too. Like stop compilation when you have left some TODO in your code (why not ?)

Here shows a tutorial on annotations and at the bottom it gives an example of defining your own annotations. Unfortunately a quick skimming of the tutorial said that those are only available in the javadoc...

Annotations Used by the Compiler There are three annotation types that are predefined by the language specification itself: @Deprecated, @Override, and @SuppressWarnings.

So it appears that all you can really do is throw in an @Deprecated tag that the compiler will print out or put a custom tag in the javadocs that tells about the hack.

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.

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

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.

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

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.")

We can do this with annotations!

To raise an error, use Messager to send a message with Diagnostic.Kind.ERROR. Short example:

processingEnv.getMessager().printMessage(
Diagnostic.Kind.ERROR, "Something happened!", element);

Here's a fairly simple annotation I wrote just to test this out.

This @Marker annotation indicates the target is a marker interface:

package marker;


import java.lang.annotation.*;


@Retention(RetentionPolicy.RUNTIME)
@Target(ElementType.TYPE)
public @interface Marker {
}

And the annotation processor causes an error if it's not:

package marker;


import javax.annotation.processing.*;
import javax.lang.model.*;
import javax.lang.model.element.*;
import javax.lang.model.type.*;
import javax.lang.model.util.*;
import javax.tools.Diagnostic;
import java.util.Set;


@SupportedAnnotationTypes("marker.Marker")
@SupportedSourceVersion(SourceVersion.RELEASE_6)
public final class MarkerProcessor extends AbstractProcessor {


private void causeError(String message, Element e) {
processingEnv.getMessager()
.printMessage(Diagnostic.Kind.ERROR, message, e);
}


private void causeError(
Element subtype, Element supertype, Element method) {
String message;
if (subtype == supertype) {
message = String.format(
"@Marker target %s declares a method %s",
subtype, method);
} else {
message = String.format(
"@Marker target %s has a superinterface " +
"%s which declares a method %s",
subtype, supertype, method);
}


causeError(message, subtype);
}


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


Elements elementUtils = processingEnv.getElementUtils();
boolean processMarker = annotations.contains(
elementUtils.getTypeElement(Marker.class.getName()));
if (!processMarker)
return false;


for (Element e : roundEnv.getElementsAnnotatedWith(Marker.class)) {
ElementKind kind = e.getKind();


if (kind != ElementKind.INTERFACE) {
causeError(String.format(
"target of @Marker %s is not an interface", e), e);
continue;
}


if (kind == ElementKind.ANNOTATION_TYPE) {
causeError(String.format(
"target of @Marker %s is an annotation", e), e);
continue;
}


ensureNoMethodsDeclared(e, e);
}


return true;
}


private void ensureNoMethodsDeclared(
Element subtype, Element supertype) {
TypeElement type = (TypeElement) supertype;


for (Element member : type.getEnclosedElements()) {
if (member.getKind() != ElementKind.METHOD)
continue;
if (member.getModifiers().contains(Modifier.STATIC))
continue;
causeError(subtype, supertype, member);
}


Types typeUtils = processingEnv.getTypeUtils();
for (TypeMirror face : type.getInterfaces()) {
ensureNoMethodsDeclared(subtype, typeUtils.asElement(face));
}
}
}

For example, these are correct uses of @Marker:

• @Marker
  interface Example {}
  

• @Marker
  interface Example extends Serializable {}
  

But these uses of @Marker will cause a compiler error:

• @Marker
  class Example {}
  

• @Marker
  interface Example {
  void method();
  }
  

  

Here's a blog post I found very helpful getting started on the subject:

• Code Generation using Annotation Processors in the Java language

Small note: what the commentor below is pointing out is that because MarkerProcessor references Marker.class, it has a compile-time dependency on it. I wrote the above example with the assumption that both classes would go in the same JAR file (say, marker.jar), but that's not always possible.

For example, suppose there's an application JAR with the following classes:

com.acme.app.Main
com.acme.app.@Ann
com.acme.app.AnnotatedTypeA (uses @Ann)
com.acme.app.AnnotatedTypeB (uses @Ann)

Then the processor for @Ann exists in a separate JAR, which is used while compiling the application JAR:

com.acme.proc.AnnProcessor (processes @Ann)

In that case, AnnProcessor would not be able to reference the type of @Ann directly, because it would create a circular JAR dependency. It would only be able to reference @Ann by String name or TypeElement/TypeMirror.

To get any warning at all to appear, I found that unused variables and custom @SuppressWarnings didn't work for me, but an unnecessary cast did:

public class Example {
public void warn() {
String fixmePlease = (String)"Hello";
}
}

Now when I compile:

$ javac -Xlint:all Example.java
ExampleTest.java:12: warning: [cast] redundant cast to String
String s = (String) "Hello!";
^
1 warning

If you're using IntelliJ. You can go to: Preferences>Editor>TODO and add "\bhack.b*" or any other pattern.

If you then make a comment like // HACK: temporary fix to work around server issues

Then in the TODO tool window, it will show up nicely, together with all your other defined patterns, while editing.