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Is there a way to get the javadoc tool to document annotations?
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I would like to document some properties of classes and methods in JavaDoc and be able to load these information at runtime. Therefore I thought it might be convenient to write a custom annotation and annotate all necessary files and methods with this annotation. With this annotation, I could load these information at runtime.
Here's a hypothetic code snippet to demonstrate my use case:
public class ImportantClass {
#DetailedDescription(description="originated from data source XYZ")
public void importantMethod() {
// snip
}
}
The String "originated from data source XYZ" should be displayed in the JavaDoc and be readable via reflections or something similar. My problem is that the JavaDoc does not contain annotation information.
Is it possible to configure the JavaDoc task (preferrably with Maven) to include annotation information?
Clarification: I'm not interested in doclets (~javadoc annotations) as they are not readable at runtime.
I finally found a nice solution without duplication. Use the #Documented annotation on the annotation interface (DetailedDescription in this case) and all instances of this annotation are documented in the JavaDoc.
See this question:
Is there a way to get the javadoc tool to document annotations?
This is more of a comment than an answer, but it is long so I need this format
Why do you want an annotation for that? The Oracle documentation says that for this kind of use, you should use both a javadoc tag and an annotation. From the link I provided:
If you need to affect both program semantics and documentation, you probably need both an annotation and a tag. For example, our guidelines now recommend using the #Deprecated annotation for alerting the compiler warning and the #deprecated tag for the comment text
Use javadocs, it provides exactly what you are trying to achieve.
Annotations provide metadata at the code level, and are meant to play role in the behaviour of your app. Should not be used for documentation.
Related
I have read a lot of tutorials about Java annotations lately and I like the idea of creating a custom one. Most articles cover the very basic idea and fairly simple implementations. I'm missing a proper pattern to process my annotation, though.
Lets say I have a custom annotation #Foobar to initialize fields. I need to pass all classes that use this annotation to my processor, let's call it FoobarProcessor:
public class AnnotatedClass {
#Foobar
private String test = "";
static {
FoobarProcessor.process(AnnotatedClass.class);
}
}
Is there any approach to overcome this drawback? Is there any single point that all classes pass, where I can easily apply my annotation processor?
A common pattern to process annotations or any language elements is the visitor pattern.
Java even includes a standard API for to this: SimpleElementVisitor7
If you need an example implementation of a processor using the pattern, take a look at the code of the PrintingProcessor. The processor traverses all kind of elements it find and prints some information. It's used for javac's non-standard Xprint option (you can try it in your command line: javac -Xprint java.lang.Object).
You need to register the processor in a META-INF file. This answer should give you more info:
What is the default annotation processors discovery process?
If you want to process your annotation at Runtime, you need to scan the classes from information of the classLoader, this answer give more information about it:
How do I read all classes from a Java package in the classpath?
It's possible to create an annotation like #deprecated, I mean, with deprecated code-style?
I'd like to create an annotation to indicates that some messages were removed from the communication protocol, so I'd not want to use the deprecated annotation because I want 1) give a better and more suggestive name for my annotation, like "message removed" 2) give parameters for my annotation, e.g. (protocol = 5) -removed since protocol 5.
But, I want to keep this strikethrough code for others know that this message has been removed just by looking to the code.
#Deprecated annotation is designed for the development environment (generally but not only). It does nothing itself. But for IDE it is the some kind of marker and when it "see" that annotation - it performs some logic (for example "strikethrough" the code). The goal of my post is to tell that it is not enough to develop and use the annotation : it must me supported by environment.
There are two things you can do:
Add the
#Deprecated annotation to the method, and
Add a
#deprecated tag to the javadoc of the method
You should do both!
Quoting the java documentation on this subject:
Starting with J2SE 5.0, you deprecate a class, method, or field by using the #Deprecated annotation. Additionally, you can use the #deprecated Javadoc tag tell developers what to use instead.
Using the annotation causes the Java compiler to generate warnings when the deprecated class, method, or field is used. The compiler suppresses deprecation warnings if a deprecated compilation unit uses a deprecated class, method, or field. This enables you to build legacy APIs without generating warnings.
You are strongly recommended to use the Javadoc #deprecated tag with appropriate comments explaining how to use the new API. This ensures developers will have a workable migration path from the old API to the new API
When looking at the source code of the Deprecated annotation, you will see that there is nothing specific that makes it appear with the strikethrough. It is a feature of the IDE to mark #Deprecated code with a strikethrough.
Here it is:
#Documented
#Retention(RetentionPolicy.RUNTIME)
public #interface Deprecated {
}
+1 for oleg.lukyrych's answer BUT you can still do something.
Create your annotation (with all the parameters you want), then add a static code analysis to your build procedure. i.e. PMD with a custom rule. PMD is well know and well integrated in various IDE and continuous build environment like hudson/jenkins. The analysis will produce you a nice report of the (mis)use of your deprecated message.
It is not as nice as having it into your editor but it make the job.
Hope this helps.
Mayur Gupta,
I created a annotation MessageRemoved:
public #interface MessageRemoved {
Protocol protocol();
}
And a Enum:
public enum Protocol {
P01, P02, P03, P04, P05
}
Using a Annotation:
#MessageRemoved(protocol = Protocol.P05)
public class OldMessage extends Message{
}
This facilitates the traceability of messages removed. Using only annotation #Deprecated this is not possible.
I am using the pluggable annotation processing api withing Java6+ to automatically create some deployment XML files. Part of these XML files contains a description of the object. The description is ALWAYS the same content as the Javadoc associated with the class itself. I could force the comment to be a field of the #Block annotation, but that duplicates the information. Is there any way during annotation processing to get the contents of the class/type comment?
In this example, I want to get "A nice description of my block" during annotation processing.
/**
* A nice description of my block
**/
#Block
public class CustomBlock {
}
I seem to always find the answer right after I post on SO.
For future reference, here is the solution
import java.util.Set;
import javax.annotation.processing.AbstractProcessor;
import javax.annotation.processing.RoundEnvironment;
import javax.lang.model.element.TypeElement;
public class CustomAnnotationProcessor extends AbstractProcessor
{
#Override
public boolean process(Set<? extends TypeElement> set, RoundEnvironment re)
{
// use the protected member, processingEnv
String comment = processingEnv.getElementUtils().getDocComment(anyElement);
}
}
The annotation processing API makes use of classes in the javax.lang.model(.*) packages. These model language constructs and said models must be generated during compilation. Since a compiler is intended to ignore comments and documentation, there doesn't seem to be anything in those packages, nor did I expect there to be, that gives you access to comments/doc.
I'm not certain how the javadoc facility performs its work, maybe that can be of help.
Kapep's answer looks interesting, but do mind that it uses stuff from a com.sun.* package, which is implementation-specific. Unless you're absolutely sure that the resources offered to your annotatation processor environment are implemented using those classes and you can safely downcast from the interfaces, it's best not to use that. It'd be a brittle solution at best.
EDIT: as an aside, I'm also using custom annotations + processor to generate metadata in XML format for wiring, validation etc. And I also need descriptions. So what I do is keep the JavaDoc for programming purposes and details that might be interesting to someone directly using the class in code, while having some description key in the annotation (or a default based on class name/other annotation values if none is given) that can be used to obtain a description from some resource file. The description is intended for the "end user" and focuses on high-level stuff, not code specifics. It has the added benefit of facilitating internationalization. I'm not certain this would be of any use to you, but there's my two cents.
There is getDocComment which sounds like it should return the comment.
Update: It got moved to the elements utitlity.
There are at the moment, two ways to mark code as depreacted in java.
Via JavaDoc
/**
* #deprecated
*/
Or as an annotation:
#Deprecated
This is my problem - I find it a bit too much to declare both, when marking a method as deprecated when using Eclipse. I really just want to use one of them.
However does using the annotation give the compiler actual useful additional information?
But only using the annotation, I cannot state why the method is deprecated - I can only do that with JavaDoc, and deprecating a method without specying why is bad.
So, can I only use one of them? Or should I really just learn to specify both?
You should use both. The Annotation allows the compiler to display a warning whenever a deprecated method is used, and the javadoc explains why. Both are important.
As per Oracle's Java Annotations tutorial:
When an element is deprecated, it should also be documented using the Javadoc #deprecated tag...
From the horse's mouth:
NOTE: The Java Language Specification
requires compilers to issue warnings
when classes, methods, or fields
marked with the #Deprecated annotation
are used. Compilers are not required
by the Java Language Specification to
issue warnings when classes, methods,
or fields marked with the #deprecated
Javadoc tag are accessed, although the
Sun compilers currently do so.
So basically, if you want a guarantee that there will be compiler warnings, you need to use the annotation. And because of some API designer's breathtaking incompetence, you need to specify the javadoc tag as well to give an explanation.
Personally, I'd say the annotation is useless and should be omitted until it's fixed, since any good compiler or IDE will display warnings with the javadoc tag as well.
You should write both.
The #Deprecated Anotation is for the Compiler and the #deprecated JavaDoc tag is for the Person who wants to know why this is deprecated.
An example can look like this:
/**
* #deprecated We dont need this Method because ...
*/
#Deprecated
public void doStuff(){..}
You should specify both.
The annotation lets the compiler know about it and trigger warnings when the method is used.
The JavaDoc attribute lets developers know about before they start using it.
These are two very different things!
This can be easily dealt with a good IDE.
Eclipse Neon, for eg. automatically adds #Deprecated annotation, when I create a javadoc #deprecated on a method or field.
So I simply write the javadoc with the appropriate explanation and let the IDE take care of adding the #Deprecated annotation, the minute I save the file.
What is the purpose of annotations in Java? I have this fuzzy idea of them as somewhere in between a comment and actual code. Do they affect the program at run time?
What are their typical usages?
Are they unique to Java? Is there a C++ equivalent?
Annotations are primarily used by code that is inspecting other code. They are often used for modifying (i.e. decorating or wrapping) existing classes at run-time to change their behavior. Frameworks such as JUnit and Hibernate use annotations to minimize the amount of code you need to write yourself to use the frameworks.
Oracle has a good explanation of the concept and its meaning in Java on their site.
Also, are they unique to Java, is there a C++ equivalent?
No, but VB and C# have attributes which are the same thing.
Their use is quite diverse. One typical Java example, #Override has no effect on the code but it can be used by the compiler to generate a warning (or error) if the decorated method doesn't actually override another method. Similarly, methods can be marked obsolete.
Then there's reflection. When you reflect a type of a class in your code, you can access the attributes and act according to the information found there. I don't know any examples in Java but in .NET this is used by the compiler to generate (de)serialization information for classes, determine the memory layout of structures and declare function imports from legacy libraries (among others). They also control how the IDE form designer works.
/EDIT: Attributes on classes are comparable to tag interfaces (like Serializable in Java). However, the .NET coding guidelines say not to use tag interfaces. Also, they only work on class level, not on method level.
Anders gives a good summary, and here's an example of a JUnit annotation
#Test(expected=IOException.class)
public void flatfileMissing() throws IOException {
readFlatFile("testfiles"+separator+"flatfile_doesnotexist.dat");
}
Here the #Test annotation is telling JUnit that the flatfileMissing method is a test that should be executed and that the expected result is a thrown IOException. Thus, when you run your tests, this method will be called and the test will pass or fail based on whether an IOException is thrown.
Java also has the Annotation Processing Tool (apt) where not only you create annotations, but decide also how do these annotations work on the source code.
Here is an introduction.
To see some cool stuff you can do with Annotations, check out my JavaBean annotations and annotation processor.
They're great for generating code, adding extra validations during your build, and I've also been using them for an error message framework (not yet published -- need to clear with the bosses...).
The first thing a newcomer to annotations will ask about annotations is: "What is an annotation?" It turns out that there is no answer to this question, in the sense that there is no common behavior which is present in all of the various kinds of java annotations. There is, in other words, nothing that binds them together into an abstract conceptual group other than the fact that they all start with an "#" symbol.
For example, there is the #Override annotation, which tells the compiler to check that this member function overrides one in the parent class. There is the #Target annotation, which is used to specify what kinds of objects a user defined annotation (a third type of construct with nothing in common with other kinds of annotation) can be attached to. These have nothing to do with one another except for starting with an # symbol.
Basically, what appears to have happened is that some committee responsible for maintaining the java language definition is gatekeeping the addition of new keywords to the java language, and therefore other developers are doing an end run around that by calling new keywords "annotations". And that's why it is hard to understand, in general what an annotation is: because there is no common feature linking all annotations that could be used to put them in a conceptual group. In other words, annotations as a concept do not exist.
Therefore I would recommend studying the behavior of every different kind of annotation individually, and do not expect understanding one kind of annotation to tell you anything about the others.
Many of the other answers to this question assume the user is asking about user defined annotations specifically, which are one kind of annotation that defines a set of integers or strings or other data, static to the class or method or variable they are attached to, that can be queried at compile time or run time. Sadly, there is no marker that distinguishes this kind of annotation from other kinds like #interface that do different things.
By literal definition an annotation adds notes to an element. Likewise, Java annotations are tags that we insert into source code for providing more information about the code. Java annotations associate information with the annotated program element. Beside Java annotations Java programs have copious amounts of informal documentation that typically is contained within comments in the source code file. But, Java annotations are different from comments they annotate the program elements directly using annotation types to describe the form of the annotations. Java Annotations present the information in a standard and structured way so that it could be used amenably by processing tools.
When do you use Java's #Override annotation and why?
The link refers to a question on when one should use the override annotation(#override)..
This might help understand the concept of annotation better.Check out.
Annotations when it comes to EJB is known as choosing Implicit middle-ware approach over an explicit middle-ware approach , when you use annotation you're customizing what you exactly need from the API
for example you need to call transaction method for a bank transfer :
without using annotation :
the code will be
transfer(Account account1, Account account2, long amount)
{
// 1: Call middleware API to perform a security check
// 2: Call middleware API to start a transaction
// 3: Call middleware API to load rows from the database
// 4: Subtract the balance from one account, add to the other
// 5: Call middleware API to store rows in the database
// 6: Call middleware API to end the transaction
}
while using Annotation your code contains no cumbersome API calls to use the middle-
ware services. The code is clean and focused on business logic
transfer(Account account1, Account account2, long amount)
{
// 1: Subtract the balance from one account, add to the other
}