I read in this article(https://www.javacodegeeks.com/2015/01/how-to-process-java-annotations.html) that Annotation Processing has multiple rounds:
Annotation processing happens in separate stages, called rounds. During each round, a processor gets a chance to process the annotations it is interested in.
The annotations to process and the elements they are present on are available via the RoundEnvironment parameter passed into the process() method.
If annotation processors generate new source or class files during a round, then the compiler will make those available for processing in the next round. This continues until no more new files are generated.
The last round contains no input, and is thus a good opportunity to release any resources the processor may have acquired.
Can someone explain to my what it means by rounds? So for example let's say we have an annotation Metrics.java, when the compiler is parsing through the java source code it will first find all the elements that use this annotation and then at the end of compilation run the annotation process for that specific Metrics annotation where the RoundEnvironment passed in will contain all the elements that have the Metrics annotation?
Or anytime the compiler sees a annotation it will immediately run the annotation if it has a retention type of Class or Source. Otherwise it will not run the annotation for that annotation until runtime since it has a retention type of Runtime?
as far as i understand, while parsing a java file compiler gathers all annotations seen in it then goes for processing them using an annotation processor that we already prepared(first round).suppose annotation processor creates a new source file(.java) and has used annotation in it,here compiler goes for parsing that file and gathering its annotations then goes for processing them using annotation processors(second round).and this continue until in last java file(which created via annotation processor or not) we have no annotation in it.
and annotation processing is a part of javac that we should ask for enabling it(that we have some ways depend on environment like maven, command line, IDEs, and so on).
annotation processing doesn't run immediately by meeting first annotation in the source file and Retention policy isn't matter too.
Related
When I press CTRL + ALT + L then every line gets breaked, except of a Controller line with annotations.
I set it up to wrap at 200 and it does so, just not on this as example:
I tried to set up the Code Style -> Java -> Wrapping and Braces.
As written in Oracle's Java tutorial
Annotations, a form of metadata, provide data about a program that is
not part of the program itself. Annotations have no direct effect on
the operation of the code they annotate.
Annotations have a number of uses, among them:
Information for the compiler — Annotations can be used by the
compiler to detect errors or suppress warnings.
Compile-time and
deployment-time processing — Software tools can process annotation
information to generate code, XML files, and so forth.
Runtime processing — Some annotations are available to be examined at
runtime.
They're not really directly executable and therefore not debuggable; for example what would you expect when putting a break point on #Override annotation?
So I have an annotation that, although it can be declared multiple times, generally needs to access the same properties file. Currently I am using a static registry in my annotation processor to track if the file has been created, and to store the writer for the file. So far this works fine, but my problem is figuring out when to save the file. Currently I am making a call to roundEnv.processingFinished (or whatever that method is) at the end of my annotation processor, and if it returns true I save the file. The problem I am having is that, if source fines are generated, and the compiler goes into a new round of processing that does not contain any of my annotation, the file will not ne saved. I was thinking about adding an entirely new processing that supports every annotation, but claims none, and making the check there. I just feel like this is a slightly clunky option, especially since the spec indicates a processor should only support annotations it will actually process. Is there some better way of doing this, or should I just go with the additional processor model?
There is a Java application and I have no permission to alter the Java code apart from annotating classes or methods (with custom or existing annotations).
Using annotations and annotations only I have to invoke code which means that every time an instance of an annotated class is created or an annotated method is called, some extra Java code must be executed (e.g a call to a REST Webservice). So my question is: how can I do this?
In order to prevent answers that I have already checked I will give you some solutions that seem to work but are not satisfying enough.
Aspect Oriented Programming (e.g AspectJ) can do this (execute code before and after the call of an annotated method) but I don't really want the runtime overhead.
Use the solution provided here which actually uses reflection. This is exactly what I need only that it alters the initial code further than just annotating and so I cannot use it.
Use annotation processor for source code generation as suggested here by the last answer. However, still this means that I will alter the source code which I don't want.
What I would really like is a way to simply include a Java file that somehow will execute some Java lines every time the annotated element will be triggered.
Why not skip annotations completely and use byteman to inject code at runtime into the entry points of your code.
I have to agree with the comment above though, that this sort of restriction is ridiculous and should be challenged.
I have a custom annotation that I've implemented and I'd like to use an annotation processor to generate a list of all the classes in my app that use that particular annotation.
I've found this tutorial which describes how to generate a class file using an annotation processor, so it should be quite easy to generate a class for each class with my annotation.
What I can't figure out is how I can collect all of that information into a single class. There doesn't seem to be a way to modify a class, so I can't append new items to the list once the class has been generated the first time.
Is there a way to use an annotation processor to generate a method that will return the list of all classes in an app that are annotated with a particular annotation?
Generated classes do not necessarily have to correspond one-to-one to the input classes being processed. Plus, you can search for classes (Elements) that are annotated with a given annotation via the RoundEnvironment:
roundEnvironment.getElementsAnnotatedWith(MyAnnotation.class)
From this you can generate a single class with a method that returns a collection of the classes found.
A couple issues around this to highlight:
Annotation processors can run along with other annotation processors and thus have to deal with classes generated at compile time. To aid this, Java annotation processing is performed in rounds to allow processors to catch the outputs of others. To be compatible with other processors you need to gracefully handle the ErrorType.
Only classes in the current compilation pass are returned from the RoundEnvironmnet methods so classes in external libraries will not be included.
IDEs (cough cough Eclipse) implement the annotation processing facilities of Java differently which can be a problem for processors that require a full non-partial compilation like I've described.
Coincidentally, I created a similar project recently that does what you are looking for:
https://github.com/johncarl81/silver
Silver is very much a WIP and uses a lot of library code to accomplish the task, but it may give you an idea for what's possible.
Can anyone please explain what the following two paragraphs mean, in simple english? (Taken from http://www.ibm.com/developerworks/java/library/j-cwt08025.html)
"Annotations are more flexible in
terms of how you use them, with
options for whether the annotation
information is to be included in class
files output by the compiler and made
available to the application at run
time"
Not sure what these means. Can Annotations be configured to optionally change the bytecode?
While annotations are ideal for
metadata that relates to a particular
component, they are not well suited to
metadata with cross-component
application.
IMHO most web applications would be cross-component ones. What is the author trying to say here?
Annotations are more flexible in terms
of how you use them, with options for
whether the annotation information is
to be included in class files output
by the compiler and made available to
the application at run time
This, I think, refers to the fact that Java5 annotations can be dropped by the compiler, whereas some can be retained in the bytecode. This is controlled by the #Retention annotation that is placed on your annotation type, e.g
#Documented
#Retention(value=RUNTIME)
public #interface Deprecated
This indicates that the #Deprecated annotation will be present in the bytecode, and will also be visible to reflection. java.lang.annotation.RetentionPolicy defines the different options.
"Annotations are more flexible in terms of how you use them, with options for whether the annotation information is to be included in class files output by the compiler and made available to the application at run time"
It is more flexible than XDoclet because:
it can be used from the source code (like XDoclet)
it can be used at runtime, when you only have the byte-code and not the source code (unlike XDoclet)
While annotations are ideal for metadata that relates to a particular component, they are not well suited to metadata with cross-component application.
Annotations (like XDoclet) have one interesting feature, as opposed to an external Xml for example :
Annotations live in the code, so it is natural for them to be applied to the code they are defined on. You don't have to specify (using some complex syntax) to what piece of code they apply. Examples:
if an annotation is defined on a method, it applies naturally to that method
if an annotation is defined on a field, it applies naturally to that field
if an annotation is defined on a class, it applies naturally to that class
if an annotation is defined on a package, it applies naturally to that package
If you want to have the same in an external Xml file, you have to use a complex syntax to identify the piece of code you refer to. So that makes them very easy to apply.
Also, in case of a code refactoring (like renaming), annotations continue to work just fine, while an external xml would have to be changed to point to the new class or method name.
I don't believe that in a web application, most things are cross-component.
If you defined Persistance (to a database) of an Entity, like what is the table where this class should be persisted, it is not something global to all Entities, it only affects the current Entity.
same for many other examples...