Develop Reference

Custom annotation processor VS processing annotation via reflection api

I want to write simple custom validation annotation and can't find proper info on difference between two ways, that i know, of processing them, which are:
Reflection API
Custom processor extending javax.annotation.processing.AbstractProcessor
Can please someone tell me the difference between these options and which is better for which scenario, any help would be appreciated

An annotation processor runs when you compile your code. At compile time, it can issue warnings, generate new code, or modify existing code. (Modifying existing code is frowned upon, but some processors such as Lombok do it.)
The reflection API is accessed when your code runs. Your code can change its behavior depending on the annotations that it reads via the reflection API.
That explains the key differences. There are too many different scenarios to list them all, but for any given scenario you can determine the pros and cons based on the differences.

Can JSR 305 annotations be used with code that will run on Android?

Can a JSR 305 annotation like javax.annotation.Nonnull be used in code that will run on Android ? It does not look like the Dalvik VM is supporting the package and those annotations have a runtime retention policy. Has anyone got it working using the javax package (or alternatives like using eclipse JDT) ?

Afaik, annotations can be missing at runtime (even with runtime retention), which will result in the annotation being silently dropped. So if your code compiles, then you're OK even if the VM that is later running your code does not know of the annotation.
In your case, you are using javax.annotation.Nonnull, which is used by static analysis tools, but usually has no effect at runtime. So yes, you can use it.
In contrast, a hypothetical other annotation whose presence is relevant to your app at runtime, must be present on the classpath.

The javax.* package is not fully supported by dalvik or ADT. You can use normal annotations however. The full package list in the official docs.
That said, anything that targets .class files will not work. There's a library called dexmaker that helps with this problem.

Why not use androidx.annotation from Android standard sdk plartform

How to hook custom compiler logic based on annotations

I want to define a few annotations that will allow extra warnings/errors to be reported during compilation (similar in concept to the #Nullable and #NotNull annotations in IntelliJ).
I would like to be able to write some compiler hooks that will also add my compilation logic based on those attributes.
I want a generic hook if possible, however since we are using Eclipse - it would also be a benefit if we had that ability.
I'd like to know:
Is it possible? (any of the options above)
Where do I start?
I had little experience with annotations so far, so if I'm going about this the wrong way - I'd like to know that and if possible get a better direction to go with.
Thanks.

You can use the Java Annotation Processor (see JSR 269: Pluggable Annotation Processing API) for that. From Annotation checking at compile time with Java Annotation Processor:
The JSR 269 states that you can
implement a plug-in for the compiler
which can handle the annotations. This
plug-in can be given as parameter at
compile time, so your code will be
called when one of your annotation
appears in source code.
The mentioned link provides an example that will get you started.
See also:
Annotation Processing Tool (apt)

Do Java annotations add notation or functionality?

Are Java annotations used for adding functionality to Java code besides just adding documentation about what's going on in the code? What's the most advanced/complex functionality you could add to your code through an annotation?

Annotation are basically not more than a tag (with optional additional data) on a class/method/field. Other code (libraries or tools) can discover these tags and execute functionality dependant on the annotations found. I don't see a real limit on the complexity of the functionality possibly added by annotations. This can for example emulate AOP (adding functionality before or after a method with an annotation).

Annotations as such only add information (metadata) to a class.
One can easily build a system that uses that metadata to provide additional functionality, however.
For example you can use apt to generate classes based on the information provided by the annotation.

An annotation needs a tool to react to it. If such a tool does not exist the annotation is merely a notation. The "tool" can be an APT based agent or some piece of code that uses reflection (for instance, JUnit's #Test).
Several annotations are recognized by the Java compiler and thus have pre-defined semantics: #Override, #Deprecated, #Target.

I would understand Annotations as a way to document your code in a machine readable way.
For example in Hibernate you can specify the whole persistence information for your objects as annotations. This is directly readable for you and not in a distant xml file. But is also readable for the tool to generate configurations, database schemes etc.

The drawbacks of annotation processing in Java?

I am considering starting a project which is used to generate code in Java using annotations (I won't get into specifics, as it's not really relevant). I am wondering about the validity and usefulness of the project, and something that has struck me is the dependence on the Annontation Processor Tool (apt).
What I'd like to know, as I can't speak from experience, is what are the drawbacks of using annotation processing in Java?
These could be anything, including the likes of:
it is hard to do TDD when writing the processor
it is difficult to include the processing on a build system
processing takes a long time, and it is very difficult to get it to run fast
using the annotations in an IDE requires a plugin for each, to get it to behave the same when reporting errors
These are just examples, not my opinion. I am in the process of researching if any of these are true (including asking this question ;-) )
I am sure there must be drawbacks (for instance, Qi4J specifically list not using pre-processors as an advantage) but I don't have the experience with it to tell what they are.
The ony reasonable alternative to using annotation processing is probably to create plugins for the relevant IDEs to generate the code (it would be something vaguely similar to override/implement methods feature that would generate all the signatures without method bodies). However, that step would have to be repeated each time relevant parts of the code changes, annotation processing would not, as far as I can tell.
In regards to the example given with the invasive amount of annotations, I don't envision the use needing to be anything like that, maybe a handful for any given class. That wouldn't stop it being abused of course.

I created a set of JavaBean annotations to generate property getters/setters, delegation, and interface extraction (edit: removed link; no longer supported)
Testing
Testing them can be quite trying...
I usually approach it by creating a project in eclipse with the test code and building it, then make a copy and turn off annotation processing.
I can then use Eclipse to compare the "active" test project to the "expected" copy of the project.
I don't have too many test cases yet (it's very tedious to generate so many combinations of attributes), but this is helping.
Build System
Using annotations in a build system is actually very easy. Gradle makes this incredibly simple, and using it in eclipse is just a matter of making a plugin specifying the annotation processor extension and turning on annotation processing in projects that want to use it.
I've used annotation processing in a continuous build environment, building the annotations & processor, then using it in the rest of the build. It's really pretty painless.
Processing Time
I haven't found this to be an issue - be careful of what you do in the processors. I generate a lot of code in mine and it runs fine. It's a little slower in ant.
Note that Java6 processors can run a little faster because they are part of the normal compilation process. However, I've had trouble getting them to work properly in a code generation capacity (I think much of the problem is eclipse's support and running multiple-phase compiles). For now, I stick with Java 5.
Error Processing
This is one of the best-thought-through things in the annotation API. The API has a "messenger" object that handles all errors. Each IDE provides an implementation that converts this into appropriate error messages at the right location in the code.
The only eclipse-specific thing I did was to cast the processing environment object so I could check if it was bring run as a build or for editor reconciliation. If editing, I exit. Eventually I'll change this to just do error checking at edit time so it can report errors as you type. Be careful, though -- you need to keep it really fast for use during reconciliation or editing gets sluggish.
Code Generation Gotcha
[added a little more per comments]
The annotation processor specifications state that you are not allowed to modify the class that contains the annotation. I suspect this is to simplify the processing (further rounds do not need to include the annotated classes, preventing infinite update loops as well)
You can generate other classes, however, and they recommend that approach.
I generate a superclass for all of the get/set methods and anything else I need to generate. I also have the processor verify that the annotated class extends the generated class. For example:
#Bean(...)
public class Foo extends FooGen
I generate a class in the same package with the name of the annotated class plus "Gen" and verify that the annotated class is declared to extend it.
I have seen someone use the compiler tree api to modify the annotated class -- this is against spec and I suspect they'll plug that hole at some point so it won't work.
I would recommend generating a superclass.
Overall
I'm really happy using annotation processors. Very well designed, especially looking at IDE/command-line build independence.
For now, I would recommend sticking with the Java5 annotation processors if you're doing code generation - you need to run a separate tool called apt to process them, then do the compilation.
Note that the API for Java 5 and Java 6 annotation processors is different! The Java 6 processing API is better IMHO, but I just haven't had luck with java 6 processors doing what I need yet.
When Java 7 comes out I'll give the new processing approach another shot.
Feel free to email me if you have questions. (scott#javadude.com)
Hope this helps!

I think if annotation processor then definitely use the Java 6 version of the API. That is the one which will be supported in the future. The Java 5 API was still in the in the non official com.sun.xyz namespace.
I think we will see a lot more uses of the annotation processor API in the near future. For example Hibernate is developing a processor for the new JPA 2 query related static meta model functionality. They are also developing a processor for validating Bean Validation annotations. So annotation processing is here to stay.
Tool integration is ok. The latest versions of the mainstream IDEs contain options to configure the annotation processors and integrate them into the build process. The main stream build tools also support annotation processing where maven can still cause some grief.
Testing I find a big problem though. All tests are indirect and somehow verify the end result of the annotation processing. I cannot write any simple unit tests which just assert simple methods working on TypeMirrors or other reflection based classes. The problem is that one cannot instantiate these type of classes outside the processors compilation cycle. I don't think that Sun had really testability in mind when designing the API.

One specific which would be helpful in answering the question would be as opposed to what? Not doing the project, or doing it not using annotations? And if not using annotations, what are the alternatives?
Personally, I find excessive annotations unreadable, and many times too inflexible. Take a look at this for one method on a web service to implement a vendor required WSDL:
#WebMethod(action=QBWSBean.NS+"receiveResponseXML")
#WebResult(name="receiveResponseXML"+result,targetNamespace = QBWSBean.NS)
#TransactionAttribute(TransactionAttributeType.NOT_SUPPORTED)
public int receiveResponseXML(
#WebParam(name = "ticket",targetNamespace = QBWSBean.NS) String ticket,
#WebParam(name = "response",targetNamespace = QBWSBean.NS) String response,
#WebParam(name = "hresult",targetNamespace = QBWSBean.NS) String hresult,
#WebParam(name = "message",targetNamespace = QBWSBean.NS) String message) {
I find that code highly unreadable. An XML configuration alternative isn't necessarily better, though.