I'm really rolling up my sleeves and trying to understand Java annotations for the first time, and have read the Sun, Oracle and Wikipedia articles on the subject. They're easy to understand conceptually, but am finding it difficult putting all the pieces of the puzzle together.
The following example is probably terrible engineering, but just humor me (it's an example!).
Let's say I have the following class:
public Widget
{
// ...
public void foo(int cmd)
{
switch(cmd)
{
case 1:
function1();
break;
case 2:
function2();
break;
case 3:
default:
function3();
break;
}
}
}
Now, somewhere else in my project, I have another class, SpaceShuttle, that has a method called blastOff():
public class SpaceShuttle
{
// ...
public void blastOff()
{
// ...
}
}
Now then, I want to configure an annotation called Widgetize so that any methods annotated with #Widgetize will have Widget::foo(int) invoked prior to their own call.
#interface Widgetize
{
int cmd() default 2;
}
So now let's revisit SpaceShuttle:
public class SpaceShuttle
{
// ...
#Widgetize(3)
public void blastOff()
{
// Since we pass a cmd of "3" to #Widgetize,
// Widget::function3() should be invoked, per
// Widget::foo()'s definition.
}
}
Alas, my questions!
I assume that somewhere I need to define an annotation processor; a Java class that will specify what to do when #Widgetize(int) annotations are encountered, yes? Or does this happen in, say, XML config files that get fed into apt (like the way ant reads build.xml files)?
Edit: If I was correct about these annotation processors in question #1 above, then how do I "map"/"register"/make known these processors to the apt?
In buildscripts, is apt typically ran before javac, so that annotation-based changes or code generation takes place prior to the compile? (This is a best practices-type question).
Thanks and I apologize for my code samples, they turned out a lot bulkier than I intended them to (!)
This sounds more like AOP (Aspect oriented programming) than annotations. The topics are often confused since AOP uses annotations to achieve it's goals. Rather than reinvent AOP from scratch, I would recommend looking up and existing AOP library such as AspectJ.
However, to answer your specific question, there are two possible approaches to achieve your goal.
Runtime Approach
This is the approach typically taken by container frameworks (like Spring). The way it works is that instead of instantiating your classes yourself, you ask a container for an instance of your class.
The container has logic to examine the class for any RuntimeAnnotations (like #Widgetize). The container will then dynamically create a proxy of your class that calls the correct Widgetize method first and then calls the target method.
The container will then return that proxy to the original requester. The requester will still thing he got the class (or interface) that he asked for and be completely unaware of the proxying behavior added by the container.
This is also the behavior used by AspectJ.
Enhancement Approach
This is the approach taken by AspectJ. To be honest, I don't know a lot of the details of how it works. Somehow, AspectJ will scan your class files (the byte code), figure out where the annotations are, and then modify the byte code itself to call the proxy class instead of the actual class.
The benefit of this approach is that you don't need to use a container. The drawback is that you now have to do this enhancement step after you compile your code.
I assume that somewhere I need to
define an annotation processor; a Java
class that will specify what to do
when #Widgetize(int) annotations are
encountered, yes? Or does this happen
in, say, XML config files that get fed
into apt (like the way ant reads
build.xml files)?
In Java 1.6, the standard way to define annotation processors its through the ServiceLoader SPI.
In buildscripts, is apt typically ran
before javac, so that annotation-based
changes or code generation takes place
prior to the compile? (This is a best
practices-type question).
APT must take place before compilation, as it operates on source files (actually on syntax trees).
I use method interceptors often with Hibernate. Hibernate requires that a transaction be started and committed round every query. Rather than have lots of duplicate code I intercept every Hibernate method and start the transaction in the interceptor.
I use AOP Alliance method interceptors in conjunction with Google Guice for this. Using these you use your Widgetise annotation and then use Guice to say where you see this annotation use this method interceptor. The following Guice snippet does this.
bindInterceptor(Matchers.any(), Matchers.annotatedWith(Transactional.class), new TransactionalInterceptor);
The interceptor catches the method, you can then call foo and then the tell the method interceptor to proceed with invocation of the original method. For example (in a simplified form):
public class Interceptor implements MethodInterceptor {
//PUT ANY DEPENDENCIES HERE IN A CONSTRUCTOR
public Object invoke(MethodInvocation invocation) throws Throwable {
//DO FOO HERE
result = invocation.proceed();
return result;
}
}
This might be a little confusing and it took me a while to get my head around but it is quite simple once you understand it.
It seems that the basis of your confusion is an incorrect belief that Annotations are something more than just Metadata. Check out this page from the JSE 1.5 language guide. Here is a relevant snippet:
Annotations do not directly affect
program semantics, but they do affect
the way programs are treated by tools
and libraries, which can in turn
affect the semantics of the running
program. Annotations can be read from
source files, class files, or
reflectively at run time.
In your code
#Widgetize(3)
public void blastOff()
does not cause Widget::function3() to execute (asside: in java we reference a member as Widget.function3()) The annotation is just the equivalent of a machine-readable comment (i.e. metadata).
Annotations can be processed both at compile time and at runtime. To process them at runtime requires using the reflection API, which will have a performance impact if it's not done smartly.
It's also possible to process annotations at compile time. The goal is to generate a class which can then be used by your code. There is a specific interface annotation processors have to satisfy, and they have to be declared in order to be executed.
Take a look at the following article for an example which is structurally simple to your use case:
Annotation processing 101
Related
Maybe this question title is not up to my queries but I am searching for the answers for so long.
After adding Lombok to a project, we can use getter, setter, Slf4j, etc. Like
#Getter
#Setter
public class Example {}
My query is After adding #Slf4j we can access an instance of a class "log".Like
#Slf4j
#Getter
#Setter
public class Example {
public void do() {
log.debug("doing");
}
}
So how can I build this kind of annotation that will provide "log" like implementation in java?
Lombok developer here: Lombok is a bit more complicated than that.
Lombok is a unique kind of tool. Yes, it is an annotation processor in the sense that you can specify lombok as being one and lombok then runs as as annotation processor and sets itself up during the init() call (javac will, once its ready to run the processors, first initialize them all).
Lombok is not an annotation processor in the sense that other than the init() step and interacting with the 'rounds' system (specifically: Causing another round to fire in certain cases, in order to ensure javac 'sees' the stuff lombok made. Usually this isn't needed, as it takes a little bit of extra time so we don't do it unless absolutely neccessary) – lombok doesn't use the annotation API at all.
If you are asking how to make your own: That's very complicated. Best route is to fork lombok on github, and have a look at e.g. both HandleSetter.java files in the source to know what to do (there's one for ecj/eclipse and one for javac). Then, you'll be ready to write your own.
Note that intellij support is done via a separate intellij plugin so if you're interested in having lombok work there, you'd also need to fork that plugin and add support there. You only need to mess with intellij if you want new 'signatures' to be visible (you want to add or remove fields or methods, or you want to change the signature parts of one of those: The name, return type, field type, parameter type, or throws line, or you want to add a new type, though that last part can be done with a simple annotation processor).
I think it's better to start with the idea I want to implement. I'm really inspired by Rust mut pointers so the same type can be immutable and mutable at same time. And I'm thinking is it possible to implement something like this for JVM.
It looks I just need to create annotation #Mutates to annotate methods that changes current class state and annotation #Mutable to annotate local variable, fields and args. Logic of processor is not too hard. For every invocation of method annotated by #Mutates I need to check that corresponding variable/field/arg is annotated by #Mutable. And if method calls any #Mutates method on fields or does assignment (= operator) that method must be annotated by #Mutates as well.
The main problem I don't know where to start as I don't have experience with annotation processing. From my limited understanding I can't use Annotation Processor as it isn't supposed analyze code of methods... So it seems I need to create some compiler plugin (two: for Java and for Kotlin).
Can somebody give me a piece of advice - is it possible at all to do that and how hard is this (so I can estimate efforts and time I will need for that and decide is it worthwhile or not)
The standard annotation processing interface does not provide access to the bodies of methods. However, other frameworks do. For example, the Checker Framework provides an API for annotation processing that descends into method bodies. Building on the Checker Framework, you can create an annotation processor that enforces your mutability rules at compile time: the annotation processor would halt compilation with a javac error message if a rule is violated.
Dozens of annotation processors have been built on the Checker Framework, and it is used daily at Amazon, Facebook, Google, Uber, etc.
Disclaimer: I am one of the maintainers of the project.
I want to know, if I've got it correct or not. I've been reading about annotations and wanted to know how all the magic happened. Here's what I think happens. Please let know if otherwise.
Firstly, let's take some annotation. Say AssertTrue. Now, the specs of the annotation is provided in javax.validation.constraints. Now, as such, AssertTrue does not really do anything. i.e. if I were to do the following
public class MyClass{
#AssertTrue(message = "MyMessage")
public myMethod(){
//something
}
}
Now, this particular piece of code is not going to do anything but save the metadata info that on this method myMethod I have some additional info i.e. message and annotation. How I make use of this info is upto me.
This is where the hibernate-validator framework comes into picture.
What it does is, that it provides a bunch of classes and it takes in the object that is supposed to be validated. On that object, it will check if AssertTrue annotation is there. If found, it will invoke the method isValid that is provided in the implementation of the AssertTrueImpl.
Is this what is happening?
Also, I have a few questions. say I have my own custom annotation and I want to provide a framework that checks something. For the sake of argument, let's say I want to do what the hibernate-validator is doing. Now how does one go about it?
Also, when the JVM encounters some annotation, what happens behind the scene? Does it look for some implementation? What exactly does it look for?
i.e. what will JVM do if it encountered AssertTrue in my code. Surely it does not have to do much but store some information, and it does not have to go looking for any implementation too, since whenever I call validator.validate() that's when it will look for hibernate-validator implementation.
let me know if I have understood it correctly.
You are correct. The annotation by itself does nothing, it's just metadata that may be used by a processing tool. In this case, Hibernate validator.
The usual procedure for those tools is to scan on the classpath what classes have metadata that they can use to build or enhance a class (by injecting a proxy, or registering an interceptor, or any other kind of operation). They either do scan the classpath, or an external configuration mechanism (xml, json, annotations on a config class [Spring way]) specifies this for the framework so that it knows where to look for annotated classes.
And so, you too, can benefit from annotations on your project, following the same discovery method. In fact, if you happen to work with CDI, you will probably use them a lot. They're quite useful for bean interception, or for providing metadata on your classes that would otherwise have to be treated with a lot of boilerplate code.
I encourage you to use them profusely.
Cheers!
You understand annotations correctly - they are just metadata on specific members (fields, methods, classes, ...). Some of them are intended for compile time only, some of them are intended for runtime. The latter will be available via Reflection API (basically forming additional metadata on annotated members). There are numerous possibilities of what you can do with this feature, where declarative validation defined by JSR-303 (and implemented by Hibernate Validator) is just one of them.
What exactly is the point of bound properties? To me they seem to be a less type-safe version of events using EventObjects - it seems a bit weak to be using string equality checking for event.getPropertyName().
Why would you use one over the other?
The whole point of Java Beans is that a system (GUI Builder, in particular) can examine a Java Bean and configure it without any previous knowledge of that component.
Although this is fairly cool, it's really only useful in this specific situation and these days annotations would work MUCH better.
So the reason they use bound properties was simply to support this drop-in GUI component technology, and I wouldn't really prefer it over events unless you need to support a reflective gui-building system.
Response to #Mike rodent
Let's say you allow your user to create a class that you will control. This class has a "Main" and can handle a couple events.
Normally you have your user do something like this:
class UserClass(someClass) {
void mainMethod() {
someClass.addEventListener(new EventListener() {
public void eventsHappen(Event e){
event1(e)
}
}
}
someClass.addDifferentEventListener(new DifferentEventListener() {
public void eventsHappen(DifferentEvent e){
event2(e)
}
}
}
}
public void event1(Event e) {
//java code for event 1
}
public void event2(DifferentEvent e) {
// java code for event 2
}
}
Anyway, you get the idea. Of course, you assume that this class is registered somewhere--probably in an xml/config file. You read it, instantiate it and execute the mainMethod(defined by agreement or by interface) and it registers itself and starts getting calls to the event handlers.
Now here's how you can accomplish the same thing with annotations: (You might recognize the pattern--it's pretty much how Junit annotates tests.)
class UserClass() {
#Event1
void event1Method(Event e) {
event1's code
}
#Event2
void event2Method(AnotherEvent e) {
event2's code
}
}
This is much more straight-froward and removes most of the boilerplate, and it also removes the need for an agreement or interface, annotations are more clearly defined and independant. (You don't actually even NEED the event annotations if you care to inspect the parameters being passed into the methods, but dragons lie in that general direction).
You still need the class registered somewhere, but this time you just scan each method for your event annotations and register them yourself. Since it's only a few lines of code to read and process a class like this, why restrict this pattern to unit testing?
Another thing I found really neat, I used this pattern for Groovy classes that were "plugged-in" to my Java program. Since I was compiling all the classes in a given directory anyway, scanning for annotations was trivial.. The effect to the user is that he drops in (or edits) a properly annotated groovy text file and my code immediately compiles, integrates and starts calling their event handlers.
I think that JavaBeans specification was designed with generic object handling in mind. By example, putting a JavaBean in a IDE and using a visual property editor to configure it. In that case the IDE will use the general PropertyChangeEvent and so on.
Or if you want to copy equal named properties from a bean to another... it's another case of bean use (BeanUtils class).
But, if you plan to do specific things, as Noel Ang says I'd recommend strong-typing.
JavaBeans is a specification. It defines a bound property as that whose modification results in a notification being emitted, and a PropertyChangeEvent is the sanctioned notification entity.
So the putative JavaBeans-spec bean editor is supposed to listen for PropertyChangeEvents. Beyond the need to work with that spec, I wouldn't use it, myself.
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
}