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.
Related
I'm having difficulties figuring out the correct way to fix the deprecation warnings on SerializationFeature.WRITE_EMPTY_JSON_ARRAYS.
Javadocs state that
Since 2.8 there are better mechanism for specifying filtering;
specifically using com.fasterxml.jackson.annotation.JsonFormat or
configuration overrides.
but I would assume that
ObjectMapper.configure(SerializationFeature.WRITE_EMPTY_JSON_ARRAYS, false);
is a configuration override, although the line above triggers a deprecation warning.
What are other alternatives that do not pollute model classes with yet another annotation? I want to configure the behaviour globally.
At the class level, you can use the #JsonInclude like:
#JsonInclude( JsonInclude.Include.NON_EMPTY )
public class MyClass ...
Also, at the mapper level you can do something like:
mapper.setSerializationInclusion(JsonInclude.Include.NON_EMPTY);
The Javadoc is just plain wrong.
"using com.fasterxml.jackson.annotation.JsonFormat": There are plenty of cases where annotations are patently NOT better, most notably when you can't annotate the POJOS because you don't own them, or when you need to override the annototations to get a different behavior than you usually want.
"configuration overrides": That would be great, but to my knowledge there are no other overrides that will take precedence over annotations.
From the user perspective, the only thing bad about the deprecated option is that it is deprecated, and thus might not be supported in the future. That is of course a real concern - you might one day have to choose between a vital security update and keeping your code working without a rewrite.
As I was trying to sharpen my Java skills by creating a Minecraft server plugin, I came across a #EventHandler in a code example for Bukkit plugin development. This is was put just over a method implementation and I was wondering what it is called.
I remember having seen some #Override somewhere else and I want to learn what it is called so I can search it online...
it is Annotation.
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.
From the doc the usage of annotations are mainly
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.
This thing is called an annotation. In Python the counterpart of Java annotations are called decorators.
OK, I will not replicate the link to what an annotation is. Rather, I will talk about how they can operate.
Annotations have both a Target and a Retention; optionally, they can also be #Documented so that you know this annotation has been present at some time in your source code. Now, onto the target and retention since these two "meta-annotations" will definitely have an influence on what you can expect:
the target determines what that annotation can be applied to; this can be a whole class, an instance variable, a method, a... Well, see the doc.
the retention determines how "long" this annotation persists in your source code. The two retention policies used in majority are SOURCE and RUNTIME.
In an annotation such as yours, it is typically an annotation with a runtime retention policy. Such an annotation can be used, at runtime, by specific processing code, to change the behaviour of the target so that its behaviour be controlled by this processing code. Such code is logically called an annotation processor.
Annotations have begun to take quite some an importance in some JSRs:
JSR 330 defines #Inject, #Provider etc as runtime annotations, so that frameworks willing to do dependency injection can rely on these annotations being present; this is the case, for instance, for dependency injection frameworks such as Dagger and Guice (since version 3.0);
JSR 305 defines #Immutable, #ThreadSafe, #NotThreadSafe, #Nullable, #Nonnull with a source retention policy, and #Documented; these annotations can be used by static code analysis tools, and of equal importance, they are typically #Documented as well. Therefore you know what to expect of them.
It is just a annotation.
#Override for ie is used in methods that is overriding method with the same name in the extended "super class"
It's an annotation.
While it usually has no direct effect on code, some object databases may use it to specify the behavior of one or more fields in regard to indexing, keying, or other functions.
I've been reading about annotations lately, and I'm a bit confused. I've used the annotation #NotNull but I don't really know how it checks if the variable is not null. No where in the code do I see anything checking values for null. That makes sense because it's an interface, so where does the actual processing happen and why isn't this location specified in the code? The examples I've seen usually just make an annotation that takes values, but doesn't do anything else, so I'm confused as to where the implementation occurs.
Annotation is just a metadata, no more no less.
When you want to specify some details about the class you put annotations on it.
Think about them (to some extent) as an alternative to the old well known XML way to define metadata.
Now, its obvious that someone reads your XML and runs the code that makes something with the metadata. The same happens with annotations: a framework to which annotation belongs to is responsible to read the annotation and make something with this information. In the case of #NotNull, its hibernate-validator project. The API exposed by java allows to access the information in the annotations by reflection (the classes like java.lang.Class, Method, Field, and so on). So somewhere inside hibernate validator there is a code that goes to your class, reads the annotations by reflection and checks whether the class adheres these annotations.
These annotation usually have retention policy 'runtime' which means that they're preserved in the bytecode and loaded along with the class that carries these annotations.
There are also annotations that should be processed by Java compiler. Like #Deprecated, #SuppressWarnings and so on. The benefit of having such annotation is that you can find some code issues during the compilation phase.
You can also put the annotation processors and 'plug' them during the compilation phase, buts its entirely different story.
Hope this clarifies a little the usage of annotations
i want to ask why are the java annotations used so much... I know that they replaced xml configuration in for example jpa, but why is this kind configuration used at all?
Consider this piece of code:
#Entity
class Ent{
// some fields
}
//... somewhere in the other file far far away
class NonEnt{
// whatever here
}
Now, when I try to put this in persistence context, with EntityManager's persist method, I get runtime error(better would be to get compile error) with trying to persist NonEnt instance. There is obvious solution for me, force the entities to implement some no-method interface instead of using #Annotations. But this isn't popular among framework designer, what is the drawback of this solution?
Thanks for answering...
When compared to marker interfaces, annotations have some advantages:
they can be parameterized
they are more fine grained - you can attach them not only to classes but also to other class elements (fields, methods, method arguments, etc)
Annotations are also supposedly less intrusive, but this point is matter of taste and debatable.
See also:
Annotations (official JDK documentation)
What is the use of marker interfaces in Java?
The use of annotations is a lot less invasive than forcing the client to implement a interface or extend a class.
There is obvious solution for me,
What you describe is called a "marker interface" and it's an abuse of the interface concept. I suspect the only reason why you consider it obvious is because of Serializable - which only exists because there were no annotations at that time.
force the entities to implement some
no-method interface instead of using
#Annotations. But this isn't popular
among framework designer, what is the
drawback of this solution?
What are its advantages? Annotations have the huge advantage that they can have parameters, and they are much more fine-grained. Marker interfaces only work at the class level.
Citing the java tutorial:
Annotations provide data about a
program that is not part of the
program itself. They 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.
Compiler-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.
As you can see, annotations are a way of specifying meta-data about your types in java, including interfaces, they are in no way a replacement for them.
Java annotation are really helpful when you want to add some additional information to your class, method or instance variable. There are a lot of libraries which use these annotations heavily. These annotations keep the code simple and readable with the power of making changes to the code at runtime.
For example if you have used lombok library, which creates setter, getter and constructor at compile time and saves you lines of code and time.
When compiler executes the code, lomok searches for all the fields marked with #Setter or #Getter annotation and add setter and getter for that field in the class.
One other example is Junit test runner. How junit differentiates between normal helper method in test class and a test. To differentiate between the two it uses #Test annotation.
This tutorial explains how you can use java annotations to create you own test runner.
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
}