We are implementing a new Java EE Project and are not very familiar with this technology so far. So we much have to learn...
Currently we are using Guava Optional in BOs to implement fields that are not mandatory.
We have a lot of discussion around this class and I don't see the advantages so far. I would prefer to using Bean Validation with the annotation #NotNull to indicate if a field is set always or not. Of course, that it's set, you can only be sure after validation.
Now to my question. Does eclipse don't show the Bean Validation Annotations? So you would better see, what you can be sure. Is there a plugin that would help?
Guava/Java 8 Optional should be used to enforce the following convention: if, according to bussiness logics, some function might return null value, then we wrap it inside Optional, so our intention is clear - users has to check for value presence. If a function returns non-optional object, we assume that is never can be null and users of that function do not have to do null checks. Hence the code will be much cleaner.
Since Optional is a part of Java 8 now, I guess this will become standard approach to handle nulls.
Some IDEs provide special annotation for the sake of showing hints. Eclipse has #NonNull annotation, similarly IntelliJ IDEA has #Nullable/#NotNull annotations, but they have nothing to do with Bean Validation annotation.
Related
This is my first question on here and I'm pretty new to Java, so please pardon me if this is kind of a silly question.
I am working on "documenting" the expected behavior of my application from within using JavaDocs and annotations, but I'm just curious if it's appropriate or good practice to use #NotNull on a method that SHOULDN'T ever return null, but may due to future developer error.
Thanks.
This really depends on what tooling you're using. The #NotNull annotation doesn't actually do anything. Actually, there are several different #NotNull annotations provided by different libraries. None of them do anything on their own, other than signalling the programmer's intent.
To make the annotation do anything, you need to combine it with a tool that will process the annotation (either at compile time or at run time) and apply some validation to your code or your data. For example:
IntelliJ IDEA contains tools which will check that your code abides by the nullability annotations you've included.
Bean validation can be used at runtime to check, for example, that user input doesn't violate the nullability constraints.
Even without the tooling, I would say that yes, these annotations can be useful to signal intent. You can always remove the annotation if it becomes untrue.
It sounds like it would be a good idea in your case for all contributors to the codebase to use a tool, like IntelliJ IDEA, that will check for compliance with these annotations at compile time. That will help you to avoid the type of developer error that you describe.
As far as I'm aware, there is not enforced standard globally to use these types of annotations. It makes sense to use them when writing a public API, but this is project specific.
In any case, feel free to do so in your project if it would help with code clarity and readability.
It's not an anti-pattern to have javadocs with annotations.
Also, if you are considering of warning the user of your method that it shouldn't return a null, but there is a chance of that happening (since you cannot enforce it), you might want to look into the Optional class.
yes it is good practice to use #NotNull. The #NotNull Annotation is, actually, an explicit contract declaring the following:
A method should not return null.
A variable (like fields, local variables, and parameters)cannot hold a null value.
So to keep it simple, it is a good practice.
Lombok's #NonNull VS javax.annotation.Nonnull
Which one is better to use for method parameters and when?
The answer here vaguely compares all annotations but has no inference on which one is best. I just to know the better of the two.
javax.annotation.Nonnull is part of JSR-305 which seems to be dead. Even if that wasn't the case said annotation would have only be used for static code analysis purposes, e.g. your IDE warning you that you're passing in a null value someplace where this is not acceptable.
lombok.NonNull is a very different story. First of all, said annotation is NOT used for validation purposes or rather static code analysis purposes. Lombok uses this annotation to fire NPEs whenever an instance variable or a parameter are null when they're not supposed to.
Lombok, being an annotation post processor will effectively pick up this annotation during the pre-compilation process and will auto generate the needed code to effectively handle these cases (by manipulating the AST).
You can read more on this here: Lombok - NonNull.
It seems that your familiarity with both the javax.annotation related functionality and Lombok's functionality is not the best, so I would suggest to have a good read on what is what.
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.
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
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
}