I would like to use Lombok's #NonNull annotation to generate the null-checking code automatically for method parameters while also using FindBugs' #NonNull to use static analysis tools and generate appropriate warnings whenver the case applies.
As of now, I need to do the following:
public void doSomething (#lombok.NonNull #edu.umd.cs.findbugs.annotations.NonNull Object parameter)
{
// Do something
}
This is quite ugly, so I would like to avoid using this syntax. I read about nested annotations (here and here), but I can't seem to find a way to create my own custom annotation with both NonNull annotations as nested annotations. Am I trying to do something that cannot work?
Here's my latest attempt:
#Documented
#Retention (RetentionPolicy.CLASS)
#Target (value={ElementType.FIELD, ElementType.METHOD, ElementType.PARAMETER, ElementType.LOCAL_VARIABLE})
public #interface MyNonNull
{
public lombok.NonNull lombokNonNull () default #lombok.NonNull;
public edu.umd.cs.findbugs.annotations.NonNull findBugsNonNull () default #edu.umd.cs.findbugs.annotations.NonNull;
}
You cannot "merge" annotations with custom annotations, however you can use #ParametersAreNonnullByDefault on the class scope, which should allow edu.umd.cs.findbugs.annotations.NonNull to be inferred.
I think neither of your links will help you. You would need to extend from both with your custom annotation, yet you cannot even extend from one, as all annotations have an implicit extends clause for Annotation.
I have the following code. I need to allow usage of this annotation (CommandName) ONLY for classes-instances of ICommand interface. How can I do it?
#Retention(RetentionPolicy.RUNTIME) // Make this annotation accessible at runtime via reflection.
#Target({ElementType.TYPE}) // This annotation can only be applied to class methods.
public #interface CommandName {
String value();
}
In compile time, you cant do that. In runtime, just check the classes - which is annotated by CommandName annotation - if they are implementing the ICommand interface.
I am not getting the #Inherited annotation in Java. If it automatically inherits the methods for you then if I need to implement the method in my own way then what about that ?
How does will it come to know my way of implementation ?
Plus it is said if I do not want to use this and do it rather in an old fashioned Java way I have to implement the the equals(), toString(), and the hashCode() methods of the Object class and also the annotation type method of the java.lang.annotation.Annotation class.
Why is that?
I have never implemented those even when I did not know about the #Inherited annotation and the programs used to work fine also .
Please somebody explain me from the scratch about this.
Just that there is no misunderstanding: You do ask about java.lang.annotation.Inherited. This is a annotation for annotations.It means that subclasses of annotated classes are considered having the same annotation as their superclass.
Example
Consider the following 2 Annotations:
#Inherited
#Target(ElementType.TYPE)
#Retention(RetentionPolicy.RUNTIME)
public #interface InheritedAnnotationType {
}
and
#Target(ElementType.TYPE)
#Retention(RetentionPolicy.RUNTIME)
public #interface UninheritedAnnotationType {
}
If three classes are annotated like this:
#UninheritedAnnotationType
class A {
}
#InheritedAnnotationType
class B extends A {
}
class C extends B {
}
running this code
System.out.println(new A().getClass().getAnnotation(InheritedAnnotationType.class));
System.out.println(new B().getClass().getAnnotation(InheritedAnnotationType.class));
System.out.println(new C().getClass().getAnnotation(InheritedAnnotationType.class));
System.out.println("_________________________________");
System.out.println(new A().getClass().getAnnotation(UninheritedAnnotationType.class));
System.out.println(new B().getClass().getAnnotation(UninheritedAnnotationType.class));
System.out.println(new C().getClass().getAnnotation(UninheritedAnnotationType.class));
will print a result similar to this (depending on the packages of the annotation):
null
#InheritedAnnotationType()
#InheritedAnnotationType()
_________________________________
#UninheritedAnnotationType()
null
null
As you can see UninheritedAnnotationType is not inherited but C inherits annotation InheritedAnnotationType from B.
I don't know what methods have to do with that.
Java annotations are marked with a #Target annotation to declare possible joinpoints which can be decorated by that annotation. Values TYPE, FIELD, METHOD, etc. of the ElementType enum are clear and simply understandable.
Question
WHY to use #Target(ANNOTATION_TYPE) value? What are the annotated annotations good for? What is their contribution? Give me an explanation of an idea how it works and why I should use it. Some already existing and well-known example of its usage would be great too.
You can use an annotated annotation to create a meta-annotation, for example consider this usage of #Transactional in Spring:
/**
* Shortcut and more descriptive "alias" for {#code #Transactional(propagation = Propagation.MANDATORY)}.
*/
#Target({ElementType.METHOD, ElementType.TYPE})
#Retention(RetentionPolicy.RUNTIME)
#Transactional(propagation = Propagation.MANDATORY)
public #interface RequiresExistingTransaction {
}
When you enable Spring to process the #Transactional annotation, it will look for classes and methods that carry #Transactional or any meta-annotation of it (an annotation that is annotated with #Transactional).
Anyway this was just one concrete example how one can make use of an annotated annotation. I guess it's mostly frameworks like Spring where it makes sense to use them.
Each annotation annotated by #Target(ElementType.ANNOTATION_TYPE) is called Meta-annotation. That means, you can define your own custom annotations that are an amalgamation of many annotations combined into one annotation to create composed annotations.
A good example from Android world is StringDef
Denotes that the annotated String element, represents a logical type and that its value should be one of the explicitly named constants.
#Retention(SOURCE)
#StringDef({POWER_SERVICE, WINDOW_SERVICE, LAYOUT_INFLATER_SERVICE})
public #interface ServicesName {}
public static final String POWER_SERVICE = "power";
public static final String WINDOW_SERVICE = "window";
public static final String LAYOUT_INFLATER_SERVICE = "layout_inflater";
Code inspector will treat #ServicesName and #WeekDays in the same way as #StringDef.
As a result we can create as much named StringDef's as we need and override set of constants. #Target(ElementType.ANNOTATION_TYPE) it is a tool that allows to extend the use of annotations.
Annotation is defined like a ordinary Java interface, but with an '#' preceding the interface keyword (i.e., #interface ). Annotations are used to provide supplemental information about a program. On the other hand, an interface can be defined as a container that stores the signatures of the methods to be implemented in the code segment.
WHY to use #Target(ANNOTATION_TYPE) value?
When there is need to apply an annotation to an another annotation. If you look at the source codes of the common Java annotations, you see often this code pattern:
#Target(ANNOTATION_TYPE)
public #interface TheAnnotation
{
...
}
For example,
#Documented
#Target({ ANNOTATION_TYPE })
#Retention(RUNTIME)
public #interface Constraint {
public Class<? extends ConstraintValidator<?, ?>>[] validatedBy();
}
What are the annotated annotations good for?
They are good or more precisely necessary if they are used to annotate other annotations.
What is their contribution?
They make possible to apply an annotation directly to an another annotation, that is a different thing than applying an annotation to a standard Java class or to method and so on.
Give me an explanation of an idea how it works and why I should use it.
For example, if you create a data model class and you may want that the program checks data validity. In that case, there might be need to create a new annotation and apply another annotations to this annotation. It is simple to add some data validity checks to this model by adding annotations to the class. For example, to check that some value is not null (#notNull) or email is valid (#ValidEmail) or length of a field is more than x characters (#Size). However, it is possible that there is not built in Java annotations for all purposes. For example, it is so if you liked to check if password and its matchingPassword are same. This is possible by creating the annotation class PasswordMatches:
#Target({TYPE})
#Retention(RUNTIME)
#Constraint(validatedBy = PasswordMatchesValidator.class)
#Documented
public #interface PasswordMatches {
String message() default "Passwords don't match";
Class<?>[] groups() default {};
Class<? extends Payload>[] payload() default {};
}
Note, there is line
#Constraint(validatedBy = PasswordMatchesValidator.class).
In other words, the annotation class Constraint, like the other annotations in this class also, must have ANNOTATION_TYPE as a value of target annotation.
Now the password equality check is easy to include to data model class simply by adding annotation #PasswordMatches:
#PasswordMatches
public class UserDto {
...
}
The PasswordMatchesValidator class could look like this:
public class PasswordMatchesValidator implements ConstraintValidator<PasswordMatches, Object> {
#Override
public void initialize(final PasswordMatches constraintAnnotation) {}
#Override
public boolean isValid(final Object obj, final ConstraintValidatorContext context) {
final UserDto user = (UserDto) obj;
return user.getPassword().equals(user.getMatchingPassword());
}
}
Some already existing and well-known example of its usage would be great too.
There is quite well-known example in item 4, but another known annotations which are applied frequently to custom annotations are #Retention, #Documented and #Target itself.
For example, if annotation looks like
#Target(ElementType.TYPE)
#Retention(RetentionPolicy.RUNTIME)
public #interface SomeAnnotation {
String description() default "This is example for class annotation";
}
the compiler will complain in this situation
#SomeAnnotation
public class SomeClass {
#SomeAnnotation // here it's complaning
public void someMethod(){}
}
If you change
#Target(ElementType.TYPE)
to
#Target({ElementType.METHOD, ElementType.TYPE})
it won't complain anymore.
Annotation are basically additional metadata (information) that goes along with your code. It can be placed along side types (Classes, Interfaces), methods, and arguments.
It is often useful during compile time and runtime. Many popular APIs such as Java EE 5+, Spring, AspectJ leverage annotation for code clarity and consistency.
Using annotation often allows code to be more readable, more easily understood.
I'd recommend you read through the annotation chapter on Java tutorial
In the past metadata are often given as an xml file, and it's difficult for someone trying to understand the code if they have to lookup a different xml configuration file. The latest Java servlet API allows mapping of servlet simply by using annotation -- as opposed of web.xml mapping:
#WebServlet("/response")
public class ResponseServlet extends HttpServlet {
// servlet code here...
}
I have this interface:
public interface IDbTable extends Serializable
{
public int getId();
}
I need to obligate all the classes that implements IDbTable, to have an annotation "#DatabaseTable" and at least one field inside class that have "#DatabaseField".
The only way of implementing IDbTable needs to be like this:
#DatabaseTable(tableName = "Something")
public static class TestTable implements IDbTable
{
#DatabaseField
public int id;
public int getId()
{
return id;
}
}
Is this possible with interface or inheritance?
Today i have an unit test that scan all the classes and check this requirements. Is this a good practice?
You can't apply annotations to an interface that will be inherited by the implementing class, according to Java documentation: http://docs.oracle.com/javase/6/docs/api/java/lang/annotation/Inherited.html
Indicates that an annotation type is automatically inherited. If an
Inherited meta-annotation is present on an annotation type
declaration, and the user queries the annotation type on a class
declaration, and the class declaration has no annotation for this
type, then the class's superclass will automatically be queried for
the annotation type. This process will be repeated until an annotation
for this type is found, or the top of the class hierarchy (Object) is
reached. If no superclass has an annotation for this type, then the
query will indicate that the class in question has no such annotation.
Note that this meta-annotation type has no effect if the annotated
type is used to annotate anything other than a class. Note also that
this meta-annotation only causes annotations to be inherited from
superclasses; annotations on implemented interfaces have no effect.
Why don't you use annotation processing, to check that the requiered annotation is present in your class when it is sent to your framework, or at build/deployment time.