I know what it means in a comment for documentation purposes, but outside of that what does it mean? (I would normally just google this but every non letter symbol shows up in results)
The # symbol denotes a Java Annotation. What a Java annotation does, is that it adds a special attribute to the variable, method, class, interface, or other language elements. (This can be configured when you declare the annotation) When you add an annotation to something, other parts of the program can check whether something has an annotation or not. It then can use this information to do whatever stuff they need.
Let me give you some examples:
The #Override annotation
public class SuperClass {
public void someInterestingMethod() {
System.out.println("Superclass!");
}
}
public class DerivedClass extends SuperClass {
public void someInterestngMethod() {
System.out.println("Derived class!");
}
}
And when you do this:
SuperClass sc = new DerivedClass();
sc.someInterestingMethod();
The someInterestingMethod() call should be dynamically dispatched, and print "Derived class!", right? Well the derived class' method was actually misspelled, so DerivedClass got its own separate method called someInterestngMethod(), totally unrelated to the superclass' someInterestingMethod(). As such, someInterestingMethod() is no longer overridden, and the superclass' implementation is invoked.
The #Override keyword is intended to help with this. It signals your intent to the compiler, that you would like the annotated method to be an overload of one of the ancestor class' methods. If it's not (such as in this typo case, or if the SuperClass API changed and renamed the method), the will fail your compilation, to alert your attention to the broken override.
The #SuppressWarnings Annotation
Here is a method:
public void someMethod() {
int i;
}
There will be a compiler warning saying that i is never used. So you can add the #SuppressWarnings to the method to suppress the warning:
#SuppressWarnings("unused")
public void someMethod() {
int i;
}
Note that there is a parameter to the #SuppressWarnings annotation. Some annotations have parameters and you can look for the them in the javadoc. But for those that don't have parameters you don't need to add () like a method.
You can also declare your own annotations and use reflection to check for them. The above 2 annotations will be checked by the compiler.
The # sign is used to specify Java Annotation.
https://en.wikipedia.org/wiki/Java_annotation
There are built-in Java Annotation and user defined Custom Annotation.
Annotations are used in various ways, such as suppress warning, associate method to URI (Servlet), associate variables to resource (JNDI) etc
The # symbol is used for annotations. In my experience, the most common annotation is #Override, which indicates that a method is declared in a superclass. Other common annotations are #Deprecated, indicating that a method should no longer be used but still exists for backwards compatibility, and #SupressWarnings, to prevent warnings from showing up in the compiler.
Note that it's actually possible to get annotations which are not included in the core Java libraries and to declare your own annotations.
The # symbol denotes Annotations. They provide information about a class, its field or method (above which they appear). They cannot perform operations. The compilers or special annotation processors use this information to make writing code less verbose.
In Java Persistence API you use them to map a Java class with database tables.
For example
#Table()
Used to map the particular Java class to the date base table.
#Entity
Represents that the class is an entity class.
Similarly you can use many annotations to map individual columns, generate ids, generate version, relationships etc.
As some other suggests, it is Java's annotation. It helps the compiler to validate your code and to notify the programmer as well.
Very simple code example:
public class SomeClass {
#Override
public String toString() {
return "SomeClass";
}
#Deprecated
public void doSomeOperation() {
// some operation...
}
}
The annotation from SomeClass#toString which is #Override helps the compiler to determine that it is an overridden function from the implicit inheritance to the class Object.
While the annotation from SomeClass#doSomeOperation will warn the programmer that the function itself is deprecated already and should be avoided to use.
The annotations are for the reader or compiler, not executable code.
Related
I am playing around with Java (javax) annotation processing.
Suppose I have an annotation for methods:
#Target(ElementType.METHOD)
public #interface MethodAnnotation { }
Now I want to process all the methods which are overridden from a type with the annotated method:
interface MyInterface() {
#MethodAnnotation
void f()
}
class MyClass implements MyInterface {
override void f() { } // <- I want to process this method
}
#Inherited meta-annotation seems not to be suitable here:
Note that this meta-annotation type has no effect if the annotated type is used to annotate anything other than a class.
Also, is it possible to process an inherited class method which is not overridden in a subclass? Like this:
class MyClass {
#MethodAnnotation
void f() { }
}
class MySubClass extends MyClass { } // <- I want to process its f()
// or at least to find out that it doesn't
// override the method
How can I access the overriden methods of a certain method within AbstractProcessor?
I guess, to achieve this I need to find subclasses of the eclosing class, but I haven't found a way to do this either.
UPD: I suppose it's possible using RoundEnvironment.getRootElements() but still found no proper way of doing this.
The short answer is that out-of-the-box annotation processing isn't going to make this easy for you, but it can be done.
Rather than using the normal dispatch mechanism for processing, you're actually going to have to process every method and do the filtering yourself.
Step 1:
Define your processor so that it supports all annotations by using "*" as its supported annotation type. This will mean that your processor will get invoked every round.
Step 2:
Use getRootElements to get the entire set of elements every round.
Step 3:
Create an ElementScanner8 to traverse any element that you find to look for ExecutableElements. If you're willing to trust that overridden methods are annotated with #Override, you can do a quick filter on those. Otherwise, just look at all of them.
Step 4:
Now you need to see if the method overrides a method with the annotation you're looking for. There's no easy way to get methods that a given method has overridden, so you need to get the enclosing element of the method, look at its superclass and implemented interfaces (recursively), get their enclosed elements, filter out the methods, and test to see if it has been overridden by the method in question. If it has, you can check the annotations to see if it has one you care about.
Step 5:
At this point, you should have the overriding method, the overridden method and the annotation mirror that you were looking for, so you should be able to implement whatever logic you wanted.
according to the javadoc of javax.annotation.processing.Processor in Jsr269-1.8
An annotation is present if it meets the definition of being present
given in AnnotatedConstruct. In brief, an annotation is considered
present for the purposes of discovery if it is directly present or
present via inheritance. An annotation is not considered present by
virtue of being wrapped by a container annotation...
The JavaDoc of AnnotatedConstruct#getAnnotationsByType says that it returns indirectly present annotations, so I think you should scan for methods and check if they indirectly have the annotation using this call. Something in the spirit of this.
Disclaimer... haven't tried it ;)
Method annotations are not inherited. Type annotations can be inherited through the use of "#Inherited" annotation.
What you could do is define a functional interface with an inherited type annotation, however I don't know if this is elegant enough for you.
If those annotations are available at runtime, and you want to reach them at runtime, you can use the Reflections library.
For example:
Collection<URL> urls = ClasspathHelper.forPackage("nl.shopname.location.domain");
Reflections reflections = new Reflections(
new ConfigurationBuilder().setUrls(urls).setScanners(new FieldAnnotationsScanner()));
Set<Field> fieldsWithAnnotation = reflections.getFieldsAnnotatedWith(MyAnnotation.class);
It is considered to be a good practice to
use #Override annotation on methods which are being overriden in
subclass.
But why is same not applied to the classes that come with Java Library. For e.g. String Class. It overrides methods of Object class but does not use #Override annotation on these methods.
Is this so to maintain backward compatibility with previous releases of Java such as 1.4 etc.
Thanks
Within an API, it does not offer much to the user (of that API). However when you implement a method, and you 'intend' do override that of a super class, it is easy to miss out on the method signature, which is supposed to match.
In this case the #Override comes to the rescue as at compile time, it will fail or give a warning when the override does not happen. Also many IDE's recognize the #Override and give you enough support to flag and correct those situations before you even compile.
So the #Override in essence declares your intention that this method overrides something. The user of the API would care less what your intent is, as long as it works.
Actually, probably the true reason is this: The Retention of the #Override annotation is set to SOURCE. Which means the #Override flag is discarded when compiled into a class file.
#Target(value=METHOD)
#Retention(value=SOURCE)
public #interface Override
That is not much more of a cosmetic annotation, it's useful when generating documentation, to give hints through your Java IDE and to explicitly state when a method is overriden.
From the runtime/standard library implementors point of view, it was not worth the effort to modify all existing classes just to add something cosmetic.
Furthermore, regarding backward compatibility of annotations in general, considering that annotations are an optional and extended attribute present in .class file (when their retention policy is either CLASS or RUNTIME and available for Class and Method as Runtime(In)VisibleAnnotations and for Parameter as Runtime(In)VisibleParameterAnnotations) previous releases of the JVM would simply ignore that attribute during the .class file parsing performed the first time that Class is needed.
But actually, that 1.4 JVM class parser will not even reach the point where those Annotation .class attribute are located inside the structure because the parsing will end abruptly when the JVM will notice that the .class version is greater than the supported one.
#override annotation is used to provide some extra information, mainly while generating documentations and also for informing the developer that the code intends to override a method from the superclass.
This is mentioned in oracle documentation.
#Override #Override annotation informs the compiler that the element
is meant to override an element declared in a superclass. Overriding
methods will be discussed in Interfaces and Inheritance.
// mark method as a superclass method // that has been
overridden #Override int overriddenMethod() { }
While it is not required to use this annotation when overriding a
method, it helps to prevent errors. If a method marked with #Override
fails to correctly override a method in one of its superclasses, the
compiler generates an error.
Refer to this discussion in SO itself.
Let's say you you have a class called Vehicle that outlines a protected (could also be abstract) method called speed. And, the sub class Car overrides that method to define its own implementation.
Now, assume that the need to have a speed method in the Vehicle class is no longer required (let's say all vehicles will be stationary for the whatever reason).
If I remove the method speed from Vehicle, I would like to throw a compile error so that so that the developer who has removed the method knows that sub class(es) are potentially relying on it to perform certain actions.
Technically speaking a compile error is not needed, but some sort of notification that acts as a hurdle when such re factoring is happening. Is there a programming pattern that can be employed to handle such a situation?
UPDATE: I am using Java 1.4 (Sorry!)
The #Override annotation is expressly for this purpose.
If you're not using Java 1.5+, then no, although you could use AOP to instrument those methods to throw an exception, or just use reflection and classpath scanning to go over all subclasses and check for the presence of said method.
If it's abstract, then there is no implementation that can be removed from the parent class, and your risk comes down to new subclasses not implementing it. If it's protected and defined in the parent, there are two cases that should already throw compiler errors if the parent implementation is removed.
1) A subclass calls that method without defining its own implementation. Method does not exist.
2) A subclass defines the method, but includes a call to super. Again, the method does not exist.
You can write super.speed() in nested classes and leave this method empty in parent. If you delete now this method in parent, you'll have an Exception. But there is a disadvantage - you must call it from all overrided methods. Try it, perhaps this will help you
Use #Override annotation for methods in subclases.
Once you remove the method from a base-class, tools like Eclipse and javac will issue a warining for those no-longer-overriding methods.
Edit: While you cannot use #Override before Java 1.5.0, there is a tool called xdoclet. Back in the days of J2EE and EJB 2.1 this was used to "simulate" annotations and do magical things with code based on javadoc-like markers. Look at it, maybe you can use it.
Edit 2: In Java 1.4.x, you can also use JavaDoc tag {#inheritDoc} for this kind of verification. Instead of annotating your method with #Override annotate it with #inheritDoc, like this:
public class MyAwesomeClass extends BaseClass
{
/** {#inheritDoc} */
protected void myAweSomeMethod()
{
//...
}
}
Now, if you change the myAweSomeMethod signature in BaseClass, or remove it, you will get warnings from JavaDoc tool, similar to this:
/home/npe/java-tests/MyAwesomeClass.java:4: warning - #inheritDoc used but myAwesomeMethod does not override or implement any method.
When you implement Serializable for your class, IDE shows a warning that The serializable class YOURCLASS does not declare a static final serialVersionUID field of type long.
How can I define an interface that forces the IDE to show this warning for my field?
public class MyClass implements Serializable
{
// this class has warning
}
public class MyClass implements MyInterface
{
// I want to have warning for this class
}
public interface MyInterface{
public static final long myField;
}
You can use aspectj to generate compiler warnings or errors. It is sometimes used to enforce architectural aspects, but might be applicable for you, too.
The Pointcut definitions itself are left as an excercise for the reader :-)
#Aspect
public class ArchitecturalEnforcement {
#Pointcut("...")
public void implementsInterface() {}
#Pointcut("...")
public void definesConstant(){}
/** Advice that defines an error when a GUI method calls a method in the DAO layer */
#DeclareWarning("implementsInterface() && !definesConstant()")
private static final String WARNING= "...";
}
Read more on AOP (Aspect Oriented Programming)
It is special case applicable only in case of classes implementing Serializable interface and configurable in eclipse. Windows--->Preference--->Compile--Error/warnings-->Potential programming problems---> This where eclipse has configuration for this. Even though you force IDE to display warning by some other means for your class, when you compile with javac, it won't force.
This doesn't have anything to with what IDE you're using, it has to do with that you declared your interface wrong. Your interface must contain method signatures and right now in your example it only contains a declaration for a constant. For your interface to be valid it would need to look something more like this:
public interface MyInterface{
public long doSomethingAndReturnALong();
public void doSomethingElseButRequireAnInt(int var);
}
If you declare any kind of variable in an interface, such as you did above, it's assumed to be static and final, also known as a constant that can't be changed by classes that implement the interface.
Hope that helps!
David
In general, this is not possible with interfaces in Java.
If you want the field to be publicly accessible, then you should provide getter and setter methods for it anyway. You can declare these methods in the interface, which will of course result in a compiler error if an implementing class does not declare them.
If you only want to force the classes to have a private field which you would at some point access by reflection, you should think if there is another solution that doesn't need the field. Perhaps its also possible to use methods?
The last option would be to use a static analysis tool like FindBugs, which allows you to define custom bug patterns. So you would write a rule which finds implementors of your interface without this field and enable it in your FindBugs configuration. There exists a good Eclipse plugin for FindBugs which displays the FindBugs warnings in your code (just like the regular Eclipse warnings).
The IDE is not the one raising the warning. It is the compiler. If you need a custom warning, you need a custom compiler. But then you should distribute the compiler with your interface.
In order to create a custom compiler you can use:
http://today.java.net/article/2008/04/09/source-code-analysis-using-java-6-apis
Basically you need to define your own annotation and annotation processor. Probably you will create an annotation for your interface that when a class implements it and it does not define a variable you will raise a warning.
I haven't done this yet, so I cannot give you more info about it.
Another possibility is to use an existing compiler plugin that defines custom warnings and annotations:
https://checkerframework.org/
I do not know if it will help you in your quest, or you should eventually write your own compiler plugin.
PS: checkout this post also:
How to intentionally cause a custom java compiler warning message?
Consider this #PointCut which gets triggered if a method is annotated with an #Secure annotation:
#Pointcut("execution(#Secure * *(..)) && #annotation(secure)")
public void accessOperation(final Access access) { }
This works perfectly well for methods like:
class Foo {
#Secure
public void secureMethod() { }
}
But is it possible to have a Pointcut which also gets triggered when the annotation only exists in a superclass/interface like this?
interface Foo {
#Secure
public void secureMethod();
}
class SubFoo implements Foo {
#Override
public void secureMethod() { // <--- execution of this method should be caught
/* .... */
}
}
EDIT:
This seems to be very closely related to: #AspectJ pointcut for subclasses of a class with an annotation
The only difference is that they use a class-level annotation, whereas I need a method-level annotation.
I don't know how AspectJ deals with annotations in such a scenario, but if he only checks the implementing class for a certain annotation, and that annotation is only found on the interface that the class implements, Java will report that infact that annoation is not present on the class method. You should annotate your annotation with #Inherited:
http://download.oracle.com/javase/6/docs/api/java/lang/annotation/Inherited.html
maybe this will do the trick (though in this case you should make sure that your Advice is not called multiple times).
I don't actually know the classes in my aspect
Considering what you are saying and the fact that #Inherited cannot be used on anything else but classes, you are implicitly hoping that AspectJ will do the job of finding out whether a method (or its overriden implementations and declarations) is annotated. This is much more than what is announced by AspectJ.
If your final objective is to secure some code, then all methods which should be secured should be properly annotated in the first place. So, the answer to your question is no, this is not possible in this context, especially if you know nothing about the classes.
However, there might be a workaround if you have access to an annotation processor. In this case, you could pick all #Secure annotation and work your way through the code with reflection to generate AspectJ code at compile time that would captures all method instances properly. Not easy, but possible.