[java] How and where are Annotations used in Java?

What are the major areas that we can use Annotations? Is the feature a replacement for XML based configuration?

Frameworks like Hibernate were lots of configuration/mapping is required uses Annotations heavily.

Take a look at Hibernate Annotations

Annotations may be used as an alternative to external configuration files, but cannot be considered a complete replacement. You can find many examples where annotationi have been used to replace configuration files, like Hibernate, JPA, EJB 3 and almost all the technologies included in Java EE.

Anyway this is not always good choice. The purpose of using configuration files is usually to separate the code from the details of the environment where the application is running. In such situations, and mostly when the configuration is used to map the application to the structure of an external system, annotation are not a good replacement for configuration file, as they bring you to include the details of the external system inside the source code of your application. Here external files are to be considered the best choice, otherwise you'll need to modify the source code and to recompile every time you change a relevant detail in the execution environment.

Annotations are much more suited to decorate the source code with extra information that instruct processing tools, both at compile time and at runtime, to handle classes and class structures in special way. @Override and JUnit's @Test are good examples of such a usage, already explained in detail in other answers.

In the end the rule is always the same: keep inside the source the things that change with the source, and keep outside the source the things that change independently from the source.

Annotations in Java, provide a mean to describe classes, fields and methods. Essentially, they are a form of metadata added to a Java source file, they can't affect the semantics of a program directly. However, annotations can be read at run-time using Reflection & this process is known as Introspection. Then it could be used to modify classes, fields or methods.

This feature, is often exploited by Libraries & SDKs (hibernate, JUnit, Spring Framework) to simplify or reduce the amount of code that a programmer would unless do in orer to work with these Libraries or SDKs.Therefore, it's fair to say Annotations and Reflection work hand-in hand in Java.

We also get to limit the availability of an annotation to either compile-time or runtime.Below is a simple example on creating a custom annotation

Driver.java

package io.hamzeen;

import java.lang.annotation.Annotation;

public class Driver {

    public static void main(String[] args) {
        Class<TestAlpha> obj = TestAlpha.class;
        if (obj.isAnnotationPresent(IssueInfo.class)) {

            Annotation annotation = obj.getAnnotation(IssueInfo.class);
            IssueInfo testerInfo = (IssueInfo) annotation;

            System.out.printf("%nType: %s", testerInfo.type());
            System.out.printf("%nReporter: %s", testerInfo.reporter());
            System.out.printf("%nCreated On: %s%n%n",
                    testerInfo.created());
        }
    }
}

TestAlpha.java

package io.hamzeen;

import io.hamzeen.IssueInfo;
import io.hamzeen.IssueInfo.Type;

@IssueInfo(type = Type.IMPROVEMENT, reporter = "Hamzeen. H.")
public class TestAlpha {

}

IssueInfo.java

package io.hamzeen;

import java.lang.annotation.ElementType;
import java.lang.annotation.Retention;
import java.lang.annotation.RetentionPolicy;
import java.lang.annotation.Target;

/**
 * @author Hamzeen. H.
 * @created 10/01/2015
 * 
 * IssueInfo annotation definition
 */
@Retention(RetentionPolicy.RUNTIME)
@Target(ElementType.TYPE)
public @interface IssueInfo {

    public enum Type {
        BUG, IMPROVEMENT, FEATURE
    }

    Type type() default Type.BUG;

    String reporter() default "Vimesh";

    String created() default "10/01/2015";
}

There are 2 views of annotations

1. user view, most of the time, annotations work like a shortcut, save you some key strokes, or make your program more readable

2. vendor view, the processor's view of annotation is more of light weighted 'interface', your program DOES confront to SOMETHING but without explicitly "implements" the particular interface(here aka the annotation)

e.g. in jpa you define something like

@Entity class Foo {...}

instead of

class Foo implements Entity {...}

both speak the same thing "Foo is an Entity class"

JPA (from Java EE 5) is an excellent example of the (over)use of annotations. Java EE 6 will also introduce annotations in lot of new areas, such as RESTful webservices and new annotations for under each the good old Servlet API.

Here are several resources:

• Sun - The Java Persistence API
• Java EE 5 tutorial - JPA
• Introducing the Java EE 6 platform (check all three pages).

It is not only the configuration specifics which are to / can be taken over by annotations, but they can also be used to control the behaviour. You see this good back in the Java EE 6's JAX-RS examples.

Is it a replacement for XML based configuration?

Not completely, but confguration that corresponds closely to code structures (such as JPA mappings or dependency injection in Spring) can often be replaced with annotations, and is then usually much less verbose, annoying and painful. Pretty much all notable frameworks have made this switch, though the old XML configuration usually remains as an option.

It attaches additional information about code by (a) compiler check or (b) code analysis

**

• Following are the Built-in annotations:: 2 types

**

Type 1) Annotations applied to java code:

@Override // gives error if signature is wrong while overriding.
Public boolean equals (Object Obj) 

@Deprecated // indicates the deprecated method
Public doSomething()....

@SuppressWarnings() // stops the warnings from printing while compiling.
SuppressWarnings({"unchecked","fallthrough"})

Type 2) Annotations applied to other annotations:

@Retention - Specifies how the marked annotation is stored—Whether in code only, compiled into the class, or available at run-time through reflection.

@Documented - Marks another annotation for inclusion in the documentation.

@Target - Marks another annotation to restrict what kind of java elements the annotation may be applied to

@Inherited - Marks another annotation to be inherited to subclasses of annotated class (by default annotations are not inherited to subclasses).

**

• Custom Annotations::

** http://en.wikipedia.org/wiki/Java_annotation#Custom_annotations

FOR BETTER UNDERSTANDING TRY BELOW LINK:ELABORATE WITH EXAMPLES

http://www.javabeat.net/2007/08/annotations-in-java-5-0/

Annotations are a form of metadata (data about data) added to a Java source file. They are largely used by frameworks to simplify the integration of client code. A couple of real world examples off the top of my head:

• JUnit 4 - you add the @Test annotation to each test method you want the JUnit runner to run. There are also additional annotations to do with setting up testing (like @Before and @BeforeClass). All these are processed by the JUnit runner, which runs the tests accordingly. You could say it's an replacement for XML configuration, but annotations are sometimes more powerful (they can use reflection, for example) and also they are closer to the code they are referencing to (the @Test annotation is right before the test method, so the purpose of that method is clear - serves as documentation as well). XML configuration on the other hand can be more complex and can include much more data than annotations can.

• Terracotta - uses both annotations and XML configuration files. For example, the @Root annotation tells the Terracotta runtime that the annotated field is a root and its memory should be shared between VM instances. The XML configuration file is used to configure the server and tell it which classes to instrument.

• Google Guice - an example would be the @Inject annotation, which when applied to a constructor makes the Guice runtime look for values for each parameter, based on the defined injectors. The @Inject annotation would be quite hard to replicate using XML configuration files, and its proximity to the constructor it references to is quite useful (imagine having to search to a huge XML file to find all the dependency injections you have set up).

Hopefully I've given you a flavour of how annotations are used in different frameworks.

Java EE 5 favors the use of annotations over XML configuration. For example, in EJB3 the transaction attributes on an EJB method are specified using annotations. They even use annotations to mark POJOs as EJBs and to specify particular methods as lifecycle methods instead of requiring that implementation of an interface.

It is useful for annotating your classes, either at the method, class, or field level, something about that class that is not quite related to the class.

You could have your own annotations, used to mark certain classes as test-use only. It could simply be for documentation purposes, or you could enforce it by filtering it out during your compile of a production release candidate.

You could use annotations to store some meta data, like in a plugin framework, e.g., name of the plugin.

Its just another tool, its has many purposes.

There are mutiple applications for Java's annotations. First of all, they may used by the compiler (or compiler extensions). Consider for example the Override annotation:

class Foo {

    @Override public boolean equals(Object other) {
        return ...;
    }
}

This one is actually built into the Java JDK. The compiler will signal an error, if some method is tagged with it, which does not override a method inherited from a base class. This annotation may be helpful in order to avoid the common mistake, where you actually intend to override a method, but fail to do so, because the signature given in your method does not match the signature of the method being overridden:

class Foo {

    @Override public boolean equals(Foo other) {  // Compiler signals an error for this one
        return ...;
    }
}

As of JDK7, annotations are allowed on any type. This feature can now be used for compiler annotations such as NotNull, like in:

public void processSomething(@NotNull String text) {
    ...
}

which allows the compiler to warn you about improper/unchecked uses of variables and null values.

Another more advanced application for annotations involves reflection and annotation processing at run-time. This is (I think) what you had in mind when you speak of annotations as "replacement for XML based configuration". This is the kind of annotation processing used, for example, by various frameworks and JCP standards (persistence, dependency injection, you name it) in order to provide the necessary meta-data and configuration information.

Following are some of the places where you can use annotations.

a. Annotations can be used by compiler to detect errors and suppress warnings
b. Software tools can use annotations to generate code, xml files, documentation etc., For example, Javadoc use annotations while generating java documentation for your class.
c. Runtime processing of the application can be possible via annotations.
d. You can use annotations to describe the constraints (Ex: @Null, @NotNull, @Max, @Min, @Email).
e. Annotations can be used to describe type of an element. Ex: @Entity, @Repository, @Service, @Controller, @RestController, @Resource etc.,
f. Annotation can be used to specify the behaviour. Ex: @Transactional, @Stateful
g. Annotation are used to specify how to process an element. Ex: @Column, @Embeddable, @EmbeddedId
h. Test frameworks like junit and testing use annotations to define test cases (@Test), define test suites (@Suite) etc.,
i. AOP (Aspect Oriented programming) use annotations (@Before, @After, @Around etc.,)
j. ORM tools like Hibernate, Eclipselink use annotations

You can refer this link for more details on annotations.

You can refer this link to see how annotations are used to build simple test suite.

Where Annotations Can Be Used

Annotations can be applied to declarations: declarations of classes, fields, methods, and other program elements. When used on a declaration, each annotation often appears, by convention, on its own line.

Java SE 8 Update: annotations can also be applied to the use of types. Here are some examples:

• Class instance creation expression:

  new @Interned MyObject();

• Type cast:

  myString = (@NonNull String) str;

• implements clause:

  class UnmodifiableList implements @Readonly List<@Readonly T> { ... }

• Thrown exception declaration:

  void monitorTemperature() throws @Critical TemperatureException { ... }