How and where are Annotations used in Java

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

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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

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Annotations are meta-meta-objects which can be used to describe other meta-objects  Meta-objects are classes  fields and methods  Asking an object for its meta-object  e g  anObj getClass     is called introspection  The introspection can go further and we can ask a meta-object what are its annotations  e g  aClass getAnnotations   Introspection and annotations belong to what is called reflection and meta-programming    An annotation needs to be interpreted in one way or another to be useful  Annotations can be interpreted at development-time by the IDE or the compiler  or at run-time by a framework    Annotation processing is a very powerful mechanism and can be used in a lot of different ways    to describe constraints or usage of an element  e g   Deprecated   Override  or  NotNull to describe the  nature  of an element  e g   Entity   TestCase   WebService to describe the behavior of an element   Statefull   Transaction  to describe how to process the element    Column   XmlElement   In all cases  an annotation is used to describe the element and clarify its meaning    Prior to JDK5  information that is now expressed with annotations needed to be stored somewhere else  and XML files were frequently used  But it is more convenient to use annotations because they will belong to the Java code itself  and are hence much easier to manipulate than XML   Usage of annotations    Documentation  e g  XDoclet Compilation IDE Testing framework  e g  JUnit IoC container e g  as Spring Serialization  e g  XML   Aspect-oriented programming  AOP   e g  Spring AOP Application servers  e g  EJB container  Web Service Object-relational mapping  ORM   e g  Hibernate  JPA and many more         have a look for instance at the project Lombok  which uses annotations to define how to generate equals or hashCode methods

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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

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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  amp  this process is known as Introspection  Then it could be used to modify classes  fields or methods   This feature  is often exploited by Libraries  amp  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 lt TestAlpha gt  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

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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

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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 lt  Readonly T          Thrown exception declaration     void monitorTemperature   throws          Critical TemperatureException

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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

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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

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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

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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

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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

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Frameworks like Hibernate were lots of configuration mapping is required uses Annotations heavily   Take a look at Hibernate Annotations

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There are 2 views of annotations    user view  most of the time  annotations work like a shortcut  save you some key strokes  or make your program more readable  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

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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

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

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