Java annotation - AbsoluteAstronomy.com

An annotation, in the Java computer programming language

Java (programming language)

Java is a programming language originally developed by James Gosling at Sun Microsystems and released in 1995 as a core component of Sun Microsystems' Java platform. The language derives much of its syntax from C and C++ but has a simpler object model and fewer low-level facilities...

, is a special form of syntactic metadata

Metadata

The term metadata is an ambiguous term which is used for two fundamentally different concepts . Although the expression "data about data" is often used, it does not apply to both in the same way. Structural metadata, the design and specification of data structures, cannot be about data, because at...

that can be added to Java

Java (programming language)

source code. Classes, methods, variables, parameters and packages may be annotated. Unlike Javadoc

Javadoc

Javadoc is a documentation generator from Sun Microsystems for generating API documentation in HTML format from Java source code.The "doc comments" format used by Javadoc is the de facto industry standard for documenting Java classes. Some IDEs, such as Netbeans and Eclipse automatically generate...

tags, Java annotations can be reflective

Reflection (computer science)

In computer science, reflection is the process by which a computer program can observe and modify its own structure and behavior at runtime....

in that they can be embedded in class files

Class (file format)

In the Java programming language, source files are compiled into machine-readable class files which have a .class extension. Since Java is a platform-independent language, source code is compiled into an output file known as bytecode, which it stores in a .class file. If a source file has more...

generated by the compiler and may be retained by the Java VM to be made retrievable at run-time.

Examples

// @Twizzle is an annotation to method toggle.
@Twizzle
public void toggle {
}

// Declares the annotation Twizzle.
public @interface Twizzle {
}

Annotations may include an optional list of key-value pairs:

// Same as: @Edible(value = true)
@Edible(true)
Item item = new Carrot;

public @interface Edible {
boolean value default false;
}

@Author(first = "Oompah", last = "Loompah")
Book book = new Book;

public @interface Author {
String first;
String last;
}

Annotations themselves may be annotated to indicate where and when they can be used:

@Retention(RetentionPolicy.RUNTIME) // Make this annotation accessible at runtime via reflection.
@Target({ElementType.METHOD}) // This annotation can only be applied to class methods.
public @interface Tweezable {
}

The compiler reserves a set of special annotations (including @Deprecated, @Override and @SuppressWarnings) for syntactic purposes.

Annotations are often used by frameworks

Software framework

In computer programming, a software framework is an abstraction in which software providing generic functionality can be selectively changed by user code, thus providing application specific software...

as a way of conveniently applying behaviours to user-defined classes and methods that must otherwise be declared in some external source (such as an XML configuration file) or programmatically (with API calls). The following, for example, is an annotated EJB 3.0 data class:

@Entity // Declares this an entity bean
@Table(name = "people") // Maps the bean to SQL table "people"
class Person implements Serializable {
@Id // Map this to the primary key column.
@GeneratedValue(strategy = GenerationType.AUTO) // Database will generate new primary keys, not us.
private Integer id;

@Column(length = 32) // Truncate column values to 32 characters.
private String name;

public Integer getId {
return id;
}

public void setId(Integer id) {
this.id = id;
}

public String getName {
return name;
}

public void setName(String name) {
this.name = name;
}
}

A complete example is given below:

package com.annotation;

import java.lang.annotation.Documented;
import java.lang.annotation.ElementType;
import java.lang.annotation.Inherited;
import java.lang.annotation.Retention;
import java.lang.annotation.RetentionPolicy;
import java.lang.annotation.Target;
@Documented
@Retention(RetentionPolicy.RUNTIME)
@Target({ElementType.TYPE,ElementType.METHOD,
ElementType.CONSTRUCTOR,ElementType.ANNOTATION_TYPE,
ElementType.PACKAGE,ElementType.FIELD,ElementType.LOCAL_VARIABLE})
@Inherited

public @interface Unfinished {
public enum Priority { LOW, MEDIUM, HIGH }
String value;
String[] changedBy default "";
String[] lastChangedBy default "";
Priority priority default Priority.MEDIUM;
String createdBy default "Aqif Hamid";
String lastChanged default "08/07/2011";
}

package com.annotation;

public @interface UnderConstruction {
String owner default "Aqifg";
String value default "Object is Under Construction.";
String createdBy default "Navin Gujarish";
String lastChanged default "08/07/2011";
}

package com.validators;

import javax.faces.application.FacesMessage;
import javax.faces.component.UIComponent;
import javax.faces.context.FacesContext;
import javax.faces.validator.Validator;
import javax.faces.validator.ValidatorException;

import com.annotation.UnderConstruction;
import com.annotation.Unfinished;
import com.annotation.Unfinished.Priority;
import com.util.Util;

@UnderConstruction(owner="Navin Gujarish")
public class DateValidator implements Validator{

public void validate(FacesContext context, UIComponent component, Object value)
throws ValidatorException
{
String date = (String) value;
String errorLabel = "Please enter a valid date.";
if(!component.getAttributes.isEmpty)
{
errorLabel = (String) component.getAttributes.get("errordisplayval");
}

if(!Util.validateAGivenDate(date))
{
@Unfinished(changedBy = "Aqif"
,value="whether to add message to context or not, confirm"
,priority=Priority.HIGH
)
FacesMessage message = new FacesMessage;
message.setSeverity(FacesMessage.SEVERITY_ERROR);
message.setSummary(errorLabel);
message.setDetail(errorLabel);
throw new ValidatorException(message);
}

}
}

The annotations are not method calls and will not, by themselves, do anything. Rather, the class object is passed to EJB implementation at run-time, which then extracts the annotations to generate an ORM

Object-relational mapping

Object-relational mapping in computer software is a programming technique for converting data between incompatible type systems in object-oriented programming languages. This creates, in effect, a "virtual object database" that can be used from within the programming language...

Processing

When Java source code is compiled, annotations can be processed by compiler plug-ins called annotation processors. Processors can produce informational messages or create additional Java source files or resources, which in turn may be compiled and processed, and also modify the annotated code itself. The Java compiler conditionally stores annotation metadata in the class files if the annotation has a RetentionPolicy of CLASS or RUNTIME. Later, the JVM or other programs can look for the metadata to determine how to interact with the program elements or change their behavior.

History

The Java platform has had various ad-hoc annotation mechanisms—for example, the transient modifier, or the @deprecated javadoc tag. The general purpose annotation (also known as metadata) facility was introduced to the Java Community Process

Java Community Process

The Java Community Process or JCP, established in 1998, is a formalized process that allows interested parties to get involved in the definition of future versions and features of the Java platform....

as JSR-175 in 2002 and approved in September 2004. Annotations became available in the language itself beginning with version 1.5 of the JDK. A provisional interface for compile-time annotation processing was provided by the apt tool in JDK version 1.5, and was formalized through JSR-269 and integrated into the javac

Javac

javac is the primary Java compiler, included in the Java Development Kit from Oracle Corporation.The compiler accepts source code conforming to the Java language specification and produces bytecode conforming to the Java Virtual Machine Specification .javac is itself written in Java...

compiler in version 1.6.

External links

The source of this article is wikipedia, the free encyclopedia. The text of this article is licensed under the GFDL.

Examples

Processing

History

See also

External links