Annotation is code about the code, that is metadata about the program
itself. In other words, organized data about the code, embedded within
the code itself. It can be parsed by the compiler, annotation processing
tools and can also be made available at run-time too.
We have basic java comments infrastructure using which we add
information about the code / logic so that in future, another programmer
or the same programmer can understand the code in a better way. Javadoc
is an additional step over it, where we add information about the
class, methods, variables in the source code. The way we need to add is
organized using a syntax. Therefore, we can use a tool and parse those
comments and prepare a javadoc document which can be distributed
separately.
Javadoc facility gives option for understanding the code in an
external way, instead of opening the code the javadoc document can be
used separately. IDE benefits using this javadoc as it is able to render
information about the code as we develop. Annotations were introduced
in JDK 1.5
Uses of Annotations
Annotations are far more powerful than java comments and javadoc
comments. One main difference with annotation is it can be carried over
to runtime and the other two stops with compilation level. Annotations
are not only comments, it brings in new possibilities in terms of
automated processing.
In java we have been passing information to compiler for long. For example take serialization,
we have the keyword transient to tell that this field is not
serializable. Now instead of having such keywords decorating an
attribute annotations provide a generic way of adding information to
class/method/field/variable. This is information is meant for
programmers, automated tools, java compiler and runtime. Transient is a
modifier and annotations are also a kind of modifiers.
More than passing information, we can generate code using these annotations. Take webservices
where we need to adhere by the service interface contract. The skeleton
can be generated using annotations automatically by a annotation
parser. This avoids human errors and decreases development time as
always with automation.
Frameworks like Hibernate, Spring, Axis make heavy use of
annotations. When a language needs to be made popular one of the best
thing to do is support development of frameworks based on the language.
Annotation is a good step towards that and will help grow Java.
When Not to Use Annotations
- Do not over use annotation as it will pollute the code.
- It is better not to try to change the behaviour of objects using
annotations. There is sufficient constructs available in oops and
annotation is not a better mechanism to deal with it.
- We should not what we are parsing. Do not try to over generalize as
it may complicate the underlying code. Code is the real program and
annotation is meta.
- Avoid using annotation to specify environment / application / database related information.
Annotation Structure
There are two main components in annotations. First is annotation
type and the next is the annotation itself which we use in the code to
add meaning. Every annotation belongs to a annotation type.
Annotation Type:
@interface <annotation-type-name> {
method declaration;
}
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Annotation type is very similar to an interface with little difference.
- We attach ‘@’ just before interface keyword.
- Methods will not have parameters.
- Methods will not have throws clause.
- Method return types are restricted to primitives, String, Class, enums, annotations, and arrays of the preceding types.
- We can assign a default value to method.
Meta Annotations
Annotations itself is meta information then what is meta annotations?
As you have rightly guessed, it is information about annotation. When
we annotate a annotation type then it is called meta annotation. For
example, we say that this annotation can be used only for methods.
@Target(ElementType.METHOD)
public @interface MethodInfo { }
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Annotation Types
- Documented
When a annotation type is annotated with @Documented then wherever this
annotation is used those elements should be documented using Javadoc
tool.
- Inherited
This meta annotation denotes that the annotation type can be inherited
from super class. When a class is annotated with annotation of type that
is annotated with Inherited, then its super class will be queried till a
matching annotation is found.
- Retention
This meta annotation denotes the level till which this annotation will
be carried. When an annotation type is annotated with meta annotation
Retention, RetentionPolicy has three possible values:
@Retention(RetentionPolicy.RUNTIME)
public @interface Developer {
String value();
}
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- Class
When the annotation value is given as ‘class’ then this annotation will be compiled and included in the class file.
- Runtime
The value name itself says, when the retention value is ‘Runtime’ this
annotation will be available in JVM at runtime. We can write custom code
using reflection package and parse the annotation. I have give an
example below.
- Source
This annotation will be removed at compile time and will not be available at compiled class.
- Target
This meta annotation says that this annotation type is applicable for
only the element (ElementType) listed. Possible values for ElementType
are, CONSTRUCTOR, FIELD, LOCAL_VARIABLE, METHOD, PACKAGE, PARAMETER,
TYPE.
@Target(ElementType.FIELD)
public @interface FieldInfo { }
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Built-in Java Annotations
@Documented, @Inherited, @Retention and @Target are the four available meta annotations that are built-in with Java.
Apart from these meta annotations we have the following annotations.
@Override
When we want to override a method, we can use this annotation to say
to the compiler we are overriding an existing method. If the compiler
finds that there is no matching method found in super class then
generates a warning. This is not mandatory to use @Override when we
override a method. But I have seen Eclipse IDE automatically adding this
@Override annotation. Though it is not mandatory, it is considered as a
best practice.
@Deprecated
When we want to inform the compiler that a method is deprecated we
can use this. So, when a method is annotated with @Deprecated and that
method is found used in some place, then the compiler generates a
warning.
@SuppressWarnings
This is like saying, “I know what I am doing, so please shut up!” We
want the compiler not to raise any warnings and then we use this
annotation.
Custom Annotations
We can create our own annotations and use it. We need to declare a
annotation type and then use the respective annotation is java classes.
Following is an example of custom annotation, where this annotation
can be used on any element by giving values. Note that I have used
@Documented meta-annotation here to say that this annotation should be
parsed by javadoc.
@Documented
public @interface Team {
int teamId();
String teamName();
String teamLead() default "[unassigned]";
String writeDate(); default "[unimplemented]";
}
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Annotation for the Above Example Type
... a java class ...
@Team(
teamId = 73,
teamName = "Rambo Mambo",
teamLead = "Yo Man",
writeDate = "3/1/2012"
)
public static void readCSV(File inputFile) { ... }
... java class continues ...
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Marker Annotations
We know what a marker interface is. Marker annotations are similar to marker interfaces, yes they don’t have methods / elements.
/**
* Code annotated by this team is supreme and need
* not be unit tested - just for fun!
*/
public @interface SuperTeam { }
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... a java class ...
@SuperTeam public static void readCSV(File inputFile) { ... }
... java class continues ...
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In the above see how this annotation is used. It will look like one
of the modifiers for this method and also note that the parenthesis ()
from annotation type is omitted. As there are no elements for this
annotation, the parenthesis can be optionally omitted.
Single Value Annotations
There is a chance that an annotation can have only one element. In such a case that element should be named value.
/**
* Developer
*/
public @interface Developer {
String value();
}
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... a java class ...
@Developer("Popeye")
public static void readCSV(File inputFile) { ... }
... java class continues ...
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How to Parse Annotation
We can use reflection package to read annotations. It is useful when
we develop tools to automate a certain process based on annotation.
Example:
package com.annotations;
import java.lang.annotation.Retention;
import java.lang.annotation.RetentionPolicy;
@Retention(RetentionPolicy.RUNTIME)
public @interface Developer {
String value();
}
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package com.annotations;
public class BuildHouse {
@Developer ("Alice")
public void aliceMethod() {
System.out.println("This method is written by Alice");
}
@Developer ("Popeye")
public void buildHouse() {
System.out.println("This method is written by Popeye");
}
}
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package com.annotations;
import java.lang.annotation.Annotation;
import java.lang.reflect.Method;
public class TestAnnotation {
public static void main(String args[]) throws SecurityException,
ClassNotFoundException {
for (Method method : Class.forName(
"com.annotations.BuildHouse").getMethods()) {
if (method
.isAnnotationPresent(com.annotations.Developer.class)) {
try {
for (Annotation anno : method.getDeclaredAnnotations()) {
System.out.println("Annotation in Method '" + method
+ "' : " + anno);
Developer a = method.getAnnotation(Developer.class);
if ("Popeye".equals(a.value())) {
System.out.println("Popeye the sailor man! "
+ method);
}
}
} catch (Throwable ex) {
ex.printStackTrace();
}
}
}
}
}
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Output:
Annotation in Method 'public void com.annotations.BuildHouse.aliceMethod()' : @com.annotations.Developer(value=Alice)
Annotation in Method 'public void com.annotations.BuildHouse.buildHouse()' : @com.annotations.Developer(value=Popeye)
Popeye the sailor man! public void com.annotations.BuildHouse.buildHouse()
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apt / javac for Annotation Processing
In previous version of JDK apt was introduced as a tool for
annotation processing. Later apt was deprecated and the capabilities are
included in javac compiler. javax.annotation.processing and
javax.lang.model contains the api for processing.
