Java Annotations provides information about the code. Java annotations have no direct effect on the code they annotate. In java annotations tutorial, we will look into the following;
Java 1.5 introduced annotations and now it’s heavily used in Java EE frameworks like Hibernate, Jersey, and Spring. Java Annotation is metadata about the program embedded in the program itself. It can be parsed by the annotation parsing tool or by the compiler. We can also specify annotation availability to either compile time only or till runtime. Before java annotations, program metadata was available through java comments or by Javadoc but annotation offers more than that. Annotations metadata can be available at runtime too and annotation parsers can use it to determine the process flow. For example, in Jersey webservice we add PATH annotation with URI string to a method and at runtime, jersey parses it to determine the method to invoke for given URI pattern.
Creating custom annotation is similar to writing an interface, except that the interface keyword is prefixed with _@_ symbol. We can declare methods in annotation. Let’s see java custom annotation example and then we will discuss its features and important points.
package com.journaldev.annotations;
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
@Target(ElementType.METHOD)
@Inherited
@Retention(RetentionPolicy.RUNTIME)
public @interface MethodInfo{
String author() default "Pankaj";
String date();
int revision() default 1;
String comments();
}
Some important points about java annotations are:
There are five types of meta annotations:
Java Provides five built-in annotations.
@Override
- When we want to override a method of Superclass, we should use this annotation to inform compiler that we are overriding a method. So when superclass method is removed or changed, compiler will show error message. Learn why we should always use java override annotation while overriding a method.@Deprecated
- when we want the compiler to know that a method is deprecated, we should use this annotation. Java recommends that in javadoc, we should provide information for why this method is deprecated and what is the alternative to use.@SuppressWarnings
- This is just to tell compiler to ignore specific warnings they produce, for example using raw types in java generics. It’s retention policy is SOURCE and it gets discarded by compiler.@FunctionalInterface
- This annotation was introduced in Java 8 to indicate that the interface is intended to be a functional interface.@SafeVarargs
- A programmer assertion that the body of the annotated method or constructor does not perform potentially unsafe operations on its varargs parameter.Let’s see a java example showing the use of built-in annotations in java as well as the use of custom annotation created by us in the above example.
package com.journaldev.annotations;
import java.io.FileNotFoundException;
import java.util.ArrayList;
import java.util.List;
public class AnnotationExample {
public static void main(String[] args) {
}
@Override
@MethodInfo(author = "Pankaj", comments = "Main method", date = "Nov 17 2012", revision = 1)
public String toString() {
return "Overriden toString method";
}
@Deprecated
@MethodInfo(comments = "deprecated method", date = "Nov 17 2012")
public static void oldMethod() {
System.out.println("old method, don't use it.");
}
@SuppressWarnings({ "unchecked", "deprecation" })
@MethodInfo(author = "Pankaj", comments = "Main method", date = "Nov 17 2012", revision = 10)
public static void genericsTest() throws FileNotFoundException {
List l = new ArrayList();
l.add("abc");
oldMethod();
}
}
I believe above java annotation example is self-explanatory and showing the use of annotations in different cases.
We will use Reflection to parse java annotations from a class. Please note that Annotation Retention Policy should be RUNTIME otherwise its information will not be available at runtime and we won’t be able to fetch any data from it.
package com.journaldev.annotations;
import java.lang.annotation.Annotation;
import java.lang.reflect.Method;
public class AnnotationParsing {
public static void main(String[] args) {
try {
for (Method method : AnnotationParsing.class.getClassLoader()
.loadClass(("com.journaldev.annotations.AnnotationExample")).getMethods()) {
// checks if MethodInfo annotation is present for the method
if (method.isAnnotationPresent(com.journaldev.annotations.MethodInfo.class)) {
try {
// iterates all the annotations available in the method
for (Annotation anno : method.getDeclaredAnnotations()) {
System.out.println("Annotation in Method '" + method + "' : " + anno);
}
MethodInfo methodAnno = method.getAnnotation(MethodInfo.class);
if (methodAnno.revision() == 1) {
System.out.println("Method with revision no 1 = " + method);
}
} catch (Throwable ex) {
ex.printStackTrace();
}
}
}
} catch (SecurityException | ClassNotFoundException e) {
e.printStackTrace();
}
}
}
Output of the above program is:
Annotation in Method 'public java.lang.String com.journaldev.annotations.AnnotationExample.toString()' : @com.journaldev.annotations.MethodInfo(author=Pankaj, revision=1, comments=Main method, date=Nov 17 2012)
Method with revision no 1 = public java.lang.String com.journaldev.annotations.AnnotationExample.toString()
Annotation in Method 'public static void com.journaldev.annotations.AnnotationExample.oldMethod()' : @java.lang.Deprecated()
Annotation in Method 'public static void com.journaldev.annotations.AnnotationExample.oldMethod()' : @com.journaldev.annotations.MethodInfo(author=Pankaj, revision=1, comments=deprecated method, date=Nov 17 2012)
Method with revision no 1 = public static void com.journaldev.annotations.AnnotationExample.oldMethod()
Annotation in Method 'public static void com.journaldev.annotations.AnnotationExample.genericsTest() throws java.io.FileNotFoundException' : @com.journaldev.annotations.MethodInfo(author=Pankaj, revision=10, comments=Main method, date=Nov 17 2012)
Reflection API is very powerful and used widely in Java, J2EE frameworks like Spring, Hibernate, JUnit, check out Reflection in Java. That’s all for the java annotations example tutorial, I hope you learned something from it. Java Annotations Updates:
Reference: Oracle website
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Hi Pankaj, What do you mean when you say Retention policy is Source or Target. Can you give an example as well?
- Sagar Vora
Hi thanks for this tutorial. Please go for writtring an annotation processor thanks alot
- hossein
All the concepts are easily explained. Post some more concepts of java 8 i need to know some basics, Thanks for sharing.
- priya
very - very nice article about java annotation. the example is also very nice and simple very useful post thank you for this post
- Anurag Singh
good sample with completed example, compared to other tutorials, which got various inconveniences to learner, like java files lumped together, or missing “import” some classes, etc. good work …
- Victor
HI Buddy thanks for sharing useful and helpful information about Annotation.The Target annotation specifies which elements of our code can have annotations of the defined type. A concrete example will help explain this concept. Add the Target annotation to the Task annotation we defined earlier in the last post, as shown here: https://www.mindstick.com/Articles/12141/annotations-in-java-target-and-retention
- Royceroy
Good explanation. Succinct and to the point without being verbose. My first working annotation! Thank you.
- Shiraz
Hey Hi, I’m new to Java world and am trying to explore more thus am not sure if my question is worth your time - i want to initialize a bean with some parameters using java annotations. I’m able to achieve the same using XML though. Background - I’m trying to fetch the student (bean) details from classs (bean) where i want to initialize the studentid, student name and student age at bean initialization time. Any help here will be a great support
- Mann
Hi Pankaj, Question: I currently have the situation that I can’t import the 3rd party package “com.journaldev.annotations” in my class so I have to load it using reflection. In this case, can you guide me how to use reflection to implement the toString() below with the @MethodInfo annotation? Thank you so much. @Override @MethodInfo(author = “Pankaj”, comments = “Main method”, date = “Nov 17 2012”, revision = 1) public String toString() { return “Overriden toString method”; }
- Key Naugh
Hi, please tell me how to do the same with method parammeters
- Palak