Java Annotations Tutorial with Custom Annotation Example and Parsing using Reflection

Java Annotations provide information about the code and they have no direct effect on the code they annotate. In this tutorial, we will learn about Java annotations, how to write custom annotation, annotations usage and how to parse annotations using reflection.

java-annotations

Annotations are introduced in Java 1.5 and now it’s heavily used in Java frameworks like Hibernate, Jersey, Spring. Annotation is metadata about the program embedded in the program itself. It can be parsed by the annotation parsing tool or by compiler. We can also specify annotation availability to either compile time only or till runtime also.

Before annotations, program metadata was available through java comments or by javadoc but annotation offers more than that. It not only contains the metadata but it can made it available to runtime 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 Annotations in Java

Creating custom annotation is similar to writing an interface, except that it interface keyword is prefixed with @ symbol. We can declare methods in annotation. Let’s see annotation example and then we will discuss it’s features.

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();
}
  • Annotation methods can’t have parameters.
  • Annotation methods return types are limited to primitives, String, Enums, Annotation or array of these.
  • Annotation methods can have default values.
  • Annotations can have meta annotations attached to them. Meta annotations are used to provide information about the annotation. There are four types of meta annotations:
    1. @Documented – indicates that elements using this annotation should be documented by javadoc and similar tools. This type should be used to annotate the declarations of types whose annotations affect the use of annotated elements by their clients. If a type declaration is annotated with Documented, its annotations become part of the public API of the annotated elements.
    2. @Target – indicates the kinds of program element to which an annotation type is applicable. Some possible values are TYPE, METHOD, CONSTRUCTOR, FIELD etc. If Target meta-annotation is not present, then annotation can be used on any program element.
    3. @Inherited – indicates that an annotation type is automatically inherited. If user queries the annotation type on a class declaration, and the class declaration has no annotation for this type, then the class’s superclass will automatically be queried for the annotation type. This process will be repeated until an annotation for this type is found, or the top of the class hierarchy (Object) is reached.
    4. @Retention – indicates how long annotations with the annotated type are to be retained. It takes RetentionPolicy argument whose Possible values are SOURCE, CLASS and RUNTIME

Java Built-in Annotations

Java Provides three built-in annotations.

  1. @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.
  2. @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.
  3. @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.

Let’s see a java example showing use of built-in annotations as well as use of custom annotation created by us in 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 example is self explanatory and showing use of annotations in different cases.

Java Annotations Parsing

We will use Reflection to parse java annotations from a class. Please note that Annotation Retention Policy should be RUNTIME otherwise it’s information will not be available at runtime and we wont 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 annotation tutorial, I hope you learned something from it.

Updates:

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