Example of Custom Annotation in Java
November 14, 2023
Find the sample code to create custom annotation.
@Target(ElementType.FIELD)
@Retention(RetentionPolicy.RUNTIME)
@interface MyAnnotation {
int maxLength();
}
The built-in annotation like
@Target is used to define that whether annotation will be applied on field or method.
@Retention defines the retention policy.
Example
Find the complete example to create custom annotation. We are creating an annotation for max length check.
package com.concretepage;
import java.lang.annotation.ElementType;
import java.lang.annotation.Retention;
import java.lang.annotation.RetentionPolicy;
import java.lang.annotation.Target;
import java.lang.reflect.Field;
@Target(ElementType.FIELD)
@Retention(RetentionPolicy.RUNTIME)
@interface MyAnnotation {
int maxLength();
}
class Person {
@MyAnnotation(maxLength = 10)
public String name;
public String getName() {
return name;
}
public void setName(String name) {
this.name = name;
}
}
class MyAnnotationHanlder {
public void handle(Object ob) throws Exception {
Field[] fields = ob.getClass().getFields();
for (Field field : fields) {
if (field.isAnnotationPresent(MyAnnotation.class)) {
MyAnnotation myAnn = field.getAnnotation(MyAnnotation.class);
int maxLen=myAnn.maxLength();
System.out.println("Max length is:"+maxLen);
if(maxLen<field.get(ob).toString().length()){
throw new Exception("You have eneterd string greater than max length."+maxLen);
}
}
}
}
}
public class CustomAnnotationTest {
public static void main(String[] args) throws Exception {
MyAnnotationHanlder parser = new MyAnnotationHanlder();
Person person= new Person();
person.setName("CONCRETEPAGE");
parser.handle(person);
}
}
Max length is:10 Exception in thread "main" java.lang.Exception: You have eneterd string greater than max length.10 at com.concretepage.MyAnnotationHanlder.handle(AnnotationTest.java:43) at com.concretepage.AnnotationTest.main(AnnotationTest.java:57)
