Java : Annotation : Example of validation with annotation (mandatory, target, maxlength…)

Hello,

I would present you an example concerning the annotation in JAVA: validation POJO with annotation.

Brief presentation
Introduced with Java SE 5, the annotations are more increasingly used in the development of applications. Annotations provide informations about a class and they have no direct effect on the annotated code. Annotations can be preserved at runtime (RetentionPolicy.RUNTIME) or are only available at development time, during the compile (RetentionPolicy.SOURCE).

There are standard annotations and Java allows the definition of custom annotations.

For example, the following standard annotations are often used in Java:
@Override:
This annotation added to a method, notifies the Java compiler to check if the annotated method really overrides a method of an interface or the extended class.

@Deprecated
This annotation added to a field, method or constructor, indicates that the annotated element should not be used anymore. It is possible to add this annotation to a class, however, this doesn’t deprecate automatically all its fields and methods.

@Target
This annotation specifies where it is legal to use an annotation type. The possible values are constants from the
java.lang.annotation.ElementType enumeration which provides a simple classification of the declared elements in a Java program.
There are the following constants:

• ANNOTATION_TYPE – Annotation type declaration – used for other annotations.
  view source

  print?

  1	@MyAnnotation

  2	public @interface AnotherAnnotation {..}

• CONSTRUCTOR – Constructor declaration
  view source

  print?

  1	public class MyAnnotatedClass {

  2	@MyAnnotation

  3	public MyAnnotatedClass() {..}

  4

  }

• FIELD – Field declaration (includes enum constants)
  view source

  print?

  1	@MyAnnotation

  2	private String myAnnotatedField;

• LOCAL_VARIABLE – Local variable declaration – It can’t be read at runtime, so it is used only for compile-time things, like the @SuppressWarnings annotation.
  view source

  print?

  1	public void someMethod() {

  2	@MyAnnotation int myLocalVariable = 0;

  3

  }

• METHOD – Method declaration
  view source

  print?

  1	@MyAnnotation

  2	public void myAnnotatedMethod() {..}

• PACKAGE – Package declaration
  view source

  print?

  1	@MyAnnotation

  2	package com.ho.myannotatedpackage;

• PARAMETER – Parameter declaration
  view source

  print?

  1	public void myMethod(@MyAnnotation param) {..}

• TYPE – Class, interface (including annotation type), or enum declaration
  view source

  print?

  1	@MyAnnotation

  2	public class MyAnnotatedClass {..}

It is possible to specify multiple ElementTypes for a given annotation:

Example : validation with annotations

When an annotation is implemented, it is necessary to tell two things to the compiler:

• where it can be applied (that’s the @Target annotation);
• what its retention policy is (aptly named @RetentionPolicy; some annotations are kept only at the source level, others at runtime).

Below, the official description of annotations retention policy:

In our example, we will use the annotations with the JDK 1.6 for validate the POJO values on two criterias ans set its name:

• an annotation Mandatory to specify if a field is mandatory or not,
• an annotation MaxLength to specify the max length of a String field,
• an annotation FiledName to set the field’s name,

STEP 1
Create a new project named test_annot_valid with the following librairies in test_annot_valid\lib folder:

• commons-beanutils-1.9.0.jar,
• commons-lang-2.6.jar,
• commons-logging-1.1.3.jar,

For information, our project structure will be:

STEP 2
We will create the definition of new annotations by creating 3 new classes in the package “com.ho.annotations.common.annotations”:

FieldName:

Mandatory:

Maxlength:

STEP 3
Then, it is necessary to create POJO on whom the previous annotations must be applied. The class AbstractEntity will contain a list of errors corresponding to validation errors:

And an error entity ErrorEntity class representing one error that occured when validating the object:

Finally, the entity class BookEntity will be validated:

So, the book entity has the following fields:

• id containing the technical ID : mandatory field named ID,
• name corresponding to the book’s name : mandatory field named NAME with a max length of 10 characters,
• price corresponding to the book’s price : mandatory field named PRICE,
• numberOfRead corresponding to the number of read of this book : no mandatory field named NB_OF_READ,
• lastReadDateTime corresponding to the last read datetime of this book : no mandatory field named LAST_READ_DATETIME,
• authors containing the book’s authors : no mandatory field named AUTHORS,

STEP 4
In this step, we create an utility class ValidatorUtil to handle the validation of entities:

The important point of this class is the part concerning the checking of annotations on entity object:

STEP 5
Finally, we are creating a main method TestValidator to check our valiation based on annotations:

The outputs are:

That’s all!!!!

Huseyin OZVEREN