How annotation check validation at runtime

Question

I am working with custom annotation building ,I am creating some custom annotation that will validating some data.I am not able to write the annotation processor for that annotation.

i.e. I created a custom annotation UserRole as :

import java.lang.annotation.ElementType;
import java.lang.annotation.Retention;
import java.lang.annotation.RetentionPolicy;
import java.lang.annotation.Target;

@Retention(RetentionPolicy.RUNTIME)
@Target({ ElementType.PARAMETER })
public @interface UserRole
{
    int[] value() default {0};
}

I will use this annotation as follow :

import java.util.List;
public class Demo
{
    public void checkValidUser(@UserRole({1,2})List<Integer> roles){
        // some code here
        }
}

I want to write Annotation processor so that I can check if given list contain any role that is specified in annotation at runtime because I will provide List at runtime. I need help to write that annotation processor.

Thanks in Advance.

Answer 1

After your last comment lets try to understand how does how does jpa works

when you use dao.find(id, class); the entity manager checks class metadata like( primary key and tablename... etc) which is parsed before .. (as i was trying tell you in the classInfo example) and adds it to entitymanager has map field

Map<Class,EntityInfo>

named as entities ( for my example it must be map must be like that Map ) when you run dao.find(id,User.class) .. it checks that map. gets the id field and table information and parsers from there ( parses sql something like this "select * from" + entity.getTableName() +" where "+ entity.getIdField().getName() +" = "+ id ) and runs query and returns the result ... i hope now you can understand what i was trying to tell you ..

How and where are Annotations used in Java? ... good answer first read what is annotation ..

Annotation describe way what the method , field or class will do .. so it is not passing parameter, it is defining a rule ... for your example if there is 3 type of user .. Admin Normal Guest and u have a method which can be call by only user types Admin .. like deleteProduct..

u have to use :

@UserRoles([Admin])
public void deleteProduct(){
///......
} 

@UserRoles([Admin,Normal])
public void commentProduct() {}

for example you have an url with
www.site.com/comments/15/likes?offset=15

and you are writing an webroute handler .. which has annotations @WebMethod (Post , Get , Put, Delete etc) , @WebParameter (POST and GET PARAMETER , @URLParam( URL PARAM handler) ;

comment/:id/likes/

you have to do that like that ..

public class Comments {
 // handles url path like  /comments/12/likes?offset=15 
 @WebMethod(MethodType.GET)
  @WebURL("comment/:id/likes/") 
  public void listLikes(@WebParameter(name="offset") int offset, @URLParam("id") long id) {
  //operatio to Do  so here offset will be 15  ,id 12
}
}

the logic of annotations something like that .. Here is an another example ..

First you have to create a holder Object ( which holds all information readed by reflection ) so you can avolute it with proxy of object ..

For example if you have annotation like @Id and @ColumnName for Fields , and @Async for method ...

you have to write to Class Object FieldInfo which has field Id ,name

For Class

 class ClassInfo {
   private  final      Set<MethodInfo> methods; 
   private final Set<FieldInfo> fields; 
}

For Fields :

 class FieldInfo {
    private   boolean id=false; 
    private Field fieldInfo ; 
    private  String columnName;
    /// More info for fields like field name  
    // constructors 
    //Getters and Setters 
         //methods to use 
} 

For Methods :

 class MethodInfo {
       private Method methodInfo; 
       private  boolean async= false;  
        /// More info for methods  like field name  
            // constructors 
            //Getters and Setters 
         //methods to use 
    }

So u can use reflection ( i do it only for Field you can do it for method and method parameters as like as you want )

  final   ClassInfo classInfo = new ClassInfo(); 
  Field[] fields =   Demo.class.getFields(); 
 for(final field :fields) {
    FieldInfo fieldInfo = new FieldInfo(field);
    if(field.hasAnnotation(Id.class)) {
      fieldInfo.setId(true); 
   } 
    if(field.hasAnnotation(ColumnName.class){
     final ColumnName col = field.getAnnotation(ColumnName.class); 
     fieldInfo.setColumnName(col.value());
    }
 }

and methods , method parameters you have parse what u have to do with it ..

i hope this information helps you ... if you have any more question , feel free to ask