[Java] Custom Annotation Example @Inject

Java Custom annotation : @Inject

 - JVM can read annotation in runtime with @Retention
 - if field has @Inject annotation, set value in runtime  

import java.lang.reflect.Field;

import static java.lang.annotation.ElementType.FIELD;

import static java.lang.annotation.RetentionPolicy.RUNTIME;

import java.lang.annotation.Retention;

import java.lang.annotation.Target;

@Target(FIELD)

@Retention(RUNTIME)

public @interface Inject {

    Class<?> adapter() default Object.class;

}

public class ExAnnotation {

@Inject

String field1;

String field2;

public static void main(String[] args) {

// TODO Auto-generated method stub

ExAnnotation inst = new ExAnnotation();

Field[] fields = inst.getClass().getDeclaredFields();

for (Field field: fields) {

//System.out.println(field.getName());

//This can be retrieve when runtime, using @Retention 

Inject inject = field.getAnnotation(Inject.class);

if (inject != null) {

System.out.println("Inject exist:" + field.getName());

Class<?> fc = field.getType();

boolean tf = String.class.isAssignableFrom(fc);

if (tf == true) {

try {

field.setAccessible(true);

field.set(inst , "insert default string value");

}

catch (IllegalArgumentException x) {

x.printStackTrace();

}

catch (IllegalAccessException x) {

x.printStackTrace();

}

}

}

}

System.out.println("field1:" + inst.field1);

System.out.println("field2:" + inst.field2);

}

}