使用自定义注释

Question

I found several related (not duplicate) question to this, but they didn't satisfy me.

I am unable to understand where and why to use custom annotations ?

I read an example of custom annotation in a book, but it was not explained thoroughly.

@interface MyAnno
{
    String str();
    int val();
}

class MyClass
{
    @MyAnno(str = "Annotation example", val = 100)
    public static void myMeth()
    {
        System.out.println("Inside myMeth()");
    }
}

class CustomAnno
{
    public static void main(String args[])
    {
        MyClass.myMeth();
    }
}

The output is as expected Inside myMeth() .

I am having few questions regarding this example.

1- How can I use String str() and int val() in this program? OR

What is the use of any abstract method of an custom annotation ?

2- Why custom annotations . I mean that what effect they are having on any code.

3- How can I create an annotation which is having effects like @override is having?(I mean any kind of effect which can be noticed)

If this example is useless for you, then please give me a suitable small example in which a custom annotation is used.

Answer 1

Three main reasons to use custom annotations are:

• To reduce the effort of writing code (a compile-time annotation processor generates code for you). Here is a tutorial: part 1 , part 2 .
• To provide additional correctness guarantees (a compile-time annotation processor warns you about errors). One nice tool for this is the Checker Framework , which prevents null pointer dereferences, concurrency errors, and more.
• To customize behavior (at run time, your code checks for the annotation using reflection and behaves differently depending on whether the annotation is present). Frameworks such as Hibernate use annotations this way; also see an Oracle article .

In each case, use of annotations reduces the likelihood of errors in your code, compared to other non-annotation approaches.

Answer 2

Here is a minimal example. The following code demonstrates use of custom annotation.

It is about Employees and Benefits. If we have a requirement such that BasicBenefits has to be applied to all types of employess then we can come up with custom annotation such as BasicBenefits, and annotate all types of Employee implementations (eg CorporateEmployee, ContractEmployee, ManagerEmployee etc. etc.) with the BasicBenefits.

Custom Annotation Class(interface)

import java.lang.annotation.*;
@Inherited
@Documented
@Target(ElementType.TYPE)
@Retention(RetentionPolicy.RUNTIME)

@interface BasicBenefits {
    String bId() default "B-101";
    String bName() default "General Class A Employee";
}

Class using the custom annotation(no need of any imports):

@BasicBenefits(bId="B-400", bName="General Plus Class A Employee")
public class Employee {
    String eId;
    String eName;
    public Employee(String eId, String eName){
        this.eId = eId;
        this.eName = eName;
    }

    public void getEmployeeDetails(){
        System.out.println("Employee ID: "+eId);
        System.out.println("Employee Name: "+eName);
    }
}

Driver class to test out the above.

import java.lang.annotation.Annotation;
public class TestCustomAnnotationBasicBenefits {
    public static void main(String[] args) throws Exception{
        Employee emp = new Employee("E-100", "user3320018");
        emp.getEmployeeDetails();
        Class reflectedClass = emp.getClass();
        Annotation hopeBenefitAnn = reflectedClass.getAnnotation(BasicBenefits.class);
        BasicBenefits bBenefits = (BasicBenefits)hopeBenefitAnn;
        System.out.println("Benefit ID: "+bBenefits.bId());
        System.out.println("Benefit Name: "+bBenefits.bName());
    }
}

Your code look almost there, just two things need to be included in the main method.

1.) Need reference to MyClass 2.) Need to get the annotation using reflection from MyClass.

Here is a bit modified code from what you have:

@Inherited
@Documented
@Target(ElementType.TYPE)
@Retention(RetentionPolicy.RUNTIME)
@interface MyAnno
{
    String str();
    int val();
}

//using above custom annotation on class level
//can also use method level
//just need to change t @Target(ElementType.METHOD)
@MyAnno(str = "Annotation example", val = 100)
class MyClass
{

    public static void myMeth()
    {
        System.out.println("Inside myMeth()");
    }
}

import java.lang.annotation.Annotation;
class CustomAnno
{
    public static void main(String args[])
    {
        //1. getting reference to the class where the custom annotation is applied.
        //2. then getting the annotation to get the values 
        MyClass myClass = new MyClass();
        Class cls = myClass.getClass();
        Annotation getMyAnno = cls.getAnnotation(MyAnno.class);
        MyAnno myAnno = (MyAnno)getMyAnno;
        MyClass.myMeth(); //left this as is.
        System.out.println("myAnno.str(): "+ myAnno.str());
        System.out.println("myAnno.str(): "+ myAnno.val());     
    }
}

Answer 3

The abstract methods of the annotation define the values you can pass to it (in your case str = "Annotation example", val = 100 ). You can access them using reflection ( Method.<T>getAnnotation(Class<T>) ). Custom annotations don't have direct impact. They are only useful if you evaluate them.

Note that you have to annotate your custom annotation with @Retention(value=RUNTIME) to be able to read it via reflection.

Answer 4

To be of any use, annotations must be parsed first. The built-in annotations (such as @Override or @FunctionalInterface , to name the most obvious ones) are parsed by the compiler itself. As for custom annotations, these guys are commonly parsed by third-party frameworks, although we can also use the reflection mechanism to demonstrate this technique in standalone code.

By way of an example, the code below changes its behaviour at run time depending on the value of the field declared in the custom annotation named @SwitchingAnnotation :

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

@Target(ElementType.METHOD)
@Retention(RetentionPolicy.RUNTIME)
@interface SwitchingAnnotation{
    boolean flag();
}

public class Worker{
    void doThis(){ System.out.println("Doing this"); }
    void doThat(){ System.out.println("Doing that"); }
    @SwitchingAnnotation(
        flag = false
    )
    public void work(boolean flag) {
        if (flag) doThis();
        else doThat();
    }
}

class Test{
    public static void main(String[] args) {
        try{
            SwitchingAnnotation sw = Worker.class.getMethod("work", boolean.class)
                                                 .getAnnotation(SwitchingAnnotation.class);

            new Worker().work(sw.flag());    // prints Doing that
        }
        catch(NoSuchMethodException nsme){
            System.out.println(nsme);
        }
    }
}