https://docs.oracle.com/javase/tutorial/java/annotations/index.html
https://docs.oracle.com/javase/tutorial/java/annotations/basics.html
https://docs.oracle.com/javase/tutorial/java/annotations/type_annotations.html
https://docs.oracle.com/javase/tutorial/java/annotations/repeating.html

Annotations, a form of metadata, provide data about a program that is not part of the program itself. Annotations have no direct effect on the operation of the code they annotate.

Annotation usages:

Information for the compiler — Annotations can be used by the compiler to detect errors or suppress warnings.
Compile-time and deployment-time processing — Software tools can process annotation information to generate code, XML files, and so forth.
Runtime processing — Some annotations are available to be examined at runtime.

The Format of an Annotation

In its simplest form, an annotation looks like the following:

@Entity

The at sign character (@) indicates to the compiler that what follows is an annotation. In the following example, the annotation's name is Override:

@Override
void mySuperMethod() { ... }

The annotation can include elements, which can be named or unnamed, and there are values for those elements:

@Author(
   name = "Benjamin Franklin",
   date = "3/27/2003"
)
class MyClass() { ... }

@SuppressWarnings(value = "unchecked")
void myMethod() { ... }

If there is just one element named value, then the name can be omitted, as in:

@SuppressWarnings("unchecked")
void myMethod() { ... }

If the annotation has no elements, then the parentheses can be omitted, as shown in the previous @Override example.

It is also possible to use multiple annotations on the same declaration:

@Author(name = "Jane Doe")
@EBook
class MyClass { ... }

If the annotations have the same type, then this is called a repeating annotation:

@Author(name = "Jane Doe")
@Author(name = "John Smith")
class MyClass { ... }

Where Annotations Can Be Used

Annotations can be applied to declarations: declarations of classes, fields, methods, and other program elements. When used on a declaration, each annotation often appears, by convention, on its own line.

Annotations can also be applied to the use of types. Here are some examples:

Class instance creation expression:
```
    new @Interned MyObject();
```
Type cast:
```
    myString = (@NonNull String) str;
```

implements clause:

    class UnmodifiableList<T> implements
        @Readonly List<@Readonly T> { ... }

Thrown exception declaration:

    void monitorTemperature() throws
        @Critical TemperatureException { ... }

This form of annotation is called a type annotation.

Declaring an Annotation Type

We want to remove the following comments:

public class Generation3List extends Generation2List {

   // Author: John Doe
   // Date: 3/17/2002
   // Current revision: 6
   // Last modified: 4/12/2004
   // By: Jane Doe
   // Reviewers: Alice, Bill, Cindy

   // class code goes here

}

Declare annotation:

The annotation type definition looks similar to an interface definition where the keyword interface is preceded by the at sign (@) (@ = AT, as in annotation type). Annotation types are a form of interface

@interface

@interface ClassPreamble {
   String author();
   String date();
   int currentRevision() default 1;
   String lastModified() default "N/A";
   String lastModifiedBy() default "N/A";
   // Note use of array
   String[] reviewers();
}

Apply the annotations declared above:

@ClassPreamble (
   author = "John Doe",
   date = "3/17/2002",
   currentRevision = 6,
   lastModified = "4/12/2004",
   lastModifiedBy = "Jane Doe",
   // Note array notation
   reviewers = {"Alice", "Bob", "Cindy"}
)
public class Generation3List extends Generation2List {

// class code goes here

}

Note: To make the information in @ClassPreamble appear in Javadoc-generated documentation, you must annotate the @ClassPreamble definition with the @Documented annotation:

// import this to use @Documented
import java.lang.annotation.*;

@Documented
@interface ClassPreamble {

   // Annotation element definitions
   
}

Predefined Annotation Types

The predefined annotation types defined in java.lang are @Deprecated, @Override, and @SuppressWarnings.

@Deprecated @Deprecated annotation indicates that the marked element is deprecated and should no longer be used. The compiler generates a warning whenever a program uses a method, class, or field with the @Deprecated annotation.

   // Javadoc comment follows

    /**
     * @deprecated
     * explanation of why it was deprecated
     */
    @Deprecated
    static void deprecatedMethod() { }

@Override @Override annotation informs the compiler that the element is meant to override an element declared in a superclass.

   // mark method as a superclass method
   // that has been overridden
   @Override 
   int overriddenMethod() { }

@SuppressWarnings @SuppressWarnings annotation tells the compiler to suppress specific warnings that it would otherwise generate. I

   // use a deprecated method and tell 
   // compiler not to generate a warning
   @SuppressWarnings("deprecation")
    void useDeprecatedMethod() {
        // deprecation warning
        // - suppressed
        objectOne.deprecatedMethod();
    }

Every compiler warning belongs to a category. The Java Language Specification lists two categories: deprecation and unchecked. The unchecked warning can occur when interfacing with legacy code written before the advent of generics. To suppress multiple categories of warnings, use the following syntax:

@SuppressWarnings({"unchecked", "deprecation"})

@SafeVarargs @SafeVarargs annotation, when applied to a method or constructor, asserts that the code does not perform potentially unsafe operations on its varargs parameter. When this annotation type is used, unchecked warnings relating to varargs usage are suppressed.

@FunctionalInterface @FunctionalInterface annotation, introduced in Java SE 8, indicates that the type declaration is intended to be a functional interface, as defined by the Java Language Specification.

Annotations That Apply to Other Annotations

@Retention @Retention annotation specifies how the marked annotation is stored:

RetentionPolicy.SOURCE – The marked annotation is retained only in the source level and is ignored by the compiler.
RetentionPolicy.CLASS – The marked annotation is retained by the compiler at compile time, but is ignored by the Java Virtual Machine (JVM).
RetentionPolicy.RUNTIME – The marked annotation is retained by the JVM so it can be used by the runtime environment.

@Documented @Documented annotation indicates that whenever the specified annotation is used those elements should be documented using the Javadoc tool. (By default, annotations are not included in Javadoc.)

@Target @Target annotation marks another annotation to restrict what kind of Java elements the annotation can be applied to. A target annotation specifies one of the following element types as its value:

ElementType.ANNOTATION_TYPE can be applied to an annotation type.
ElementType.CONSTRUCTOR can be applied to a constructor.
ElementType.FIELD can be applied to a field or property.
ElementType.LOCAL_VARIABLE can be applied to a local variable.
ElementType.METHOD can be applied to a method-level annotation.
ElementType.PACKAGE can be applied to a package declaration.
ElementType.PARAMETER can be applied to the parameters of a method.
ElementType.TYPE can be applied to any element of a class.

@Inherited @Inherited annotation indicates that the annotation type can be inherited from the super class. (This is not true by default.) When the user queries the annotation type and the class has no annotation for this type, the class' superclass is queried for the annotation type. This annotation applies only to class declarations.

@Repeatable @Repeatable annotation, introduced in Java SE 8, indicates that the marked annotation can be applied more than once to the same declaration or type use.

Type Annotations and Pluggable Type Systems

Before the Java SE 8 release, annotations could only be applied to declarations.

You want to ensure that a particular variable in your program is never assigned to null; you want to avoid triggering a NullPointerException. You can write a custom plug-in to check for this. You would then modify your code to annotate that particular variable, indicating that it is never assigned to null. The variable declaration might look like this:

@NonNull String str;

When you compile the code, including the NonNull module at the command line, the compiler prints a warning if it detects a potential problem, allowing you to modify the code to avoid the error. After you correct the code to remove all warnings, this particular error will not occur when the program runs.

You can use multiple type-checking modules where each module checks for a different kind of error. In this way, you can build on top of the Java type system, adding specific checks when and where you want them.

Repeating Annotations

There are some situations where you want to apply the same annotation to a declaration or type use. As of the Java SE 8 release, repeating annotations enable you to do this.
For example, you are writing code to use a timer service that enables you to run a method at a given time or on a certain schedule, similar to the UNIX cron service. Now you want to set a timer to run a method, doPeriodicCleanup, on the last day of the month and on every Friday at 11:00 p.m. To set the timer to run, create an @Schedule annotation and apply it twice to the doPeriodicCleanup method. The first use specifies the last day of the month and the second specifies Friday at 11p.m., as shown in the following code example:

@Schedule(dayOfMonth="last")
@Schedule(dayOfWeek="Fri", hour="23")
public void doPeriodicCleanup() { ... }

The previous example applies an annotation to a method. You can repeat an annotation anywhere that you would use a standard annotation. For example, you have a class for handling unauthorized access exceptions. You annotate the class with one @Alert annotation for managers and another for admins:

@Alert(role="Manager")
@Alert(role="Administrator")
public class UnauthorizedAccessException extends SecurityException { ... }

For compatibility reasons, repeating annotations are stored in a container annotation that is automatically generated by the Java compiler. In order for the compiler to do this, two declarations are required in your code.

Step 1: Declare a Repeatable Annotation Type

The annotation type must be marked with the @Repeatable meta-annotation. The following example defines a custom @Schedule repeatable annotation type:

import java.lang.annotation.Repeatable;

@Repeatable(Schedules.class)
public @interface Schedule {
  String dayOfMonth() default "first";
  String dayOfWeek() default "Mon";
  int hour() default 12;
}

The value of the @Repeatable meta-annotation, in parentheses, is the type of the container annotation that the Java compiler generates to store repeating annotations. In this example, the containing annotation type is Schedules, so repeating @Schedule annotations is stored in an @Schedules annotation.
Applying the same annotation to a declaration without first declaring it to be repeatable results in a compile-time error.

Step 2: Declare the Containing Annotation Type

The containing annotation type must have a value element with an array type. The component type of the array type must be the repeatable annotation type. The declaration for the Schedules containing annotation type is the following:

public @interface Schedules {
    Schedule[] value();
}

Retrieving Annotations

There are several methods available in the Reflection API that can be used to retrieve annotations. The behavior of the methods that return a single annotation, such asAnnotatedElement.getAnnotation(Class<T>), are unchanged in that they only return a single annotation if one annotation of the requested type is present. If more than one annotation of the requested type is present, you can obtain them by first getting their container annotation. In this way, legacy code continues to work. Other methods were introduced in Java SE 8 that scan through the container annotation to return multiple annotations at once, such as AnnotatedElement.getAnnotationsByType(Class<T>). See the AnnotatedElement class specification for information on all of the available methods.

Design Considerations

When designing an annotation type, you must consider the cardinality of annotations of that type. It is now possible to use an annotation zero times, once, or, if the annotation's type is marked as @Repeatable, more than once. It is also possible to restrict where an annotation type can be used by using the @Target meta-annotation. For example, you can create a repeatable annotation type that can only be used on methods and fields. It is important to design your annotation type carefully to ensure that the programmer using the annotation finds it to be as flexible and powerful as possible.

Quiz:

Questions

Question: What is wrong with the following interface:

public interface House {
    @Deprecated
    public void open();
    public void openFrontDoor();
    public void openBackDoor();
}

Answer The documentation should reflect why open is deprecated and what to use instead. For example:

public interface House { 
    /**
     * @deprecated use of open 
     * is discouraged, use
     * openFrontDoor or 
     * openBackDoor instead.
     */
    @Deprecated
    public void open(); 
    public void openFrontDoor();
    public void openBackDoor();
}

Question: Consider this implementation of the House interface, shown in Question 1.

public class MyHouse implements House {
    public void open() {}
    public void openFrontDoor() {}
    public void openBackDoor() {}
}

If you compile this program, the compiler produces a warning because open was deprecated (in the interface). What can you do to get rid of that warning?
Answer: You can deprecate the implementation of open:

public class MyHouse implements House { 
    // The documentation is 
    // inherited from the interface.
    @Deprecated
    public void open() {} 
    public void openFrontDoor() {}
    public void openBackDoor() {}
}

Alternatively, you can suppress the warning:

public class MyHouse implements House { 
    @SuppressWarnings("deprecation")
    public void open() {} 
    public void openFrontDoor() {}
    public void openBackDoor() {}
}

Will the following code compile without error? Why or why not?
```
public @interface Meal { ... }

@Meal("breakfast", mainDish="cereal")
@Meal("lunch", mainDish="pizza")
@Meal("dinner", mainDish="salad")
public void evaluateDiet() { ... }
```
Answer: The code fails to compile. Before JDK 8, repeatable annotations are not supported. As of JDK 8, the code fails to compile because the Meal annotation type was not defined to be repeatable. It can be fixed by adding the @Repeatable meta-annotation and defining a container annotation type:
```
@java.lang.annotation.Repeatable(MealContainer.class)
public @interface Meal { ... }

public @interface MealContainer {
    Meal[] value();
}
```

Exercises

Exercise: Define an annotation type for an enhancement request with elements id, synopsis, engineer, and date. Specify the default value as unassigned for engineer and unknown for date.Answer:

/**
 * Describes the Request-for-Enhancement (RFE) annotation type.
 */
public @interface RequestForEnhancement {
    int id();
    String synopsis();
    String engineer() default "[unassigned]";
    String date() default "[unknown]";
}

Return Saturday's Notes

Sunday, February 18, 2018

Java Annotations