I denne vejledning lærer vi om forskellige typer Java-annoteringer ved hjælp af eksempler.
Java-annoteringer er metadata (data om data) for vores programkildekode. Der er flere foruddefinerede annoteringer leveret af Java SE. Desuden kan vi også oprette brugerdefinerede annoteringer efter vores behov.
Hvis du ikke ved, hvad annoteringer er, skal du besøge Java-annotationsvejledningen.
Disse kommentarer kan kategoriseres som:
1. Foruddefinerede kommentarer
@Deprecated@Override@SuppressWarnings@SafeVarargs@FunctionalInterface
2. Brugerdefinerede kommentarer
3. Metanoteringer
@Retention@Documented@Target@Inherited@Repeatable
Foruddefinerede annoteringstyper
1. @Forældet
Den @Deprecatedannotation er en markør annotation, der angiver elementet (klasse, metode, felt, etc.) er forældet og er blevet erstattet af en nyere element.
Dens syntaks er:
@Deprecated accessModifier returnType deprecatedMethodName() (… )
Når et program bruger det element, der er erklæret udfaset, genererer compileren en advarsel.
Vi bruger Javadoc- @deprecatedtag til dokumentation af det forældede element.
/** * @deprecated * why it was deprecated */ @Deprecated accessModifier returnType deprecatedMethodName() (… )
Eksempel 1: @Forældet annoteringseksempel
class Main ( /** * @deprecated * This method is deprecated and has been replaced by newMethod() */ @Deprecated public static void deprecatedMethod() ( System.out.println("Deprecated method"); ) public static void main(String args()) ( deprecatedMethod(); ) )
Produktion
Forældet metode
2. @ Override
De @Overrideannotation angiver, at en metode til en underklasse tilsidesætter metoden til superklassen med den samme metode navn, returtype, og parameter listen.
Det er ikke obligatorisk at bruge, @Overridenår en metode tilsidesættes. Men hvis vi bruger det, giver compileren en fejl, hvis noget er galt (såsom forkert parametertype), mens metoden tilsidesættes.
Eksempel 2: @Overstyr annoteringseksempel
class Animal ( // overridden method public void display()( System.out.println("I am an animal"); ) ) class Dog extends Animal ( // overriding method @Override public void display()( System.out.println("I am a dog"); ) public void printMessage()( display(); ) ) class Main ( public static void main(String() args) ( Dog dog1 = new Dog(); dog1.printMessage(); ) )
Produktion
Jeg er en hund
I dette eksempel kan vi ved at lave et objekt hund1 af hundeklassen kalde dets metode printMessage (), som derefter udfører display()udsagnet.
Da display()er defineret i begge klasser, tilsidesætter metoden med underklasse Hund metoden til superklasse Animal. Derfor display()kaldes underklassen.
3. @SuppressWarnings
Som navnet antyder, @SuppressWarningsbeder kommentaren kompilatoren om at undertrykke advarsler, der genereres, mens programmet udføres.
We can specify the type of warnings to be suppressed. The warnings that can be suppressed are compiler-specific but there are two categories of warnings: deprecation and unchecked.
To suppress a particular category of warning, we use:
@SuppressWarnings("warningCategory")
For example,
@SuppressWarnings("deprecated")
To suppress multiple categories of warnings, we use:
@SuppressWarnings(("warningCategory1", "warningCategory2"))
For example,
@SuppressWarnings(("deprecated", "unchecked"))
Category deprecated instructs the compiler to suppress warnings when we use a deprecated element.
Category unchecked instructs the compiler to suppress warnings when we use raw types.
And, undefined warnings are ignored. For example,
@SuppressWarnings("someundefinedwarning")
Example 3: @SuppressWarnings annotation example
class Main ( @Deprecated public static void deprecatedMethod() ( System.out.println("Deprecated method"); ) @SuppressWarnings("deprecated") public static void main(String args()) ( Main depObj = new Main(); depObj. deprecatedMethod(); ) )
Output
Deprecated method
Here, deprecatedMethod() has been marked as deprecated and will give compiler warnings when used. By using the @SuppressWarnings("deprecated") annotation, we can avoid compiler warnings.
4. @SafeVarargs
The @SafeVarargs annotation asserts that the annotated method or constructor does not perform unsafe operations on its varargs (variable number of arguments).
We can only use this annotation on methods or constructors that cannot be overridden. This is because the methods that override them might perform unsafe operations.
Before Java 9, we could use this annotation only on final or static methods because they cannot be overridden. We can now use this annotation for private methods as well.
Example 4: @SafeVarargs annotation example
import java.util.*; class Main ( private void displayList(List… lists) ( for (List list : lists) ( System.out.println(list); ) ) public static void main(String args()) ( Main obj = new Main(); List universityList = Arrays.asList("Tribhuvan University", "Kathmandu University"); obj.displayList(universityList); List programmingLanguages = Arrays.asList("Java", "C"); obj.displayList(universityList, programmingLanguages); ) )
Warnings
Type safety: Potential heap pollution via varargs parameter lists Type safety: A generic array of List is created for a varargs parameter
Output
Note: Main.java uses unchecked or unsafe operations. (Tribhuvan University, Kathmandu University) (Tribhuvan University, Kathmandu University) (Java, C)
Here, List … lists specifies a variable-length argument of type List. This means that the method displayList() can have zero or more arguments.
The above program compiles without errors but gives warnings when @SafeVarargs annotation isn't used.
When we use @SafeVarargs annotation in the above example,
@SafeVarargs private void displayList(List… lists) (… )
We get the same output but without any warnings. Unchecked warnings are also suppressed when we use this annotation.
5. @FunctionalInterface
Java 8 first introduced this @FunctionalInterface annotation. This annotation indicates that the type declaration on which it is used is a functional interface. A functional interface can have only one abstract method.
Example 5: @FunctionalInterface annotation example
@FunctionalInterface public interface MyFuncInterface( public void firstMethod(); // this is an abstract method )
If we add another abstract method, let's say
@FunctionalInterface public interface MyFuncInterface( public void firstMethod(); // this is an abstract method public void secondMethod(); // this throws compile error )
Now, when we run the program, we will get the following warning:
Unexpected @FunctionalInterface annotation @FunctionalInterface MyFuncInterface is not a functional interface multiple non-overriding abstract methods found in interface MyFuncInterface
It is not mandatory to use @FunctionalInterface annotation. The compiler will consider any interface that meets the functional interface definition as a functional interface.
We use this annotation to make sure that the functional interface has only one abstract method.
However, it can have any number of default and static methods because they have an implementation.
@FunctionalInterface public interface MyFuncInterface( public void firstMethod(); // this is an abstract method default void secondMethod() (… ) default void thirdMethod() (… ) )
Custom Annotations
It is also possible to create our own custom annotations.
Its syntax is:
(Access Specifier) @interface ( DataType () (default value); )
Here is what you need to know about custom annotation:
- Annotations can be created by using
@interfacefollowed by the annotation name. - The annotation can have elements that look like methods but they do not have an implementation.
- The default value is optional. The parameters cannot have a null value.
- The return type of the method can be primitive, enum, string, class name or array of these types.
Example 6: Custom annotation example
@interface MyCustomAnnotation ( String value() default "default value"; ) class Main ( @MyCustomAnnotation(value = "programiz") public void method1() ( System.out.println("Test method 1"); ) public static void main(String() args) throws Exception ( Main obj = new Main(); obj.method1(); ) )
Output
Test method 1
Meta Annotations
Meta-annotations are annotations that are applied to other annotations.
1. @Retention
The @Retention annotation specifies the level up to which the annotation will be available.
Its syntax is:
@Retention(RetentionPolicy)
There are 3 types of retention policies:
- RetentionPolicy.SOURCE - The annotation is available only at the source level and is ignored by the compiler.
- RetentionPolicy.CLASS - The annotation is available to the compiler at compile-time, but is ignored by the Java Virtual Machine (JVM).
- RetentionPolicy.RUNTIME - The annotation is available to the JVM.
For example,
@Retention(RetentionPolicy.RUNTIME) public @interface MyCustomAnnotation(… )
2. @Documented
By default, custom annotations are not included in the official Java documentation. To include our annotation in the Javadoc documentation, we use the @Documented annotation.
For example,
@Documented public @interface MyCustomAnnotation(… )
3. @Target
We can restrict an annotation to be applied to specific targets using the @Target annotation.
Its syntax is:
@Target(ElementType)
The ElementType can have one of the following types:
| Element Type | Target |
|---|---|
ElementType.ANNOTATION_TYPE | Annotation type |
ElementType.CONSTRUCTOR | Constructors |
ElementType.FIELD | Fields |
ElementType.LOCAL_VARIABLE | Local variables |
ElementType.METHOD | Methods |
ElementType.PACKAGE | Package |
ElementType.PARAMETER | Parameter |
ElementType.TYPE | Any element of class |
For example,
@Target(ElementType.METHOD) public @interface MyCustomAnnotation(… )
In this example, we have restricted the use of this annotation to methods only.
Note: If the target type is not defined, the annotation can be used for any element.
4. @Inherited
By default, an annotation type cannot be inherited from a superclass. However, if we need to inherit an annotation from a superclass to a subclass, we use the @Inherited annotation.
Its syntax is:
@Inherited
For example,
@Inherited public @interface MyCustomAnnotation (… ) @MyCustomAnnotation public class ParentClass(… ) public class ChildClass extends ParentClass (… )
5. @Repeatable
An annotation that has been marked by @Repeatable can be applied multiple times to the same declaration.
@Repeatable(Universities.class) public @interface University ( String name(); )
Den værdi, der er defineret i @Repeatablekommentaren, er container-kommentaren. Containerannotationen har en variabel værdi af arraytypen i ovenstående gentagelige annotation. Her Universitieser den indeholdende annoteringstype.
public @interface Universities ( University() value(); )
Nu kan @Universitykommentaren bruges flere gange på den samme erklæring.
@University(name = "TU") @University(name = "KU") private String uniName;
Hvis vi har brug for at hente annoteringsdataene, kan vi bruge Reflection API.
For at hente annoteringsværdier bruger vi getAnnotationsByType()eller getAnnotations()metoden defineret i Reflection API.
