Java advanced custom annotations

A list,

Annotations were introduced in Java 1.5 to provide additional information about your code and are now widely used. In addition to built-in Java annotations, we can also customize annotations.

Here is a simple example of a custom annotation:

@target (elementtype.type) // Annotations for types (classes, interfaces, annotations, enumerations) @retention (retentionPolicy.runtime) The @inherited // annotation will be applied to the @documented // subclass of the class to generate a Javadoc file public @interface JsAnnotation {String DEFAULT_VALUE ="JS"; /** * Create this value method so that annotations can be passed directly, such as @jsannotation ("msg"* * @)return
     */
    String value() default DEFAULT_VALUE;

}
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Key points of custom annotations:

• The @interface keyword defines annotations
• Annotations can be modified by other annotations, the most important of which are meta-annotations
• Annotations, like interfaces, can define constants and methods internally
• Methods defined by annotations have some limitations: methods cannot have parameters; Return values can only be primitive types, strings, classes, enumerations, annotations, and arrays of the above types. You can include default values

2. Introduction of meta-notes

A meta-annotation is the annotation that defines the annotation.

Including @target, @Retention, @Inherited, and @documented

1. @ Target

Describe the purpose of the annotation.

The source code is:

@Documented
@Retention(RetentionPolicy.RUNTIME)
@Target({ElementType.ANNOTATION_TYPE})
public @interface Target {
    ElementType[] value();
}
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The annotation method returns the value ElementType[], which is an enumeration type. The enumeration value is the value that the @target annotation can take on.

Value, which eliminates the need to specify a method name when using annotations.

Desirable values are:

Note: The default can be applied to any of these targets.

2, @ Retention

Describe the annotation lifecycle.

The source code is:

@Documented
@Retention(RetentionPolicy.RUNTIME)
@Target({ElementType.ANNOTATION_TYPE})
public @interface Retention {
    RetentionPolicy value();
}
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The annotation method returns a value of the enumerated type RetentionPolicy, which is the available value for the @Retention annotation.

Desirable values are:

Default retentionPolicy.class value.

3, @ Documented

The description annotation can be documented and is a markup annotation.

When javadoc is generated, there is no annotation, but if the annotation is qualified with @documented, the resulting document contains that annotation.

The source code is:

@Documented
@Retention(RetentionPolicy.RUNTIME)
@Target({ElementType.ANNOTATION_TYPE})
public @interface Documented {
}
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4, @ Inherited

Markup annotations. When an annotation is applied to a class using the @Inherited annotation, the annotation will be applied to the children of that class.

The source code is:

@Documented
@Retention(RetentionPolicy.RUNTIME)
@Target({ElementType.ANNOTATION_TYPE})
public @interface Inherited {
}
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Custom annotation examples

1. Define annotations

@target (elementtype.type) // Annotations for types (classes, interfaces, annotations, enumerations) @retention (retentionPolicy.runtime) The @inherited // annotation will be applied to the @documented // subclass of the class to generate a Javadoc file public @interface JsAnnotation {String DEFAULT_VALUE ="JS"; /** * @author peida */ enum Color {BULE, RED, GREEN}"msg"* * @)return*/ String value() default DEFAULT_VALUE; /** * This method returns an enumeration type, when using annotations such as @jsannotation (color= color.blue) ** @return
     */
    Color color() default Color.BULE;

    int num() default -1;
}
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Where: three methods are defined, all of which have default return values. Defines a Color enumeration (because annotated methods can return enumeration types)

Note here:

• Value method: Pass the notation directly when using annotations, such as @jsannotation (” MSG “)
• Normal methods: When using annotations, such as @jsannotation (color= color.blue)

2. Use annotations

@JsAnnotation("jia shuai")
public class AnnotationTest {
    
}
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or

@JsAnnotation(num = 100)
public class AnnotationTest {
    
}
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or

@JsAnnotation(color = Color.BLUE)
public class AnnotationTest {
    
}
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3. Get annotations

public static void main(String[] args) {
    AnnotationTest test = new AnnotationTest();
    Class tClass = test.getClass();
    JsAnnotation jsAnnotation = (JsAnnotation) tClass.getAnnotation(JsAnnotation.class);

    System.out.println(jsAnnotation.value());
    System.out.println(jsAnnotation.color());
    System.out.println(jsAnnotation.num());
}
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