Master the technical principles of the "Annotations" framework in the Java library

Master the technical principles of the "Annotations" framework in the Java class library introduction: In the Java library, Annotations is a way to represent metadata. It provides a flexible method to add metad data and description information to the code.Comment plays an important role in Java programming. They can add labels, configurations and descriptions to the code, and can be used to check, runtime processing, and generate documents during compilation.This article will introduce the technical principles of the Annotations framework in the Java class library, and deepen understanding through some Java code examples. The basic concept and grammar of the annotation 1. Basic concept of annotation: Note is a special interface in Java.It uses the@`symbol as prefix and provides a set of meta -nNOTATION to help developers define and use custom annotations.Note can be applied to Java elements such as bags, classes, methods, fields. 2. Annotated grammar: The syntax structure of the annotation consists of the annotation name, a pair of brackets and optional elemental values.The annotation name is the type used to identify the annotation. The elemental value of the annotation can be specified in parentheses, and multiple elemental values are separated by a comma. Example: @Note Name (element name 1 = value 1, element name 2 = value 2, ...) 2. Customized annotation The Java class library provides some commonly used built -in annotations (such as `@deprecated`,` `@suppressWarnings, etc.), but sometimes we need to define our annotations based on specific business scenarios.Custom annotations can be defined using the keywords of `@interface`. Example: import java.lang.annotation.Retention; import java.lang.annotation.RetentionPolicy; @Retention(RetentionPolicy.RUNTIME) public @interface MyAnnotation { String value() default ""; int count() default 0; } In the above example, the `@Retention` annotation is used to specify the retention strategy of the annotation.`` @RETENTION (RetentionPolicy.runtime) `indicates that the annotation will be retained at runtime, so that we can obtain the annotation and processed accordingly through the reflection mechanism. Third, Yuan Note Metropolitan annotations are used to define annotations, which provide some metadata for customized annotations.The Java class library provides several commonly used meta -solutions, including `@Retention`,@Target`,` `@documented`,` `@inherited`, etc. -` `` 注 -Note: `@Retention` The retention strategy used to specify the annotation, its value can be `RetentionPolicy.source`,` RetentionPolicy.class`, and `RetentionPolicy.runtime`.`RetentionPolicy.runtime` is the most commonly used retaining strategy, indicating that the annotation will be retained at runtime and can be obtained by reflection. -`@target` yuan annotation: `@Target` is used to specify the type of target element that can be applied, including `Elementtype.Type`,` ElementType.field`, `Elementtype.method`,` ElementType.parameter`, etc.Multiple target types can be used for grouping. -`@Documented` yuan annotation: `@Documented` is used to specify whether the annotation is included in the generated Java document. -`@inherited` yuan Note: `@Inherited` is used to specify whether the annotation can be inherited.If an annotation with a `@inherited` yuan annotation, and this annotation is applied to the parent class, it will also be applied to the subclass. Fourth, use annotations The use annotation is achieved by adding annotations and its elemental values at the right location.Some of the built -in annotations in the Java class library, such as `@& oVERRIDE` and@SUPPRESSWARNINININGS`, provide some functions of compilation and warning suppression.The customized annotations can define the corresponding functions according to specific needs. Example: @MyAnnotation(value = "example", count = 3) public class MyClass { @MyAnnotation(value = "field", count = 1) private String myField; @MyAnnotation(value = "method", count = 2) public void myMethod() { // do something } } The above example shows how to use custom annotations on classes, fields and methods.The value of the annotation can be set according to business needs. Five, the processing of annotations If you want to achieve annotations, you can use the reflection mechanism to obtain the annotation information.The Java class library provides some tools for processing annotations, such as `java.lang.reflet.annotatedelement`,` java.lang.annotation.annotation`, etc. Example: import java.lang.reflect.Field; public class AnnotationProcessor { public static void processAnnotations(Class<?> cls) { if (cls.isAnnotationPresent(MyAnnotation.class)) { MyAnnotation annotation = cls.getAnnotation(MyAnnotation.class); System.out.println("Class annotation: " + annotation.value() + ", " + annotation.count()); } Field[] fields = cls.getDeclaredFields(); for (Field field : fields) { if (field.isAnnotationPresent(MyAnnotation.class)) { MyAnnotation annotation = field.getAnnotation(MyAnnotation.class); System.out.println("Field annotation: " + annotation.value() + ", " + annotation.count()); } } } } In the above examples, the `ProcessNotations` method obtains the annotation information by reflects and processes it accordingly.Corresponding processing logic can be written according to business needs. in conclusion: Through the introduction of this article, we understand the technical principles of the Annotations framework in the Java class library.Note is a way to represent metadata. It provides a convenient method to add metad data and description information to the code.By customized annotations, we can add markers, configurations and descriptions to the code according to the specific business needs, and to achieve annotations through the reflection mechanism.Mastering the technical principles of the Annotations framework will help us write more flexible and maintainable Java code.