In -depth understanding of the "annotation" framework design ideas in the Java library

In -depth understanding of the "annotation" framework design ideas in the Java library introduction: Annotion in the Java class library is a special label that can be added to the code.The annotation framework is a programming style widely used in the Java language, which provides a way to declare and use annotations.This article will deeply explore the design ideas in the JAVA library and explain it through the Java code example. 1. The basic concept of annotation Note is a new feature introduced from Java 5. It is essentially a special interface. The method in the interface is called the annotation member.Note can be used to mark program elements such as class, methods, fields, and add additional information to these elements. Comments in Java are usually used in the following aspects: 1. Provide information to the compiler: Through the annotation, you can tell the compiler how to handle the elements of the annotation mark, thereby changing the behavior of the compiler. 2. Generate auxiliary code in the code: By defining customized annotations processors, additional code can be generated during compilation. 3. Analysis and processing during runtime: Through the Java reflection mechanism, the coding solution can be parsed during runtime to achieve corresponding logic. The expression of the annotation is very simple, starting with the "@" symbol, followed by the annotation name and a pair of brackets.There can be some parameters in parentheses and empty.The example is as follows: @Entity public class Person { // class body // ... @Id private Long id; @Column(name="name") private String name; @Transient private int age; // Construct function, method, etc. // ... } In the above examples, `@Entity`,`@ID` and `@Column` are common annotations in the Java class library, used to mark the Person classes and their members.Through annotations, the code can make the code more concise, clear and easy to understand. 2. Principles of the implementation of annotations The annotations in Java are achieved through the reflection mechanism.When compiling, scan the source code through the annotation processor, and process the logic related to the annotation, such as generating auxiliary code and modifying the compiler behavior.When running, the relevant information of the annotation can be obtained through the reflection mechanism to achieve the corresponding logic. Java provides some built -in annotations, such as `@& oVERRIDE`,@defrecated` and`@suppressWarnings` and so on.In addition, developers can also define their own annotations and process them through the need to process the processor. Example: Definition custom annotation `@myannotation` import java.lang.annotation.*; @Retention(RetentionPolicy.RUNTIME) @Target(ElementType.METHOD) public @interface MyAnnotation { String value() default ""; int count() default 1; } In the above examples, a annotation is customized `@myannotation`, and specify that its retention strategy is the method of the target range when running.The annotation contains two members: `Value` and` Count`, corresponding to the default values of a string and an integer type, respectively. Third, the application scenario of the annotation 1. Code level mark and metadata: You can add meta data to the code through annotations to provide additional information for subsequent processing.For example, the annotation of `@& oVERRIDE` is used for the label method to cover the parent class, reminding the developer to check whether the method is correctly rewritten. 2. Code check and prompt during compilation: By customized annotations and corresponding annotation processors, you can check the code during compilation, find some potential questions and give warning or error prompts. 3. Generate auxiliary code: By customized annotations and annotations, auxiliary code can be generated during compilation to simplify the development process.For example, using the annotation label database table structure, the corresponding SQL statement is generated through the annotation processor. 4. Analysis and processing during runtime: The reflection mechanism can be parsed and cooked at runtime to achieve corresponding logic.For example, the JUNIT framework is to implement automated test case executions by parsing the annotation of the `@test`. Fourth, the annotation of JDK comes Java SE provides some common annotations that can be used directly in development.Here are some common annotations and their usage scenarios: 1. `@Override` Function: The method is the method of covering the parent. Example: `` `` `` `` `` `...}` 2. `@Deprecated` Function: Mark the outdated method or class. Example: `` p `` `` {{...} ` 3. `@SuppressWarnings` Role: Inhibit a specific compiler warning. 示例：`@SuppressWarnings("unchecked") List<String> list = new ArrayList();` 5. Customized annotations and annotation processors Java allows developers to customize annotations and corresponding annotations to achieve more flexible annotations.The custom annotation is defined using the keywords of `@interface`, and the annotation processor uses the` javax.annotation.processing` package for processing. Example: Custom annotation `@myannotation` and annotation processor` myannotationProcessor` import java.lang.annotation.ElementType; import java.lang.annotation.Retention; import java.lang.annotation.RetentionPolicy; import java.lang.annotation.Target; @Retention(RetentionPolicy.SOURCE) @Target(ElementType.TYPE) public @interface MyAnnotation { String value() default ""; } import javax.annotation.processing.AbstractProcessor; import javax.annotation.processing.Processor; import javax.annotation.processing.RoundEnvironment; import javax.annotation.processing.SupportedAnnotationTypes; import javax.lang.model.element.Element; import javax.lang.model.element.TypeElement; import java.util.Set; @SupportedAnnotationTypes("MyAnnotation") public class MyAnnotationProcessor extends AbstractProcessor { @Override public boolean process(Set<? extends TypeElement> annotations, RoundEnvironment roundEnv) { for (Element element : roundEnv.getElementsAnnotatedWith(MyAnnotation.class)) { // Treatment logic } return true; } } In the above examples, `@myannotation` is a custom annotation, used for marking.`MyannotationProcessor` is an annotation processor that is used to process the category of labeling`@myannotation`. 6. Summary The annotation framework is a design idea in the Java class library. It can add additional information to the program element by using annotations and process these annotations through the annotation processor.Comments are widely used in code marks, assisted code, and inspection during compilation, which brings greater flexibility and convenience to Java development. Through the introduction of this article, we have a deeper understanding of the design thought of the annotation framework in the Java library.By defining and using custom annotations, and combined with the corresponding annotation processor, we can better use the annotation mechanism in Java development to improve development efficiency and code quality.