Introduction

Introduction In the Java library, "Annotion" is a special grammar data data that can add additional information to code elements (such as classes, methods, fields, etc.).By using annotations, metadata can be added to the code without modifying the code to provide more information, configuration or behavior. Note on the wide application of the Java platform includes the Java framework, third -party library and tools.In the Java library, the annotation framework provides a mechanism for statement and processing information, allowing developers to customize annotations and use these annotations in some way. There are the following types of annotations in some common Java libraries: 1. JDK comes with annotation framework: -@Ooverride: The method used to identify the method of covering the parent class. -@DePRECATED: Used to annotate code elements that have been outdated. -@SuppressWarnings: Used to inhibit the compiler warning. 2. Spring framework annotation framework: -@Autowired: For automatic assembly dependency relationship. -@Component: Used to declare the Spring component. -@RequestMapping: Used to mappore URL and processor methods. 3. Hibernate framework annotation framework: -@Entity: Used to mark the POJO class as a physical class. -@Table: Used to specify the database table corresponding to the physical class. -@Column: Used to mappore the attributes and database fields. In addition to these common annotation frameworks, developers can also add custom annotations to their code through the annotation framework.Here are examples of creating custom annotations: import java.lang.annotation.*; @Retention(RetentionPolicy.RUNTIME) @Target(ElementType.METHOD) public @interface MyAnnotation { String value() default ""; int count() default 0; } In the above examples, Java's built -in `@Retention` and@Target` annotations specify the scope of the retaining strategy and role of custom annotations, and define two member variables` value` and `count` respectively, respectivelySet the default value. Example of using custom annotations: public class MyClass { @MyAnnotation(value = "Hello", count = 5) public void myMethod() { // do something } } By using the ‘@myannotation` annotation, the two parameters of" Hello "and 5 can be bound to the method of` mymethod (). The annotation framework in the Java class library adds additional meta -data to the code, so that developers can program in a more flexible and easy -to -configure manner.Through reasonable use of annotations, the readability and maintenance of the code can be improved, and the writing of model code can be reduced.The annotation framework provides more tools and opportunities for Java developers to expand the functions and behaviors of the code.