Java annotation
Java annotation is a special syntax element in the Java language, which can be used to provide additional information for program code. Annotations themselves do not directly affect the running results of the program, but the content of annotations can be obtained through reflection mechanisms during compilation or runtime, and specific processing can be carried out based on annotations. Annotations can be used on various program elements such as classes, methods, and fields, providing a declarative and business logic independent way to describe metadata. Java annotation define a series of meta annotations to describe the behavior of other annotations. Common meta annotations include: 1. @ Target: Used to declare which program elements annotations can be applied to, including classes, methods, fields, etc. 2. @ Retention: Used to declare the lifecycle of annotations, that is, when annotations take effect, which can be at compile time, runtime, or in source code. 3. @ Documented: Used to specify whether to include annotations in Java documents. 4. @ Inherited: Used to specify whether annotations can be inherited. By default, annotations are not inheritable. The following is an example code for a simple custom annotation: ```java import java.lang.annotation.ElementType; import java.lang.annotation.Retention; import java.lang.annotation.RetentionPolicy; import java.lang.annotation.Target; //Define a custom annotation @Target(ElementType.METHOD) @Retention(RetentionPolicy.RUNTIME) @interface MyAnnotation { String value() default ""; int count() default 0; } //Use custom annotations class MyClass { @MyAnnotation(value = "example", count = 10) public void myMethod() { System.out.println("My method is invoked!"); } } public class AnnotationDemo { public static void main(String[] args) throws NoSuchMethodException { //Obtain annotation information MyAnnotation annotation = MyClass.class.getMethod("myMethod").getAnnotation(MyAnnotation.class); System.out.println("Value: " + annotation.value()); System.out.println("Count: " + annotation.count()); } } ``` In the above code, a custom annotation '@ MyAnnotation' is defined that can be applied to methods`@ MyAnnotation 'has two attributes, namely' value 'and' count ', and both have default values` The 'myMethod' method in the MyClass' class uses the '@ MyAnnotation' annotation, which has attribute values of 'value' and 'count'. In the 'AnnotationDemo' class, annotations on the 'myMethod' method can be obtained through reflection, and the attribute values in the annotations can be obtained. Summary: Java annotation is a mechanism used to provide additional information for program elements. It belongs to metadata and has nothing to do with business logic. Based on the mechanism of meta annotation, Java annotation can be used to define the behavior of custom annotations. The use of annotations is to apply annotations to the target program elements and obtain the information of annotations through reflection mechanisms. Annotations can take effect at compile time or runtime and have a certain lifecycle. Annotations can be applied to different program elements such as classes, methods, and fields to describe additional information about these elements.