Advanced usage and skills of the annotation framework

Note is a mechanism that provides metadata information in Java.It can be marked on program, methods, fields and other procedures, and add additional description information to these elements.The annotation framework is an annotation -based programming mode that can achieve some advanced usage and techniques by custom annotations and processors.This article will introduce the advanced usage of the annotation framework and some related Java code examples. 1. Custom annotation: The custom annotation was created by using the Java's metajuction.Yuan Note is a special annotation in Java, which is used to annotate other annotations.We can use meta -withdrawal to specify the scope, retaining strategy and other attributes of custom annotations. import java.lang.annotation.ElementType; import java.lang.annotation.Retention; import java.lang.annotation.RetentionPolicy; import java.lang.annotation.Target; @Retention(RetentionPolicy.RUNTIME) @Target(ElementType.METHOD) public @interface MyAnnotation { String value() default ""; int priority() default 0; } The above code demonstrates the custom annotation `myannotation`, which contains a` value` property and a `priority` attribute. 2. Note processor: The annotation processor is a tool for processing annotations when compiling or runtime.You can read and analyze the annotation information by writing a customized annotation processor, and perform code generation or other processing operations according to the annotation information. import java.lang.reflect.Method; public class MyAnnotationProcessor { public static void processAnnotations(Object obj) { Class<?> clazz = obj.getClass(); for (Method method : clazz.getMethods()) { MyAnnotation annotation = method.getAnnotation(MyAnnotation.class); if (annotation != null) { String value = annotation.value(); int priority = annotation.priority(); System.out.println("Method: " + method.getName() + ", Value: " + value + ", Priority: " + priority); } } } } The above code demonstrates a simple annotation processor `myannotationProcessor`. It can read whether the method in the class method exists in the method of` myannotion` and output the relevant information of the annotation. 3. Use the annotation framework: Using the annotation framework can simplify the writing and maintenance of code.The following is an example that shows how to use the above custom annotations and annotation processors: public class MyClass { @MyAnnotation(value = "Hello", priority = 1) public void myMethod() { // Do some treatment } public static void main(String[] args) { MyClass obj = new MyClass(); MyAnnotationProcessor.processAnnotations(obj); } } Run the above example code, and will get the following output: Method: myMethod, Value: Hello, Priority: 1 This example shows how to use the `Myannotation` annotation on the method` mymethod`, and read and output the relevant information of the annotation through the annotation processor `myannotationProcessor`. Advanced usage and techniques of the annotation framework usually involve more complicated and more flexible annotations, the use of meta annotations, the writing of the annotation processor, and the runtime reflection of the annotation.By using the annotation framework reasonably, we can achieve some powerful functions, such as dependency injection, ORM mapping, code generation, etc.