Analyze the "annotation" framework principle in the Java class library
Annotion is a special label in the Java class library that provides the ability to insert metadata information in the code.In the Java language, the annotation can be used to add additional meta -data information to the class, methods, fields, parameters, etc., so as to analyze and process it during compilation, runtime or through the reflection mechanism.The annotations are mainly achieved through meta -nnotation and Annotion Processor.
1. Yuan Note
Java provides some built -in meta -injects to define the tags of custom annotations:
1. @Target: Used to specify the target types that can be applied to custom annotations.The types that can be included are class, methods, fields, etc.
2. @Retention: Life cycle used to specify custom annotations.The annotation can exist in the source code, compilation, and during runtime. The value of the annotation of the meta is Source, Class, and Runtime.
3. @Documented: It is used to specify whether the custom annotation is included in Javadoc.
4. @inherited: It is used to specify whether the custom annotation can be inherited by the subclass.
2. Comment processor
Java's annotation processor is analyzed by the reflection mechanism, and specific processing is performed according to the definition of the annotation.Generally speaking, the annotation processor can be performed through the Java APT (Annotation Processing Tool) tool.
The workflow of the annotation processor is as follows:
1. Scan source code: The annotation processor will first scan the source code to find elements containing target annotations.
2. Analysis annotation: The annotation processor analyzes the annotation through the reflection mechanism and obtains the value of the annotation.
3. Generate code: Corresponding logical operations are performed according to the annotation value, such as generating code, check -in constraints, etc.
4. Output results: After processing is completed, the annotation processor can generate new source code files, binary files or log information.
For example, the following is an example of a customized annotation and its annotation processor:
import java.lang.annotation.*;
import java.lang.reflect.*;
@Retention(RetentionPolicy.RUNTIME)
@Target(ElementType.METHOD)
public @interface MyAnnotation {
String value();
}
public class MyAnnotationProcessor {
public static void main(String[] args) throws IllegalAccessException, IllegalArgumentException, InvocationTargetException, NoSuchMethodException, SecurityException {
Class<MyAnnotationUsage> obj = MyAnnotationUsage.class;
Method[] methods = obj.getMethods();
for(Method method : methods) {
if(method.isAnnotationPresent(MyAnnotation.class)) {
MyAnnotation annotation = method.getAnnotation(MyAnnotation.class);
System.out.println("Method: " + method.getName() + ", Annotation value: " + annotation.value());
}
}
}
}
public class MyAnnotationUsage {
@MyAnnotation("Hello")
public void myMethod() {
System.out.println("My Method");
}
}
In the above code, we first define a custom annotation `@myannotation`, which can be applied to the method and has a` value` attribute.Then, we define an annotation processor `myannotationProcessor` to analyze the coding solution through the reflection mechanism and output the value of the annotation.Finally, we used custom annotations in the `myannotationusage` class`@myannotation` and set the annotation value on the `MyMethod` method.When we run the `MyannotationProcessor`, it will output" Method: MyMethod, Annotation Value: Hello ".It can be seen from the output results that we have successfully parsed the annotation and obtained the value of the annotation.
Summarize:
Annotion is a special label in the Java library that can add metad data information to the code.The annotations are implemented by meta -annotations and annotations. The meta -commentary is used to define and describe the attributes of the annotation. The annotation processor analyzes the annotation through the reflection mechanism and is processed according to the definition of the annotation.By using annotations, we can add more semantic information to our code to improve the readability and flexibility of code.
