Detailed explanation of the annotation and processing process in the Java library
Note is a meta -programming feature provided by the Java language, which can add additional metadata information to the code.In the Java library, annotations are widely used for annotations during and runtime, and are used for code generation, configuration management, and inspection constraints.This article will explain the annotations and processing processes in the Java library, and provide relevant Java code examples.
1. Definition and use of annotations
In Java, the annotation is defined by the keywords of the `@Internet, which can define its own types of annotation.The following is an example of a custom annotation:
import java.lang.annotation.*;
@Retention(RetentionPolicy.RUNTIME)
@Target(ElementType.METHOD)
public @interface MyAnnotation {
String value() default "";
int priority() default 0;
}
In the above code, we specify the retention strategy of the annotation through the annotation of `@RETENTION` to ensure that the annotation is still visible at runtime; the target of the annotation is specified by the `@target` annotation.
Using annotations can be completed by adding a@使用 `` to the code, as shown below:
@MyAnnotation(value = "Hello", priority = 1)
public void myMethod() {
// do something
}
2. Comment processor
The annotation processor is a tool for processing the annotation, which is located in the annotation process.The annotation processor in the Java class library is responsible for associating the annotation with the relevant code during the compilation and runtime period.Developers can be based on the definition and extension annotations provided by the API provided by the API provided in the `javax.annotation.processing` package.
The following is an example of a simple annotation processor, which is used to handle the custom annotation in the above example:
import javax.annotation.processing.*;
import javax.lang.model.SourceVersion;
import javax.lang.model.element.Element;
import javax.lang.model.element.TypeElement;
import javax.tools.Diagnostic;
import java.util.Set;
@SupportedAnnotationTypes("MyAnnotation")
@SupportedSourceVersion(SourceVersion.RELEASE_11)
public class MyAnnotationProcessor extends AbstractProcessor {
@Override
public boolean process(Set<? extends TypeElement> annotations, RoundEnvironment roundEnv) {
for (TypeElement annotation : annotations) {
for (Element element : roundEnv.getElementsAnnotatedWith(annotation)) {
MyAnnotation myAnnotation = element.getAnnotation(MyAnnotation.class);
String value = myAnnotation.value();
int priority = myAnnotation.priority();
processingEnv.getMessager().printMessage(Diagnostic.Kind.NOTE, "Value: " + value + ", Priority: " + priority, element);
}
}
return true;
}
}
In the above code, the annotation of the annotation of the annotation processing processing processor is specified that the annotation of the annotation processor is `Myannotation`;
3. Comment processing process
The annotations in Java generally include the following steps:
1. Write and define your own types of annotations.
2. Write the annotation processor to implement the abstraction class of `AbstractProcessor`, and rewrite the` Process` method.
3. In the `process` method of the annotation processor, obtain the annotated marked marked marked markedly markedly markedly markedly markedly markedly marked markedly markedly markedly marked.
4. The element obtained by traversing is obtained by the `Element#Getannotation" method and processed the annotation.
5. In the process of processing the annotation, you can use the `ProcessingenV#GetMessager` method to print messages or report errors.
6. When compiling the source code that contains the annotation with the Java compiler, the annotation processor will be automatically triggered.
Fourth, call processor call
The processing process of the annotation can exist during the compilation period, runtime or both.In the compilation period, the annotation processor is automatically called during the Java compilation stage, and executed through the `javac` command or constructing tools (such as Maven or Gradle).
When running, use the Java's reflection mechanism to obtain and process annotations.The following is an example of the annotation of the annotation during runtime:
import java.lang.reflect.Method;
public class AnnotationDemo {
public static void main(String[] args) {
Class<?> clazz = AnnotationDemo.class;
Method[] methods = clazz.getDeclaredMethods();
for (Method method : methods) {
if (method.isAnnotationPresent(MyAnnotation.class)) {
MyAnnotation myAnnotation = method.getAnnotation(MyAnnotation.class);
String value = myAnnotation.value();
int priority = myAnnotation.priority();
System.out.println("Value: " + value + ", Priority: " + priority);
}
}
}
}
In the above code, we obtain all the methods in the `AnnotationDemo` class through the reflection mechanism, and then determine whether the method is marked with the method of` Method#IsannotationPreSent`.
Through the above examples and explanations, we can clearly understand the annotations and processing processes in the Java class library.The annotation is a very useful meta -programming feature in Java development, which can make our code more flexible and readable.
