How to use the AnnotationProcessor function for annotation processing in Java

In Java programming, annotations can add additional information to the program and provide a simpler and clearer code structure. However, in some cases we need to process annotations during compilation in order to generate the necessary code or perform a specific operation. At this time, you need to use the AnnotationProcessing tool to process annotations.

AnnotationProcessing is a tool provided by the AnnotationProcessor API in the Java compiler, which can process annotations encountered by the compiler and generate additional Java source code and other files. In this article, we will explore how to use the AnnotationProcessor function to process annotations.

1. Create the Annotation class

First, we need to create the annotation class. Annotated classes in Java must be decorated with the @Retention(RetentionPolicy.SOURCE) annotation to ensure that it is discarded during the compiler process and is not included in the final class file.

The following is a simple annotation class example:

import java.lang.annotation.ElementType;
import java.lang.annotation.Retention;
import java.lang.annotation.RetentionPolicy;
import java.lang.annotation.Target;

@Retention(RetentionPolicy.SOURCE)
@Target(ElementType.TYPE)
public @interface MyAnnotation {
    String value() default "";
}

2. Create the AnnotationProcessor class

Next, we need to create an AnnotationProcessor class, which will Used to process annotations. The AnnotationProcessor class must implement the process() method in the javax.annotation.processing.Processor interface, which will be called when the compiler encounters an annotation.

import javax.annotation.processing.AbstractProcessor;
import javax.annotation.processing.RoundEnvironment;
import javax.lang.model.SourceVersion;
import javax.lang.model.element.TypeElement;
import java.util.Set;

public class MyAnnotationProcessor extends AbstractProcessor {
    @Override
    public boolean process(Set<? extends TypeElement> annotations,
                           RoundEnvironment roundEnv) {
        // 处理注解
        return true;
    }

    @Override
    public Set<String> getSupportedAnnotationTypes() {
        return Set.of(MyAnnotation.class.getName());
    }

    @Override
    public SourceVersion getSupportedSourceVersion() {
        return SourceVersion.latestSupported();
    }
}

The process() method in the AnnotationProcessor class contains two parameters, one is a set collection containing the currently processed annotation types, and the other is an element containing elements used to access and process current and previous annotations. RoundEnvironment object. In this method we can write code to handle the supported annotations.

3. Configuring AnnotationProcessor

We also need to configure a metadata file for AnnotationProcessor to ensure that the compiler can find and load our AnnotationProcessor when needed. The metadata file should be named META-INF/services/javax.annotation.processing.Processor and contain the fully qualified class name of the AnnotationProcessor class.

You can create it using the following command line:

$ mkdir -p META-INF/services
$ echo 'com.example.MyAnnotationProcessor' > META-INF/services/javax.annotation.processing.Processor

4. Using Annotation in Code

Now that we have created the annotation class and the AnnotationProcessor class, and made relevant configurations. Next, we need to use annotations in Java code and trigger the processing of AnnotationProcessor.

@MyAnnotation(value = "Hello, Annotation Processor!")
public class MyClass {
    public static void main(String[] args) {
        System.out.println("Hello, World!");
    }
}

In this example, we mark the MyClass class as MyAnnotation, and then we will process the annotation in the AnnotationProcessor.

5. Execute AnnotationProcessor

Finally, we need to ensure that AnnotationProcessor is executed at compile time. To do this, we can use the annotation processor command line option provided by the Java compiler -javax.annotation.processing.Processor to specify the fully qualified class name of the AnnotationProcessor.

$ javac -cp /path/to/annotation-processor.jar -processor com.example.MyAnnotationProcessor MyClass.java

In the above example, the -cp option is used to specify the dependency of AnnotationProcessing, the -processor option is used to specify the fully qualified class name of the AnnotationProcessor to be used, and MyClass.java is the source code file to be compiled.

The above is a simple example of using the AnnotationProcessor function for annotation processing. Of course, AnnotationProcessing can perform more complex processing operations and generate more code.

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