编译时注解 5 步走

前言

简单记录一下，编写编译时注解的几个步骤。

新建Java Module

引入auto-service(自动生成services下文件)和javapoet（生成java文件）

1 2	compile 'com.google.auto.service:auto-service:1.0-rc2' compile 'com.squareup:javapoet:1.8.0'

编写注解

@Target(ElementType.TYPE)
@Retention(RetentionPolicy.SOURCE)
@Inherited
public @interface Api {

    String scope();

    String name();

    String version();

}

创建AbstractProcessor实现类

重写getSupportedSourceVersion，getSupportedAnnotationTypes和process方法。注意在类上加入注解AutoService，便会自动生成META-INF下的services文件

@AutoService(Processor.class)
public class AnnotationProcessor extends AbstractProcessor {

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

    @Override
    public Set<String> getSupportedAnnotationTypes() {
        Set<String> supportedAnnotationTypes = new HashSet<>();
        supportedAnnotationTypes.add(Api.class.getCanonicalName());
        return supportedAnnotationTypes;
    }

    @Override
    public boolean process(Set<? extends TypeElement> annotations, RoundEnvironment roundEnv) {
        return false;
    }


}

实现process方法，根据注解生成对应代码

public boolean process(Set<? extends TypeElement> annotations, RoundEnvironment roundEnv) {
    Set<? extends Element> elements = roundEnv.getElementsAnnotatedWith(Api.class);
    for (Element e : elements) {
    	//获得对应的注解
        Api api = e.getAnnotation(Api.class);
        System.out.println("*");
        //使用javapoet开始生成代码
        TypeSpec clazz = TypeSpec.classBuilder(api.name().toUpperCase() + "Request")
                .addJavadoc("Generated request for network\n")
                .addSuperinterface(Serializable.class)
                .build();

        JavaFile javaFile = JavaFile.builder("com.funcknmb.api", clazz)
                .build();

        //生成的代码写入文件
        try {
            JavaFileObject fileObject = processingEnv.getFiler().createSourceFile("com.funcknmb.api." + api.name().toUpperCase() + "Request");
            processingEnv.getMessager().printMessage(Diagnostic.Kind.NOTE, "Creating " + fileObject.toUri());
            Writer writer = fileObject.openWriter();
            javaFile.writeTo(writer);
            writer.close();
        } catch (IOException x) {
            processingEnv.getMessager().printMessage(Diagnostic.Kind.ERROR, x.toString());
        }
    }
    return false;
}

总结

编写的注解以及process的实现只是做个模板，具体情况视情况而定。