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Annotation Programming 簡介

meta programming

  • 編譯時期 Processor
  • 執行時期 Reflection, InvocationHandler

這邊主要討論編譯時期 JSR 269 javax.annotation.processing.AbstractProcessor

從常見的 ORM annotations 、json2pojo(Gson, Jackson) 、Dagger1/2、Mockito、Retrofit、AutoValue、AutoParcel、ButterKnife、AndroidAnnotations、RoboGuice 等函式庫，大量利用 annotations 來精簡聚焦、解決煩冗的例行性撰寫程序。

像是 AutoValue 就節省了煩冗的 getter 、 setter 、builder 等例行撰寫程序。

除了解析 annotations 還包括解析 abstract methods, methods, interfaces, fields 等，緊接著是產生 java file 的 template language 以及物件導向的產生器。

解析 Annotation

@Example
public class ExampleClass implements Runnable {
    public final String name;
    public ExampleClass(String name) {
        this.name = name;
    }
    @Override
    public void run() {
        System.out.println(name);
    }
}
public ExampleProcessor extends AbstractProcessor {
    // ...
    @Override
    public boolean process(Set<? extends TypeElement> annotations, RoundEnvironment env) {
        Set<? extends Element> elements = env.getElementAnnotatedWith(Example.class);
        for (Element e : elements) { // 可列出 @Example
            System.out.println(e);
        }
        return false;
    }
}

產生器的撰寫方法

依據解析結果產生 java file。

Template Language:

Apache Velocity Template Language, *.vm

For AVTL example:

#if (!$pkg.empty)
package $pkg;
#end

#foreach ($i in $imports)
import $i;
#end

@${generated}("com.google.auto.value.processor.AutoAnnotationProcessor")
final class $className implements $annotationName {

## Fields

#foreach ($m in $members)
  #if ($params.containsKey($m.toString()))

  private final $m.type $m;

  #else

  private static final $m.type $m = $m.defaultValue;

  #end
#end

Square JavaPoet: https://github.com/square/javapoet

For JavaPoet example:

package com.example.helloworld;

public final class HelloWorld {
  public static void main(String[] args) {
    System.out.println("Hello, JavaPoet!");
  }
}
MethodSpec main = MethodSpec.methodBuilder("main")
    .addModifiers(Modifier.PUBLIC, Modifier.STATIC)
    .returns(void.class)
    .addParameter(String[].class, "args")
    .addStatement("$T.out.println($S)", System.class, "Hello, JavaPoet!")
    .build();

TypeSpec helloWorld = TypeSpec.classBuilder("HelloWorld")
    .addModifiers(Modifier.PUBLIC, Modifier.FINAL)
    .addMethod(main)
    .build();

JavaFile javaFile = JavaFile.builder("com.example.helloworld", helloWorld)
    .build();

javaFile.emit(System.out);

TypeMirror -> Annoation

Annotation annotation = ((DeclaredType) typeMirror).asElement().getAnnotation(AutoValue.Field.class);

TODO

https://github.com/yongjhih/JavaPoetic:

JavaFile javaFile = JavaFile.package("com.example.helloworld").class(
    JavaClass.public().final().name("HelloWorld").method(
            JavaMethod.public().static().void().name("main").parameter(String[].class, "args").statement(
                JavaStatement("$T.out.println($S)", System.class, "Hello, JavaPoet!"),
                JavaStatement("$T.out.println($S)", System.class, "Hello, JavaPoet!")
            )
        )
    )
);

名詞解釋

APT, Annotation-Processing Tool

bytecode weaving

  • ASM
  • AspectJ
  • Javasist

See also