JVM进阶 -- 浅谈注解处理器

@Target(ElementType.METHOD)
@Retention(RetentionPolicy.SOURCE)
public @interface Override {
}
// 元注解@Target：用来限定目标注解所能标注的Java结构
// 元注解@Retention：用来限定目标注解的生命周期
//      SOURCE：源代码，一旦源代码被编译为字节码，注解便会被擦除
//      CLASS：源代码+字节码
//      RUNTIME：源代码+字节码+运行时

import java.lang.annotation.*;

@Target({ElementType.TYPE, ElementType.FIELD})
@Retention(RetentionPolicy.SOURCE)
public @interface CheckGetter {
}

public interface Processor {
    void init(ProcessingEnvironment processingEnv);
    Set<String> getSupportedAnnotationTypes();
    SourceVersion getSupportedSourceVersion();
    boolean process(Set<? extends TypeElement> annotations, RoundEnvironment roundEnv);
    ...
}

import javax.annotation.processing.*;
import javax.lang.model.SourceVersion;
import javax.lang.model.element.*;
import javax.lang.model.util.ElementFilter;
import javax.tools.Diagnostic;
import java.util.Set;

@SupportedAnnotationTypes("CheckGetter")
@SupportedSourceVersion(SourceVersion.RELEASE_8)
public class CheckGetterProcessor extends AbstractProcessor {
    @Override
    public boolean process(Set<? extends TypeElement> annotations, RoundEnvironment roundEnv) {
        // annotations：该注解处理器所能处理的注解类型
        // roundEnv：囊括当前轮生成的抽象语法树RoundEnvironment
        // roundEnv.getElementsAnnotatedWith(CheckGetter.class) -> 获取所有被@CheckGetter注解的类
        for (TypeElement annotatedClass : ElementFilter.typesIn(roundEnv.getElementsAnnotatedWith(CheckGetter.class))) {
            for (VariableElement field : ElementFilter.fieldsIn(annotatedClass.getEnclosedElements())) {
                if (!containsGetter(annotatedClass, field.getSimpleName().toString())) {
                    processingEnv.getMessager().printMessage(Diagnostic.Kind.ERROR,
                            String.format("getter not fund for '%s.%s'.", annotatedClass.getSimpleName(),
                                    field.getSimpleName()));
                }
            }
        }
        return false;
    }

    private static boolean containsGetter(TypeElement typeElement, String name) {
        // 拼接getter名称
        String getter = "get" + name.substring(0, 1).toUpperCase() + name.substring(1).toLowerCase();
        for (ExecutableElement executableElement : ElementFilter.methodsIn(typeElement.getEnclosedElements())) {
            if (!executableElement.getModifiers().contains(Modifier.STATIC)
                    && executableElement.getSimpleName().toString().equals(getter)
                    && executableElement.getParameters().isEmpty()) {
                return true;
            }
        }
        return false;
    }
}

// package me.zhongmingmao.advanced.annotation; // PackageElement

@CheckGetter
public class Foo { // TypeElement
    int a; // VariableElement
    static int b; // VariableElement

    Foo() { // ExecutableElement
    }

    void setA( // ExecutableElement
               int newA // VariableElement
    ) {
    }
}

$ ll
total 24
-rw-r--r--  1 zhongmingmao  staff   147B  1  7 22:53 CheckGetter.java
-rw-r--r--  1 zhongmingmao  staff   2.0K  1  7 22:54 CheckGetterProcessor.java
-rw-r--r--  1 zhongmingmao  staff   316B  1  7 22:55 Foo.java

$ javac CheckGetter.java CheckGetterProcessor.java

$ javac -cp . -processor CheckGetterProcessor Foo.java
错误: getter not fund for 'Foo.a'.
错误: getter not fund for 'Foo.b'.
2 个错误

$ ll
total 40
-rw-r--r--  1 zhongmingmao  staff   385B  1  7 22:57 CheckGetter.class
-rw-r--r--  1 zhongmingmao  staff   147B  1  7 22:53 CheckGetter.java
-rw-r--r--  1 zhongmingmao  staff   3.1K  1  7 22:57 CheckGetterProcessor.class
-rw-r--r--  1 zhongmingmao  staff   2.0K  1  7 22:54 CheckGetterProcessor.java
-rw-r--r--  1 zhongmingmao  staff   316B  1  7 22:55 Foo.java