java基础之编译器注解

上一篇我们讲到了如何实现编译器的注解,本期让我们来看一下源文件注解的创建和使用.

在JAVA1.6后,以后不再默认包含APT了,并且相关的资源都被移除了比如Mirror相关类.提供了新的插件化注解处理API(Pluggable Annotation Processing API).

毕竟现在大家都是1.8或以上版本(嗯 是时候搞个jdk11了),本文就介绍新的插件化注解处理API(原理差不多),若对原有的APT处理有兴趣的小伙伴可以参照java编程思想注解部分学习(这么经典的书,不可能没有是不是,真没有?小哈提供了电子版下载地址关注公众号私聊小哈获取)

什么是插件化注解处理API

插件化注解处理(Pluggable Annotation Processing)APIJSR 269提供一套标准API来处理AnnotationsJSR 175.实际上JSR 269不仅仅用来处理Annotation，我觉得更强大的功能是它建立了Java 语言本身的一个模型,它把method、package、constructor、type、variable、enum、annotation等Java语言元素映射为Types和Elements，从而将Java语言的语义映射成为对象，我们可以在javax.lang.model包下面可以看到这些类。所以我们可以利用JSR 269提供的API来构建一个功能丰富的元编程(metaprogramming)环境。JSR 269用Annotation Processor在编译期间而不是运行期间处理Annotation(源文件注解), Annotation Processor相当于编译器的一个插件,所以称为插入式注解处理.如果Annotation Processor处理Annotation时(执行process()方法)产生了新的Java代码，编译器会再调用一次Annotation Processor，如果第二次处理还有新代码产生，就会接着调用Annotation Processor，直到没有新代码产生为止(可以理解为递归调用到无可调用)。每执行一次process()方法被称为一个round，这样整个Annotation processing过程可以看作是一个round的序列。JSR 269主要被设计成为针对Tools或者容器的API。这个特性虽然在JavaSE 6已经存在，但是很少人知道它的存在.(lombok就是使用这个特性实现编译期的代码插入的)

Pluggable Annotation Processing API的核心是Annotation Processor即注解处理器，一般需要继承抽象类javax.annotation.processing.AbstractProcessor。注意，与运行时注解RetentionPolicy.RUNTIME不同，注解处理器只会处理编译期注解，也就是RetentionPolicy.SOURCE的注解类型，处理的阶段位于Java代码编译期间

使用步骤

• 自定义源注解:@Retention(RetentionPolicy.SOURCE)

• 自定义任务处理器(Annotation Processor):

 继承javax.annotation.processing.AbstractProcessor，并覆写process方法.Annotation Processor中使用javax.annotation.processing.SupportedAnnotationTypes指定注解类型的名称(注意需要全类名，"包名.注解类型名称"，否则会不生效).Annotation Processor中使用javax.annotation.processing.SupportedSourceVersion指定编译版本.Annotation Processor中使用javax.annotation.processing.SupportedOptions 指定编译参数 可选参数

• 使用:在类上添加自定义注解

demo1 基础版

• 创建注解

@Target({ElementType.METHOD})
@Retention(RetentionPolicy.SOURCE)
public @interface SourceTest {public String value() default "默认值";
}

• 定义一个注解处理器

@SupportedAnnotationTypes(value = {"test2.SourceTest"})
@SupportedSourceVersion(value = SourceVersion.RELEASE_8)
public class SourceTestAnnotationProcessor extends AbstractProcessor {@Overridepublic boolean process(Set<? extends TypeElement> annotations, RoundEnvironment roundEnv) {System.out.println("Log in AnnotationProcessor.process");for (TypeElement typeElement : annotations) {System.out.println(typeElement);}System.out.println(roundEnv);return true;}
}

• 编写测试主类

public class SourceTestMain {public static void main(String[] args) throws Exception {System.out.println("开始插入式注解测试");test();}@SourceTest(value = "测试编译器注解")public static void test()throws Exception{}
}

编译方式

1. 直接使用编译参数指定，例如：javac -processor test2.SourceTestAnnotationProcessor SourceTestMain.java。

2. 通过服务注册指定，就是META-INF/services/javax.annotation.processing.Processor文件中添加test2.SourceTestAnnotationProcessor.

3. 通过Maven的编译插件的配置指定如下:

 <plugin><groupId>org.apache.maven.plugins</groupId><artifactId>maven-compiler-plugin</artifactId><version>3.5.1</version><configuration><source>1.8</source><target>1.8</target><encoding>UTF-8</encoding><annotationProcessors><annotationProcessor>test2.SourceTestAnnotationProcessor</annotationProcessor></annotationProcessors></configuration></plugin>

注意

• 使用idea时需要开启注解支持,如图所示

   

  

• 使用前注解处理器SourceTestAnnotationProcessor必须先被编译,不然会得到下面的报错信息

  

运行成功得到结果

Log in AnnotationProcessor.process
[errorRaised=false, rootElements=[club.throwable.processor.Test,club.throwable.processor.Main, club.throwable.processor.AnnotationProcessor, processingOver=false]
Log in AnnotationProcessor.process
[errorRaised=false, rootElements=[], processingOver=false]
Log in AnnotationProcessor.process
[errorRaised=false, rootElements=[], processingOver=true]

进阶版 类lombok注解

设置一个@Builder注解使类可以使用Build模式创建

• 创建@Builder注解

@Target({ElementType.METHOD})
@Retention(RetentionPolicy.SOURCE)
public @interface Builder {
}

• 创建实体类Person

public class Person {private Integer age;private String name;public Integer getAge() {return age;}@Builderpublic void setAge(Integer age) {this.age = age;}public String getName() {return name;}@Builderpublic void setName(String name) {this.name = name;}
}

• 创建注解处理器BuilderProcessor

@SupportedAnnotationTypes(value = {"club.throwable.processor.builder.Builder"})
@SupportedSourceVersion(value = SourceVersion.RELEASE_8)
public class BuilderProcessor extends AbstractProcessor {@Overridepublic boolean process(Set<? extends TypeElement> annotations, RoundEnvironment roundEnv) {for (TypeElement typeElement : annotations) {Set<? extends Element> annotatedElements = roundEnv.getElementsAnnotatedWith(typeElement);Map<Boolean, List<Element>> annotatedMethods= annotatedElements.stream().collect(Collectors.partitioningBy(element -> ((ExecutableType) element.asType()).getParameterTypes().size() == 1&& element.getSimpleName().toString().startsWith("set")));List<Element> setters = annotatedMethods.get(true);List<Element> otherMethods = annotatedMethods.get(false);otherMethods.forEach(element ->processingEnv.getMessager().printMessage(Diagnostic.Kind.ERROR,"@Builder must be applied to a setXxx method "+ "with a single argument", element));Map<String, String> setterMap = setters.stream().collect(Collectors.toMap(setter -> setter.getSimpleName().toString(),setter -> ((ExecutableType) setter.asType()).getParameterTypes().get(0).toString()));String className = ((TypeElement) setters.get(0).getEnclosingElement()).getQualifiedName().toString();try {writeBuilderFile(className, setterMap);} catch (IOException e) {e.printStackTrace();}}return true;}private void writeBuilderFile(String className, Map<String, String> setterMap)throws IOException {String packageName = null;int lastDot = className.lastIndexOf('.');if (lastDot > 0) {packageName = className.substring(0, lastDot);}String simpleClassName = className.substring(lastDot + 1);String builderClassName = className + "Builder";String builderSimpleClassName = builderClassName.substring(lastDot + 1);JavaFileObject builderFile = processingEnv.getFiler().createSourceFile(builderClassName);try (PrintWriter out = new PrintWriter(builderFile.openWriter())) {if (packageName != null) {out.print("package ");out.print(packageName);out.println(";");out.println();}out.print("public class ");out.print(builderSimpleClassName);out.println(" {");out.println();out.print("    private ");out.print(simpleClassName);out.print(" object = new ");out.print(simpleClassName);out.println("();");out.println();out.print("    public ");out.print(simpleClassName);out.println(" build() {");out.println("        return object;");out.println("    }");out.println();setterMap.forEach((methodName, argumentType) -> {out.print("    public ");out.print(builderSimpleClassName);out.print(" ");out.print(methodName);out.print("(");out.print(argumentType);out.println(" value) {");out.print("        object.");out.print(methodName);out.println("(value);");out.println("        return this;");out.println("    }");out.println();});out.println("}");}}
}

• 提起编译注解处理器

javac BuilderProcessor.java

• 编译

META-INF/services/javax.annotation.processing.Processor文件中添加test3.BuilderProcessor执行mvn compile

• 结果生成PersonBuilder类

public class PersonBuilder {private Person object = new Person();public PersonBuilder() {}public Person build() {return this.object;}public PersonBuilder setName(String value) {this.object.setName(value);return this;}public PersonBuilder setAge(Integer value) {this.object.setAge(value);return this;}
}

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