java冷知识:javac AbstractProcessor详解

// 源于javax.annotation.processing;
public abstract class AbstractProcessor implements Processor {
    // 集合中指定支持的注解类型的名称（这里必须时完整的包名+类名)
    public Set<String> getSupportedAnnotationTypes() {
            SupportedAnnotationTypes sat = this.getClass().getAnnotation(SupportedAnnotationTypes.class);
            if  (sat == null) {
                if (isInitialized())
                    processingEnv.getMessager().printMessage(Diagnostic.Kind.WARNING,
                                                             "No SupportedAnnotationTypes annotation " +
                                                             "found on " + this.getClass().getName() +
                                                             ", returning an empty set.");
                return Collections.emptySet();
            }
            else
                return arrayToSet(sat.value());
        }
    // 指定当前正在使用的Java版本
    public SourceVersion getSupportedSourceVersion() {
        SupportedSourceVersion ssv = this.getClass().getAnnotation(SupportedSourceVersion.class);
        SourceVersion sv = null;
        if (ssv == null) {
            sv = SourceVersion.RELEASE_6;
            if (isInitialized())
                processingEnv.getMessager().printMessage(Diagnostic.Kind.WARNING,
                                                         "No SupportedSourceVersion annotation " +
                                                         "found on " + this.getClass().getName() +
                                                         ", returning " + sv + ".");
        } else
            sv = ssv.value();
        return sv;
    }
    // 初始化处理器
    public synchronized void init(ProcessingEnvironment processingEnv) {
        if (initialized)
            throw new IllegalStateException("Cannot call init more than once.");
        Objects.requireNonNull(processingEnv, "Tool provided null ProcessingEnvironment");
 
        this.processingEnv = processingEnv;
        initialized = true;
    }
    
    public abstract boolean process(Set<? extends TypeElement> annotations,
                                    RoundEnvironment roundEnv);
}

<build>
    <plugins>
        <plugin>
            <groupId>org.apache.maven.plugins</groupId>
            <artifactId>maven-compiler-plugin</artifactId>
            <configuration>
                <source>1.8</source>
                <target>1.8</target>
                <!--Disable annotation processing for ourselves-->
                <!--<compilerArgument>-proc:none</compilerArgument>-->
            </configuration>
        </plugin>
    </plugins>
</build>

@Retention(RetentionPolicy.SOURCE)
@Target({ElementType.TYPE, ElementType.METHOD})
public @interface ApiAnnotation {
    String author() default "alex.chen";
    String date();
    int version() default 1;
}
@SupportedAnnotationTypes({"com.kxtx.annotation.ApiAnnotation"})
@SupportedSourceVersion(SourceVersion.RELEASE_8)
//@AutoService(Processor.class)
public class MyProcessor extends AbstractProcessor {
    //类名的前缀、后缀
    public static final String SUFFIX = "AutoGenerate";
    public static final String PREFIX = "My_";
    @Override
    public boolean process(Set<? extends TypeElement> annotations, RoundEnvironment env) {
        Messager messager = processingEnv.getMessager();
        for (TypeElement typeElement : annotations) {
            for (Element e : env.getElementsAnnotatedWith(typeElement)) {
                //打印
                messager.printMessage(Diagnostic.Kind.WARNING, "Printing:" + e.toString());
                messager.printMessage(Diagnostic.Kind.WARNING, "Printing:" + e.getSimpleName());
                messager.printMessage(Diagnostic.Kind.WARNING, "Printing:" + e.getEnclosedElements().toString());
 
                //获取注解
                ApiAnnotation annotation = e.getAnnotation(ApiAnnotation.class);
                //获取元素名并将其首字母大写
                String name = e.getSimpleName().toString();
                char c = Character.toUpperCase(name.charAt(0));
                name = String.valueOf(c + name.substring(1));
                //包裹注解元素的元素, 也就是其父元素, 比如注解了成员变量或者成员函数, 其上层就是该类
                Element enclosingElement = e.getEnclosingElement();
                //获取父元素的全类名,用来生成报名
                String enclosingQualifiedname;
                if (enclosingElement instanceof PackageElement) {
                    enclosingQualifiedname = ((PackageElement) enclosingElement).getQualifiedName().toString();
                } else {
                    enclosingQualifiedname = ((TypeElement) enclosingElement).getQualifiedName().toString();
                }
                try {
                    //生成包名
                    String generatePackageName = enclosingQualifiedname.substring(0, enclosingQualifiedname.lastIndexOf("."));
                    // 生成的类名
                    String genarateClassName = PREFIX + enclosingElement.getSimpleName() + SUFFIX;
                    //创建Java 文件
                    JavaFileObject f = processingEnv.getFiler().createSourceFile(genarateClassName);
                    // 在控制台输出文件路径
                    messager.printMessage(Diagnostic.Kind.WARNING, "Printing: " + f.toUri());
                    Writer w = f.openWriter();
                    try {
                        PrintWriter pw = new PrintWriter(w);
                        pw.println("package " + generatePackageName + ";");
                        pw.println("\npublic class " + genarateClassName + " { ");
                        pw.println("\n    ");
                        pw.println("    public static void print" + name + "() {");
                        pw.println("        // 注解的父元素: " + enclosingElement.toString());
                        pw.println("        System.out.println(\"代码生成的路径: " + f.toUri() + "\");");
                        pw.println("        System.out.println(\"注解的元素: " + e.toString() + "\");");
                        pw.println("        System.out.println(\"注解的版本: " + annotation.version() + "\");");
                        pw.println("        System.out.println(\"注解的作者: " + annotation.author() + "\");");
                        pw.println("        System.out.println(\"注解的日期: " + annotation.date() + "\");");
 
                        pw.println("    }");
                        pw.println("}");
                        pw.flush();
                    } finally {
                        w.close();
                    }
                } catch (IOException e1) {
                    processingEnv.getMessager().printMessage(Diagnostic.Kind.ERROR,
                            e1.toString());
                }
            }
        }
        return true;
    }
}

public class My_feignAutoGenerate {
    public My_feignAutoGenerate() {
    }
    public static void printStartUp() {
        System.out.println("代码生成的路径: file:/C:/Users/Administrator/Desktop/feign-async-master/target/generated-sources/annotations/My_feignAutoGenerate.java");
        System.out.println("注解的元素: com.GitHub.feign.StartUp");
        System.out.println("注解的版本: 1");
        System.out.println("注解的作者: alex");
        System.out.println("注解的日期: 2019-03-6");
    }
}

<dependency>
    <groupId>com.google.auto.service</groupId>
    <artifactId>auto-service</artifactId>
    <version>1.0-rc2</version>
</dependency>

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.writeTo(System.out);