1.新建Java Library-- 命名为annotation, 新建注解ARouter

import java.lang.annotation.ElementType;
import java.lang.annotation.Retention;
import java.lang.annotation.RetentionPolicy;
import java.lang.annotation.Target;

@Target(ElementType.TYPE)
@Retention(RetentionPolicy.CLASS)
public @interface ARouter {
    String path();
    String group() default "";
}

2.新建注解处理器Java Library 命名为compiler, 用来编译时生成代码。

import javax.annotation.processing.AbstractProcessor;
import javax.annotation.processing.Filer;
import javax.annotation.processing.Messager;
import javax.annotation.processing.ProcessingEnvironment;
import javax.annotation.processing.Processor;
import javax.annotation.processing.RoundEnvironment;
import javax.annotation.processing.SupportedAnnotationTypes;
import javax.annotation.processing.SupportedOptions;
import javax.annotation.processing.SupportedSourceVersion;
import javax.lang.model.SourceVersion;
import javax.lang.model.element.Element;
import javax.lang.model.element.Modifier;
import javax.lang.model.element.TypeElement;
import javax.lang.model.util.Elements;
import javax.lang.model.util.Types;
import javax.tools.Diagnostic;


@AutoService(Processor.class)
@SupportedAnnotationTypes({"com.XXX.annotation.ARouter"})
@SupportedSourceVersion(SourceVersion.RELEASE_7)
@SupportedOptions("maValue")
public class ArouterProcesser extends AbstractProcessor {

    private Elements mElementsTool;

    private Types mTypesTool;
    private Messager messager;
    private Filer mFiler;

    @Override
    public synchronized void init(ProcessingEnvironment processingEnvironment) {
        super.init(processingEnvironment);

        mElementsTool = processingEnvironment.getElementUtils();
        mTypesTool = processingEnvironment.getTypeUtils();
        messager = processingEnvironment.getMessager();
        mFiler = processingEnvironment.getFiler();

        String value = processingEnvironment.getOptions().get("maValue");

        messager.printMessage(Diagnostic.Kind.NOTE,"------------->" + value);

    }

    @Override
    public boolean process(Set<? extends TypeElement> set, RoundEnvironment roundEnvironment) {

        if(set.isEmpty()){
            messager.printMessage(Diagnostic.Kind.NOTE,"没有发现 被@ARouter注解的地方呀" + set.size());
            return false;
        }

        Set<? extends Element> elements = roundEnvironment.getElementsAnnotatedWith(ARouter.class);

        for(Element element:elements){
            //包名
            String packageName = mElementsTool.getPackageOf(element).getQualifiedName().toString();
            //类名
            String className =  element.getSimpleName().toString();
            messager.printMessage(Diagnostic.Kind.NOTE,"被注解的类名>>>>>" + className);


            ARouter aRouter = element.getAnnotation(ARouter.class);

            /**
             * package com.example.helloworld;
             *
             * public final class HelloWorld {
             *   public static void main(String[] args) {
             *     System.out.println("Hello, JavaPoet!");
             *   }
             * }
             */

//            MethodSpec  methodSpec = MethodSpec.methodBuilder("main")
//                    .addModifiers(Modifier.PUBLIC,Modifier.STATIC)
//                    .returns(void.class)
//                    .addParameter(String[].class,"args")
//                    .addStatement("$T.out.println($S)",System.class,"hello world")
//                    .build();
//
//            TypeSpec typeSpec = TypeSpec.classBuilder("HelloWorld")
//                    .addModifiers(Modifier.PUBLIC,Modifier.FINAL)
//                    .addMethod(methodSpec)
//                    .build();
//
//            JavaFile javaFile = JavaFile.builder("com.example.helloworld",typeSpec)
//                    .build();
//
//            try {
//                javaFile.writeTo(mFiler);
//            } catch (IOException e) {
//                e.printStackTrace();
//                messager.printMessage(Diagnostic.Kind.ERROR,"failed to create....");
//            }

            // =====================================================

            /**
             * public class MainActivity$$$$$$$$$ARouter {
             *   public static Class findTargetClass(String path) {
             *     return path.equals("app/MainActivity") ? MainActivity.class : null;
             *   }
             * }
             */

            MethodSpec methodSpec = MethodSpec.methodBuilder("findTargetClass")
                    .addModifiers(Modifier.PUBLIC,Modifier.STATIC)
                    .returns(Class.class)
                    .addParameter(String.class,"path")
                    .addStatement("return path.equals($S) ? $T.class : null",aRouter.path(), ClassName.get((TypeElement) element))
                    .build();
            String finalClsName = className + "$$ARouter";

            TypeSpec typeSpec = TypeSpec.classBuilder(finalClsName)
                    .addModifiers(Modifier.PUBLIC)
                    .addMethod(methodSpec)
                    .build();

            JavaFile javaFile = JavaFile.builder(packageName,typeSpec).build();

            try {
                javaFile.writeTo(mFiler);
            } catch (IOException e) {
                e.printStackTrace();
            }


        }

        return true;
    }
}

build.gradle代码：

apply plugin: 'java-library'

dependencies {
    implementation fileTree(dir: 'libs', include: ['*.jar'])

    // 构建 -------》 【编译时期.....】 ------》打包 ----》安装
    // AS3.4.1 + Gradle 5.1.1 + auto-service:1.0-rc4
    compileOnly'com.google.auto.service:auto-service:1.0-rc4'
    annotationProcessor'com.google.auto.service:auto-service:1.0-rc4'

    implementation project(':annotation')
    implementation "com.squareup:javapoet:1.9.0"
}

tasks.withType(JavaCompile){
    options.encoding="UTF-8"
}

sourceCompatibility = "7"
targetCompatibility = "7"

3.在主Moudle app的MainActivity加上注解@ARouter

@ARouter(path = "/app/MainActivity")
public class MainActivity extends AppCompatActivity {

    @Override
    protected void onCreate(Bundle savedInstanceState) {
        super.onCreate(savedInstanceState);
        setContentView(R.layout.activity_main);
    }
}

主module app的build.gradle

apply plugin: 'com.android.application'

android {
    compileSdkVersion 29
    buildToolsVersion "29.0.3"
    defaultConfig {
        applicationId "com.XXX.majavapoet"
        minSdkVersion 21
        targetSdkVersion 29
        versionCode 1
        versionName "1.0"
        testInstrumentationRunner "androidx.test.runner.AndroidJUnitRunner"

        javaCompileOptions{
            annotationProcessorOptions{
                //给注解传入参数
                arguments = [maValue:'hello  Javapoet'];
            }
        }

    }
    buildTypes {
        release {
            minifyEnabled false
            proguardFiles getDefaultProguardFile('proguard-android-optimize.txt'), 'proguard-rules.pro'
        }
    }
}

dependencies {
    implementation fileTree(dir: 'libs', include: ['*.jar'])
    implementation 'androidx.appcompat:appcompat:1.1.0'
    implementation 'androidx.constraintlayout:constraintlayout:1.1.3'
    testImplementation 'junit:junit:4.12'
    androidTestImplementation 'androidx.test.ext:junit:1.1.1'
    androidTestImplementation 'androidx.test.espresso:espresso-core:3.2.0'
    //依赖注解
    implementation project(":annotation")
   //依赖注解处理器
    annotationProcessor project(":compiler")
}

4.在如下目录就可以看到编译时生成的代码了

JavaPoet.png