public class BindingView {
 
    public static void init(Activity activity) {
        Field[] fields = activity.getClass().getDeclaredFields();
        for (Field field : fields) {
            //獲取 被注解
            BindView annotation = field.getAnnotation(BindView.class);
            if (annotation != null) {
                int viewId = annotation.value();
                field.setAccessible(true);
                try {
                    field.set(activity, activity.findViewById(viewId));
                } catch (IllegalAccessException e) {
                    e.printStackTrace();
                }
            }
 
        }
 
    }
}

public class BindingView {
 
    public static void init(Activity activity) {
        try {
            Class bindingClass = Class.forName(activity.getClass().getCanonicalName() + "ViewBinding");
            Constructor constructor = bindingClass.getDeclaredConstructor(activity.getClass());
            constructor.newInstance(activity);
        } catch (ClassNotFoundException | NoSuchMethodException e) {
            e.printStackTrace();
        } catch (IllegalAccessException e) {
            e.printStackTrace();
        } catch (InstantiationException e) {
            e.printStackTrace();
        } catch (InvocationTargetException e) {
            e.printStackTrace();
        }
    }
}

@Retention(RetentionPolicy.SOURCE)
@Target(ElementType.FIELD)
public @interface BindView {
    int value();
}
public class BindingProcessor extends AbstractProcessor {
 
    Messager messager;
 
    @Override
    public synchronized void init(ProcessingEnvironment processingEnvironment) {
        messager = processingEnvironment.getMessager();
        messager.printMessage(Diagnostic.Kind.NOTE, " BindingProcessor init");
        super.init(processingEnvironment);
    }
 
    @Override
    public boolean process(Set<? extends TypeElement> set, RoundEnvironment roundEnvironment) {
        return false;
    }
 
    //要支持哪些注解
    @Override
    public Set<String> getSupportedAnnotationTypes() {
        return Collections.singleton(BindView.class.getCanonicalName());
    }
}

public class BindingProcessor extends AbstractProcessor {
 
    Messager messager;
    Filer filer;
    Elements elementUtils;
 
    @Override
    public synchronized void init(ProcessingEnvironment processingEnvironment) {
        //主要是輸出一些重要的日志使用
        messager = processingEnvironment.getMessager();
        //你就理解成最終我們寫java文件 要用到的重要 輸出參數(shù)即可
        filer = processingEnvironment.getFiler();
        //一些方便的utils方法
        elementUtils = processingEnvironment.getElementUtils();
        //這里要注意的是Diagnostic.Kind.ERROR 是可以讓編譯失敗的 一些重要的參數(shù)校驗(yàn)可以用這個(gè)來提示用戶你哪里寫的不對
        messager.printMessage(Diagnostic.Kind.NOTE, " BindingProcessor init");
        super.init(processingEnvironment);
    }
 
    private void generateCodeByStringBuffer(String className, List<Element> elements) throws IOException {
 
        //你要生成的類 要和 注解的類 同屬一個(gè)package 所以還要取 package的名稱
        String packageName = elementUtils.getPackageOf(elements.get(0)).getQualifiedName().toString();
        StringBuffer sb = new StringBuffer();
        // 每個(gè)java類 的開頭都是package sth...
        sb.append("package ");
        sb.append(packageName);
        sb.append(";\n");
 
        // public class XXXActivityViewBinding {
        final String classDefine = "public class " + className + "ViewBinding { \n";
        sb.append(classDefine);
 
        //定義構(gòu)造函數(shù)的開頭
        String constructorName = "public " + className + "ViewBinding(" + className + " activity){ \n";
        sb.append(constructorName);
 
        //遍歷所有element 生成諸如 activity.tv=activity.findViewById(R.id.xxx) 之類的語句
        for (Element e : elements) {
            sb.append("activity." + e.getSimpleName() + "=activity.findViewById(" + e.getAnnotation(BindView.class).value() + ");\n");
        }
 
        sb.append("\n}");
        sb.append("\n }");
 
        //文件內(nèi)容確定以后 直接生成即可
        JavaFileObject sourceFile = filer.createSourceFile(className + "ViewBinding");
        Writer writer = sourceFile.openWriter();
        writer.write(sb.toString());
        writer.close();
    }
 
    @Override
    public boolean process(Set<? extends TypeElement> set, RoundEnvironment roundEnvironment) {
 
        // key 就是使用注解的class的類名 element就是使用注解本身的元素 一個(gè)class 可以有多個(gè)使用注解的field
        Map<String, List<Element>> fieldMap = new HashMap<>();
        // 這里 獲取到 所有使用了 BindView 注解的 element
        for (Element element : roundEnvironment.getElementsAnnotatedWith(BindView.class)) {
            //取到 這個(gè)注解所屬的class的Name
            String className = element.getEnclosingElement().getSimpleName().toString();
            //取到值以后 判斷map中 有沒有 如果沒有就直接put 有的話 就直接在這個(gè)value中增加一個(gè)element
            if (fieldMap.get(className) != null) {
                List<Element> elementList = fieldMap.get(className);
                elementList.add(element);
            } else {
                List<Element> elements = new ArrayList<>();
                elements.add(element);
                fieldMap.put(className, elements);
            }
        }
 
        //遍歷map，開始生成輔助類
        for (Map.Entry<String, List<Element>> entry : fieldMap.entrySet()) {
            try {
                generateCodeByStringBuffer(entry.getKey(), entry.getValue());
            } catch (IOException e) {
                e.printStackTrace();
            }
        }
        return false;
    }
 
    //要支持哪些注解
    @Override
    public Set<String> getSupportedAnnotationTypes() {
        return Collections.singleton(BindView.class.getCanonicalName());
    }
}

package com.smart.annotationlib_2;//PackageElement |表示一個(gè)包程序元素
//  TypeElement 表示一個(gè)類或接口程序元素。
public class VivoTest {
    //VariableElement |表示一個(gè)字段、enum 常量、方法或構(gòu)造方法參數(shù)、局部變量或異常參數(shù)。
    int a;
 
    //VivoTest 這個(gè)方法 :ExecutableElement|表示某個(gè)類或接口的方法、構(gòu)造方法或初始化程序（靜態(tài)或?qū)嵗?，包括注釋類型元素?
    //int b 這個(gè)函數(shù)參數(shù): TypeParameterElement |表示一般類、接口、方法或構(gòu)造方法元素的形式類型參數(shù)。
    public VivoTest(int b ) {
        this.a = b;
    }
}

private void generateCodeByJavapoet(String className, List<Element> elements) throws IOException {
 
    //聲明構(gòu)造方法
    MethodSpec.Builder constructMethodBuilder =
            MethodSpec.constructorBuilder().addModifiers(Modifier.PUBLIC).addParameter(ClassName.bestGuess(className), "activity");
    //構(gòu)造方法里面 增加語句
    for (Element e : elements) {
        constructMethodBuilder.addStatement("activity." + e.getSimpleName() + "=activity.findViewById(" + e.getAnnotation(BindView.class).value() + ");");
    }
 
    //聲明類
    TypeSpec viewBindingClass =
            TypeSpec.classBuilder(className + "ViewBinding").addModifiers(Modifier.PUBLIC).addMethod(constructMethodBuilder.build()).build();
    String packageName = elementUtils.getPackageOf(elements.get(0)).getQualifiedName().toString();
     
    JavaFile build = JavaFile.builder(packageName, viewBindingClass).build();
    build.writeTo(filer);
}