先建三个module，分别为Butterknife ButterKnife-Annotions ButterKnife-compiler,其中butterknife为Android Module其余的都是Java Module
。

ButterKnife-Annotions：提供注解。
ButterKnife-compiler: 依赖ButterKnife-Annotions,生成代码的module。
Butterknife：应用直接使用的库。

Butterknife-Annotions添加注解

@Retention(CLASS)
@Target(FIELD)
public @interface BindView {
int value();
}

ButterKnife-compiler添加依赖

dependencies {
implementation fileTree(include: ['*.jar'], dir: 'libs')
implementation 'com.google.auto.service:auto-service:1.0-rc3'
implementation 'com.squareup:javapoet:1.9.0'
implementation project(':ButterKnife-Annotions')
}
写一个生成代码的类ButterKnifeProcessor继承AbstractProcessor

public class ButterKnifeProcessor extends AbstractProcessor {
@Override
public boolean process(Set<? extends TypeElement> annotations, RoundEnvironment roundEnv) {
return false;
}
}

我们的步骤是:

添加注解@AutoService(Processor.class)
确定好我们需要生成代码的JAVA版本
确定生成代码
生成代码
import com.zzw.butterknife_annotions.BindView;

import java.lang.annotation.Annotation;
import java.util.LinkedHashSet;
import java.util.Set;

import javax.annotation.processing.AbstractProcessor;
import javax.annotation.processing.RoundEnvironment;
import javax.lang.model.SourceVersion;
import javax.lang.model.element.TypeElement;

//1. 添加注解 @AutoService(Processor.class)
@AutoService(Processor.class)
public class ButterKnifeProcessor extends AbstractProcessor {

//2. 指定处理的版本 >1.7

@Override
public SourceVersion getSupportedSourceVersion() {
    //最高版本
    return SourceVersion.latestSupported();
}

//3. 指定处理的注解
@Override
public Set<String> getSupportedAnnotationTypes() {
    Set<String> types = new LinkedHashSet<>();
    for (Class<? extends Annotation> annotation : getSupportedAnnotations()) {
        types.add(annotation.getCanonicalName());
    }
    return types;
}

//4. 在这执行生成类
@Override
public boolean process(Set<? extends TypeElement> annotations, RoundEnvironment roundEnv) {
    return false;
}


private Set<Class<? extends Annotation>> getSupportedAnnotations() {
    Set<Class<? extends Annotation>> annotations = new LinkedHashSet<>();
    annotations.add(BindView.class);
    return annotations;
}

}

我们确定一下这个环境编译是否是正确的
Butterknife里面添加ButterKnife-compiler依赖

api project(':ButterKnife-compiler')

app添加Butterknife依赖和compiler生成代码依赖

implementation project(':Butterknife')
annotationProcessor project(':ButterKnife-compiler')

在app的MainActivity添加注解

@BindView(R.id.text_view)
TextView textView;
这个时候运行程序报错了提示

C:\Users\Swartz\Desktop\githubdemo\AptDemo\ButterKnife-compiler\src\main\java\com\zzw\butterknife_compiler\ButterKnifeProcessor.java
Error:(21, 20) 错误: 编码GBK的不可映射字符
Error:(25, 12) 错误: 编码GBK的不可映射字符
Error:(25, 17) 错误: 编码GBK的不可映射字符
Error:(29, 20) 错误: 编码GBK的不可映射字符
Error:(39, 20) 错误: 编码GBK的不可映射字符
Error:Execution failed for task ':Butterknife:javaPreCompileDebug'.

Annotation processors must be explicitly declared now. The following dependencies on the compile classpath are found to contain annotation processor. Please add them to the annotationProcessor configuration.
- ButterKnife-compiler.jar (project :ButterKnife-compiler)
Alternatively, set android.defaultConfig.javaCompileOptions.annotationProcessorOptions.includeCompileClasspath = true to continue with previous behavior. Note that this option is deprecated and will be removed in the future.
See https://developer.android.com/r/tools/annotation-processor-error-message.html for more details.

这个是因为我们在ButterKnifeProcessor这个类使用了中文，所以我们在ButterKnife-compiler中指定编码集

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

这个时候我们在ButterKnifeProcessor里面process函数添加打印

System.out.print("11111");
然后随便修改一下MainActivity的代码，重新编译在Gradle Console里面就可以看到打印了。这里需要修改MainActivity代码是因为我们是编译时生成，as为了节省时间是重新编译修改了的类，如果不修改的话可以能会不编译。

环境ok了，这下我们就需要重点放在生成代码这块了，这块又分为以下步骤:

因为这getSupportedAnnotationTypes添加的所有注解都会走到process函数，所以我们要将注解按照类来分离。比如AActivity对应那些有那些注解，BActivity里面有那些注解。
生成代码

package com.zzw.butterknife_compiler;

import com.google.auto.service.AutoService;
import com.squareup.javapoet.ClassName;
import com.squareup.javapoet.JavaFile;
import com.squareup.javapoet.MethodSpec;
import com.squareup.javapoet.TypeSpec;
import com.zzw.butterknife_annotions.BindView;

import java.io.IOException;
import java.lang.annotation.Annotation;
import java.util.ArrayList;
import java.util.LinkedHashMap;
import java.util.LinkedHashSet;
import java.util.List;
import java.util.Map;
import java.util.Set;

import javax.annotation.processing.AbstractProcessor;
import javax.annotation.processing.Filer;
import javax.annotation.processing.ProcessingEnvironment;
import javax.annotation.processing.Processor;
import javax.annotation.processing.RoundEnvironment;
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;

//1. 添加注解 @AutoService(Processor.class)
@AutoService(Processor.class)
public class ButterKnifeProcessor extends AbstractProcessor {
//决定生成的文件在哪个文件夹
private Filer mFiler;
//决定生成的生成包名用
private Elements mElementUtils;

@Override
public synchronized void init(ProcessingEnvironment processingEnv) {
    super.init(processingEnv);
    mFiler = processingEnv.getFiler();
    mElementUtils = processingEnv.getElementUtils();
}


//2. 指定处理的版本 >1.7

@Override
public SourceVersion getSupportedSourceVersion() {
    //最高版本
    return SourceVersion.latestSupported();
}

//3. 指定处理的注解
@Override
public Set<String> getSupportedAnnotationTypes() {
    Set<String> types = new LinkedHashSet<>();
    for (Class<? extends Annotation> annotation : getSupportedAnnotations()) {
        types.add(annotation.getCanonicalName());
    }
    return types;
}

//4. 在这执行生成类
@Override
public boolean process(Set<? extends TypeElement> annotations, RoundEnvironment roundEnv) {
    //4.1 注解分类
    //拿到所有的注解标记的元素
    Set<? extends Element> elements=  roundEnv.getElementsAnnotatedWith(BindView.class);
    //分类集合
    Map<Element , List<Element>>  elementMap= new LinkedHashMap<>();
    //进行分类
    for (Element element : elements) {
        //拿到属于哪个类
        Element  enclosingElement = element.getEnclosingElement();
        List<Element>  viewBindElements =  elementMap.get(enclosingElement);
        if(viewBindElements ==null){
            viewBindElements = new ArrayList<>();
            elementMap.put(enclosingElement,viewBindElements);
        }
        viewBindElements.add(element);
    }
    //4.2 生成代码
    for (Map.Entry<Element,List<Element>> entry : elementMap.entrySet()) {
        Element enclosingElement =   entry.getKey();
        List<Element> viewBindElements =  entry.getValue();

        //类信息
        //  public final class xxxActivity_ViewBinding implements Unbinder
        final String activityNameStr = enclosingElement.getSimpleName().toString();
        ClassName activityClassName =   ClassName.bestGuess(activityNameStr);
        TypeSpec.Builder classBuilder = TypeSpec.classBuilder( activityNameStr +"_ViewBinding")
                .addModifiers(Modifier.FINAL,Modifier.PUBLIC)
                //添加实现
                .addSuperinterface(ClassName.get("com.zzw.butterknife","Unbinder"))
                //添加属性
                .addField(activityClassName,"target",Modifier.PRIVATE);

        //方法信息
        //实现unbinder方法
        MethodSpec.Builder unbindMethodBuilder  = MethodSpec.methodBuilder("unbind")
                .addAnnotation(Override.class)
                .addModifiers(Modifier.PUBLIC);

        unbindMethodBuilder.addStatement("$T target = this.target",activityClassName);
        unbindMethodBuilder.addStatement("if (target == null) throw new IllegalStateException(\"Bindings already cleared.\");");

        //构造函数
        MethodSpec.Builder constructorMethodBuilder = MethodSpec.constructorBuilder()
                .addModifiers(Modifier.PUBLIC)
                .addParameter(activityClassName,"target");
        constructorMethodBuilder.addStatement("this.target = target");


        //findViewById
        for (Element viewBindElement : viewBindElements) {
            //target.textView1 = Utils.findViewById(target,R.id.text_view1);
            String filedName =  viewBindElement.getSimpleName().toString();
            ClassName utilsClassName = ClassName.get("com.zzw.butterknife","Utils");
            int resId = viewBindElement.getAnnotation(BindView.class).value();
            constructorMethodBuilder.addStatement("target.$L = $T.findViewById(target,$L)",filedName,utilsClassName,resId);

            unbindMethodBuilder.addStatement("target.$L = null" ,filedName);
        }


        //添加构造函数到类里面
        classBuilder.addMethod(constructorMethodBuilder.build());
        //添加unbinder方法到类里面
        classBuilder.addMethod(unbindMethodBuilder.build());

        try {
            //拿到包名
            String packageName =mElementUtils.getPackageOf(enclosingElement).getQualifiedName().toString();
            JavaFile.builder(packageName,classBuilder.build())
                    .addFileComment("butterknife 自动生成")//注释
                    .build()
                    .writeTo(mFiler);
        } catch (IOException e) {
            e.printStackTrace();
            System.out.print("异常");
        }
    }
    return false;
}


private Set<Class<? extends Annotation>> getSupportedAnnotations() {
    Set<Class<? extends Annotation>> annotations = new LinkedHashSet<>();
    annotations.add(BindView.class);
    return annotations;
}

}

生成的代码如下：

// butterknife 自动生成
package com.zzw.app;

import com.zzw.butterknife.Unbinder;
import com.zzw.butterknife.Utils;
import java.lang.Override;

public final class MainActivity_ViewBinding implements Unbinder {
private MainActivity target;

public MainActivity_ViewBinding(MainActivity target) {
this.target = target;
target.textView1 = Utils.findViewById(target,2131165301);
target.textView2 = Utils.findViewById(target,2131165302);
}

@Override
public void unbind() {
MainActivity target = this.target;
if (target == null) throw new IllegalStateException("Bindings already cleared.");;
target.textView1 = null;
target.textView2 = null;
}
}

我们可以自动生成代码了，这个时候我们可以通过这个来实现findViewById
我们在Butterknife module里面建Butterknife这个类

package com.zzw.butterknife;

import android.app.Activity;

import java.lang.reflect.Constructor;

/**

• Des:
• Created by zzw on 2018/1/29.
  */

public class Butterknife {

public static Unbinder bind(Activity activity){
    //xxxActivity_ViewBinding viewBinding = new xxxActivity_ViewBinding(activity);
    try{
      Class<? extends Unbinder> bindClass= (Class<? extends Unbinder>) Class.forName(activity.getClass().getName()+"_ViewBinding");
      Constructor<? extends Unbinder> bindConstructor=  bindClass.getDeclaredConstructor(activity.getClass());
        return bindConstructor.newInstance(activity);
    }catch (Exception e){
        e.printStackTrace();
        return Unbinder.EMPTY;
    }
}

}

MainActivity

package com.zzw.app;

import android.os.Bundle;
import android.support.v7.app.AppCompatActivity;
import android.widget.TextView;

import com.zzw.aptdemo.R;
import com.zzw.butterknife.Butterknife;
import com.zzw.butterknife.Unbinder;
import com.zzw.butterknife_annotions.BindView;

public class MainActivity extends AppCompatActivity {

@BindView(R.id.text_view1)
TextView textView1;
@BindView(R.id.text_view2)
TextView textView2;

private Unbinder mUnbinder;

@Override
protected void onCreate(Bundle savedInstanceState) {
    super.onCreate(savedInstanceState);
    setContentView(R.layout.activity_main);
    mUnbinder = Butterknife.bind(this);

    textView1.setText("你好 大哥");
}

@Override
protected void onDestroy() {
    super.onDestroy();
    mUnbinder.unbind();
}

}
该文中Butterknife module用到的类:

import android.support.annotation.UiThread;

/**

• Des:
• Created by zzw on 2018/1/29.
  */

public interface Unbinder {
@UiThread
void unbind();

Unbinder EMPTY = new Unbinder() {
    @Override public void unbind() { }
};

}

package com.zzw.butterknife;

import android.app.Activity;
import android.view.View;

/**

• Des:
• Created by zzw on 2018/1/29.
  */

public class Utils {
public static <T extends View>T findViewById(Activity activity,int viewId){
return activity.findViewById(viewId);
}
}