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Android |《看完不忘系列》之Retrofit

嗨,我是哈利迪~《看完不忘系列》将以从树干到细枝的思路分析一些技术框架,本文将对开源项目Retrofit进行介绍。

本文约2800字,阅读大约8分钟。

Retrofit源码基于最新版本2.9.0

预备

Retrofit使得网络调用可以像RESTful设计风格一样简洁,如:

interface WanApi {
    //用注解标记网络请求方式get、post,参数path、query等
    @GET("article/list/{page}/json")
    Call<WanArticleBean> articleList(@Path("page") int page);
}
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又如,后端的Spring Boot框架通过约定大于配置思想省去了很多配置,其在网络接口RestController上也运用了这种风格,

@RestController
public class ActivityController {
    @Autowired
    private ActivityService activityService;

    //用注解标记网络请求方式和入参
    @GetMapping("/goods")
    public ResultEntity queryGoods(@RequestParam("page") int page) {
        return activityService.queryGoods(page);
    }
}
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Retrofit的底层网络实现基于okhttp,自身的类不是很多,最核心的点就是动态代理了。代理模式简单来说,就是为对象提供一个增强控制其访问的代理。下面我们先来了解下静态代理动态代理~

静态代理

编译期就完成代理

  • 源码级:手动编写代理类、APT生成代理类
  • 字节码级:编译期生成字节码、

举个栗子,

interface 赚钱 {
    void makeMoney(int income);
}

class 小鲜肉 implements 赚钱 { //委托类
    @Override
    public void makeMoney(int income) {
        System.out.println("开拍,赚个" + income);
    }
}

class 经纪人 implements 赚钱 { //代理类
    赚钱 xxr;

    public 经纪人(赚钱 xxr) {
        this.xxr = xxr;
    }

    @Override
    public void makeMoney(int income) {
        if (income < 1000_0000) { //控制访问
            System.out.println("才" + income + ",先回去等通知吧");
        } else {
            xxr.makeMoney(income);
        }
    }
}

public static void main(String[] args) {
    赚钱 xxr = new 小鲜肉();
    赚钱 jjr = new 经纪人(xxr);
    jjr.makeMoney(100_0000); //输出:才1000000,先回去等通知吧
    jjr.makeMoney(1000_0000); //输出:开拍,赚个10000000
}
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为什么代理类委托类要实现相同接口?是为了尽可能保证代理类的内部结构和委托类一致,这样对代理类的操作都可以转移到委托类上,代理类只关注增强控制

动态代理

运行期生成字节码,如Proxy.newProxyInstance、CGLIB

Proxy.newProxyInstance是java自带,只能对接口代理(因为生成的类已经继承了Proxy,java没法多继承)

CGLIB则更强大,还能对普通类代理,底层基于ASM(ASM使用类似SAX解析器逐行扫描来提高性能)

举个栗子,

class 合作标准 implements InvocationHandler {
    赚钱 xxr;

    public 合作标准(赚钱 xxr) {
        this.xxr = xxr;
    }

    @Override
    public Object invoke(Object proxy, Method method, Object[] args) throws Throwable {
        int income = (int) args[0];
        if (income < 1000_0000) { //控制访问
            System.out.println("才" + income + ",先回去等通知吧");
            return null;
        } else {
            return method.invoke(xxr, args);
        }
    }
}

public static void main(String[] args) {
    赚钱 xxr = new 小鲜肉();
    合作标准 standard = new 合作标准(xxr);
    //生成类(字节码):class $Proxy0 extends Proxy implements 赚钱
    //然后反射创建其实例bd,即来一场临时的商务拓展
    赚钱 bd = (赚钱) Proxy.newProxyInstance(赚钱.class.getClassLoader(),
                                        new Class[]{赚钱.class}, 
                                        standard);
    //调用makeMoney,内部转发给了合作标准的invoke
    bd.makeMoney(100_0000);
    bd.makeMoney(1000_0000);
}
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通过栗子可以看出,动态代理不需要提前创建具体的代理类(如经纪人经纪公司)去实现赚钱接口,而是先拟一份合作标准(InvocationHandler),等到运行期才创建代理类$Proxy0(字节码),然后反射创建其实例商务拓展,这样显得更为灵活。

了解完动态代理,就可以开始Retrofit之旅了~

树干

简单使用

引入依赖,

implementation 'com.squareup.okhttp3:okhttp:3.14.9'
implementation 'com.squareup.retrofit2:retrofit:2.9.0'
implementation 'com.squareup.retrofit2:converter-gson:2.9.0'
implementation 'com.google.code.gson:gson:2.8.6'
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定义接口WanApi

interface WanApi {
    //用注解标记网络请求类型get,参数path
    @GET("article/list/{page}/json")
    Call<WanArticleBean> articleList(@Path("page") int page);
}
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发起请求,

class RetrofitActivity extends AppCompatActivity {
    final String SERVER = "https://www.xxx.com/";

    @Override
    protected void onCreate(Bundle savedInstanceState) {
        Retrofit retrofit = new Retrofit.Builder()
            .baseUrl(SERVER) //指定服务器地址
            .addConverterFactory(GsonConverterFactory.create()) //用gson将数据反序列化成实体
            .build();
        //运行期生成一个实现WanApi接口的类(字节码),并反射创建其实例
        WanApi wanApi = retrofit.create(WanApi.class);
        //得到Retrofit的call,他封装了okhttp的call
        Call<WanArticleBean> call = wanApi.articleList(0);
        //请求入队
        call.enqueue(new Callback<WanArticleBean>() {
            @Override
            public void onResponse(Call<WanArticleBean> call, Response<WanArticleBean> response) {
                //得到数据实体
                WanArticleBean bean = response.body();
                //不同于okhttp,Retrofit已经用Handler帮我们切回主线程了
                mBinding.tvResult.setText("" + bean.getData().getDatas().size());
            }

            @Override
            public void onFailure(Call<WanArticleBean> call, Throwable t) {}
        });
    }
}
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实现原理

由于Retrofit底层基于okhttp,哈迪在《看完不忘系列》之okhttp已经对网络流程做了分析,所以本文忽略网络实现只关注Retrofit自身的一些处理,Retrofit对象的构建就是简单的builder模式,我们直接看create,

//Retrofit.java
public <T> T create(final Class<T> service) {
    //验证
    validateServiceInterface(service);
    return (T)
        //动态代理
        Proxy.newProxyInstance(
        service.getClassLoader(), //类加载器
        new Class<?>[] {service}, //一组接口
        new InvocationHandler() {
            //判断android和jvm平台及其版本
            private final Platform platform = Platform.get();

            @Override
            public Object invoke(Object proxy, Method method, Object[] args){
                //如果该方法是Object的方法,直接执行不用管
                if (method.getDeclaringClass() == Object.class) {
                    return method.invoke(this, args);
                }
                //isDefaultMethod:检查是否是java8开始支持的接口默认方法
                return platform.isDefaultMethod(method)
                    ? platform.invokeDefaultMethod(method, service, proxy, args)
                    : loadServiceMethod(method).invoke(args); //我们关注这里
            }
        });
}
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Proxy.newProxyInstance动态代理,运行期会生成一个类(字节码)如$ProxyN,实现传入的接口即WanApi,重写接口方法然后转发给InvocationHandler的invoke,如下(伪代码),

class $ProxyN extends Proxy implements WanApi{
    Call<WanArticleBean> articleList(@Path("page") int page){
        //转发给invocationHandler
        invocationHandler.invoke(this,method,args);
    }
}
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我们先看validateServiceInterface验证逻辑,

//Retrofit.java
private void validateServiceInterface(Class<?> service) {
    //检查:WanApi不是接口就抛异常...
    //检查:WanApi不能有泛型参数,不能实现其他接口...
    if (validateEagerly) { //是否进行严格检查,默认关闭
        Platform platform = Platform.get();
        for (Method method : service.getDeclaredMethods()) { //遍历WanApi方法
            //不是默认方法,并且不是静态方法
            if (!platform.isDefaultMethod(method) && !Modifier.isStatic(method.getModifiers())) {
                //把方法提前加载进来(检查下有没有问题)
                loadServiceMethod(method);
            }
        }
    }
}
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如果开了validateEagerly,会一次性把接口WanApi的所有方法都检查一遍并加载进来,可以在debug模式下开启,提前发现错误写法,比如在@GET请求设置了@Body这种错误就会抛出异常:

java.lang.IllegalArgumentException: Non-body HTTP method cannot contain @Body.

loadServiceMethod

然后是loadServiceMethod(method).invoke(args),看名字可知是先找方法,然后执行,

//Retrofit.java
//缓存,用了线程安全ConcurrentHashMap
final Map<Method, ServiceMethod<?>> serviceMethodCache = new ConcurrentHashMap<>();

ServiceMethod<?> loadServiceMethod(Method method) {
    ServiceMethod<?> result = serviceMethodCache.get(method);
    //WanApi的articleList方法已缓存,直接返回
    if (result != null) return result;
    synchronized (serviceMethodCache) {
        result = serviceMethodCache.get(method);
        if (result == null) {
            //解析articleList的注解,创建ServiceMethod并缓存起来
            result = ServiceMethod.parseAnnotations(this, method);
            serviceMethodCache.put(method, result);
        }
    }
    return result;
}
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跟进ServiceMethod.parseAnnotations,

//ServiceMethod.java
static <T> ServiceMethod<T> parseAnnotations(Retrofit retrofit, Method method) {
    //1.
    RequestFactory requestFactory = RequestFactory.parseAnnotations(retrofit, method);
    //检查:articleList方法返回类型不能用通配符和void...
    //2.
    return HttpServiceMethod.parseAnnotations(retrofit, method, requestFactory);
}
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先看1. RequestFactory.parseAnnotations,

//RequestFactory.java
static RequestFactory parseAnnotations(Retrofit retrofit, Method method) {
    return new Builder(retrofit, method).build();
}

class Builder {
    RequestFactory build() {
        //解析方法注解如GET
        for (Annotation annotation : methodAnnotations) {
            parseMethodAnnotation(annotation);
        }
        //省略各种检查...
        //解析参数注解如Path
        int parameterCount = parameterAnnotationsArray.length;
        parameterHandlers = new ParameterHandler<?>[parameterCount];
        for (int p = 0, lastParameter = parameterCount - 1; p < parameterCount; p++) {
            parameterHandlers[p] =
                parseParameter(p, parameterTypes[p], parameterAnnotationsArray[p], p == lastParameter);
        }
        //省略各种检查...
        return new RequestFactory(this);
    }
}
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得到RequestFactory后,看2. HttpServiceMethod.parseAnnotations,HttpServiceMethod负责适配和转换处理,将接口方法的调用调整为HTTP调用,

//HttpServiceMethod.java
//ResponseT响应类型如WanArticleBean,ReturnT返回类型如Call
static <ResponseT, ReturnT> HttpServiceMethod<ResponseT, ReturnT> parseAnnotations(
    Retrofit retrofit, Method method, RequestFactory requestFactory) {
    //省略kotlin协程逻辑...
    Annotation[] annotations = method.getAnnotations();
    //遍历找到合适的适配器
    CallAdapter<ResponseT, ReturnT> callAdapter =
        createCallAdapter(retrofit, method, adapterType, annotations);
    //得到响应类型,如WanArticleBean
    Type responseType = callAdapter.responseType();
    //遍历找到合适的转换器
    Converter<ResponseBody, ResponseT> responseConverter =
        createResponseConverter(retrofit, method, responseType);
    okhttp3.Call.Factory callFactory = retrofit.callFactory;
    return new CallAdapted<>(requestFactory, callFactory, responseConverter, callAdapter);
}
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可见最终返回了一个CallAdapted,看到CallAdapted,

//CallAdapted extends HttpServiceMethod extends ServiceMethod
class CallAdapted<ResponseT, ReturnT> extends HttpServiceMethod<ResponseT, ReturnT> {
    private final CallAdapter<ResponseT, ReturnT> callAdapter;

    CallAdapted(
        RequestFactory requestFactory,
        okhttp3.Call.Factory callFactory,
        Converter<ResponseBody, ResponseT> responseConverter,
        CallAdapter<ResponseT, ReturnT> callAdapter) {
        super(requestFactory, callFactory, responseConverter);
        this.callAdapter = callAdapter;
    }

    @Override
    protected ReturnT adapt(Call<ResponseT> call, Object[] args) {
        //适配器
        return callAdapter.adapt(call);
    }
}
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那这个CallAdapter实例到底是谁呢,我们先回到Retrofit.Builder,

//Retrofit.Builder.java
public Retrofit build() {
    Executor callbackExecutor = this.callbackExecutor;
    //如果没设置线程池,则给android平台设置一个默认的MainThreadExecutor(用Handler将回调切回主线程)
    if (callbackExecutor == null) {
        callbackExecutor = platform.defaultCallbackExecutor();
    }
    List<CallAdapter.Factory> callAdapterFactories = new ArrayList<>(this.callAdapterFactories);
    //添加默认的DefaultCallAdapterFactory
    callAdapterFactories.addAll(platform.defaultCallAdapterFactories(callbackExecutor));
}
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DefaultCallAdapterFactory这个工厂创建具体的CallAdapter实例,

//DefaultCallAdapterFactory.java
public CallAdapter<?, ?> get(Type returnType, Annotation[] annotations, Retrofit retrofit) {
    final Type responseType = Utils.getParameterUpperBound(0, (ParameterizedType) returnType);
    //如果指定了SkipCallbackExecutor注解,就表示不需要切回主线程
    final Executor executor =
        Utils.isAnnotationPresent(annotations, SkipCallbackExecutor.class)
        ? null
        : callbackExecutor;
    return new CallAdapter<Object, Call<?>>() {
        @Override
        public Type responseType() {
            return responseType;
        }

        @Override
        public Call<Object> adapt(Call<Object> call) {
            //默认情况下,返回用主线程池包装的Call,他的enqueue会使用主线程池的execute
            return executor == null ? call : new ExecutorCallbackCall<>(executor, call);
        }
    };
}
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invoke

前边loadServiceMethod得到了CallAdapted,然后执行invoke,实现在父类HttpServiceMethod里,

//HttpServiceMethod.java
final ReturnT invoke(Object[] args) {
    //终于见到okhttp了!
    Call<ResponseT> call = new OkHttpCall<>(requestFactory, args, callFactory, responseConverter);
    return adapt(call, args);
}

class CallAdapted<ResponseT, ReturnT> extends HttpServiceMethod<ResponseT, ReturnT> {
    private final CallAdapter<ResponseT, ReturnT> callAdapter;

    @Override
    protected ReturnT adapt(Call<ResponseT> call, Object[] args) {
        //用前边得到的适配器,把OkHttpCall包成ExecutorCallbackCall
        return callAdapter.adapt(call);
    }
}
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然后是请求入队,ExecutorCallbackCall.enqueue -> OkHttpCall.enqueue,

//ExecutorCallbackCall.java
void enqueue(final Callback<T> callback) {
    delegate.enqueue(
        new Callback<T>() {
            @Override
            public void onResponse(Call<T> call, final Response<T> response) {
                //将回调切回主线程
                callbackExecutor.execute(
                    () -> {
                        callback.onResponse(ExecutorCallbackCall.this, response);
                    });
                //...
            }

            @Override
            public void onFailure(Call<T> call, final Throwable t) {}
        });
}

//OkHttpCall.java
void enqueue(final Callback<T> callback) {
    //okhttp逻辑
    okhttp3.Call call;
    call.enqueue(new okhttp3.Callback() {
        void onResponse(okhttp3.Call call, okhttp3.Response rawResponse) {
            callback.onResponse(OkHttpCall.this, response);
        }
    })
}
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总算把流程跑通了~回到前边再看一遍流程图,就豁然开朗了~

细枝

CallAdapter

CallAdapter适配器用于适配返回类型,比如还可以支持Rxjava、协程的使用,

interface WanApi {
    //Call
    @GET("article/list/{page}/json")
    Call<WanArticleBean> articleList(@Path("page") int page);

    //Rxjava,需要 addCallAdapterFactory(RxJavaCallAdapterFactory.create())
    @GET("article/list/{page}/json")
    Observable<WanArticleBean> articleListRx(@Path("page") int page);
}
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Converter

Converter转换器用于转换参数类型,比如把Long时间戳格式化成string再传给后端,

interface WanApi {
    //Long cur 当前时间
    @GET("article/list/{page}/json")
    Call<WanArticleBean> articleList(@Path("page") int page, @Query("cur") Long cur);
}

class TimeConverter implements Converter<Long, String> {
    private SimpleDateFormat mFormat = new SimpleDateFormat("yyyy-MM-dd-HHmmss");

    @Override
    public String convert(Long value) throws IOException {
        if (value > 1_000_000_000_000L) {//毫秒,不是很严谨 - -
            return mFormat.format(new Date(value));
        }
        return String.valueOf(value);
    }
}

class TimeConverterFactory extends Converter.Factory {

    @Override
    public Converter<?, String> stringConverter(Type type, Annotation[] annotations, Retrofit retrofit) {
        if (type == Long.class) {
            //使用自定义TimeConverter
            return new TimeConverter();
        }
        return super.stringConverter(type, annotations, retrofit);
    }

    public static Converter.Factory create() {
        return new TimeConverterFactory();
    }
}

//再设置一下就行了,addConverterFactory(TimeConverterFactory.create())
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动态替换url

在构建Retrofit时传入HttpUrl对象,之后这个实例就一直存在不会更改,所以可以反射修改他的字段比如host,来实现动态替换服务端地址,

String SERVER = "https://www.xxx.com/";
HttpUrl httpUrl = HttpUrl.get(SERVER);
Retrofit retrofit = new Retrofit.Builder()
    //.baseUrl(SERVER)
    .baseUrl(httpUrl) //使用HttpUrl
    .build();
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尾声

咱们下期见~😆

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参考资料


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