Android网络之Retrofit2.0使用和解析

#Android网络之Retrofit2.0使用和解析

##Retrofit2在项目中的使用

###Android studio项目添加依赖

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compile 'com.squareup.retrofit2:retrofit:2.0.1'

###项目中使用样例

####定义HTTP API使用接口

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public interface GitHubService {
@GET("users/{user}/repos")
Call<List<Repo>> listRepos(@Path("user") String user);
}

  • 通过在接口上添加注解的方式来表示如何处理网络请求。
  • Retrofit支持5中类型的注解:GET,POST,PUT,DELETE和HEAD.
  • 可以使用不带参数的url
    @GET("users/list")```,也可以使用带参数的url```java @GET("users/list?sort=desc")```
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    ####构造Retrofit实例
    ```java
    Retrofit retrofit = new Retrofit.Builder()
    .baseUrl("https://api.github.com/")
    .build();
    GitHubService service = retrofit.create(GitHubService.class);

####创建同步或异步HTTP请求到远程网络服务器

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Call<List<Repo>> repos = service.listRepos("octocat");

####定制数据类型转换器

  • Gson: com.squareup.retrofit2:converter-gson
  • Jackson: com.squareup.retrofit2:converter-jackson
  • Moshi: com.squareup.retrofit2:converter-moshi
  • Protobuf: com.squareup.retrofit2:converter-protobuf
  • Wire: com.squareup.retrofit2:converter-wire
  • Simple XML: com.squareup.retrofit2:converter-simplexml
  • Scalars (primitives, boxed, and String): com.squareup.retrofit2:converter-scalars
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Retrofit retrofit = new Retrofit.Builder()
.baseUrl("https://api.github.com")
.addConverterFactory(GsonConverterFactory.create())
.build();

GitHubService service = retrofit.create(GitHubService.class);

###Retrofit使用扩展

####自定义Gson类型转换器

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/**
{
"resultcode":0,
"resultmsg":"请求成功",
"result":{}
}
*/

public class Wrapper {
public int resultcode ;
public String resultmsg ;
public Object result ;
public static class JsonAdapter implements JsonDeserializer<Wrapper02> {
@Override
public Wrapper deserialize(JsonElement json, Type typeOfT, JsonDeserializationContext context) throws JsonParseException {
try {
String jsonRoot = json.getAsJsonObject().toString() ;
Wrapper wrapper = new Wrapper() ;
JSONObject jsobRespData = new JSONObject(jsonRoot) ;
wrapper.resultcode = jsobRespData.getInt("resultcode") ;
wrapper.resultmsg = jsobRespData.getString("resultmsg") ;
wrapper.result = jsobRespData.get("result") ;
return wrapper;
} catch (JSONException e) {
throw new JsonParseException(e) ;
}
}
}

}

添加到Retrofit当中

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Gson gson = new GsonBuilder()
.registerTypeAdapter(Wrapper.class, new Wrapper.JsonAdapter())
.create() ;
Retrofit retrofit = new Retrofit.Builder()
.baseUrl("https://api.github.com")
.addConverterFactory(GsonConverterFactory.create(gson))
.build();

GitHubService service = retrofit.create(GitHubService.class);

####请求时添加head信息
在定义请求接口时添加:

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@Headers("Cache-Control: max-age=640000")
@GET("widget/list")
Call<List<Widget>> widgetList();

@Headers({
"Accept: application/vnd.github.v3.full+json",
"User-Agent: Retrofit-Sample-App"
})
@GET("users/{username}")
Call<User> getUser(@Path("username") String username);

@GET("user")
Call<User> getUser(@Header("Authorization") String authorization)

Retrofit依赖
如果所示在Retrofit2.0中只支持okhttp,所以另一种方法是在okhttp的拦截器中addheader。

##Retrofit2源码解析

Retrofit请求框架实现了高度的解耦,通过解析注解的得到的代理类生成http请求,然后将请求交给OkHttp。通过在Retrofit创建时生成的Converter再将OkHttp返回的数据进行类型转换得到自己需要的数据。现在Rxjava响应式编程已经广泛应用,在使用Retrofit时也会结合RxJava使编码更加简单、高效。

一张图简单描述一下Retrofit的工作原理:
Retrofit工作原理

###定义网络请求接口

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public interface GitHubService {
@GET("users/{user}/repos")
Call<List<Repo>> listRepos(@Path("user") String user);
}

###创建Retrofit实例

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Retrofit retrofit = new Retrofit.Builder()
.baseUrl("https://api.github.com")
//支持RxJava
.addCallAdapterFactory(RxJavaCallAdapterFactory.create())
.addConverterFactory(ByteArrayConverterFactory.create())
.addConverterFactory(JSONObjectResponseConverterFactory.create())
.addConverterFactory(StringResponseConverterFactory.create())
//支持对象转json串
.addConverterFactory(GsonConverterFactory.create())
.build();

  • 设置BaseUrl
  • 添加CallAdapterFactory
  • 添加converterFactory
  • 此时也可以设置自定义的okHttpclient

接下来我们看

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GitHubService service = retrofit.create(GitHubService.class);

###Retrofit.create方法的详细介绍

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public <T> T create(final Class<T> service) {
//判断定义的接口服务是否可用
Utils.validateServiceInterface(service);
if (validateEagerly) {
eagerlyValidateMethods(service);
}
return (T) Proxy.newProxyInstance(service.getClassLoader(), new Class<?>[] { service },
new InvocationHandler() {
//判断Android,IOS,java8
private final Platform platform = Platform.get();

@Override public Object invoke(Object proxy, Method method, Object... args)
throws Throwable {

//如果是对象里的方法直接调用
if (method.getDeclaringClass() == Object.class) {
return method.invoke(this, args);
}
/**
* 对java8做兼容,android和ios值恒为false
*/

if (platform.isDefaultMethod(method)) {
return platform.invokeDefaultMethod(method, service, proxy, args);
}
//主要看这三行代码
/**
* 1、生成获取缓存中的method对应的ServiceMethod或者生产method对应的ServiceMethod
* 2、生成OkHttpCall的实例
* 3、根据生成的ServiceMethod实例中的callAdapter对象,调用callAdapter.adapt方法创建
* 对应的Observable
*/

ServiceMethod serviceMethod = loadServiceMethod(method);
OkHttpCall okHttpCall = new OkHttpCall<>(serviceMethod, args);
return serviceMethod.callAdapter.adapt(okHttpCall);
}
});
}

###Platfrom的获取
Platfrom

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class Platform {
//这个方法Android中为Plafrom默认的
//Java8返回的是method.isDefault(),熟悉Java8的都知道这是Java8的新特性。。
boolean isDefaultMethod(Method method) {
return false;
}
//这个方法Android中有自己默认的实现MainThreadExecutor
Executor defaultCallbackExecutor() {
return null;
}
static class Android extends Platform {
@Override
public Executor defaultCallbackExecutor() {
return new MainThreadExecutor();
}

@Override
CallAdapter.Factory defaultCallAdapterFactory(Executor callbackExecutor) {
return new ExecutorCallAdapterFactory(callbackExecutor);
}
//Rx默认请求方式都是同步请求,所以我们在发出请求和请求结果回来的时候切换线程
static class MainThreadExecutor implements Executor {
private final Handler handler = new Handler(Looper.getMainLooper());
@Override
public void execute(Runnable r) {
handler.post(r);
}
}
}
}

###ServiceMethod对象的生成
先看一张我debug时候的ServiceMethod的图
ServiceMethod

####ServiceMethod的获取

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/**Retrofit.java
* 首先从serviceMethodCache缓存中获取,如果为null则创建
*/

ServiceMethod loadServiceMethod(Method method) {
ServiceMethod result;
synchronized (serviceMethodCache) {
result = serviceMethodCache.get(method);
if (result == null) {
result = new ServiceMethod.Builder(this, method).build();
serviceMethodCache.put(method, result);
}
}
return result;
}

####ServiceMethod的创建

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final class ServiceMethod<T> {
//部分代码省略
ServiceMethod(Builder<T> builder) {
this.callFactory = builder.retrofit.callFactory();
this.callAdapter = builder.callAdapter;
this.baseUrl = builder.retrofit.baseUrl();
this.responseConverter = builder.responseConverter;
this.httpMethod = builder.httpMethod;
this.relativeUrl = builder.relativeUrl;
this.headers = builder.headers;
this.contentType = builder.contentType;
this.hasBody = builder.hasBody;
this.isFormEncoded = builder.isFormEncoded;
this.isMultipart = builder.isMultipart;
this.parameterHandlers = builder.parameterHandlers;
}
static final class Builder<T> {
public Builder(Retrofit retrofit, Method method) {
this.retrofit = retrofit;
this.method = method;
this.methodAnnotations = method.getAnnotations();
this.parameterTypes = method.getGenericParameterTypes();
this.parameterAnnotationsArray = method.getParameterAnnotations();
}

public ServiceMethod build() {
//创建CallAdapter<?>
callAdapter = createCallAdapter();
responseType = callAdapter.responseType();
if (responseType == Response.class || responseType == okhttp3.Response.class) {
throw methodError("'"
+ Utils.getRawType(responseType).getName()
+ "' is not a valid response body type. Did you mean ResponseBody?");
}
//创建Converter<ResponseBody, T>
responseConverter = createResponseConverter();
for (Annotation annotation : methodAnnotations) {
parseMethodAnnotation(annotation);
}
/********/
return new ServiceMethod<>(this);
}

private CallAdapter<?> createCallAdapter() {
Type returnType = method.getGenericReturnType();
/*******/
Annotation[] annotations = method.getAnnotations();
try {
//调用retrofit中的方法进行创建
return retrofit.callAdapter(returnType, annotations);
} catch (RuntimeException e) { // Wide exception range because factories are user code.
throw methodError(e, "Unable to create call adapter for %s", returnType);
}
}
private Converter<ResponseBody, T> createResponseConverter() {
Annotation[] annotations = method.getAnnotations();
try {
//调用retrofit中的方法进行创建
return retrofit.responseBodyConverter(responseType, annotations);
} catch (RuntimeException e) { // Wide exception range because factories are user code.
throw methodError(e, "Unable to create converter for %s", responseType);
}
}
}
}

#####CallAdapter的创建

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public CallAdapter<?> callAdapter(Type returnType, Annotation[] annotations) {
return nextCallAdapter(null, returnType, annotations);
}

public CallAdapter<?> nextCallAdapter(CallAdapter.Factory skipPast, Type returnType,
Annotation[] annotations) {
checkNotNull(returnType, "returnType == null");
checkNotNull(annotations, "annotations == null");
int start = adapterFactories.indexOf(skipPast) + 1;
for (int i = start, count = adapterFactories.size(); i < count; i++) {
CallAdapter<?> adapter = adapterFactories.get(i).get(returnType, annotations, this);
if (adapter != null) {
return adapter;
}
}
/********************/
}

在创建Retrofit的时候我们添加过addCallAdapterFactory(RxJavaCallAdapterFactory.create()),这是我们会调用RxJavaCallAdapterFactory.get方法获取CallAdapter,通过源代码我们可以找到其返回的是new SimpleCallAdapter(observableType, scheduler)。

#####Converter的创建

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public <T> Converter<T, RequestBody> requestBodyConverter(Type type,
Annotation[] parameterAnnotations, Annotation[] methodAnnotations)
{

return nextRequestBodyConverter(null, type, parameterAnnotations, methodAnnotations);
}

public <T> Converter<T, RequestBody> nextRequestBodyConverter(Converter.Factory skipPast,
Type type, Annotation[] parameterAnnotations, Annotation[] methodAnnotations)
{

checkNotNull(type, "type == null");
checkNotNull(parameterAnnotations, "parameterAnnotations == null");
checkNotNull(methodAnnotations, "methodAnnotations == null");
int start = converterFactories.indexOf(skipPast) + 1;
for (int i = start, count = converterFactories.size(); i < count; i++) {
Converter.Factory factory = converterFactories.get(i);
Converter<?, RequestBody> converter =
factory.requestBodyConverter(type, parameterAnnotations, methodAnnotations, this);
if (converter != null) {
//noinspection unchecked
return (Converter<T, RequestBody>) converter;
}
}
/******/
}

相同的在创建Retrofit的时候我们也添加过许多的ConverterFactory,在寻找相匹配的Converter时我们是通过遍历在寻找到第一个合适的Converter返回。什么叫做合适的Converter,即该ConverterFactory能产生出匹配服务接口注解和返回类型。

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public static final class Builder {
private Platform platform;
private okhttp3.Call.Factory callFactory;
private HttpUrl baseUrl;
private List<Converter.Factory> converterFactories = new ArrayList<>();
private List<CallAdapter.Factory> adapterFactories = new ArrayList<>();
private Executor callbackExecutor;
private boolean validateEagerly;

Builder(Platform platform) {
this.platform = platform;
// Add the built-in converter factory first. This prevents overriding its behavior but also
// ensures correct behavior when using converters that consume all types.
//自动回添加加默认的转化器
converterFactories.add(new BuiltInConverters());
}
/****************/
public Retrofit build() {
if (baseUrl == null) {
throw new IllegalStateException("Base URL required.");
}

okhttp3.Call.Factory callFactory = this.callFactory;
if (callFactory == null) {
callFactory = new OkHttpClient();
}

Executor callbackExecutor = this.callbackExecutor;
if (callbackExecutor == null) {
callbackExecutor = platform.defaultCallbackExecutor();
}

// Make a defensive copy of the adapters and add the default Call adapter.
List<CallAdapter.Factory> adapterFactories = new ArrayList<>(this.adapterFactories);
//添加一个默认的适配器(Android、IOS、Java8)
adapterFactories.add(platform.defaultCallAdapterFactory(callbackExecutor));

// Make a defensive copy of the converters.
List<Converter.Factory> converterFactories = new ArrayList<>(this.converterFactories);

return new Retrofit(callFactory, baseUrl, converterFactories, adapterFactories,
callbackExecutor, validateEagerly);
}

###OkHttpCall的创建

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OkHttpCall(ServiceMethod<T> serviceMethod, Object[] args) {
this.serviceMethod = serviceMethod;
this.args = args;
}

###网络请求

####请求的准备

对应SimpleCallAdapter.adapt
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```java
static final class SimpleCallAdapter implements CallAdapter<Observable<?>> {
private final Type responseType;
private final Scheduler scheduler;
SimpleCallAdapter(Type responseType, Scheduler scheduler) {
this.responseType = responseType;
this.scheduler = scheduler;
}
@Override public Type responseType() {
return responseType;
}
@Override public <R> Observable<R> adapt(Call<R> call) {
//创建请求的观察者,返回我们需要的Ovservable
Observable<R> observable = Observable.create(new CallOnSubscribe<>(call)) //
.lift(OperatorMapResponseToBodyOrError.<R>instance());
if (scheduler != null) {
return observable.subscribeOn(scheduler);
}
return observable;
}
}

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static class Android extends Platform {
@Override
public Executor defaultCallbackExecutor() {
return new MainThreadExecutor();
}
@Override
CallAdapter.Factory defaultCallAdapterFactory(Executor callbackExecutor) {
return new ExecutorCallAdapterFactory(callbackExecutor);
}
static class MainThreadExecutor implements Executor {
private final Handler handler = new Handler(Looper.getMainLooper());
@Override
public void execute(Runnable r) {
handler.post(r);
}
}
}
/*
* 因为默认的执行线程为主线程,所以我们要切换到subscribeOn(Schedulers.io())线程从而达到异步的目的。
* 然后通过observeOn(AndroidSchedulers.mainThread())将线程切回UI线程。
* 当Okhttp请求完数据并进行相应的convert之后,就可以在UI处理相应的逻辑。
*/

service.listRepos("octocat")
.observeOn(AndroidSchedulers.mainThread())
.subscribeOn(Schedulers.io())
.subscribe(list->{
if(list!=null){
//TODO 取得数据后逻辑处理
}
});

####请求的发起
回到CallAdapt方法,创建Observable,而new CallOnSubscribe<>(call)生成了一个OnSubscribe()的实例,而OnSubscribe继承自Action1,其只包含一个call()方法,而这个call是在CallOnSubscribe中实现:

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static final class CallOnSubscribe<T> implements Observable.OnSubscribe<Response<T>> {
private final Call<T> originalCall;
CallOnSubscribe(Call<T> originalCall) {
this.originalCall = originalCall;
}
@Override public void call(final Subscriber<? super Response<T>> subscriber) {
// Since Call is a one-shot type, clone it for each new subscriber.
Call<T> call = originalCall.clone();

// Wrap the call in a helper which handles both unsubscription and backpressure.
RequestArbiter<T> requestArbiter = new RequestArbiter<>(call, subscriber);
subscriber.add(Subscriptions.create(requestArbiter));
subscriber.setProducer(requestArbiter);
}
}

首先clone了一份Call,然后生成了RequestArbiter,他继承自AtomicBoolean,实现了Subscription, Producer接口,Producer只有一个request方法;一般实现该接口的类,都会包含一个Subscriber对象和一个待处理的数据:

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static final class RequestArbiter<T> extends AtomicBoolean implements Action0,
Producer {

private final Call<T> call;
private final Subscriber<?super Response<T>> subscriber;
RequestArbiter(Call<T> call, Subscriber<?super Response<T>> subscriber) {
this.call = call;
this.subscriber = subscriber;
}
@Override
public void request(long n) {
if (n < 0) {
throw new IllegalArgumentException("n < 0: " + n);
}
if (n == 0) {
return; // Nothing to do when requesting 0.
}
if (!compareAndSet(false, true)) {
return; // Request was already triggered.
}
try {
//进行网络请求
Response<T> response = call.execute();
if (!subscriber.isUnsubscribed()) {
subscriber.onNext(response);
}
} catch (Throwable t) {
Exceptions.throwIfFatal(t);
if (!subscriber.isUnsubscribed()) {
subscriber.onError(t);
}
return;
}
if (!subscriber.isUnsubscribed()) {
subscriber.onCompleted();
}
}

@Override
public void call() {
call.cancel();
}
}

####请求的执行
回顾创建Retrofit.create()代码中

就是OkHttpCall.execute
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```java
public Response<T> execute() throws IOException{
okhttp3.Call call;
synchronized (this) {
if ( executed )
throw new IllegalStateException( "Already executed." );
executed = true;
if ( creationFailure != null ){
if ( creationFailure instanceof IOException ){
throw (IOException) creationFailure;
} else {
throw (RuntimeException) creationFailure;
}
}
call = rawCall;
if ( call == null ){
try {
//获取okhttp实例
call = rawCall = createRawCall();
} catch ( IOException | RuntimeException e ) {
creationFailure = e;
throw e;
}
}
}
if ( canceled ){
call.cancel();
}
//执行okhttp请求
return(parseResponse( call.execute() ) );
}

private okhttp3.Call createRawCall() throws IOException{
Request request = serviceMethod.toRequest( args );
//serviceMethod构造中this.callFactory = builder.retrofit.callFactory();
okhttp3.Call call = serviceMethod.callFactory.newCall( request );
if ( call == null ){
throw new NullPointerException( "Call.Factory returned null." );
}
return(call);
}

####请求的OkHttpClient实例获取

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public okhttp3.Call.Factory callFactory() {
return callFactory;
}
public Builder callFactory(okhttp3.Call.Factory factory) {
this.callFactory = checkNotNull(factory, "factory == null");
return this;
}
//使用自定义OkHttpClient
public Builder client(OkHttpClient client) {
return callFactory(checkNotNull(client, "client == null"));
}

public Retrofit build(){
if ( baseUrl == null ){
throw new IllegalStateException( "Base URL required." );
}
okhttp3.Call.Factory callFactory = this.callFactory;
//没有自定义OkHttpClient,则会新创建一个
if ( callFactory == null ){
callFactory = new OkHttpClient();
}
Executor callbackExecutor = this.callbackExecutor;
if ( callbackExecutor == null ){
callbackExecutor = platform.defaultCallbackExecutor();
}
List<CallAdapter.Factory> adapterFactories = new ArrayList<>( this.adapterFactories );
adapterFactories.add( platform.defaultCallAdapterFactory( callbackExecutor ) );
List<Converter.Factory> converterFactories = new ArrayList<>( this.converterFactories );
return(new Retrofit( callFactory, baseUrl, converterFactories, adapterFactories,
callbackExecutor, validateEagerly ) );
}
}

####请求结果的处理

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Response<T> parseResponse( okhttp3.Response rawResponse ) throws IOException{
ResponseBody rawBody = rawResponse.body();
rawResponse = rawResponse.newBuilder()
.body( new NoContentResponseBody( rawBody.contentType(), rawBody.contentLength() ) )
.build();
int code = rawResponse.code();
if ( code < 200 || code >= 300 ){
try {
ResponseBody bufferedBody = Utils.buffer( rawBody );
return(Response.error( bufferedBody, rawResponse ) );
} finally {
rawBody.close();
}
}
if ( code == 204 || code == 205 ){
return(Response.success( null, rawResponse ) );
}
ExceptionCatchingRequestBody catchingBody = new ExceptionCatchingRequestBody( rawBody );
try {
//使用Converter将返回结果转化为接口返回的数据格式类型
T body = serviceMethod.toResponse( catchingBody );
//包装成Response并返回
return(Response.success( body, rawResponse ) );
} catch ( RuntimeException e ) {
catchingBody.throwIfCaught();
throw e;
}
}

还记得创建Observable时 Observable<R> observable = Observable.create(new CallOnSubscribe<>(call)).lift(OperatorMapResponseToBodyOrError.<R>instance()); ,OperatorMapResponseToBodyOrError将包装的Response中的body取出来并进行发射。

##总结
现在随着Rxjava响应式编程越来越多的程序猿使用,自己也开始接触和使用。Retrofit+Rxjava+okhttp是时下比较受欢迎的网络请求框架,其源代码并不是很多,其底层网络通信时交由 OkHttp来完成的,但是Retrofit运用了大量的设计模式,代码逻辑很清晰,笔者以前用的是AsyncHttpClient作为app的网络请求框架,其源码也没自己的研究过。但看完Retrofit代码之后觉得收获很大,建议如果感兴趣可以抽时间仔细的阅读。

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