Retrofit源码解析

1 使用方式
创建Retrofit对象

Retrofit retrofit = new Retrofit.Builder().baseUrl("https://api.example.com").addConverterFactory(GsonConverterFactory.create().build();

创建代理对象

public interface MyApi {    @GET("/api/columns/{user} ")    Call<String> getUser(@Path("user") String user);}

MyApi api = retrofit.create(MyApi.class);

通过代理去调用请求的方法

Call<String> call = api.getUser("");

请求结果处理

call.enqueue(new Callback<String>() {            @Override            public void onResponse(Call<String> call, Response<String> response) {            }            @Override            public void onFailure(Call<String> call, Throwable t) {            }        });

2 构建Retrofit对象的参数
Platform

 public Builder() {      this(Platform.get());    }

 static Platform get() {    return PLATFORM;  }

 private static final Platform PLATFORM = findPlatform();

private static Platform findPlatform() {    try {      Class.forName("android.os.Build");      if (Build.VERSION.SDK_INT != 0) {        return new Android();      }    }

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);      }    }  }

HttpUr

private HttpUrl baseUrl;

public Builder baseUrl(String baseUrl) {      checkNotNull(baseUrl, "baseUrl == null");      HttpUrl httpUrl = HttpUrl.parse(baseUrl);      if (httpUrl == null) {        throw new IllegalArgumentException("Illegal URL: " + baseUrl);      }      return baseUrl(httpUrl);    }

 public Builder baseUrl(HttpUrl baseUrl) {      checkNotNull(baseUrl, "baseUrl == null");      List<String> pathSegments = baseUrl.pathSegments();      if (!"".equals(pathSegments.get(pathSegments.size() - 1))) {        throw new IllegalArgumentException("baseUrl must end in /: " + baseUrl);      }      this.baseUrl = baseUrl;      return this;    }

Converter.Factory

 private List<Converter.Factory> converterFactories = new ArrayList<>();

 public Builder addConverterFactory(Converter.Factory factory) {      converterFactories.add(checkNotNull(factory, "factory == null"));      return this;    }

Executor

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

 @Override public Executor defaultCallbackExecutor() {      return new MainThreadExecutor();    }

static class MainThreadExecutor implements Executor {      private final Handler handler = new Handler(Looper.getMainLooper());      @Override public void execute(Runnable r) {        handler.post(r);      }    }

CallAdapter.Factory

private List<CallAdapter.Factory> adapterFactories = new ArrayList<>();

设置添加CallAdapter.Factory对象

public Builder addCallAdapterFactory(CallAdapter.Factory factory) {      adapterFactories.add(checkNotNull(factory, "factory == null"));      return this;    }

默认的CallAdapter.Factory对象

List<CallAdapter.Factory> adapterFactories = new ArrayList<>(this.adapterFactories);      adapterFactories.add(platform.defaultCallAdapterFactory(callbackExecutor));

@Override CallAdapter.Factory defaultCallAdapterFactory(Executor callbackExecutor) {      return new ExecutorCallAdapterFactory(callbackExecutor);    }

从这里可以看出，默认发挥的CallAdapter.Factory对象是ExecutorCallAdapterFactory。

3 创建代理对象

MyApi api = retrofit.create(MyApi.class);

public <T> T create(final Class<T> service) {    Utils.validateServiceInterface(service);    if (validateEagerly) {      eagerlyValidateMethods(service);    }    return (T) Proxy.newProxyInstance(service.getClassLoader(), new Class<?>[] { service },        new InvocationHandler() {          private final Platform platform = Platform.get();          @Override public Object invoke(Object proxy, Method method, Object... args)              throws Throwable {            // If the method is a method from Object then defer to normal invocation.            if (method.getDeclaringClass() == Object.class) {              return method.invoke(this, args);            }            if (platform.isDefaultMethod(method)) {              return platform.invokeDefaultMethod(method, service, proxy, args);            }            ServiceMethod serviceMethod = loadServiceMethod(method);            OkHttpCall okHttpCall = new OkHttpCall<>(serviceMethod, args);            return serviceMethod.callAdapter.adapt(okHttpCall);          }        });  }

首先要保证必须是接口，而且接口不能实现其他接口

Utils.validateServiceInterface(service);

static <T> void validateServiceInterface(Class<T> service) {    if (!service.isInterface()) {      throw new IllegalArgumentException("API declarations must be interfaces.");    }    // Prevent API interfaces from extending other interfaces. This not only avoids a bug in    // Android (http://b.android.com/58753) but it forces composition of API declarations which is    // the recommended pattern.    if (service.getInterfaces().length > 0) {      throw new IllegalArgumentException("API interfaces must not extend other interfaces.");    }  }

然后看出直接创建一个代理并返回。

return (T) Proxy.newProxyInstance(service.getClassLoader(), new Class<?>[] { service },        new InvocationHandler());

4 通过代理调用接口方法

Call<String> call = api.getUser("");

用代理对象api调用方法时将会执行下面方法

 @Override public Object invoke(Object proxy, Method method, Object... args)              throws Throwable {            // If the method is a method from Object then defer to normal invocation.            if (method.getDeclaringClass() == Object.class) {              return method.invoke(this, args);            }            if (platform.isDefaultMethod(method)) {              return platform.invokeDefaultMethod(method, service, proxy, args);            }            ServiceMethod serviceMethod = loadServiceMethod(method);            OkHttpCall okHttpCall = new OkHttpCall<>(serviceMethod, args);            return serviceMethod.callAdapter.adapt(okHttpCall);          }        });

加载请求的方法

ServiceMethod serviceMethod = loadServiceMethod(method);

ServiceMethod loadServiceMethod(Method method) {    ServiceMethod result;    synchronized (serviceMethodCache) {      //如果缓存中有 直接用      result = serviceMethodCache.get(method);      if (result == null) {        //缓存中没有 就创建ServiceMethod对象        result = new ServiceMethod.Builder(this, method).build();        serviceMethodCache.put(method, result);      }    }    return result;  }

在下面这一行代码中，主要是根据method对象中的注解，参数等。来构造一个ServiceMethod对象。

 result = new ServiceMethod.Builder(this, method).build();

创建OkHttpCall对象

OkHttpCall okHttpCall = new OkHttpCall<>(serviceMethod, args);

代理对象返回ExecutorCallAdapterFactory

return serviceMethod.callAdapter.adapt(okHttpCall);

在ServiceMethod的build()方法中

callAdapter = createCallAdapter();

 private CallAdapter<?> createCallAdapter() {      Type returnType = method.getGenericReturnType();      if (Utils.hasUnresolvableType(returnType)) {        throw methodError(            "Method return type must not include a type variable or wildcard: %s", returnType);      }      if (returnType == void.class) {        throw methodError("Service methods cannot return void.");      }      Annotation[] annotations = method.getAnnotations();      try {        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);      }    }

这段代码中的

return retrofit.callAdapter(returnType, annotations);

在Retrofit中

 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;      }    }

主要是这一句

CallAdapter<?> adapter = adapterFactories.get(i).get(returnType, annotations, this);

由上文中的CallAdapter.Factory参数可知。adapterFactories.get(i)将会返回ExecutorCallAdapterFactory对象。所以将会调用这个对象中的get方法。

 @Override  public CallAdapter<Call<?>> get(Type returnType, Annotation[] annotations, Retrofit retrofit) {    if (getRawType(returnType) != Call.class) {      return null;    }    final Type responseType = Utils.getCallResponseType(returnType);    return new CallAdapter<Call<?>>() {      @Override public Type responseType() {        return responseType;      }      @Override public <R> Call<R> adapt(Call<R> call) {        return new ExecutorCallbackCall<>(callbackExecutor, call);      }    };  }

所以可以看出，这里的CallAdapter是匿名内部类。所以

return serviceMethod.callAdapter.adapt(okHttpCall);

将会调用

 @Override public <R> Call<R> adapt(Call<R> call) {        return new ExecutorCallbackCall<>(callbackExecutor, call);      }

callBackExecutor
但是这里还有一个参数，callbackExecutor。这个参数是构造ExecutorCallAdapterFactory对象时传进来的。

ExecutorCallAdapterFactory(Executor callbackExecutor) {    this.callbackExecutor = callbackExecutor;  }

而在这段代码

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);      }    }  }

中的这一段

 @Override CallAdapter.Factory defaultCallAdapterFactory(Executor callbackExecutor) {      return new ExecutorCallAdapterFactory(callbackExecutor);    }

构造了ExecutorCallAdapterFactory对象。继续追踪callbackExecutor.发现是在Retrofit的build方法中传入的。

adapterFactories.add(platform.defaultCallAdapterFactory(callbackExecutor));

继续追，发现在Retrofit的buil()方法中

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

追踪到继承自PlatFomr的子类Android

@Override public Executor defaultCallbackExecutor() {      return new MainThreadExecutor();    }

static class MainThreadExecutor implements Executor {      private final Handler handler = new Handler(Looper.getMainLooper());      @Override public void execute(Runnable r) {        handler.post(r);      }    }

从这里可以得出结论。callBackExecutor就是MainThreadExecutor。
而Retrofit中的create()方法最终返回的就是ExecutorCallbackCall对象。

5 执行请求

 call.enqueue(new Callback<String>() {            @Override            public void onResponse(Call<String> call, Response<String> response) {            }            @Override            public void onFailure(Call<String> call, Throwable t) {            }        });

从上文可以知道，当用代理对象调用方法时，返回一个ExecutorCallbackCall。即执行请求调用的是ExecutorCallbackCall中的enqueue方法。

在ExecutorCallbackCall中的equeue()方法中：

@Override public void enqueue(final Callback<T> callback) {      if (callback == null) throw new NullPointerException("callback == null");      delegate.enqueue(new Callback<T>() {        @Override public void onResponse(Call<T> call, final Response<T> response) {          callbackExecutor.execute(new Runnable() {            @Override public void run() {              if (delegate.isCanceled()) {                // Emulate OkHttp's behavior of throwing/delivering an IOException on cancellation.                callback.onFailure(ExecutorCallbackCall.this, new IOException("Canceled"));              } else {                callback.onResponse(ExecutorCallbackCall.this, response);              }            }          });        }        @Override public void onFailure(Call<T> call, final Throwable t) {          callbackExecutor.execute(new Runnable() {            @Override public void run() {              callback.onFailure(ExecutorCallbackCall.this, t);            }          });        }      });    }

可以看出相当于一个代理。直接调用了delegate.enqueue（）方法。由上文可知，delegate即为OkHttpCall对象。
在OkHttpCall中的enqueue()方法如下：

 @Override public void enqueue(final Callback<T> callback) {    if (callback == null) throw new NullPointerException("callback == null");    okhttp3.Call call;    Throwable failure;    synchronized (this) {      if (executed) throw new IllegalStateException("Already executed.");      executed = true;      call = rawCall;      failure = creationFailure;      if (call == null && failure == null) {        try {          call = rawCall = createRawCall();        } catch (Throwable t) {          failure = creationFailure = t;        }      }    }    if (failure != null) {      callback.onFailure(this, failure);      return;    }    if (canceled) {      call.cancel();    }    call.enqueue(new okhttp3.Callback() {      @Override public void onResponse(okhttp3.Call call, okhttp3.Response rawResponse)          throws IOException {        Response<T> response;        try {          response = parseResponse(rawResponse);        } catch (Throwable e) {          callFailure(e);          return;        }        callSuccess(response);      }      @Override public void onFailure(okhttp3.Call call, IOException e) {        try {          callback.onFailure(OkHttpCall.this, e);        } catch (Throwable t) {          t.printStackTrace();        }      }      private void callFailure(Throwable e) {        try {          callback.onFailure(OkHttpCall.this, e);        } catch (Throwable t) {          t.printStackTrace();        }      }      private void callSuccess(Response<T> response) {        try {          callback.onResponse(OkHttpCall.this, response);        } catch (Throwable t) {          t.printStackTrace();        }      }    });  }

可以看出，直接通过call.equeue()来执行请求，
真正执行请求的RealCall
上面这段代码中的call对象是什么。可以看到

 call = rawCall = createRawCall();

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

可以看到call对象即为RealCall对象。

@Override public Call newCall(Request request) {    return new RealCall(this, request);  }

至此，Retrofit的请求过程结束。