Okhttp3源碼解析(4)-攔截器與設計模式

### 前言回顧： [Okhttp的基本用法](https://www.jianshu.com/p/8e404d9c160f) [Okhttp3源碼解析(1)-OkHttpClient分析](https://www.jianshu.com/p/bf1d01b79ce7) [Okhttp3源碼解析( ...

### 前言回顧： [Okhttp的基本用法](https://www.jianshu.com/p/8e404d9c160f) [Okhttp3源碼解析(1)-OkHttpClient分析](https://www.jianshu.com/p/bf1d01b79ce7) [Okhttp3源碼解析(2)-Request分析](https://www.jianshu.com/p/5a85345c8ea7) [Okhttp3源碼解析(3)-Call分析(整體流程)](https://www.jianshu.com/p/4ed79472797a) 上節我們講了okhttp的整體的流程，裡面的核心方法之一是`getResponseWithInterceptorChain()` ，這個方法應該知道吧？通過攔截器層層處理返回Response；這個方法中其實應用了責任鏈設計模式。今天主要講一下它是如何應用的！ ### 責任鏈設計模式 ###### 責任鏈模式的定義在責任鏈模式里，很多對象由每一個對象對其下家的引用而連接起來形成一條鏈。請求在這個鏈上傳遞，直到鏈上的某一個對象決定處理此請求。發出這個請求的客戶端並不知道鏈上的哪一個對象最終處理這個請求，這使得系統可以在不影響客戶端的情況下動態地重新組織和分配責任。模型： ![](https://img2018.cnblogs.com/blog/1312938/201908/1312938-20190827082427829-283762395.png) 1.優點耦合度降低，請求和處理是分開的 2.缺點責任鏈太長或者每條鏈判斷處理的時間太長會影響性能。特別是遞歸迴圈的時候不一定被處理，每個職責類的職責很明確，這就需要對寫預設的處理了 **責任鏈模式重要的兩點：分離職責，動態組合** 對責任鏈設計模式不明白的可以去網上那個找找實例看看，這裡就不舉例子了。 ### 源碼中的責任鏈話不多說，直接上`getResponseWithInterceptorChain()` 源碼 ``` Response getResponseWithInterceptorChain() throws IOException { // Build a full stack of interceptors. List interceptors = new ArrayList<>(); interceptors.addAll(client.interceptors()); //自定義 interceptors.add(retryAndFollowUpInterceptor); //錯誤與跟蹤攔截器 interceptors.add(new BridgeInterceptor(client.cookieJar())); //橋攔截器 interceptors.add(new CacheInterceptor(client.internalCache())); //緩存攔截器 interceptors.add(new ConnectInterceptor(client)); //連接攔截器 if (!forWebSocket) { interceptors.addAll(client.networkInterceptors()); //網路攔截器 } interceptors.add(new CallServerInterceptor(forWebSocket)); //調用伺服器攔截器 Interceptor.Chain chain = new RealInterceptorChain(interceptors, null, null, null, 0, originalRequest, this, eventListener, client.connectTimeoutMillis(), client.readTimeoutMillis(), client.writeTimeoutMillis()); return chain.proceed(originalRequest); } ``` 方法中大部分上節已經說了，就是 `List`添加自定義、cookie等等的攔截器，今天我們主要看看後半部分： ``` Interceptor.Chain chain = new RealInterceptorChain(interceptors, null, null, null, 0, originalRequest, this, eventListener, client.connectTimeoutMillis(), client.readTimeoutMillis(), client.writeTimeoutMillis()); return chain.proceed(originalRequest); ``` 首先初始化了 `RealInterceptorChain`，`RealInterceptorChain`是`Interceptor.Chain`的實現類 ![](https://img2018.cnblogs.com/blog/1312938/201908/1312938-20190827082428677-935973782.png) 先看一下`Interceptor.Chain`： ``` public interface Interceptor { Response intercept(Chain chain) throws IOException; interface Chain { Request request(); Response proceed(Request request) throws IOException; //部分代碼省略.... } } ``` 生成了RealInterceptorChain的實例，調用了`proceed()`，返回了最後的Response 我們看下 `RealInterceptorChain`類中的`proceed()`： ``` @Override public Response proceed(Request request) throws IOException { return proceed(request, streamAllocation, httpCodec, connection); } public Response proceed(Request request, StreamAllocation streamAllocation, HttpCodec httpCodec, RealConnection connection) throws IOException { if (index >= interceptors.size()) throw new AssertionError(); calls++; // If we already have a stream, confirm that the incoming request will use it. if (this.httpCodec != null && !this.connection.supportsUrl(request.url())) { throw new IllegalStateException("network interceptor " + interceptors.get(index - 1) + " must retain the same host and port"); } // If we already have a stream, confirm that this is the only call to chain.proceed(). if (this.httpCodec != null && calls > 1) { throw new IllegalStateException("network interceptor " + interceptors.get(index - 1) + " must call proceed() exactly once"); } // Call the next interceptor in the chain. RealInterceptorChain next = new RealInterceptorChain(interceptors, streamAllocation, httpCodec, connection, index + 1, request, call, eventListener, connectTimeout, readTimeout, writeTimeout); Interceptor interceptor = interceptors.get(index); Response response = interceptor.intercept(next); // Confirm that the next interceptor made its required call to chain.proceed(). if (httpCodec != null && index + 1 < interceptors.size() && next.calls != 1) { throw new IllegalStateException("network interceptor " + interceptor + " must call proceed() exactly once"); } // Confirm that the intercepted response isn't null. if (response == null) { throw new NullPointerException("interceptor " + interceptor + " returned null"); } if (response.body() == null) { throw new IllegalStateException( "interceptor " + interceptor + " returned a response with no body"); } return response; } ``` 經過一系列的判斷，看下`proceed()`的核心 ``` // Call the next interceptor in the chain. RealInterceptorChain next = new RealInterceptorChain(interceptors, streamAllocation, httpCodec, connection, index + 1, request, call, eventListener, connectTimeout, readTimeout, writeTimeout); Interceptor interceptor = interceptors.get(index); Response response = interceptor.intercept(next); ``` 如果對責任鏈模式有認識的朋友看到上面代碼，應該直接就能看出來了，**並能分析出：** - 如果是責任鏈模式，那個`intercept()`一定是關鍵， `Interceptor`是介面，interceptors集合中的攔截器類肯定都實現了 `Interceptor`以及 `interceptor.intercept()` - 每個攔截器 `intercept() `方法中的chain，都在上一個 `chain`實例的 `chain.proceed() `中被初始化，並傳遞了攔截器List與 `index `，直接最後一個 `chain實例 `執行即停止。 ![](https://img2018.cnblogs.com/blog/1312938/201908/1312938-20190827082428850-1579043948.png) ### 總結：每個`chain`的`interceptors`與`index`都是由上一個`chain`初始化傳遞過來的，在`chain.proceed()`中獲取對應的`interceptor實例` 並初始化下一個`chain`，直到最後一個`chain`被執行。這樣就清晰了吧？責任鏈模式的重點在“鏈上”，由一條鏈去處理相似的請求，在鏈中決定誰來處理這個請求，並返回相應的結果。 ![](https://img2018.cnblogs.com/blog/1312938/201908/1312938-20190827082429260-1507882191.png) 這節就說到這，希望對大家有所幫助..... 大家可以關註我的微信公眾號：「秦子帥」一個有質量、有態度的公眾號！ ![公眾號](https://img2018.cnblogs.com/blog/1312938/201908/1312938-20190827082429414-2044533379.jpg)