责任链模式应用之Okhttp的拦截器机制

责任链模式的概念

责任链模式定义如下：
Avoid coupling the sender of a request to its receiver by giving more than one object a chance to handle the request.Chain the receiving objects and pass the request along the chain until an object handles it.(翻一下：使多个对象都有机会处理请求，从而避免了请求的发送者和接收者之间的耦合关系。将这些对象连成一条链，并沿着这条链传递该请求，直到有对象处理为止。)
用简单的类图表示如下：

从概念上和类图上可以推断责任链会带来最大的优点，就是将请求和处理分开，请求者完全不用care是谁处理，是谁在加工。模式的核心在于创建一条chain，而处理具体事务的是chain的节点ConcreteHandler。研究这个模式，okhttp的拦截器机制是很好的研究点。

OKhttp的拦截器机制

这里展示一下demo，简单输出一下url，demo只是为了演示用法，不具有实际应用性。

      OkHttpClient okHttpClient = new OkHttpClient.Builder()
        .addInterceptor(hostSelectionInterceptor)
        .addInterceptor(new Interceptor() {
          @Override
          public okhttp3.Response intercept(Chain chain) throws IOException {
            Request request = chain.request();
            System.out.println("request url intercept " + request.url().toString());
            return chain.proceed(request);
          }
        })
        .build();
    Request request = new Request.Builder().url("http://www.github.com/").build();
    Response response = okHttpClient.newCall(request).execute();

控制台会打印:“request url intercept http://www.github.com/”，request和interceptor解耦，request不用care intercept 怎么处理，只关心repose的内容。
这个具体实现原理到底是啥呢,这里不扒okhttp的运行原理，只讲重点部分。
OkhttpClient.newCall(request)返回的对象的类是RealCall,我们直接看这个类的execute方法,

    public Response execute() throws IOException {
        synchronized(this) {
            if (this.executed) {
                throw new IllegalStateException("Already Executed");
            }

            this.executed = true;
        }

        this.transmitter.timeoutEnter();
        this.transmitter.callStart();

        Response var1;
        try {
            this.client.dispatcher().executed(this);
            var1 = this.getResponseWithInterceptorChain();
        } finally {
            this.client.dispatcher().finished(this);
        }

        return var1;
    }

关键看this.getResponseWithInterceptorChain()，在这个方法里处理了拦截器机制的主要逻辑。

    Response getResponseWithInterceptorChain() throws IOException {
        List interceptors = new ArrayList();
        interceptors.addAll(this.client.interceptors());
        interceptors.add(new RetryAndFollowUpInterceptor(this.client));
        interceptors.add(new BridgeInterceptor(this.client.cookieJar()));
        interceptors.add(new CacheInterceptor(this.client.internalCache()));
        interceptors.add(new ConnectInterceptor(this.client));
        if (!this.forWebSocket) {
            interceptors.addAll(this.client.networkInterceptors());
        }

        interceptors.add(new CallServerInterceptor(this.forWebSocket));
        Chain chain = new RealInterceptorChain(interceptors, this.transmitter, (Exchange)null, 0, this.originalRequest, this, this.client.connectTimeoutMillis(), this.client.readTimeoutMillis(), this.client.writeTimeoutMillis());
        boolean calledNoMoreExchanges = false;

        Response var5;
        try {
            Response response = chain.proceed(this.originalRequest);
            if (this.transmitter.isCanceled()) {
                Util.closeQuietly(response);
                throw new IOException("Canceled");
            }

            var5 = response;
        } catch (IOException var9) {
            calledNoMoreExchanges = true;
            throw this.transmitter.noMoreExchanges(var9);
        } finally {
            if (!calledNoMoreExchanges) {
                this.transmitter.noMoreExchanges((IOException)null);
            }

        }

        return var5;
    }

先创建了拦截器的数组interceptors，this.client.interceptors()返回的是我们自己定义的拦截器，经常使用的还有处理cookie的BridgeInterceptor，其他都是框架内置的拦截器。然后创建了链对象chain,类型是RealInterceptorChain，RealInterceptorChain的构造方法：

    public RealInterceptorChain(List interceptors, Transmitter transmitter, @Nullable Exchange exchange, int index, Request request, Call call, int connectTimeout, int readTimeout, int writeTimeout) {
        this.interceptors = interceptors;
        this.transmitter = transmitter;
        this.exchange = exchange;
        this.index = index;
        this.request = request;
        this.call = call;
        this.connectTimeout = connectTimeout;
        this.readTimeout = readTimeout;
        this.writeTimeout = writeTimeout;
    }

chain保存interceptors数组，其他参数如transmitter,exchange 暂且不提，这里主要理解责任链模式。这个数组其实就是所以处理具体事务的节点。

最后调用chain.proceed方法。

    public Response proceed(Request request, Transmitter transmitter, @Nullable Exchange exchange) throws IOException {
        if (this.index >= this.interceptors.size()) {
            throw new AssertionError();
        } else {
            ++this.calls;
            if (this.exchange != null && !this.exchange.connection().supportsUrl(request.url())) {
                throw new IllegalStateException("network interceptor " + this.interceptors.get(this.index - 1) + " must retain the same host and port");
            } else if (this.exchange != null && this.calls > 1) {
                throw new IllegalStateException("network interceptor " + this.interceptors.get(this.index - 1) + " must call proceed() exactly once");
            } else {
                RealInterceptorChain next = new RealInterceptorChain(this.interceptors, transmitter, exchange, this.index + 1, request, this.call, this.connectTimeout, this.readTimeout, this.writeTimeout);
                Interceptor interceptor = (Interceptor)this.interceptors.get(this.index);
                Response response = interceptor.intercept(next);
                if (exchange != null && this.index + 1 < this.interceptors.size() && next.calls != 1) {
                    throw new IllegalStateException("network interceptor " + interceptor + " must call proceed() exactly once");
                } else if (response == null) {
                    throw new NullPointerException("interceptor " + interceptor + " returned null");
                } else if (response.body() == null) {
                    throw new IllegalStateException("interceptor " + interceptor + " returned a response with no body");
                } else {
                    return response;
                }
            }
        }
    }

传入request，还记得前面构造RealInterceptorChain对象，exchange对象是null，所以在proceed方法中，逻辑会走到else分支，就是获取下一个拦截器（下一个节点）来处理这个请求，调用拦截器的interceptos方法,传入新创建的节点。这里和标准的责任链不一样，第一个是节点处理顺序是用数组元素的顺序，第二个不同点每次调用下一个节点的时候，是重新创建的，没有一开始就把链创建好，但其思想是一样的，如果调用的是execute，那么拦截器的执行就在调用的线程，如果是enqueue，那么是在线程池创建的线程里，这个不详细说明，可以看源码，okhttp的执行流程还是很清晰的。最后返回的是response对象。此时我们可以画一个简单的类图。

在这里拦截器起这么一个作用，执行下一个节点的proceed方法或者直接返回响应的response，在执行前后给request或者response加工。在okhttp中直接返回response的拦截器就是CallServerInterceptor，可以看下这个拦截器的proceed方法，就是最终处理了request。下一篇文章实战下，分析下自带的BridgeInterceptor将cookie添加到request中。

总结

okhttp的拦截器机制是通过责任链模式来设计，虽然不是标准的责任链，但实现这个模式的主要思想。RealCall发起了请求，拦截器处理请求并调用下一个节点的处理，形成一条完美的chain，最后返回请求结果response