前言

安卓技术学习图谱（持续更新中，欢迎关注）
https://github.com/Sakuragi/android-summary

实现需求

无网的时候读取本地缓存，有网的情况下更新缓存。
注：本文只是给了post的缓存的一种解决思路，具体的本地持久化策略还需读者自己实现

解决方案

默认情况下，一般认为使用GET通过HTTP访问的请求可以缓存，也应当缓存。使用HTTPS或POST访问的请求不应当缓存。OKHttp也是这样设计的，OKHttp默认不支持Post缓存。那么我们该如何来设计使OKHttp支持POST缓存呢？

大致有这么几种持久化方式：

• Sqlite ：网络正常时缓存响应信息到数据库，在没有网络的时候读出数据。
• DiskLruCache ：通过文件缓存到本地。

结合OKHttp的源码，我们发现OKHttp也是使用了DiskLruCache进行GET请求缓存的。我们首先来看看OkHttp是怎么进行缓存的。

当OKHttp执行excute时：

  @Override public Response execute() throws IOException {
    synchronized (this) {
      if (executed) throw new IllegalStateException("Already Executed");
      executed = true;
    }
    captureCallStackTrace();
    try {
      client.dispatcher().executed(this);
      Response result = getResponseWithInterceptorChain();
      if (result == null) throw new IOException("Canceled");
      return result;
    } finally {
      client.dispatcher().finished(this);
    }
  }

通过源码可以看到它通过getResponseWithInterceptorChain()方法得到响应，这个方法是通过一个一个拦截器，以责任链模式发起请求获得响应，来进一步看这个方法：

  Response getResponseWithInterceptorChain() throws IOException {
    // Build a full stack of interceptors.
    List<Interceptor> interceptors = new ArrayList<>();
    interceptors.addAll(client.interceptors());
    interceptors.add(retryAndFollowUpInterceptor);
    interceptors.add(new BridgeInterceptor(client.cookieJar()));
    interceptors.add(new CacheInterceptor(client.internalCache()));
    interceptors.add(new ConnectInterceptor(client));
    if (!forWebSocket) {
      interceptors.addAll(client.networkInterceptors());
    }
    interceptors.add(new CallServerInterceptor(forWebSocket));

    Interceptor.Chain chain = new RealInterceptorChain(
        interceptors, null, null, null, 0, originalRequest);
    return chain.proceed(originalRequest);
  }

在这里面我们可以看到添加了一个缓存拦截器interceptors.add(new CacheInterceptor(client.internalCache()))，继续跟踪进去：

public final class CacheInterceptor implements Interceptor {
  final InternalCache cache;

  public CacheInterceptor(InternalCache cache) {
    this.cache = cache;
  }

  @Override public Response intercept(Chain chain) throws IOException {
    Response cacheCandidate = cache != null
        ? cache.get(chain.request())
        : null;

    long now = System.currentTimeMillis();

    CacheStrategy strategy = new CacheStrategy.Factory(now, chain.request(), cacheCandidate).get();
    Request networkRequest = strategy.networkRequest;
    Response cacheResponse = strategy.cacheResponse;

    if (cache != null) {
      cache.trackResponse(strategy);
    }

    if (cacheCandidate != null && cacheResponse == null) {
      closeQuietly(cacheCandidate.body()); // The cache candidate wasn't applicable. Close it.
    }

    // If we're forbidden from using the network and the cache is insufficient, fail.
    if (networkRequest == null && cacheResponse == null) {
      return new Response.Builder()
          .request(chain.request())
          .protocol(Protocol.HTTP_1_1)
          .code(504)
          .message("Unsatisfiable Request (only-if-cached)")
          .body(Util.EMPTY_RESPONSE)
          .sentRequestAtMillis(-1L)
          .receivedResponseAtMillis(System.currentTimeMillis())
          .build();
    }

    // If we don't need the network, we're done.
    if (networkRequest == null) {
      return cacheResponse.newBuilder()
          .cacheResponse(stripBody(cacheResponse))
          .build();
    }

    Response networkResponse = null;
    try {
      networkResponse = chain.proceed(networkRequest);
    } finally {
      // If we're crashing on I/O or otherwise, don't leak the cache body.
      if (networkResponse == null && cacheCandidate != null) {
        closeQuietly(cacheCandidate.body());
      }
    }

    // If we have a cache response too, then we're doing a conditional get.
    if (cacheResponse != null) {
      if (networkResponse.code() == HTTP_NOT_MODIFIED) {
        Response response = cacheResponse.newBuilder()
            .headers(combine(cacheResponse.headers(), networkResponse.headers()))
            .sentRequestAtMillis(networkResponse.sentRequestAtMillis())
            .receivedResponseAtMillis(networkResponse.receivedResponseAtMillis())
            .cacheResponse(stripBody(cacheResponse))
            .networkResponse(stripBody(networkResponse))
            .build();
        networkResponse.body().close();

        // Update the cache after combining headers but before stripping the
        // Content-Encoding header (as performed by initContentStream()).
        cache.trackConditionalCacheHit();
        cache.update(cacheResponse, response);
        return response;
      } else {
        closeQuietly(cacheResponse.body());
      }
    }

    Response response = networkResponse.newBuilder()
        .cacheResponse(stripBody(cacheResponse))
        .networkResponse(stripBody(networkResponse))
        .build();

    if (cache != null) {
      if (HttpHeaders.hasBody(response) && CacheStrategy.isCacheable(response, networkRequest)) {
        // Offer this request to the cache.
        CacheRequest cacheRequest = cache.put(response);
        return cacheWritingResponse(cacheRequest, response);
      }

      if (HttpMethod.invalidatesCache(networkRequest.method())) {
        try {
          cache.remove(networkRequest);
        } catch (IOException ignored) {
          // The cache cannot be written.
        }
      }
    }

    return response;
  }

  private static Response stripBody(Response response) {
    return response != null && response.body() != null
        ? response.newBuilder().body(null).build()
        : response;
  }

  /**
   * Returns a new source that writes bytes to {@code cacheRequest} as they are read by the source
   * consumer. This is careful to discard bytes left over when the stream is closed; otherwise we
   * may never exhaust the source stream and therefore not complete the cached response.
   */
  private Response cacheWritingResponse(final CacheRequest cacheRequest, Response response)
      throws IOException {
    // Some apps return a null body; for compatibility we treat that like a null cache request.
    if (cacheRequest == null) return response;
    Sink cacheBodyUnbuffered = cacheRequest.body();
    if (cacheBodyUnbuffered == null) return response;

    final BufferedSource source = response.body().source();
    final BufferedSink cacheBody = Okio.buffer(cacheBodyUnbuffered);

    Source cacheWritingSource = new Source() {
      boolean cacheRequestClosed;

      @Override public long read(Buffer sink, long byteCount) throws IOException {
        long bytesRead;
        try {
          bytesRead = source.read(sink, byteCount);
        } catch (IOException e) {
          if (!cacheRequestClosed) {
            cacheRequestClosed = true;
            cacheRequest.abort(); // Failed to write a complete cache response.
          }
          throw e;
        }

        if (bytesRead == -1) {
          if (!cacheRequestClosed) {
            cacheRequestClosed = true;
            cacheBody.close(); // The cache response is complete!
          }
          return -1;
        }

        sink.copyTo(cacheBody.buffer(), sink.size() - bytesRead, bytesRead);
        cacheBody.emitCompleteSegments();
        return bytesRead;
      }

      @Override public Timeout timeout() {
        return source.timeout();
      }

      @Override public void close() throws IOException {
        if (!cacheRequestClosed
            && !discard(this, HttpCodec.DISCARD_STREAM_TIMEOUT_MILLIS, MILLISECONDS)) {
          cacheRequestClosed = true;
          cacheRequest.abort();
        }
        source.close();
      }
    };

    return response.newBuilder()
        .body(new RealResponseBody(response.headers(), Okio.buffer(cacheWritingSource)))
        .build();
  }

  /** Combines cached headers with a network headers as defined by RFC 2616, 13.5.3. */
  private static Headers combine(Headers cachedHeaders, Headers networkHeaders) {
    Headers.Builder result = new Headers.Builder();

    for (int i = 0, size = cachedHeaders.size(); i < size; i++) {
      String fieldName = cachedHeaders.name(i);
      String value = cachedHeaders.value(i);
      if ("Warning".equalsIgnoreCase(fieldName) && value.startsWith("1")) {
        continue; // Drop 100-level freshness warnings.
      }
      if (!isEndToEnd(fieldName) || networkHeaders.get(fieldName) == null) {
        Internal.instance.addLenient(result, fieldName, value);
      }
    }

    for (int i = 0, size = networkHeaders.size(); i < size; i++) {
      String fieldName = networkHeaders.name(i);
      if ("Content-Length".equalsIgnoreCase(fieldName)) {
        continue; // Ignore content-length headers of validating responses.
      }
      if (isEndToEnd(fieldName)) {
        Internal.instance.addLenient(result, fieldName, networkHeaders.value(i));
      }
    }

    return result.build();
  }

  /**
   * Returns true if {@code fieldName} is an end-to-end HTTP header, as defined by RFC 2616,
   * 13.5.1.
   */
  static boolean isEndToEnd(String fieldName) {
    return !"Connection".equalsIgnoreCase(fieldName)
        && !"Keep-Alive".equalsIgnoreCase(fieldName)
        && !"Proxy-Authenticate".equalsIgnoreCase(fieldName)
        && !"Proxy-Authorization".equalsIgnoreCase(fieldName)
        && !"TE".equalsIgnoreCase(fieldName)
        && !"Trailers".equalsIgnoreCase(fieldName)
        && !"Transfer-Encoding".equalsIgnoreCase(fieldName)
        && !"Upgrade".equalsIgnoreCase(fieldName);
  }
}

我们来看这个缓存拦截器的实现方式：主要分三步：

第一步：首先判断用户是否设置缓存，如果有的话读出缓存
第二部：根据CacheStrategy缓存策略，判断是否需要网络请求，如果networkRequest为空则说明不需要网络请求，直接返回当前缓存
第三步：获取网络请求，如果缓存不为空，调用validate进行验证，是否需要更新缓存，如果缓存为空，则保存当前缓存。

根据这个实现步骤，我们可以模仿拦截器实现网络缓存：

public class PostCacheInterceptor implements Interceptor {

    final DiskLruCacheHelper cache;

    public PostCacheInterceptor(DiskLruCacheHelper cache) {
        this.cache = cache;
    }

    private final int REQUEST_URL = 0;
    private final int REQUEST_METHOD = 1;
    private final int REQUESTCONTENTTYPE = 2;
    private final int PROTOCAL = 3;
    private final int CODE = 4;
    private final int MESSAGE = 5;
    private final int REPONSE_BODY = 6;
    private final int MEDIA_TYPE = 7;
    private final int SETN_REQUEST_AT_MILLIS = 8;
    private final int RECEIVE_REPONSE_AT_MILLIS = 9;
    private final int CACHE_LENGTH = 10;


    @Override
    public Response intercept(Interceptor.Chain chain) throws IOException {
        if (!isNeedCache(chain.request().url().toString())) {
            return chain.proceed(chain.request());
        }

        //获取缓存
        String key = createKey(chain.request());
        LogUtil.d("cache key: " + key);
        Response cacheResponse = null;
        String cacheRes = cache != null&&key!=null
                ? cache.getAsString(key)
                : null;

        if (!TextUtils.isEmpty(cacheRes)) {
            LogUtil.d("cacheRes: " + cacheRes);
            cacheResponse = combineCacheToResponse(cacheRes);
        }

        //没有网络连接的时候读取缓存
        if (!NetStateUtils.isNetWorkConnected()) {
            LogUtil.d("no network connected jujge cache available");
            if (cacheResponse != null) {
                LogUtil.d("no network connected, return cache： " + cacheResponse);
                return cacheResponse;
            }
        }

        LogUtil.d("waiting for network response...");
        //获取网络响应
        Request netWorkRequest = chain.request();
        Response networkResponse = null;
        try {
            networkResponse = chain.proceed(netWorkRequest);
        } finally {
            if (networkResponse == null) {
                LogUtil.d("close cache response...");
                if (cacheResponse!=null&&HttpHeaders.hasBody(cacheResponse)){
                    closeQuietly(cacheResponse.body());
                }
                return chain.proceed(netWorkRequest);
            }
        }
        LogUtil.d("prepare update cache response...");
        //更新缓存
        if (cacheResponse != null) {
            Response response = null;
            response = networkResponse.newBuilder()
                    .request(new Request.Builder()
                            .method("GET", null)
                            .url(netWorkRequest.url())
                            .headers(netWorkRequest.headers())
                            .tag(netWorkRequest.tag())
                            .build())
                    .build();
            LogUtil.d("update cache response");
            if (key!=null){
                cache.put(key, createCache(response));
            }
            if (cacheResponse!=null&&HttpHeaders.hasBody(cacheResponse)){
                closeQuietly(cacheResponse.body());
            }
            return networkResponse;

        }

        Request newRequest = new Request.Builder()
                .method("GET", null)
                .url(netWorkRequest.url())
                .headers(netWorkRequest.headers())
                .tag(netWorkRequest.tag())
                .build();

        Response newResponse = networkResponse.newBuilder()
                .request(newRequest)
                .build();

        LogUtil.d("init cache response");
        //放入缓存
        if (cache != null) {
            LogUtil.d("url: " + netWorkRequest.url().toString());
            if (HttpHeaders.hasBody(newResponse)) {
                try {
                    LogUtil.d("chain request url: " + newResponse.request().url());
                    if (key!=null){
                        cache.put(key, createCache(newResponse));
                        LogUtil.d("put cache response key: " + key);
                    }
//                    String resp1 = cache.getAsString(key);
//                    LogUtil.d("resp1: " + resp1);
                    return networkResponse;
                } catch (Exception e) {
                    LogUtil.d("put cache exception: " + e);
                }finally {
                    if (cacheResponse != null && HttpHeaders.hasBody(cacheResponse)) {
                        closeQuietly(cacheResponse.body());
                    }
                }
            }
        }
        return networkResponse;
    }


    private String createKey(Request request) {
        RequestBody requestBody = request.body();
        Charset charset = Charset.forName("UTF-8");
        String url = request.url().toString();
        StringBuilder sb = new StringBuilder();
        sb.append(url + "&");
        MediaType type = requestBody.contentType();
        if (type != null) {
            charset = type.charset() == null ? charset : type.charset();
        }
        Buffer buffer = new Buffer();
        try {
            requestBody.writeTo(buffer);
            sb.append(buffer.readString(charset));
        } catch (Exception e) {
            LogUtil.d("read request error: " + e);
        } finally {
            buffer.close();
        }
        if (url.startsWith(BuildConfig.SERVER_URL + "your own url")) {
            return //这里可以根据url来定制化key
        } 
        return sb.toString();
    }

    //根据键返回索引
    private int[] getIndexofKeyValue(String str, String originStr) {
        int[] indexs = new int[2];
        indexs[0] = originStr.indexOf(str);
        indexs[1] = originStr.indexOf("&", indexs[0]) >= 0 ? originStr.indexOf("&", indexs[0]) : originStr.length();
        LogUtil.d("index0: " + indexs[0] + " index1: " + indexs[1]);
        return indexs;
    }

   

    private boolean isNeedCache(String url) {
        //这里可以根据Url来判断是否需要缓存
    }

    private Response combineCacheToResponse(String cache) {
        String[] caches = cache.split("&#&#");
        if (caches == null || caches.length <= 0) {
            return null;
        }
        Request request = new Request.Builder()
                .url(caches[REQUEST_URL])
                .method(caches[REQUEST_METHOD], null)
                .build();
        Response.Builder builder = new Response.Builder();
        try {
            builder.protocol(Protocol.get(caches[PROTOCAL]));
        } catch (IOException e) {
            e.printStackTrace();
        }
        return builder.message(caches[MESSAGE])
                .code(Integer.valueOf(caches[CODE]))
                .request(request)
                .receivedResponseAtMillis(Long.valueOf(caches[RECEIVE_REPONSE_AT_MILLIS]))
                .sentRequestAtMillis(Long.valueOf(caches[SETN_REQUEST_AT_MILLIS]))
                .body(ResponseBody.create(MediaType.parse(caches[MEDIA_TYPE]), caches[REPONSE_BODY]))
                .build();
    }

    private String createCache(Response response) {
        String[] caches = new String[CACHE_LENGTH];
        caches[REQUEST_URL] = response.request().url().toString();
        caches[REQUEST_METHOD] = response.request().method();
        if (response.request().body() != null && response.request().body().contentType() != null) {
            caches[REQUESTCONTENTTYPE] = response.request().body().contentType().toString();
        } else {
            caches[REQUESTCONTENTTYPE] = "application/x-www-form-urlencoded";
        }
        caches[PROTOCAL] = response.protocol().toString();
        caches[CODE] = response.code() + "";
        caches[MESSAGE] = response.message();
        if (response.body() != null && response.body().contentType() != null) {
            caches[MEDIA_TYPE] = response.body().contentType().toString();
        } else {
            caches[MEDIA_TYPE] = "application/x-www-form-urlencoded";
        }
        caches[SETN_REQUEST_AT_MILLIS] = response.sentRequestAtMillis() + "";
        caches[RECEIVE_REPONSE_AT_MILLIS] = response.receivedResponseAtMillis() + "";
        if (HttpHeaders.hasBody(response)) {
            BufferedSource source = response.body().source();
            Buffer buffer = null;
            try {
                source.request(Long.MAX_VALUE);
                buffer = source.buffer();
                Charset charset = response.body().contentType().charset();
                if (charset == null) {
                    charset = Charset.forName("UTF-8");
                }
                caches[REPONSE_BODY] = buffer.clone().readString(charset);
            } catch (IOException e) {
                e.printStackTrace();
            }finally {
//                closeQuietly(response.body());
            }
        }
        String cache = "";
        for (String str : caches) {
            cache += str + "&#&#";
        }
        return cache;
    }

    static boolean isEndToEnd(String fieldName) {
        return !"Connection".equalsIgnoreCase(fieldName)
                && !"Keep-Alive".equalsIgnoreCase(fieldName)
                && !"Proxy-Authenticate".equalsIgnoreCase(fieldName)
                && !"Proxy-Authorization".equalsIgnoreCase(fieldName)
                && !"TE".equalsIgnoreCase(fieldName)
                && !"Trailers".equalsIgnoreCase(fieldName)
                && !"Transfer-Encoding".equalsIgnoreCase(fieldName)
                && !"Upgrade".equalsIgnoreCase(fieldName);
    }

    private String subString(String str, int[] index) {
        if (index == null || index.length < 2) {
            return null;
        }
        if (index[0] < 0 || index[1] < 0) {
            return null;
        }
        return str.substring(index[0], index[1]);
    }
}