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关于Okhttp3（六）-CacheInterceptor

来源：要发发知识网

现在的app没有几个是不联网的了，在流量费用很高、速度一般的今天给用户合理节省流量，以及提高响应速度就显得尤为重要了。所以一个优秀的app都会在发展到一定程度后就会开始引入缓存，什么是缓存呢？

百度百科：

缓存就是数据交换的缓冲区（称作Cache），当某一硬件要读取数据时，会首先从缓存中查找需要的数据，如果找到了则直接执行，找不到的话则从内存中找。由于缓存的运行速度比内存快得多，故缓存的作用就是帮助硬件更快地运行。

通俗一点就是：接杯水放在手边，渴了直接喝，没有去饮水机取。

原理

使用DiskLruCache缓存策略

注意点

  1. 目前只支持GET方式，其他请求方式需要自己实现
  2. 需要服务器配合，通过header相关的头来控制缓存
  3. 创建okhttpclient时候需要配置Cache

流程

1、如果配置缓存，则从缓存中取一次，不保证存在
2、缓存策略
3、缓存监测
4、禁止使用网络（根据缓存策略），缓存又无效，直接返回
5、缓存有效，不使用网络
6、缓存无效，执行下一个拦截器
7、本地有缓存，根具条件选择使用哪个响应
8、使用网络响应
9、 缓存到本地

源码

@Override 
public Response intercept(Chain chain) throws IOException {
  // 1、如果配置缓存，则从缓存中取一次，不保证存在
  Response cacheCandidate = cache != null
      ? cache.get(chain.request())
      : null;

  long now = System.currentTimeMillis();

  // 2、缓存策略
  CacheStrategy strategy = new CacheStrategy.Factory(now, chain.request(), cacheCandidate).get();
  Request networkRequest = 
  Response cacheResponse = strategy.cacheResponse;

  // 3、缓存监测
  if (cache != null) {
    cache.trackResponse(strategy);
  }

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

  // 4、禁止使用网络（根据缓存策略），缓存又无效，直接返回
  // 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();
  }

  // 5、缓存有效，不使用网络
  // If we don't need the network, we're done.
  if (networkRequest == null) {
    return cacheResponse.newBuilder()
        .cacheResponse(stripBody(cacheResponse))
        .build();
  }
  // 6、缓存无效，执行下一个拦截器
  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());
    }
  }

  // 7、本地有缓存，根具条件选择使用哪个响应
  // 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))
          
          .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());
    }
  }

  // 8、使用网络响应
  Response response = networkResponse.newBuilder()
      .cacheResponse(stripBody(cacheResponse))
      
      .build();
 // 9、 缓存到本地
  if (HttpHeaders.hasBody(response)) {
    CacheRequest cacheRequest = maybeCache(response, networkResponse.request(), cache);
    response = cacheWritingResponse(cacheRequest, response);
  }

  return response;
}

步骤分析

读取缓存
// 入口
Response cacheCandidate = cache != null
    ? cache.get(chain.request())
    : null;
// 主要是Cache类
  1. 通过url生成key（MD5、HEX）
  2. 通过key从内存中读取包装实体类Entry，内存中使用LinkedHashMap<String, Entry>
  3. 通过实体得到一个Snapshot，关联起文件系统中的缓存文件（缓存文件有多个，请求头文件、响应提文件），然后生成流（Source，Okio中的类，时间上就是inputStream）
  4. 通过快照得到一个Response实例
  5. 匹配是否是符合要求的，是返回响应，否关闭
// 位置 okhttp3/Cache
Response get(Request request) {
  // 1、
  String key = key(request.url());
  DiskLruCache.Snapshot snapshot;
  Entry entry;
  try {
     // 2、
    snapshot = cache.get(key);
    if (snapshot == null) {
      return null;
    }
  } catch (IOException e) {
    // Give up because the cache cannot be read.
    return null;
  }

  try {
     // 3、
    entry = new Entry(snapshot.getSource(ENTRY_METADATA));
  } catch (IOException e) {
    Util.closeQuietly(snapshot);
    return null;
  }

   // 4、
  Response response = entry.response(snapshot);

   // 5、
  if (!entry.matches(request, response)) {
    Util.closeQuietly(response.body());
    return null;
  }

  return response;
}
缓存策略的配置

如果上一步能够得到缓存响应，则配置策略，主要是解析缓存中与响应有关的头（Date\Expires\Last-Modified\ETag\Age）

// 2、缓存策略
CacheStrategy strategy = new CacheStrategy.Factory(now, chain.request(), cacheCandidate).get();
Request networkRequest = 
Response cacheResponse = strategy.cacheResponse;

  1. 解析缓存中与缓存有关的头

    
public Factory(long nowMillis, Request request, Response cacheResponse) {
  this.nowMillis = nowMillis;
  this.request = request;
  this.cacheResponse = cacheResponse;
 // 有缓存响应
  if (cacheResponse != null) {
    this.sentRequestMillis = cacheResponse.sentRequestAtMillis();
    this.receivedResponseMillis = cacheResponse.receivedResponseAtMillis();
    Headers headers = cacheResponse.headers();
    for (int i = 0, size = headers.size(); i < size; i++) {
      String fieldName = headers.name(i);
      String value = headers.value(i);
      if ("Date".equalsIgnoreCase(fieldName)) {
        servedDate = HttpDate.parse(value);
        servedDateString = value;
      } else if ("Expires".equalsIgnoreCase(fieldName)) {
        expires = HttpDate.parse(value);
      } else if ("Last-Modified".equalsIgnoreCase(fieldName)) {
        lastModified = HttpDate.parse(value);
        lastModifiedString = value;
      } else if ("ETag".equalsIgnoreCase(fieldName)) {
        etag = value;
      } else if ("Age".equalsIgnoreCase(fieldName)) {
        ageSeconds = HttpHeaders.parseSeconds(value, -1);
      }
    }
  }
}

  2. 根据一些条件实例一个CacheStrategy（get()）

    private CacheStrategy getCandidate() {
  // No cached response.
  // 1、没有缓存响应，返回一个没有响应的策略
  if (cacheResponse == null) {
    return new CacheStrategy(request, null);
  }

  // 2、如果是https，丢失了握手缓存则，返回一个没有响应的策略
  // Drop the cached response if it's missing a required handshake.
  if (request.isHttps() && cacheResponse.handshake() == null) {
    return new CacheStrategy(request, null);
  }

  // 3、不能被缓存
  // If this response shouldn't have been stored, it should never be used
  // as a response source. This check should be redundant as long as the
  // persistence store is well-behaved and the rules are constant.
  if (!isCacheable(cacheResponse, request)) {
    return new CacheStrategy(request, null);
  }

  // 4、缓存控制
  CacheControl requestCaching = request.cacheControl();
  if (requestCaching.noCache() || hasConditions(request)) {
    return new CacheStrategy(request, null);
  }

  // 5、根据响应头
  long ageMillis = cacheResponseAge();
  long freshMillis = computeFreshnessLifetime();

  if (requestCaching.maxAgeSeconds() != -1) {
    freshMillis = Math.min(freshMillis, SECONDS.toMillis(requestCaching.maxAgeSeconds()));
  }

  long minFreshMillis = 0;
  if (requestCaching.minFreshSeconds() != -1) {
    minFreshMillis = SECONDS.toMillis(requestCaching.minFreshSeconds());
  }

  long maxStaleMillis = 0;
  CacheControl responseCaching = cacheResponse.cacheControl();
  if (!responseCaching.mustRevalidate() && requestCaching.maxStaleSeconds() != -1) {
    maxStaleMillis = SECONDS.toMillis(requestCaching.maxStaleSeconds());
  }

  if (!responseCaching.noCache() && ageMillis + minFreshMillis < freshMillis + maxStaleMillis) {
    Response.Builder builder = cacheResponse.newBuilder();
    if (ageMillis + minFreshMillis >= freshMillis) {
      builder.addHeader("Warning", "110 HttpURLConnection \"Response is stale\"");
    }
    long oneDayMillis = 24 * 60 * 60 * 1000L;
    if (ageMillis > oneDayMillis && isFreshnessLifetimeHeuristic()) {
      builder.addHeader("Warning", "113 HttpURLConnection \"Heuristic expiration\"");
    }
    return new CacheStrategy(null, builder.build());
  }

  // Find a condition to add to the request. If the condition is satisfied, the response body
  // will not be transmitted.
  String conditionName;
  String conditionValue;
  if (etag != null) {
    conditionName = "If-None-Match";
    conditionValue = etag;
  } else if (lastModified != null) {
    conditionName = "If-Modified-Since";
    conditionValue = lastModifiedString;
  } else if (servedDate != null) {
    conditionName = "If-Modified-Since";
    conditionValue = servedDateString;
  } else {
    return new CacheStrategy(request, null); // No condition! Make a regular request.
  }

  Headers.Builder conditionalRequestHeaders = request.headers().newBuilder();
  Internal.instance.addLenient(conditionalRequestHeaders, conditionName, conditionValue);

  Request conditionalRequest = request.newBuilder()
      .headers(conditionalRequestHeaders.build())
      .build();
  return new CacheStrategy(conditionalRequest, cacheResponse);
}

只有一种情况是会有正常的缓存被使用：所有的缓存头符合要求，即第5条。

缓存监测
// 3、缓存监测
if (cache != null) {
  cache.trackResponse(strategy);
}

此处记录缓存使用情况

synchronized void trackResponse(CacheStrategy cacheStrategy) {
  requestCount++;

  if  != null) {
    // If this is a conditional request, we'll increment hitCount if/when it hits.
    networkCount++;
  } else if (cacheStrategy.cacheResponse != null) {
    // This response uses the cache and not the network. That's a cache hit.
    hitCount++;
  }
}
禁止使用网络（根据缓存策略），缓存又无效，直接返回

根据上面缓存策略的配置，这种情况不会发生，不清楚为什么有这个逻辑

缓存有效，不使用网络

通过缓存策略，如果符合要求将会把Request置空，Response不为空，所以直接使用缓存

// 5、缓存有效，不使用网络
// If we don't need the network, we're done.
if (networkRequest == null) {
  return cacheResponse.newBuilder()
      .cacheResponse(stripBody(cacheResponse))
      .build();
}
缓存无效，执行下一个拦截器

如果缓存无效，将会执行下一个拦截器，等待响应结果

本地有缓存，根具条件选择使用哪个响应
// 本地有缓存，响应结果没有修改，合并两个响应
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))
        
        .build();
    networkResponse.body().close();

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

以上都不符合，只能使用网络响应

Response response = networkResponse.newBuilder()
    .cacheResponse(stripBody(cacheResponse))
    
    .build();
缓存到本地
// 9、 缓存到本地
// 1.
if (HttpHeaders.hasBody(response)) {
  // 2.
  CacheRequest cacheRequest = maybeCache(response, networkResponse.request(), cache);
  response = cacheWritingResponse(cacheRequest, response);
}

  1. 根据头判断是否支持缓存

    1. 必须有响应体
    2. 内容有变化

  2. 是否符合缓存要求，根据策略

    
private CacheRequest maybeCache(Response userResponse, Request networkRequest,
    InternalCache responseCache) throws IOException {
  // 1、没有响应体 不缓存
  if (responseCache == null) return null;

  // 2、是否支持
  // Should we cache this response for this request?
  // 2.1、根据头
  if (!CacheStrategy.isCacheable(userResponse, networkRequest)) {
    // 2.2、根据请求方式，有请求体的方式都不支持
    if (HttpMethod.invalidatesCache(networkRequest.method())) {
      try {
        responseCache.remove(networkRequest);
      } catch (IOException ignored) {
        // The cache cannot be written.
      }
    }
    return null;
  }

  // 写入缓存
  // Offer this request to the cache.
  return responseCache.put(userResponse);
}

根据请求方式，有请求体的方式都不支持缓存

  1. 通过配置好的cache写入缓存

    写入缓存和读取缓存使用的方式类似，都是通过Cache，DiskLruCache

    CacheRequest put(Response response) {
  String requestMethod = response.request().method();

  if (HttpMethod.invalidatesCache(response.request().method())) {
    try {
      remove(response.request());
    } catch (IOException ignored) {
      // The cache cannot be written.
    }
    return null;
  }
  if (!requestMethod.equals("GET")) {
    // Don't cache non-GET responses. We're technically allowed to cache
    // HEAD requests and some POST requests, but the complexity of doing
    // so is high and the benefit is low.
    return null;
  }

  if (HttpHeaders.hasVaryAll(response)) {
    return null;
  }

  Entry entry = new Entry(response);
  DiskLruCache.Editor editor = null;
  try {
    editor = cache.edit(key(response.request().url()));
    if (editor == null) {
      return null;
    }
    // 提交缓存
    entry.writeTo(editor);
    return new CacheRequestImpl(editor);
  } catch (IOException e) {
    abortQuietly(editor);
    return null;
  }
}

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