okhttp 之 CacheInterceptor（缓存）拦截

还是先进入intercept这个方法

 @Override public Response intercept(Chain chain) throws IOException {
//从缓存里面拿
Response cacheCandidate = cache != null
    ? cache.get(chain.request())
    : null;

long now = System.currentTimeMillis();
 //构建一个缓存策略
CacheStrategy strategy = new CacheStrategy.Factory(now, chain.request(), cacheCandidate).get();
//做了一系列处理
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.
}
  //如果请求的request为空 且处理过的也为空  则返回504 
// 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();
}
    //如果网络的返回空 则获取缓存的
// If we don't need the network, we're done.
if (networkRequest == null) {
  return cacheResponse.newBuilder()
      .cacheResponse(stripBody(cacheResponse))
      .build();
}

Response networkResponse = null;
//否则的话 则 传给下面的拦截器
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());
  }
}
     //处理304情况
// If we have a cache response too, then we're doing a conditional get.
if (cacheResponse != null) {
   //如果服务器数据没有变化 则还是可以从缓存获取数据
  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());
  }
}
//如果数据变化了  则返回
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;
}

我们接着进入CacheStrategy.Factory(now, chain.request(), cacheCandidate).get();
这个Factory方法里面看下

  public Factory(long nowMillis, Request request, Response cacheResponse) {
  this.nowMillis = nowMillis;
  this.request = request;
  this.cacheResponse = cacheResponse;

  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);
      //这是里面的一个内部类 解析之前缓存的头部信息
      if ("Date".equalsIgnoreCase(fieldName)) {
        servedDate = HttpDate.parse(value);
        servedDateString = value;
       //Expires 缓存的过期时间
      } else if ("Expires".equalsIgnoreCase(fieldName)) {
        expires = HttpDate.parse(value);
        //上次的更新时间
      } 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);
      }
    }
  }
}

接着我们进入.get();

/**
 * Returns a strategy to satisfy {@code request} using the a cached response {@code   response}.
 */
 public CacheStrategy get() {
  CacheStrategy candidate = getCandidate();
      //判断networkRequest ！null  并且只从缓存中获取 则全设置为空
  if (candidate.networkRequest != null && request.cacheControl().onlyIfCached()) {
    // We're forbidden from using the network and the cache is insufficient.
    return new CacheStrategy(null, null);
  }

  return candidate;
}

我们接着进入getCandidate(); 这个方法中看下

 private CacheStrategy getCandidate() {
  // No cached response.
  //进行判断 设置  strategy.networkRequest;strategy.cacheResponse;这两个对象
  if (cacheResponse == null) {
    return new CacheStrategy(request, null);
  }
     //对Https判断
  // Drop the cached response if it's missing a required handshake.
  if (request.isHttps() && cacheResponse.handshake() == null) {
    return new CacheStrategy(request, null);
  }

  // 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.
  //判断要不要缓存 根据缓存策略来的
  if (!isCacheable(cacheResponse, request)) {
    return new CacheStrategy(request, null);
  }

  CacheControl requestCaching = request.cacheControl();
   //判断是否不缓存
  if (requestCaching.noCache() || hasConditions(request)) {
    return new CacheStrategy(request, null);
  }

  CacheControl responseCaching = cacheResponse.cacheControl();
  if (responseCaching.immutable()) {
    return new CacheStrategy(null, cacheResponse);
  }

  long ageMillis = cacheResponseAge();
  long freshMillis = computeFreshnessLifetime();

  if (requestCaching.maxAgeSeconds() != -1) {
    freshMillis = Math.min(freshMillis, SECONDS.toMillis(requestCaching.maxAgeSeconds()));
  }
      //计算过期时间
  long minFreshMillis = 0;
  if (requestCaching.minFreshSeconds() != -1) {
    minFreshMillis = SECONDS.toMillis(requestCaching.minFreshSeconds());
  }

  long maxStaleMillis = 0;
  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());
  }
    //到了这一步 代表缓存已经过期  需要把上一次的请求返回服务器 在下次请求返回过去
    //If-Modified-Since
  // 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);
}