CacheInterceptor

CacheInterceptor 即缓存拦截器，和请求的缓存控制相关；

查看 CacheInterceptor 的 intercept 方法；

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

    long now = System.currentTimeMillis();

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

    if (cache != null) { // cache 不为空
      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) { // 无网络且没有缓存，返回 504 响应无效 
      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) { // 不需要调用请求，且有缓存，返回缓存
      return cacheResponse.newBuilder()
          .cacheResponse(stripBody(cacheResponse))
          .build();
    }

    Response networkResponse = null;
    try {
      networkResponse = chain.proceed(networkRequest);// 调用下一个拦截器
    } 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) { // 缓存不为null
      if (networkResponse.code() == HTTP_NOT_MODIFIED) { //网络请求返回标志 缓存无需改变 存储原来缓存即可，同时更新相关时间
        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(); // 创建 Response

    if (cache != null) { // 设置 request 和 response 到 cache 中
      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.
        }
      }
    }

CacheInterceptor 的 intercept 方法首先会去获取 cache 里面的 Request 请求；

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

  InternalCache internalCache() {
    return cache != null ? cache.internalCache : internalCache;
  }

在 OkHttp3 中 InternalCache 默认使用的 DiskLruCache 方式进行实现；

public final class Cache implements Closeable, Flushable {
  private static final int VERSION = 201105;
  private static final int ENTRY_METADATA = 0;
  private static final int ENTRY_BODY = 1;
  private static final int ENTRY_COUNT = 2;

  final InternalCache internalCache = new InternalCache() {
    @Override public Response get(Request request) throws IOException {
      return Cache.this.get(request);
    }

    @Override public CacheRequest put(Response response) throws IOException {
      return Cache.this.put(response);
    }

    @Override public void remove(Request request) throws IOException {
      Cache.this.remove(request);
    }

    @Override public void update(Response cached, Response network) {
      Cache.this.update(cached, network);
    }

    @Override public void trackConditionalCacheHit() {
      Cache.this.trackConditionalCacheHit();
    }

    @Override public void trackResponse(CacheStrategy cacheStrategy) {
      Cache.this.trackResponse(cacheStrategy);
    }
  };

  final DiskLruCache cache;
}

所以 这里的 Cache 默认使用 DiskLruCache 存储相关 Request 和 Response 进行实现；

当通过 Cache 获取到 cacheCandidate 不为空的时候，默认会进行创建 CacheStrategy 实例；

继续查看 CacheStrategy 是什么策略来控制缓存？

CacheStrategy

    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);
          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);
          }
        }
      }
    }

如果此时获取到 Request 对应的缓存不为空，获取到 Last-Modified 、 ETag等相关数据进行赋值，这些数据后续用来判断缓存是否已经过期；

get 方法；

    public CacheStrategy get() {
      CacheStrategy candidate = getCandidate(); // 获取 CacheStrategy 

      if (candidate.networkRequest != null && request.cacheControl().onlyIfCached()) { // 没有网络的情况 而且 请求禁止缓存
        // We're forbidden from using the network and the cache is insufficient.
        return new CacheStrategy(null, null);
      }

      return candidate;
    }

    /** Returns a strategy to use assuming the request can use the network. */
    private CacheStrategy getCandidate() {
      // No cached response.
      if (cacheResponse == null) { // 对应 Request 无缓存
        return new CacheStrategy(request, null);
      }

      // Drop the cached response if it's missing a required handshake.
      if (request.isHttps() && cacheResponse.handshake() == null) { // 缺少握手
        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.
      if (!isCacheable(cacheResponse, request)) { // 客户端设置不允许缓存 Response
        return new CacheStrategy(request, null);
      }

      CacheControl requestCaching = request.cacheControl();
      if (requestCaching.noCache() || hasConditions(request)) {// 请求没有缓存
        return new CacheStrategy(request, null);
      }

      CacheControl responseCaching = cacheResponse.cacheControl();
      if (responseCaching.immutable()) { // 可以一直使用缓存
        return new CacheStrategy(null, cacheResponse);
      }

      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;
      if (!responseCaching.mustRevalidate() && requestCaching.maxStaleSeconds() != -1) {
        maxStaleMillis = SECONDS.toMillis(requestCaching.maxStaleSeconds());
      }
       //响应支持缓存
       //持续时间+最短刷新时间<上次刷新时间+最大验证时间 则可以缓存
      //现在时间(now)-已经过去的时间（sent）+可以存活的时间<最大存活时间(max-age)
      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());
      }
      //  拿到上一次缓存的相应过期标签，添加到 Request 中
      // 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);
    }

CacheStrategy 主要去做了哪些事情呢？
1 、定义了一些不会缓存的情况，例如：响应没有握手、客户端不允许缓存等，返回 response 为 null；
2、通过上一次的响应情况，根据 E-tag、lastModified等定义缓存策略，重新设置 Request 的 Builder 进行返回；

拿到了 CacheStrategy 策略实例，再次回到 intercept 方法，根据返回分析就更容易理解缓存策略了；
1、networkRequest 和 cacheResponse 都为空，说明没有网络也没有缓存，返回 504 的响应状态；
2、networkRequest 为 null ，但是 cacheResponse 不为 null，说明可以直接使用缓存返回；
3、除掉上述两种情况，进行网络请求，同时对比返回的 Response 是否需要更新缓存进行相应的处理，存入到 Cache；

Cache 流程大致示意图如下：

Cache.png