(Android) OkHttp3.10 原始碼學習筆記 8 Cahce get put分析&& CacheInterceptor分析
今天我們來分析OkHttp中的cache攔截器。使用cache的方簡單如下
OkHttpClient client = new OkHttpClient().newBuilder().cache(new Cache(new File("cache"), 24 * 1024 * 1024)).build();
進入cache類看一下,裡面有一個InternalCache, 它實現了InternalCache這個介面,實現均呼叫了cache類的方法。
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); } }
Put方法
@Nullable 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; } entry.writeTo(editor); return new CacheRequestImpl(editor); } catch (IOException e) { abortQuietly(editor); return null; } }
可以看出,OkHttp快取的核心就是DiskLruCache。首先,put方法得到了request的request的method,如果不是get方法,則不去快取。接著建立了Entry例項,這個Entry封裝了一些url,protocol等等欄位。
Entry(Response response) { this.url = response.request().url().toString(); this.varyHeaders = HttpHeaders.varyHeaders(response); this.requestMethod = response.request().method(); this.protocol = response.protocol(); this.code = response.code(); this.message = response.message(); this.responseHeaders = response.headers(); this.handshake = response.handshake(); this.sentRequestMillis = response.sentRequestAtMillis(); this.receivedResponseMillis = response.receivedResponseAtMillis(); }
接著,以url的md5值為key,寫入了快取。這裡我們有一個疑問,這裡只儲存了url,header等資訊,那最關鍵的response body儲存在哪裡了呢?我們看最後
return new CacheRequestImpl(editor);
看一下這個類,這裡儲存了body,同時這裡面還有一個lruCache
CacheRequestImpl(final DiskLruCache.Editor editor) {
this.editor = editor;
this.cacheOut = editor.newSink(ENTRY_BODY);
this.body = new ForwardingSink(cacheOut) {
@Override public void close() throws IOException {
synchronized (Cache.this) {
if (done) {
return;
}
done = true;
writeSuccessCount++;
}
super.close();
editor.commit();
}
};
}
Get方法
先看看程式碼
@Nullable Response get(Request request) {
String key = key(request.url());
DiskLruCache.Snapshot snapshot;
Entry entry;
try {
snapshot = cache.get(key);
if (snapshot == null) {
return null;
}
} catch (IOException e) {
// Give up because the cache cannot be read.
return null;
}
try {
entry = new Entry(snapshot.getSource(ENTRY_METADATA));
} catch (IOException e) {
Util.closeQuietly(snapshot);
return null;
}
Response response = entry.response(snapshot);
if (!entry.matches(request, response)) {
Util.closeQuietly(response.body());
return null;
}
return response;
}
這裡定義了一個快取快照,snapshot,表示某一時刻的快取物件。還是老套路,根據key值獲取快取物件,儲存在snapshot。然後賦值一個Entry物件,根據Entry獲取我們的快取物件,我們看一下entry的response方法
public Response response(DiskLruCache.Snapshot snapshot) {
String contentType = responseHeaders.get("Content-Type");
String contentLength = responseHeaders.get("Content-Length");
Request cacheRequest = new Request.Builder()
.url(url)
.method(requestMethod, null)
.headers(varyHeaders)
.build();
return new Response.Builder()
.request(cacheRequest)
.protocol(protocol)
.code(code)
.message(message)
.headers(responseHeaders)
.body(new CacheResponseBody(snapshot, contentType, contentLength))
.handshake(handshake)
.sentRequestAtMillis(sentRequestMillis)
.receivedResponseAtMillis(receivedResponseMillis)
.build();
}
}
這裡首先進行了一些賦值,然後根據snapshot物件構造出了body,然後返回,這樣get方法就解釋完了。
CacheInterceptor
現在我們分析主要部分,快取攔截器,主要看intercept方法。
@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;
}
首先通過request獲取ca'che,然後獲取了時間戳,然後構造了一個快取策略,看看這個快取策略是如何獲取的
public CacheStrategy get() {
CacheStrategy candidate = getCandidate();
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;
}
這個方法也是一個空殼,去看實際的getCandidate方法
/** Returns a strategy to use assuming the request can use the network. */
private CacheStrategy getCandidate() {
// No cached response.
if (cacheResponse == null) {
return new CacheStrategy(request, null);
}
// 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.
if (!isCacheable(cacheResponse, request)) {
return new CacheStrategy(request, null);
}
CacheControl requestCaching = request.cacheControl();
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()));
}
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());
}
// 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);
}
這個方法比較長,最上面,根據cacheResponse是否為null,來決定是重新請求還是取快取,後面進行了一系列判斷,來決定是重新請求還是取cacheResponse。最後,根據條件添加了一些快取header,然後返回了快取策略。
接著回去intercept方法,在取得快取策略以後,執行了 cache.trackResponse(strategy),來更新了快取命中統計。
在禁止使用網路的情況下,有如下兩種種可能
如果這裡cache是不完整的,則構造一個504錯誤的response。
如果快取正常,則返回快取response
如果需要網路請求的時候,則執行proceed方法,將網路請求交給下一個攔截器。
1.在得到response後,如果響應是304(Not modify),則從快取中讀取資料
2.判斷是否有響應體且快取策略允許寫入快取,則寫入快取,為下一次取快取準備
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); }
3. 判斷請求方式,如果是POST,PATCH等請求,則移除快取
if (HttpMethod.invalidatesCache(networkRequest.method())) { try { cache.remove(networkRequest); } catch (IOException ignored) { // The cache cannot be written. } }
雖然這個方法比較長,但邏輯比較清晰,看完原始碼後很容易理解全部過程。