Android之网络请求8————OkHttp源码5:缓存相关
Android之网络请求8————OkHttp源码5:缓存相关
文章目录
- Android之网络请求8————OkHttp源码5:缓存相关
- 一.前言
- 二.Cache-Control
- 1.HTTP中的Cache-Control首部
- 2. OkHttp中的CacheControl类
- 三.Cache类
- 1.缓存响应
- 2.获取缓存
- 3.Entry
- 4.小结
- 四.缓存的使用
- 五.CacheInterceptor
- 1. intercept
- 2.缓存策略
- 六.总结
- 七.参考资料
- 八.文章索引
一.前言
这是OKHttp的源码分析第五篇,主要分析的是OKHttp的缓存相关。在前面的文章中,我们也简单写过OKHttp的缓存相关。在 Android之网络请求2————OkHttp的基本使用 中写了如何使用缓存。在Android之网络请求6————OkHttp源码3:拦截器链 中写了缓存拦截器,并在其中分析了缓存策略相关的源码。
在这里我们来详细的分析一下OKHttp的缓存相关。
二.Cache-Control
OkHttp根据HTTP头部中的CacheControl进行缓存控制。
1.HTTP中的Cache-Control首部
HTTP中关于缓存部分的可以查看这篇博客彻底弄懂HTTP缓存机制及原理,看完之后对OKHttp中关于缓存的部分有很大的帮助。
HTTP头部中的Cache-Control首部可以指示对应请求该如何获取响应,比如应该直接使用缓存的响应还是应该从网络获取响应;可以指示响应该如何缓存,比如是否应该缓存下来还是设置一个过期时间等。Cache-Control首部的一些值既可以用于请求首部又可以用于响应首部。具体的值有no-cache、nostore、max-age、s-maxage、only-if-cached等。
2. OkHttp中的CacheControl类
CacheControl类是对HTTP的Cache-Control首部的描述。CacheControl没有公共的构造方法,内部通过一个Builder进行设置值,获取值可以通过CacheControl对象进行获取。Builder中具体有如下设置方法:
CacheControl(Builder builder) { this.noCache = builder.noCache; this.noStore = builder.noStore; this.maxAgeSeconds = builder.maxAgeSeconds; this.sMaxAgeSeconds = -1; this.isPrivate = false; this.isPublic = false; this.mustRevalidate = false; this.maxStaleSeconds = builder.maxStaleSeconds; this.minFreshSeconds = builder.minFreshSeconds; this.onlyIfCached = builder.onlyIfCached; this.noTransform = builder.noTransform; this.immutable = builder.immutable; }
- noCache()
对应于“no-cache”,如果出现在响应首部,不是表示不允许对响应进行缓存,而是表示客户端需要与服务器进行再验证,进行一个额外的GET请求得到最新的响应;如果出现在请求首部,表示不适用缓存响应,即进行网络请求得到响应 - noStore()
对应于“no-store”,只能出现在响应首部,表明该响应不应该被缓存 - maxAge(int maxAge, TimeUnit timeUnit)
对应于“max-age”,设置缓存响应的最大存活时间。如果缓存响应达到了最大存活时间,那么将不会再使用而会进行网络请求 - maxStale(int maxStale,TimeUnit timeUnit)
对应于“max-stale”,缓存响应可以接受的最大过期时间,如果没有指定该参数,那么过期缓存响应将不会使用。 - minFresh(int minFresh,TimeUnit timeUnit)
对应于“min-fresh”,设置一个响应将会持续刷新的最小秒数。如果一个响应当minFresh过去后过期了,那么缓存响应不会再使用了,会进行网络请求。 - onlyIfCached()
对应于“onlyIfCached”,用于请求首部,表明该请求只接受缓存中的响应。如果缓存中没有响应,那么返回一个状态码为504的响应。
CacheControl类中还有其他方法,这里就不一一介绍了。想了解的可以去API文档查看。
三.Cache类
Cache中很多方法都是通过DiskLruCache实现的,对于DiskLruCache的使用可以参考下面两篇博客。
Android DiskLruCache完全解析,硬盘缓存的最佳方案
Android DiskLruCache 源码解析 硬盘缓存的绝佳方案
OKHttp在DiskLruCache的基础上进行了修改,将IO操作改成了OKio
在OkHttp中Cache负责将响应缓存到文件中,以便可以重用和减少带宽。
在Cache类内部又一个InternalCache的实现了类
//根据请求得到响应 final InternalCache internalCache = new InternalCache() { @Override public Response get(Request request) throws IOException { return Cache.this.get(request); } //缓存响应 @Override public CacheRequest put(Response response) throws IOException { return Cache.this.put(response); }//移出响应 @Override public void remove(Request request) throws IOException { Cache.this.remove(request); }//更新响应 @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); } };
在代码中可以看出来,ternalCache接口中的每个方法的实现都交给了外部类Cache,所以主要看Cache类中的各个方法,而Cache类的这些方法又主要交给了DiskLruCache来实现。
1.缓存响应
首先来看缓存响应的Put方法
@Nullable CacheRequest put(Response response) { //得到请求的方法 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; } //不缓存非GET方法 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; } //如果请求头中如果含有星号,也不进行缓存 if (HttpHeaders.hasVaryAll(response)) { return null; } //使用DiskLruCache进行缓冲 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; } }
上面的代码对请求进行判断,如果满足条件,则使用响应创建一个Entry,然后使用DiskLruCache写入缓存,最终返回一个CacheRequestImpl对象。cache是DiskLruCache的实例,调用edit方法传入响应的key值。下面是Key方法的实现:
public static String key(HttpUrl url) { return ByteString.encodeUtf8(url.toString()).md5().hex(); //对其请求的Url做MD5,然后获得其值。 }
然后查看edit方法
/** * Returns an editor for the entry named {@code key}, or null if another edit is in progress. */ public @Nullable Editor edit(String key) throws IOException { return edit(key, ANY_SEQUENCE_NUMBER); } synchronized Editor edit(String key, long expectedSequenceNumber) throws IOException { initialize(); //初始化 checkNotClosed(); validateKey(key); Entry entry = lruEntries.get(key);//通过key获得entry if (expectedSequenceNumber != ANY_SEQUENCE_NUMBER && (entry == null || entry.sequenceNumber != expectedSequenceNumber)) { return null; // Snapshot is stale. } if (entry != null && entry.currentEditor != null) { return null; // Another edit is in progress. // 当前cache entry正在被其他对象操作 } if (mostRecentTrimFailed || mostRecentRebuildFailed) { // The OS has become our enemy! If the trim job failed, it means we are storing more data than // requested by the user. Do not allow edits so we do not go over that limit any further. If // the journal rebuild failed, the journal writer will not be active, meaning we will not be // able to record the edit, causing file leaks. In both cases, we want to retry the clean up // so we can get out of this state! executor.execute(cleanupRunnable); return null; } // 日志接入DIRTY记录 // Flush the journal before creating files to prevent file leaks. journalWriter.writeUtf8(DIRTY).writeByte(' ').writeUtf8(key).writeByte('\n'); journalWriter.flush(); if (hasJournalErrors) { return null; // Don't edit; the journal can't be written. } if (entry == null) { entry = new Entry(key); lruEntries.put(key, entry); } Editor editor = new Editor(entry); entry.currentEditor = editor; return editor; }
在这里获得Editor后,然后调用editor.writeTo(editor),将editor写入
public void writeTo(DiskLruCache.Editor editor) throws IOException { BufferedSink sink = Okio.buffer(editor.newSink(ENTRY_METADATA)); //缓存请求有关信息 sink.writeUtf8(url) .writeByte('\n'); sink.writeUtf8(requestMethod) .writeByte('\n'); sink.writeDecimalLong(varyHeaders.size()) .writeByte('\n'); for (int i = 0, size = varyHeaders.size(); i < size; i++) { sink.writeUtf8(varyHeaders.name(i)) .writeUtf8(": ") .writeUtf8(varyHeaders.value(i)) .writeByte('\n'); } //缓存Http响应行 sink.writeUtf8(new StatusLine(protocol, code, message).toString()) .writeByte('\n'); //缓存响应首部 sink.writeDecimalLong(responseHeaders.size() + 2) .writeByte('\n'); for (int i = 0, size = responseHeaders.size(); i < size; i++) { sink.writeUtf8(responseHeaders.name(i)) .writeUtf8(": ") .writeUtf8(responseHeaders.value(i)) .writeByte('\n'); } sink.writeUtf8(SENT_MILLIS) .writeUtf8(": ") .writeDecimalLong(sentRequestMillis) .writeByte('\n'); sink.writeUtf8(RECEIVED_MILLIS) .writeUtf8(": ") .writeDecimalLong(receivedResponseMillis) .writeByte('\n'); //是Https请求,缓存握手,证书信息 if (isHttps()) { sink.writeByte('\n'); sink.writeUtf8(handshake.cipherSuite().javaName()) .writeByte('\n'); writeCertList(sink, handshake.peerCertificates()); writeCertList(sink, handshake.localCertificates()); sink.writeUtf8(handshake.tlsVersion().javaName()).writeByte('\n'); } sink.close(); }
上面的代码里面有,将响应头的头部信息还有请求头的部分信息(URL 请求方法 请求头部)进行缓存。同时对于一个请求和响应而言,缓存中的key值是请求的URL的MD5值,而value包括请求和响应部分。Entry的writeTo()方法只把请求的头部和响应的头部保存了,最关键的响应主体部分在哪里保存呢?它就在put方法的返回体CacheRequestImpl,下面是这个类的实现:
private final class CacheRequestImpl implements CacheRequest { private final DiskLruCache.Editor editor; private Sink cacheOut; private Sink body; boolean done; 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(); } }; } @Override public void abort() { synchronized (Cache.this) { if (done) { return; } done = true; writeAbortCount++; } Util.closeQuietly(cacheOut); try { editor.abort(); } catch (IOException ignored) { } } @Override public Sink body() { return body; } }
中close,abort方法会调用editor.abort和editor.commit来更新日志,editor.commit还会将dirtyFile重置为cleanFile作为稳定可用的缓存,先看adort方法
public void abort() throws IOException { synchronized (DiskLruCache.this) { if (done) { throw new IllegalStateException(); } if (entry.currentEditor == this) { completeEdit(this, false); } done = true; } }
继续来看 completeEdit()方法
synchronized void completeEdit(Editor editor, boolean success) throws IOException { Entry entry = editor.entry; if (entry.currentEditor != editor) { throw new IllegalStateException(); } // If this edit is creating the entry for the first time, every index must have a value. //如果这辑第一次创建条目,那么每个索引都必须有一个值。 if (success && !entry.readable) { for (int i = 0; i < valueCount; i++) { if (!editor.written[i]) { editor.abort(); throw new IllegalStateException("Newly created entry didn't create value for index " + i); } if (!fileSystem.exists(entry.dirtyFiles[i])) { editor.abort(); return; } } } for (int i = 0; i < valueCount; i++) { File dirty = entry.dirtyFiles[i]; if (success) { if (fileSystem.exists(dirty)) { File clean = entry.cleanFiles[i]; fileSystem.rename(dirty, clean); long oldLength = entry.lengths[i]; long newLength = fileSystem.size(clean); entry.lengths[i] = newLength; size = size - oldLength + newLength; } } else { fileSystem.delete(dirty);//若失败则删除dirtyfile } } redundantOpCount++; entry.currentEditor = null; //更新日志 if (entry.readable | success) { entry.readable = true; journalWriter.writeUtf8(CLEAN).writeByte(' '); journalWriter.writeUtf8(entry.key); entry.writeLengths(journalWriter); journalWriter.writeByte('\n'); if (success) { entry.sequenceNumber = nextSequenceNumber++; } } else { lruEntries.remove(entry.key); journalWriter.writeUtf8(REMOVE).writeByte(' '); journalWriter.writeUtf8(entry.key); journalWriter.writeByte('\n'); } journalWriter.flush(); if (size > maxSize || journalRebuildRequired()) { executor.execute(cleanupRunnable); } }
2.获取缓存
获取缓存在get方法里
@Nullable Response get(Request request) { //获得key值 String key = key(request.url()); //从DiskLruCache中得到缓存 DiskLruCache.Snapshot snapshot; Entry entry; try { snapshot = cache.get(key); if (snapshot == null) { //如果没有找到 return null; } } catch (IOException e) { // Give up because the cache cannot be read. return null; } try { entry = new Entry(snapshot.getSource(ENTRY_METADATA)); //创建entry对象 } catch (IOException e) { Util.closeQuietly(snapshot); return null; } Response response = entry.response(snapshot); //获得响应对象 if (!entry.matches(request, response)) { //如果请求和响应不匹配 Util.closeQuietly(response.body()); return null; } return response; }
3.Entry
首先来看其构造方法
Entry(Source in) throws IOException { try { BufferedSource source = Okio.buffer(in); //读取请求相关的信息 url = source.readUtf8LineStrict(); requestMethod = source.readUtf8LineStrict(); Headers.Builder varyHeadersBuilder = new Headers.Builder(); int varyRequestHeaderLineCount = readInt(source); for (int i = 0; i < varyRequestHeaderLineCount; i++) { varyHeadersBuilder.addLenient(source.readUtf8LineStrict()); } varyHeaders = varyHeadersBuilder.build(); //读响应状态行 StatusLine statusLine = StatusLine.parse(source.readUtf8LineStrict()); protocol = statusLine.protocol; code = statusLine.code; message = statusLine.message; //读响应行状态 Headers.Builder responseHeadersBuilder = new Headers.Builder(); int responseHeaderLineCount = readInt(source); for (int i = 0; i < responseHeaderLineCount; i++) { responseHeadersBuilder.addLenient(source.readUtf8LineStrict()); } String sendRequestMillisString = responseHeadersBuilder.get(SENT_MILLIS); String receivedResponseMillisString = responseHeadersBuilder.get(RECEIVED_MILLIS); responseHeadersBuilder.removeAll(SENT_MILLIS); responseHeadersBuilder.removeAll(RECEIVED_MILLIS); sentRequestMillis = sendRequestMillisString != null ? Long.parseLong(sendRequestMillisString) : 0L; receivedResponseMillis = receivedResponseMillisString != null ? Long.parseLong(receivedResponseMillisString) : 0L; responseHeaders = responseHeadersBuilder.build(); //是Https协议,读握手,证书信息 if (isHttps()) { String blank = source.readUtf8LineStrict(); if (blank.length() > 0) { throw new IOException("expected \"\" but was \"" + blank + "\""); } String cipherSuiteString = source.readUtf8LineStrict(); CipherSuite cipherSuite = CipherSuite.forJavaName(cipherSuiteString); List<Certificate> peerCertificates = readCertificateList(source); List<Certificate> localCertificates = readCertificateList(source); TlsVersion tlsVersion = !source.exhausted() ? TlsVersion.forJavaName(source.readUtf8LineStrict()) : TlsVersion.SSL_3_0; handshake = Handshake.get(tlsVersion, cipherSuite, peerCertificates, localCertificates); } else { handshake = null; } } finally { in.close(); } }
在put方法中我们知道了缓存中保存了请求的信息和响应的信息,这个构造方法主要用于从缓存中解析出各个字段。当获得这些信息后,就可以用过response() (get方法最后的调用)获得对应的响应
public Response response(DiskLruCache.Snapshot snapshot) { String contentType = responseHeaders.get("Content-Type"); String contentLength = responseHeaders.get("Content-Length"); Request cacheRequest = new Request.Builder() //缓存的请求 .url(url) .method(requestMethod, null) .headers(varyHeaders) .build(); return new Response.Builder()//缓存的响应 .request(cacheRequest) .protocol(protocol) .code(code) .message(message) .headers(responseHeaders) .body(new CacheResponseBody(snapshot, contentType, contentLength)) //获得请求体 .handshake(handshake) .sentRequestAtMillis(sentRequestMillis) .receivedResponseAtMillis(receivedResponseMillis) .build(); }
查看 CacheResponseBody类的构造方法
CacheResponseBody(final DiskLruCache.Snapshot snapshot, String contentType, String contentLength) { this.snapshot = snapshot; this.contentType = contentType; this.contentLength = contentLength; Source source = snapshot.getSource(ENTRY_BODY); bodySource = Okio.buffer(new ForwardingSource(source) { @Override public void close() throws IOException { snapshot.close(); super.close(); } }); }
上面那个是get方法是的构造方法,Entry还有一种构造方法,即将响应里的内容保存起来。
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(); }
4.小结
上面的代码,基本将cache里的内容看了个差不多,分析了缓存的取出和存入,当然还有其他的方法,都比较简单。这里就不过多分析,关于里面涉及的DiskLruCache类,我看的不是很多,之后应该会去看看它以及Okio相关的内容。
四.缓存的使用
在okHttp中,如何应用缓存。可以参考我的这篇博客 Android之网络请求2————OkHttp的基本使用
Cache的设置均在OkHttpClient的Builder中设置,有两个方法可以设置,分别是setInternalCache()和cache()方法,如下:
/** Sets the response cache to be used to read and write cached responses. */ void setInternalCache(InternalCache internalCache) { this.internalCache = internalCache; this.cache = null; } public Builder cache(Cache cache) { this.cache = cache; this.internalCache = null; return this; }
从代码中可以看出,这两个方法会互相消除彼此。在之前讲到的InternalCache类,该类是一个接口,文档中说应用不应该实现该类,所以这儿,我也明白为什么OkHttpClient为什么还提供这样一个接口。
当设置好Cache后,我们再来看下Cache的构造方法:
public Cache(File directory, long maxSize) { this(directory, maxSize, FileSystem.SYSTEM); } Cache(File directory, long maxSize, FileSystem fileSystem) { this.cache = DiskLruCache.create(fileSystem, directory, VERSION, ENTRY_COUNT, maxSize); }
可以看到暴露对外的构造方法只有两个参数,一个目录,一个最大尺寸,而其内部使用的DiskLruCache的create静态工厂方法。这里面FileSystem.SYSTEM是FileSystem接口的一个实现类,该类的各个方法使用Okio对文件I/O进行封装。
DiskLruCache的create()方法中传入的目录将会是缓存的父目录,其中ENTRY_COUNT表示每一个缓存实体中的值的个数,这儿是2。(第一个是请求头部和响应头部,第二个是响应主体部分)至此,Cache和其底层的DiskLruCache创建成功了。
五.CacheInterceptor
在Okhttp缓存的具体执行时机是在缓存拦截器中,关于这一部分在Android之网络请求6————OkHttp源码3:拦截器链 中,有比较详细的描述,这里我在简单的写一下
1. intercept
@Override public Response intercept(Chain chain) throws IOException { //得到候选缓存响应,可能为空 Response cacheCandidate = cache != null ? cache.get(chain.request()) : null; long now = System.currentTimeMillis(); //得到缓存策略 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. } // 只要缓存响应,但是缓存响应不存在,返回504错误 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(EMPTY_BODY) .sentRequestAtMillis(-1L) .receivedResponseAtMillis(System.currentTimeMillis()) .build(); } // 不使用网络,直接返回缓存响应 if (networkRequest == null) { return cacheResponse.newBuilder() .cacheResponse(stripBody(cacheResponse)) .build(); } //进行网络操作获取响应 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()); } } // 如果也有缓存响应,则需要检查缓存响应是否需要进行更新 if (cacheResponse != null) { //需要更新 if (validate(cacheResponse, networkResponse)) { Response response = cacheResponse.newBuilder() .headers(combine(cacheResponse.headers(), networkResponse.headers())) .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(); //保存缓存 if (HttpHeaders.hasBody(response)) { CacheRequest cacheRequest = maybeCache(response, networkResponse.request(), cache); response = cacheWritingResponse(cacheRequest, response); } return response; }
2.缓存策略
进入查看CacheStrategy中的Factory类(工厂类)
//CacheStrategy.Factory类//构造方法 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(); //获取响应头的各种信息 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); } } } }
继续查看Factory的get方法
//CacheStrategy.Factory类 public CacheStrategy get() { CacheStrategy candidate = getCandidate(); //如果设置取消缓存 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; }
继续查看getCandidate()方法,可以看出,在这个方法里,就是最终决定缓存策略的方法
//CacheStrategy.Factory类private CacheStrategy getCandidate() { // No cached response. //如果没有response的缓存,那就使用请求。 if (cacheResponse == null) { return new CacheStrategy(request, null); } // Drop the cached response if it's missing a required handshake. //如果请求是https的并且没有握手,那么重新请求。 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. //如果response是不该被缓存的,就请求,isCacheable()内部是根据状态码判断的。 if (!isCacheable(cacheResponse, request)) { return new CacheStrategy(request, null); } //如果请求指定不使用缓存响应,或者是可选择的,就重新请求。 CacheControl requestCaching = request.cacheControl(); if (requestCaching.noCache() || hasConditions(request)) { return new CacheStrategy(request, null); } //强制使用缓存 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()); } //如果response有缓存,并且时间比较近,添加一些头部信息后,返回request = null的策略 /(意味着虽过期,但可用,只是会在响应头添加warning) 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; //流程走到这,说明缓存已经过期了 //添加请求头:If-Modified-Since或者If-None-Match //etag与If-None-Match配合使用 //lastModified与If-Modified-Since配合使用 //前者和后者的值是相同的 //区别在于前者是响应头,后者是请求头。 //后者用于服务器进行资源比对,看看是资源是否改变了。 // 如果没有,则本地的资源虽过期还是可以用的 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的构造方法
CacheStrategy(Request networkRequest, Response cacheResponse) { this.networkRequest = networkRequest; this.cacheResponse = cacheResponse; }
六.总结
OKHttp的缓存部分,一个是设置缓存这一方面由用户(程序员自己调用),还有进行缓存的时机,在缓存拦截器中发生。在获取缓存时,主要是缓存的存和取,这两个是由DiskLruCache+Okio一同实现的,同时在缓存拦截器跟据请求来进行不同的缓存策略。
七.参考资料
深入理解OkHttp源码(四)——缓存
OkHttp 3.7源码分析(四)——缓存策略
八.文章索引
Android之网络请求1————HTTP协议
Android之网络请求2————OkHttp的基本使用
Android之网络请求3————OkHttp的拦截器和封装
Android之网络请求4————OkHttp源码1:框架
Android之网络请求5————OkHttp源码2:发送请求
Android之网络请求6————OkHttp源码3:拦截器链
Android之网络请求7————OkHttp源码4:网络操作
Android之网络请求8————OkHttp源码5:缓存相关
Android之网络请求9————Retrofit的简单使用
Android之网络请求10————Retrofit的进阶使用
Android之网络请求11————Retrofit的源码分析
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