HTTP的缓存规则
可分为两大类:
- 强制缓存
- 对比缓存
不同点就是:强制缓存如果生效(有缓存数据且未失效),不需要再和服务器发生交互,,而对比缓存不管是否生效,都需要与服务端发生交互。两类缓存规则可以同时存在,强制缓存优先级高于对比缓存,也就是说,当执行强制缓存的规则时,如果缓存生效,直接使用缓存,不再执行对比缓存规则。
强制缓存
在没有缓存数据的时候,浏览器向服务器请求数据时,服务器会将数据和缓存规则一并返回,缓存规则信息包含在响应header中。响应header中会有两个字段来标明失效规则(Expires/Cache-Control)。
Expires
到期时间,它是http 1.0的东西,到期时间是由服务端生成的,但是客户端时间可能跟服务端时间有误差,这就会导致缓存命中的误差,所以http 1.1使用Cache-Control替代。(现在默认浏览器使用http 1.1)
Cache-Control
OkHttp根据HTTP头部中的CacheControl进行缓存控制。
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。
private: 客户端可以缓存 (默认为private)
public: 客户端和代理服务器都可缓存(前端的同学,可以认为public和private是一样的)
对比缓存
浏览器第一次请求数据时,服务器会将缓存标识与数据一起返回给客户端,客户端将二者备份至缓存数据库中。
再次请求数据时,客户端将从缓存数据库得到的备份的缓存标识发送给服务器,服务器根据缓存标识进行判断,判断成功后,返回304状态码,通知客户端比较成功,可以使用缓存数据。
在对比缓存生效时,服务端在进行标识比较后,只返回header部分,通过状态码通知客户端使用缓存,不再需要将报文主体部分返回给客户端。
缓存标识
缓存标识在请求header和响应header间传递,一般分为两种标识传递;Last-Modified / If-Modified-Since和Etag / If-None-Match(优先级高)
Last-Modified
服务器在响应请求时,告诉浏览器资源的最后修改时间。
If-Modified-Since
再次请求服务器时,通过此字段通知服务器上次请求时,服务器返回的资源最后修改时间。
服务器收到请求后发现有头If-Modified-Since 则与被请求资源的最后修改时间进行比对。
若资源的最后修改时间大于If-Modified-Since,说明资源又被改动过,则响应整片资源内容,返回状态码200;
若资源的最后修改时间小于或等于If-Modified-Since,说明资源无新修改,则响应HTTP 304,告知浏览器继续使用所保存的cache。
Etag
服务器响应请求时,告诉浏览器当前资源在服务器的唯一标识(生成规则由服务器决定)。
If-None-Match
再次请求服务器时,通过此字段通知服务器客户端缓存数据的唯一标识。
服务器收到请求后发现有头If-None-Match 则与被请求资源的唯一标识进行比对,
不同,说明资源又被改动过,则响应整片资源内容,返回状态码200;
相同,说明资源无新修改,则响应HTTP 304,告知浏览器继续使用所保存的cache。
Cache类
将响应缓存到文件系统中。
Cache中很多方法都是通过DiskLruCache实现的,OkHttp在DiskLruCache的基础上进行修改,将IO操作改成了Okio。
为了测量缓存效率,Cache类跟踪三个数据:
- 请求数量
- 网络命中数:需要请求进行网络请求的请求数量
- 缓存命中数:响应由缓存提供的请求的数量
Cache类的内部有一个InternalCache(本身是一个接口)的实现类。
// 接口使用匿名内部类
final InternalCache internalCache = new InternalCache() {
// 得到缓存
@Override public @Nullable Response get(Request request) throws IOException {
return Cache.this.get(request);
}
// 缓存响应
@Override public @Nullable CacheRequest put(Response response) throws IOException {
return Cache.this.put(response);
}
// 移除缓存
@Override public void remove(Request request) throws IOException {
Cache.this.remove(request);
}
// 更新缓存
@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);
}
};
InternalCache接口中每个方法的实现都交给外部类Cache来完成,看Cache类的各个方法(主要交给DiskLruCache)。
Cache # put()
缓存响应
@Nullable CacheRequest put(Response response) {
String requestMethod = response.request().method();
// 对request的方法进行校验
if (HttpMethod.invalidatesCache(response.request().method())) {
try {
// 若method为POST PATCH PUT DELETE MOVE其中一个,删除现有缓存并结束
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.
// 虽然技术上允许缓存POST请求及HEAD请求,但这样实现较为复杂且收益不高
// 因此OkHttp只允许缓存GET请求
return null;
}
// 如果响应头中含有星号,也不进行缓存
if (HttpHeaders.hasVaryAll(response)) {
return null;
}
// 使用DiskLruCache进行缓存
// 使用响应创建一个Entry(Cache中的),将响应中的内容保存起来
Entry entry = new Entry(response);
// 这个类也是不能new的,需要调用edit()来获取实例
DiskLruCache.Editor editor = null;
try {
// cache是DiskLruCache的实例
editor = cache.edit(key(response.request().url()));
if (editor == null) {
return null;
}
// 写入请求头部分信息和响应头
entry.writeTo(editor);
// 返回CacheRequestImpl对象(写入响应主体)
return new CacheRequestImpl(editor);
} catch (IOException e) {
abortQuietly(editor);
return null;
}
}
Cache # key()
public static String key(HttpUrl url) {
// 对其请求的url做MD5,然后获得其值
return ByteString.encodeUtf8(url.toString()).md5().hex();
}
DiskLruCache # edit()
来获取Editor实例
/** * 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 {
// 初始化
initialize();
checkNotClosed();
validateKey(key);
// 通过key获得Entry
Entry entry = lruEntries.get(key);
if (expectedSequenceNumber != ANY_SEQUENCE_NUMBER && (entry == null
|| entry.sequenceNumber != expectedSequenceNumber)) {
return null; // Snapshot is stale. 过期的
}
// 当前disklrucache entry正在被其他对象操作
if (entry != null && entry.currentEditor != null) {
return null; // Another edit is in progress.
}
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) {
// DiskLruCache中的Entry
entry = new Entry(key);
lruEntries.put(key, entry);
}
Editor editor = new Editor(entry);
entry.currentEditor = editor;
return editor;
}
Entry # writeTo()
只是写入请求头部分信息和响应头,响应主体主要在CacheRequestImpl中保存。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。
public void writeTo(DiskLruCache.Editor editor) throws IOException {
BufferedSink sink = Okio.buffer(editor.newSink(ENTRY_METADATA));
// 在editor的输出流中写入请求的相关信息
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');
}
// 写入响应行
sink.writeUtf8(new StatusLine(protocol, code, message).toString())
.writeByte('\n');
// 写入响应头
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,写入握手、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();
}
CacheRequestImpl
CacheRequestImpl实现了CacheRequest接口,通过这个CacheRequest接口暴露给CacheInterceptor 缓存拦截器的,然后缓存拦截器就可以直接通过CacheRequestImpl实现类来更新和写入缓存数据(响应体)。
private final class CacheRequestImpl implements CacheRequest {
private final DiskLruCache.Editor editor;
private Sink cacheOut;
private Sink body; // 写入响应体的流
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++;
}
// ForwardingSink.close()
super.close();
// editor.commit还会将dirtyFile重置为cleanFile作为稳定可用的缓存
editor.commit();
}
};
}
@Override public void abort() {
synchronized (Cache.this) {
if (done) {
return;
}
done = true;
writeAbortCount++;
}
Util.closeQuietly(cacheOut);
try {
// 调用abort()方法的话则表示放弃此次写入。
editor.abort();
} catch (IOException ignored) {
}
}
@Override public Sink body() {
return body;
}
}
Editor # commit()
更新日志,将dirtyFile重置为cleanFile作为稳定可用的缓存
public void commit() throws IOException {
synchronized (DiskLruCache.this) {
if (done) {
throw new IllegalStateException();
}
if (entry.currentEditor == this) {
// abort()第二个参数为false
completeEdit(this, true);
}
done = true;
}
}
DiskLruCache # completeEdit()
journalFile日志文件对cache每一次读写都对应一条日志记录。DiskLruCache中的每个Entry(key,cleanFiles/dirtyFiles,currentEditor)对应多个文件,其对应的文件数由DiskLruCache.valueCount指定。当前在OkHttp中valueCount为2。即每个cache对应2个cleanFiles,2个dirtyFiles。第一个cleanFiles/dirtyFiles记录cache的meta数据(如URL,创建时间,SSL握手记录等等),第二个文件记录cache的真正内容。cleanFiles记录处于稳定状态的cache结果,dirtyFiles记录处于创建或更新状态的cache
外部访问到的cache快照均为CLEAN状态,更新和创建都只操作DIRTY状态副本,实现了Cache的读写分离。
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.
// 如果这是第一次编辑条目,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;
}
}
}
// 将dirtyFile重置为cleanFile作为稳定可用的缓存
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 {
// 如果是abort()则删除dirtyfile
fileSystem.delete(dirty);
}
}
redundantOpCount++;
entry.currentEditor = null;
// 更新日志,防止日志过分膨胀定时执行日志精简
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);
}
}
Cache # get()
获取缓存
@Nullable Response get(Request request) {
// 获得key值
String key = key(request.url());
// 从DiskLruCache.Snapshot中得到缓存文件的输入流
DiskLruCache.Snapshot snapshot;
Entry entry;
try {
// cache为DiskLruCache
snapshot = cache.get(key);
// 如果没有找到
if (snapshot == null) {
return null;
}
} catch (IOException e) {
// Give up because the cache cannot be read.
return null;
}
try {
// 创建entry对象
entry = new Entry(snapshot.getSource(ENTRY_METADATA));
} catch (IOException e) {
Util.closeQuietly(snapshot);
return null;
}
// 得到response对象
Response response = entry.response(snapshot);
// 如果请求和响应不匹配
if (!entry.matches(request, response)) {
Util.closeQuietly(response.body());
return null;
}
return response;
}
DisLruCache # get()
得到对应key的snapshot
/** * Returns a snapshot of the entry named {@code key}, or null if it doesn't exist is not currently * readable. If a value is returned, it is moved to the head of the LRU queue. */
public synchronized Snapshot get(String key) throws IOException {
initialize();
checkNotClosed();
validateKey(key);
// 得到相应key的Entry对象
Entry entry = lruEntries.get(key);
if (entry == null || !entry.readable) return null;
// 得到entry的snapshot
Snapshot snapshot = entry.snapshot();
if (snapshot == null) return null;
redundantOpCount++;
// 更新日志
journalWriter.writeUtf8(READ).writeByte(' ').writeUtf8(key).writeByte('\n');
if (journalRebuildRequired()) {
// 清理线程,用于重建精简日志
executor.execute(cleanupRunnable);
}
return snapshot;
}
Cache # Entry(source)
Okio.source 封装 InputStream,读出缓存文件中的信息保存到Entry
Entry(Source in) throws IOException {
try {
BufferedSource source = Okio.buffer(in);
// 读取请求的相关信息
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;
// 读响应首部
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();
// 如果是https,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();
}
}
DisLruCache # Entry
下面是它的成员变量
private final class Entry {
final String key;
/** Lengths of this entry's files. */
final long[] lengths;
final File[] cleanFiles;
final File[] dirtyFiles;
/** True if this entry has ever been published. */
boolean readable;
/** The ongoing edit or null if this entry is not being edited. */
Editor currentEditor;
/** The sequence number of the most recently committed edit to this entry. */
long sequenceNumber;
}
Cache # Entry(response)
将响应中的内容保存起来。(没有响应体)
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();
}
Cache.Entry # response()
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() 获得对应的响应
public Response response(DiskLruCache.Snapshot snapshot) {
// 从Entry中的变量获取
String contentType = responseHeaders.get("Content-Type");
String contentLength = responseHeaders.get("Content-Length");
// 缓存的请求
Request cacheRequest = new Request.Builder()
.url(url)
.method(requestMethod, null)
.headers(varyHeaders)
.build();
// 缓存的响应
return new Response.Builder()
.request(cacheRequest)
.protocol(protocol)
.code(code)
.message(message)
.headers(responseHeaders)
// 响应体
.body(new CacheResponseBody(snapshot, contentType, contentLength))
.handshake(handshake)
.sentRequestAtMillis(sentRequestMillis)
.receivedResponseAtMillis(receivedResponseMillis)
.build();
}
CacheResponseBody
构造方法,获得响应体
CacheResponseBody(final DiskLruCache.Snapshot snapshot,
String contentType, String contentLength) {
this.snapshot = snapshot;
this.contentType = contentType;
this.contentLength = contentLength;
// 获得body的输入流
Source source = snapshot.getSource(ENTRY_BODY);
bodySource = Okio.buffer(new ForwardingSource(source) {
@Override public void close() throws IOException {
snapshot.close();
super.close();
}
});
}
缓存的使用
OkHttpClient # internalCache()
@Nullable InternalCache internalCache() {
return cache != null ? cache.internalCache : internalCache;
}
cache域在我们构造OkHttpClient的时候是没有被初始化的,因此如果我们没有通过调用Builder的cache方法设置cache值的话,该方法返回的对象实际上是一个不支持任何缓存操作的对象(接口),该对象的所有方法为空。因此如果需要OkHttpClient支持缓存,需要我们写一个Cache对象并在构造OkHttpClient的时候将其传给OkHttpClient。
构建cache
OkHttpClient client = new OkHttpClient.Builder()
.connectTimeout(5, TimeUnit.SECONDS)
.cache(new Cache(new File(this.getExternalCacheDir(), "okhttpcache"), 10 * 1024 * 1024))
.build();
OkHttpClient.Builder # cache()
public Builder cache(@Nullable Cache cache) {
this.cache = cache;
this.internalCache = null;
return this;
}
cache的构造方法
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(文件),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)
public Cache(File directory, long maxSize) {
// 文件目录,文件最大大小
this(directory, maxSize, FileSystem.SYSTEM);
}
Cache(File directory, long maxSize, FileSystem fileSystem) {
this.cache = DiskLruCache.create(fileSystem, directory, VERSION, ENTRY_COUNT, maxSize);
}