時間一晃而過，今天想給大家?guī)鞳kHttp的zuihou最后一篇文章，主要講一下OkHttp的緩存機制。OkHttp的責任鏈中有一個攔截器就是專門應(yīng)對OkHttp的緩存的，那就是CacheInterceptor攔截器。
CacheInterceptor
其對應(yīng)的方法如下，我們就從這個方法講起：
public Response intercept(Chain chain) throws IOException {
//假如有緩存，會得到拿到緩存，否則為null
Response cacheCandidate = cache != null
? cache.get(chain.request())
: null;
long now = System.currentTimeMillis();
//獲取緩存策略
CacheStrategy strategy = new CacheStrategy.Factory(now, chain.request(), cacheCandidate).get();
//緩存策略請求
Request networkRequest = strategy.networkRequest;
//緩存策略響應(yīng)
Response cacheResponse = strategy.cacheResponse;
//緩存非空判斷
if (cache != null) {
cache.trackResponse(strategy);
}
//本地緩存不為空并且緩存策略響應(yīng)為空
if (cacheCandidate != null && cacheResponse == null) {
closeQuietly(cacheCandidate.body()); // The cache candidate wasn't applicable. Close it.
}
//緩存策略請求和緩存策略響應(yīng)為空,禁止使用網(wǎng)絡(luò)直接返回
// 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();
}
//緩存策略請求為空，即緩存有效則直接使用緩存不使用網(wǎng)絡(luò)
// If we don't need the network, we're done.
if (networkRequest == null) {
return cacheResponse.newBuilder()
.cacheResponse(stripBody(cacheResponse))
.build();
}
//緩存無效，則執(zhí)行下一個攔截器以獲取請求
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());
}
}
//假如本地也有緩存，則根據(jù)條件選擇使用哪個響應(yīng)
// 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();
//更新緩存
cache.update(cacheResponse, response);
return response;
} else {
closeQuietly(cacheResponse.body());
}
}
//沒有緩存，則直接使用網(wǎng)絡(luò)響應(yīng)
Response response = networkResponse.newBuilder()
.cacheResponse(stripBody(cacheResponse))
.networkResponse(stripBody(networkResponse))
.build();
if (cache != null) {
//緩存到本地
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;
}
這就是整個緩存攔截器的主要方法，首先會從cache去拿緩存，沒有則返回null,然后通過CacheStrategy來獲取緩存策略，CacheStrategy根據(jù)之前緩存的結(jié)果與當前將要發(fā)送Request的header進行策略，并得出是否進行請求的結(jié)果。由于篇幅關(guān)系，這一塊不細講因為涉及網(wǎng)絡(luò)協(xié)議，最終他的得出的規(guī)則如下如:
image.png
因為我把注釋流程都寫在代碼了，大家可以看上面方法代碼理解，其整體緩存流程如下：
- 如果有緩存，則取出緩存否則為null
- 根據(jù)CacheStrategy拿到它的緩存策略請求和響應(yīng)
- 緩存策略請求和緩存策略響應(yīng)為空,禁止使用網(wǎng)絡(luò)直接返回
- 緩存策略請求為空，即緩存有效則直接使用緩存不使用網(wǎng)絡(luò)
- 緩存無效，則執(zhí)行下一個攔截器以獲取請求
- 假如本地也有緩存，則根據(jù)條件選擇使用哪個響應(yīng)，更新緩存
- 沒有緩存，則直接使用網(wǎng)絡(luò)響應(yīng)
- 添加緩存
到這里我們可以看到，緩存的“增刪改查”都是cache(Cache)類來進行操作的。下面讓我們來看一下這個類吧。
Cache
Cache的“增刪改查”其實都是基于DiskLruCache，下面我們會繼續(xù)講，先來看一下“增刪改查”的各個方法吧
- 添加緩存
CacheRequest put(Response response) {
String requestMethod = response.request().method();
//如果請求是"POST","PUT","PATCH","PROPPATCH","REPORT"則移除這些緩存
if (HttpMethod.invalidatesCache(response.request().method())) {
try {
remove(response.request());
} catch (IOException ignored) {
}
return null;
}
//僅支持GET的請求緩存，其他請求不緩存
if (!requestMethod.equals("GET")) {
return null;
}
//判斷請求中的http數(shù)據(jù)包中headers是否有符號"*"的通配符，有則不緩存
if (HttpHeaders.hasVaryAll(response)) {
return null;
}
//把response構(gòu)建成一個Entry對象
Entry entry = new Entry(response);
DiskLruCache.Editor editor = null;
try {
//生成DiskLruCache.Editor對象
editor = cache.edit(key(response.request().url()));
if (editor == null) {
return null;
}
//對緩存進行寫入
entry.writeTo(editor);
//構(gòu)建一個CacheRequestImpl類，包含Ok.io的Sink對象
return new CacheRequestImpl(editor);
} catch (IOException e) {
abortQuietly(editor);
return null;
}
}
- 得到緩存
Response get(Request request) {
//獲取url轉(zhuǎn)換過來的key
String key = key(request.url());
DiskLruCache.Snapshot snapshot;
Entry entry;
try {
//根據(jù)key獲取對應(yīng)的snapshot
snapshot = cache.get(key);
if (snapshot == null) {
return null;
}
} catch (IOException e) {
return null;
}
try {
//創(chuàng)建一個Entry對象,并由snapshot.getSource()獲取Sink
entry = new Entry(snapshot.getSource(ENTRY_METADATA));
} catch (IOException e) {
Util.closeQuietly(snapshot);
return null;
}
//通過entry和response生成respson，通過Okio.buffer獲取請求體，然后封裝各種請求信息
Response response = entry.response(snapshot);
if (!entry.matches(request, response)) {
//對request和Response進行比配檢查，成功則返回該Response。
Util.closeQuietly(response.body());
return null;
}
return response;
}
- 更新緩存
void update(Response cached, Response network) {
//用Respon構(gòu)建一個Entry
Entry entry = new Entry(network);
//從緩存中獲取DiskLruCache.Snapshot
DiskLruCache.Snapshot snapshot = ((CacheResponseBody) cached.body()).snapshot;
DiskLruCache.Editor editor = null;
try {
//獲取DiskLruCache.Snapshot.edit對象
editor = snapshot.edit(); // Returns null if snapshot is not current.
if (editor != null) {
//將entry寫入editor中
entry.writeTo(editor);
editor.commit();
}
} catch (IOException e) {
abortQuietly(editor);
}
}
- 刪除緩存
void remove(Request request) throws IOException {
//通過url轉(zhuǎn)化成的key去刪除緩存
cache.remove(key(request.url()));
}
Cache的"增刪改查"大體通過注釋代碼的方式給出，Cache還有一個更重要的緩存處理類就是DiskLruCache。
DiskLruCache
不仔細看還以為這個類和JakeWharton寫的DiskLruCache:[https://link.jianshu.com/t=https://github.com/JakeWharton/DiskLruCache(https://link.jianshu.com/t=https://github.com/JakeWharton/DiskLruCache)是一樣的，其實主體架構(gòu)差不多，只不過OkHttp的DiskLruCache結(jié)合了Ok.io，用Ok.io處理數(shù)據(jù)文件的儲存.
我們可以看到上面的DiskLruCache有shang三個內(nèi)部類，分別是Entry，Snapshot，Editor。
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;
Entry(String key) {
this.key = key;
lengths = new long[valueCount];
cleanFiles = new File[valueCount];
dirtyFiles = new File[valueCount];
// The names are repetitive so re-use the same builder to avoid allocations.
StringBuilder fileBuilder = new StringBuilder(key).append('.');
int truncateTo = fileBuilder.length();
for (int i = 0; i < valueCount; i++) {
fileBuilder.append(i);
cleanFiles[i] = new File(directory, fileBuilder.toString());
fileBuilder.append(".tmp");
dirtyFiles[i] = new File(directory, fileBuilder.toString());
fileBuilder.setLength(truncateTo);
}
}
//省略
......
實際上只是用于存儲緩存數(shù)據(jù)的實體類，一個url對應(yīng)一個實體,在Entry還有Snapshot對象，代碼如下:
Snapshot snapshot() {
if (!Thread.holdsLock(DiskLruCache.this)) throw new AssertionError();
Source[] sources = new Source[valueCount];
long[] lengths = this.lengths.clone(); // Defensive copy since these can be zeroed out.
try {
for (int i = 0; i < valueCount; i++) {
sources[i] = fileSystem.source(cleanFiles[i]);
}
return new Snapshot(key, sequenceNumber, sources, lengths);
} catch (FileNotFoundException e) {
// A file must have been deleted manually!
for (int i = 0; i < valueCount; i++) {
if (sources[i] != null) {
Util.closeQuietly(sources[i]);
} else {
break;
}
}
// Since the entry is no longer valid, remove it so the metadata is accurate (i.e. the cache
// size.)
try {
removeEntry(this);
} catch (IOException ignored) {
}
return null;
}
}
即一個Entry對應(yīng)著一個Snapshot對象，在看一下Snapshot的內(nèi)部代碼：
public final class Snapshot implements Closeable {
private final String key;
private final long sequenceNumber;
private final Source[] sources;
private final long[] lengths;
Snapshot(String key, long sequenceNumber, Source[] sources, long[] lengths) {
this.key = key;
this.sequenceNumber = sequenceNumber;
this.sources = sources;
this.lengths = lengths;
}
public String key() {
return key;
}
/**
* Returns an editor for this snapshot's entry, or null if either the entry has changed since
* this snapshot was created or if another edit is in progress.
*/
public @Nullable Editor edit() throws IOException {
return DiskLruCache.this.edit(key, sequenceNumber);
}
/** Returns the unbuffered stream with the value for {@code index}. */
public Source getSource(int index) {
return sources[index];
}
/** Returns the byte length of the value for {@code index}. */
public long getLength(int index) {
return lengths[index];
}
public void close() {
for (Source in : sources) {
Util.closeQuietly(in);
}
}
}
初始化的Snapshot僅僅只是存儲了一些變量而已。
Editor
在Editor的初始化中要傳入Editor，其實Editor就是編輯entry的類。源碼如下:
public final class Editor {
final Entry entry;
final boolean[] written;
private boolean done;
Editor(Entry entry) {
this.entry = entry;
this.written = (entry.readable) ? null : new boolean[valueCount];
}
void detach() {
if (entry.currentEditor == this) {
for (int i = 0; i < valueCount; i++) {
try {
fileSystem.delete(entry.dirtyFiles[i]);
} catch (IOException e) {
// This file is potentially leaked. Not much we can do about that.
}
}
entry.currentEditor = null;
}
}
//返回指定index的cleanFile的讀入流
public Source newSource(int index) {
synchronized (DiskLruCache.this) {
if (done) {
throw new IllegalStateException();
}
if (!entry.readable || entry.currentEditor != this) {
return null;
}
try {
return fileSystem.source(entry.cleanFiles[index]);
} catch (FileNotFoundException e) {
return null;
}
}
}
//向指定index的dirtyFiles文件寫入數(shù)據(jù)
public Sink newSink(int index) {
synchronized (DiskLruCache.this) {
if (done) {
throw new IllegalStateException();
}
if (entry.currentEditor != this) {
return Okio.blackhole();
}
if (!entry.readable) {
written[index] = true;
}
File dirtyFile = entry.dirtyFiles[index];
Sink sink;
try {
sink = fileSystem.sink(dirtyFile);
} catch (FileNotFoundException e) {
return Okio.blackhole();
}
return new FaultHidingSink(sink) {
@Override protected void onException(IOException e) {
synchronized (DiskLruCache.this) {
detach();
}
}
};
}
}
//這里執(zhí)行的工作是提交數(shù)據(jù)，并釋放鎖，最后通知DiskLruCache刷新相關(guān)數(shù)據(jù)
public void commit() throws IOException {
synchronized (DiskLruCache.this) {
if (done) {
throw new IllegalStateException();
}
if (entry.currentEditor == this) {
completeEdit(this, true);
}
done = true;
}
}
//終止編輯，并釋放鎖
public void abort() throws IOException {
synchronized (DiskLruCache.this) {
if (done) {
throw new IllegalStateException();
}
if (entry.currentEditor == this) {
completeEdit(this, false);
}
done = true;
}
}
//除非正在編輯，否則終止
public void abortUnlessCommitted() {
synchronized (DiskLruCache.this) {
if (!done && entry.currentEditor == this) {
try {
completeEdit(this, false);
} catch (IOException ignored) {
}
}
}
}
}
各個方法對應(yīng)作用如下：
- Source newSource(int index)：返回指定index的cleanFile的讀入流
- Sink newSink(int index)：向指定index的dirtyFiles文件寫入數(shù)據(jù)
- commit()：這里執(zhí)行的工作是提交數(shù)據(jù)，并釋放鎖，最后通知DiskLruCache刷新相關(guān)數(shù)據(jù)
- abort()：終止編輯，并釋放鎖
- abortUnlessCommitted()：除非正在編輯，否則終止
剩下關(guān)鍵來了，還記得上面我們講Cache添加有一行代碼entry.writeTo(editor);，里面操作如下：
public void writeTo(DiskLruCache.Editor editor) throws IOException {
BufferedSink sink = Okio.buffer(editor.newSink(ENTRY_METADATA));
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');
}
sink.writeUtf8(new StatusLine(protocol, code, message).toString())
.writeByte('\n');
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');
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();
}
上面的都是Ok.io的操作了，不懂OK.io的可以去看一下相關(guān)知識。BufferedSink sink = Okio.buffer(editor.newSink(ENTRY_METADATA));editor.newSink拿到ok.io版的OutputStream(Sink)生成Ok.io的輸入類，剩下的就是把數(shù)據(jù)用ok.io寫入文件，然后關(guān)閉輸出類。
同理我看們可以一下上面Cache獲取緩存的代碼 Response response = entry.response(snapshot);，在response方法里又有一個方法：CacheResponseBody()就是獲取緩存的方法，代碼如下：
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();
}
});
}
new ForwardingSource(source)相當于傳入ok.io版的InputStream(Source)生成Ok.io的讀取類，剩下的都是讀取緩存數(shù)據(jù)然后生成Response.
而上面Cache的Update()方法，其寫入過程也和上面的添加是一樣的，不同的只不過先構(gòu)造成一個就得Entry然后再把新的緩存寫上去更新而已，因為涉及我重要的Ok.io是一樣的，所以不細講。
剩下就是刪除了，在Cache的delete方法里，在removeEntry就是執(zhí)行刪除操作，代碼如下：
boolean removeEntry(Entry entry) throws IOException {
//省略
journalWriter.writeUtf8(REMOVE).writeByte(' ').writeUtf8(entry.key).writeByte('\n');
lruEntries.remove(entry.key);
//省略
return true;
}
上面這兩句代碼就是刪除的關(guān)鍵， journalWriter.writeUtf8表示在DiskLruCache的本地緩存清單列表里刪除，lruEntries.remove表示在緩存內(nèi)存里刪除。
到此增刪給查的流程基本結(jié)束，其實DiskLruCache還有很多可以講，但是我的重心是OKhttp的緩存底層是用Ok.io,為此在這里點到為止。
內(nèi)容有點多，如有錯誤請多多指出
