OKHTTP分享二缓存策略

与缓存有关的Header

• Expires
  Expires: Thu, 12 Jan 2017 11:01:33 GMT
  表示到期时间，一般用在response报文中，当超过此时间响应将被认为是无效的而需要网络连接，反之直接使用缓存

• 条件GET
  客户端发送条件get请求，如果缓存是有效的，则返回304 Not Modifiled，否则才返回body。

• ETag
  ETag是对资源文件的一种摘要，当客户端第一次请求某个对象，服务器在响应头返回
  ETag: “5694c7ef-24dc”
  客户端再次请求时，通过发送
  If-None-Match:”5694c7ef-24dc”
  交给服务器进行判断，如果仍然可以缓存使用，服务器就直接返回304　Not Modifiled

• Vary
  Vary: *
  告诉客户端和缓存服务器不要缓存任何信息
  Vary: header-name, header-name, …
  逗号分隔的一系列http头部名称，用于确定缓存是否可用
  作用：动态服务，防止客户端误使用了用于pc端的缓存。即使请求的是相同资源，因Vary指定的首部字段不同，也必须从源服务器请求

• Cache Control
  客户端可以在HTTP**请求**中使用的标准 Cache-Control 指令
  Cache-Control: max-age=
  Cache-Control: max-stale[=]
  Cache-Control: min-fresh=
  Cache-control: no-cache
  Cache-control: no-store
  Cache-control: no-transform
  Cache-control: only-if-cached
  服务器可以在响应中使用的标准 Cache-Control 指令。
  Cache-control: must-revalidate
  Cache-control: no-cache
  Cache-control: no-store
  Cache-control: no-transform
  Cache-control: public
  Cache-control: private
  Cache-control: proxy-revalidate
  Cache-Control: max-age=
  Cache-control: s-maxage=

1) 可缓存性
public
表明其他用户也可以利用缓存。
private
表明缓存只对单个用户有效，不能作为共享缓存。
no-cache
强制所有缓存了该响应的缓存用户，在使用已存储的缓存数据前，发送带验证的请求到原始服务器
no-store
缓存不应存储有关客户端请求或服务器响应的任何内容。
only-if-cached
表明客户端只接受已缓存的响应，并且不要向原始服务器检查是否有更新的拷贝(相当于禁止使用网络连接)
2) 到期
max-age=
设置缓存存储的最大周期，超过这个时间缓存被认为过期(单位秒)。与Expires相反，时间是相对于请求的时间。
s-maxage=
覆盖max-age 或者 Expires 头，但是仅适用于共享缓存(比如各个代理)，并且私有缓存中它被忽略。
max-stale[=]
表明客户端愿意接收一个已经过期的资源，且可选地指定响应不能超过的过时时间。
min-fresh=
表示客户端希望在指定的时间内获取最新的响应。
3) 有效性
must-revalidate
缓存必须在使用之前验证旧资源的状态，并且不可使用过期资源。
proxy-revalidate
与must-revalidate作用相同，但它仅适用于共享缓存（例如代理），并被私有缓存忽略。
immutable
表示响应正文不会随时间而改变。资源（如果未过期）在服务器上不发生改变，因此客户端不应发送重新验证请求头（例如If-None-Match或If-Modified-Since）来检查更新，即使用户显式地刷新页面。

缓存策略

CacheInterceptor

@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 (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;
  }

主要做三件事：

• 根据Request和之前缓存的Response得到CacheStrategy
• 根据CacheStrategy决定是请求网络还是直接返回缓存
• 如果2中决定请求网络，则在这一步将返回的网络响应和本地缓存对比，对本地缓存进行增删改操作

CacheStrategy

public final class CacheStrategy {
  /** The request to send on the network, or null if this call doesn't use the network. */
  public final @Nullable Request networkRequest;

  /** The cached response to return or validate; or null if this call doesn't use a cache. */
  public final @Nullable Response cacheResponse;

  CacheStrategy(Request networkRequest, Response cacheResponse) {
    this.networkRequest = networkRequest;
    this.cacheResponse = cacheResponse;
  }
  ...
}

• 作用：将“请求”和旧的缓存进行分析比较，决定是发起网络请求还是直接使用缓存。具体来说，根据networkRequest和cacheResponse是否为空执行不同的动作

networkRequest	cacheResponse	结果
null	null	禁止进行网络请求，但缓存不存在或者过期，只能返回503错误
null	non-null	缓存可以使用，直接返回缓存，不用请求网络
non-null	null	缓存不存在或者过期，直接访问网络
non-null	non-null	条件get，请求网络

CacheStrategy的加工过程

主要是读取请求头和响应头中有关缓存的HTTP 字段生成CacheStrategy对象，可结合开头与缓存有关的Header查看源码，这里不再赘述

缓存UML类图

Cache

• OkHttp的缓存“门面”，对外提供增删改查方法
• 通过内部的DiskLruCache来管理缓存对象
• 每一条缓存记录的key是url的md5值

OkHttp的缓存文件，如下图

  public Cache(File directory, long maxSize) {
    this(directory, maxSize, FileSystem.SYSTEM);
  }

    //OkHttpClient指定Cache
    public Builder cache(@Nullable Cache cache) {
      this.cache = cache;
      this.internalCache = null;
      return this;
    }

使用时需指定缓存目录和缓存大小上限

DiskLruCache

Cache内部通过DiskLruCache管理cache在文件系统层面的创建，读取，自动清理等工作

public final class DiskLruCache implements Closeable, Flushable {
  static final String JOURNAL_FILE = "journal";
  static final String JOURNAL_FILE_TEMP = "journal.tmp";
  static final String JOURNAL_FILE_BACKUP = "journal.bkp";
  static final String MAGIC = "libcore.io.DiskLruCache";
  static final String VERSION_1 = "1";
  static final long ANY_SEQUENCE_NUMBER = -1;
  static final Pattern LEGAL_KEY_PATTERN = Pattern.compile("[a-z0-9_-]{1,120}");
  private static final String CLEAN = "CLEAN";//缓存记录的4种状态
  private static final String DIRTY = "DIRTY";
  private static final String REMOVE = "REMOVE";
  private static final String READ = "READ";
    final FileSystem fileSystem;
  final File directory;
  private final File journalFile;//缓存日志
  private final File journalFileTmp;
  private final File journalFileBackup;
  private final int appVersion;
  private long maxSize;
  final int valueCount;
  private long size = 0;
  BufferedSink journalWriter;
  final LinkedHashMap lruEntries = new LinkedHashMap<>(0, 0.75f, true);//Entry是缓存文件的描述
  ...
  }

DiskLruCache通过journal文件和lruEntries（LinkedHashMap）共同管理缓存文件

journal

作用：
- 在进程启动时重建DiskLruCache（lruEntries），将磁盘中的缓存文件和url对应关系加载到内存中
- 记录和跟踪缓存文件的状态
- 保证对缓存文件读写操作的原子性

• 前5行固定不变，分别为：常量：libcore.io.DiskLruCache；diskLruCache版本；应用程序版本；valueCount(表示一个Entry对应的文件数量，在Cache中为2)，空行
• 接下来每一行对应一个cache entry的一次状态记录，其格式为：[状态（DIRTY,CLEAN,READ,REMOVE），key（url的md5值），文件大小(两个文件：响应头和响应体)]。中间以空格隔开。
• DIRTY 表示缓存正在被插入、更新或删除，在磁盘中操作成功后会有一条对应的CLEAN或REMOVE记录。否则该DIRTY记录无效（操作未成功，被异常中断过）。相当于DIRTY对应的只是临时文件。
• CLEAN 表示该条缓存是一个有效的记录，可以正常读取(get)。
• READ 表示该条缓存最近被读取过
• REMOVE 表示该条记录对应的缓存文件已经被删除了

DiskLruCache.Entry

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;

• Entry是缓存文件在文件系统层面的引用
• key是url的md5值
• 每个Entry可以对应多个文件，具体由DiskLruCache的valueCount决定，默认是2（响应头和响应体）
• cleanFiles代表正常有效的可读缓存，dirtyFiles表示缓存文件正在被创建或更新（但还没完成，只是临时文件），操作完成后会将dirtyFiles重命名为cleanFiles，并将旧的cleanFiles删除

    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);
      }
    }

• 生成Entry对象的同时，生成以url的md5命名的File对象（但缓存文件还未写入磁盘）。文件名如下图：
  

  DiskLruCache的初始化

public synchronized void initialize() throws IOException {
    assert Thread.holdsLock(this);

    if (initialized) {
      return; // Already initialized.
    }

    // If a bkp file exists, use it instead.
    if (fileSystem.exists(journalFileBackup)) {
      // If journal file also exists just delete backup file.
      if (fileSystem.exists(journalFile)) {
        fileSystem.delete(journalFileBackup);
      } else {
        fileSystem.rename(journalFileBackup, journalFile);
      }
    }

    // Prefer to pick up where we left off.
    if (fileSystem.exists(journalFile)) {
      try {
        readJournal();
        processJournal();
        initialized = true;
        return;
      } catch (IOException journalIsCorrupt) {
        Platform.get().log(WARN, "DiskLruCache " + directory + " is corrupt: "
            + journalIsCorrupt.getMessage() + ", removing", journalIsCorrupt);
      }

      // The cache is corrupted, attempt to delete the contents of the directory. This can throw and
      // we'll let that propagate out as it likely means there is a severe filesystem problem.
      try {
        delete();
      } finally {
        closed = false;
      }
    }

    rebuildJournal();

    initialized = true;
  }

• 根据journal文件重建lruEntries，并删除dirty缓存
• 如果journal文件初始化失败会重建journal文件

private void readJournal() throws IOException {
    BufferedSource source = Okio.buffer(fileSystem.source(journalFile));
    try {
      String magic = source.readUtf8LineStrict();
      String version = source.readUtf8LineStrict();
      String appVersionString = source.readUtf8LineStrict();
      String valueCountString = source.readUtf8LineStrict();
      String blank = source.readUtf8LineStrict();
      if (!MAGIC.equals(magic)
          || !VERSION_1.equals(version)
          || !Integer.toString(appVersion).equals(appVersionString)
          || !Integer.toString(valueCount).equals(valueCountString)
          || !"".equals(blank)) {
        throw new IOException("unexpected journal header: [" + magic + ", " + version + ", "
            + valueCountString + ", " + blank + "]");
      }

      int lineCount = 0;
      while (true) {
        try {
          readJournalLine(source.readUtf8LineStrict());
          lineCount++;
        } catch (EOFException endOfJournal) {
          break;
        }
      }
      redundantOpCount = lineCount - lruEntries.size();

      // If we ended on a truncated line, rebuild the journal before appending to it.
      if (!source.exhausted()) {
        rebuildJournal();
      } else {
        journalWriter = newJournalWriter();
      }
    } finally {
      Util.closeQuietly(source);
    }
  }

readJournal方法开头校验Journal文件的前面５行是否正确，如果不正确抛出异常，并在之后重建Journal文件。然后通过readJournalLine方法，逐行读取Journal文件的每条缓存记录，并更新对应Entry

  private void processJournal() throws IOException {
    fileSystem.delete(journalFileTmp);
    for (Iterator i = lruEntries.values().iterator(); i.hasNext(); ) {
      Entry entry = i.next();
      if (entry.currentEditor == null) {
        for (int t = 0; t < valueCount; t++) {
          size += entry.lengths[t];
        }
      } else {
        entry.currentEditor = null;
        for (int t = 0; t < valueCount; t++) {
          fileSystem.delete(entry.cleanFiles[t]);
          fileSystem.delete(entry.dirtyFiles[t]);
        }
        i.remove();
      }
    }
  }

processJournal方法将DIRTY（且无对应CLEAN）的entry从内存和磁盘中一并删除

Response缓存的添加

//CacheInterceptor中intercept方法
if (HttpHeaders.hasBody(response) && CacheStrategy.isCacheable(response, networkRequest)) {
        // Offer this request to the cache.
        CacheRequest cacheRequest = cache.put(response);//缓存响应头
        return cacheWritingResponse(cacheRequest, response);//缓存响应体
}

响应头的缓存

@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;
    }
  }

• 检查请求头的方法，非”get”的响应不缓存
• 响应头中包含“Vary:*”的不缓存
• 通过DiskLruCache.Editor将响应头信息缓存到磁盘中
• 生成CacheRequest对象为下一步将响应体信息缓存到磁盘中做准备

  public @Nullable Editor edit(String key) throws IOException {
    return edit(key, ANY_SEQUENCE_NUMBER);
  }

synchronized Editor edit(String key, long expectedSequenceNumber) throws IOException {
    initialize();

    checkNotClosed();
    validateKey(key);
    Entry entry = lruEntries.get(key);
    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.
    }
    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;
    }

    // Flush the journal before creating files to prevent file leaks.
    journalWriter.writeUtf8(DIRTY).writeByte(' ').writeUtf8(key).writeByte('\n');
    journalWriter.flush();//在jounar文件上添加DIRTY记录，表示该文件正在被加入缓存

    if (hasJournalErrors) {
      return null; // Don't edit; the journal can't be written.
    }

    if (entry == null) {
      entry = new Entry(key);//建立和url对应的Entry对象
      lruEntries.put(key, entry);//将Entry保存到LinkedHashMap中
    }
    Editor editor = new Editor(entry);
    entry.currentEditor = editor;
    return editor;
  }

• 在journal文件上写入DIRTY记录，表示该条缓存正在被写入
• 新建和url对应的Entry对象，将Entry保存到LinkedHashMap中
• 返回Editor对象，方便下一步真正写入文件流

DiskLruCache.Editor

  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];
    }

• 每一个DiskLruCache.Editor对象对应一个DiskLruCache.Entry对象
• 负责返回和DIRTY FILE对应的output stream(new sink(int index))及commit修改

Cache.Entry

    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();
    }

• 将Response中除了Responsebody外的信息提取出来

public void writeTo(DiskLruCache.Editor editor) throws IOException {
      BufferedSink sink = Okio.buffer(editor.newSink(ENTRY_METADATA));//获取输出流

      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();
    }

• 将除了响应体外的信息通过输出流写入到磁盘中

 private static final int ENTRY_METADATA = 0;//对应响应头
 private static final int ENTRY_BODY = 1;//对应响应体

public Sink newSink(int index) {//在valuecount为2时，index为0或1
      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);//获取临时文件xxx.0.tmp对应的输出流
        } catch (FileNotFoundException e) {
          return Okio.blackhole();
        }
        return new FaultHidingSink(sink) {
          @Override protected void onException(IOException e) {
            synchronized (DiskLruCache.this) {
              detach();
            }
          }
        };
      }
    }

获取和指定File对应的输出流

Blockquote
Sink是okio对OutputStream的封装，可简单理解为OutputStream,与之对应的还有source,是okio对InputStream的封装

CacheRequestImpl(final DiskLruCache.Editor editor) {
      this.editor = editor;
      this.cacheOut = editor.newSink(ENTRY_BODY);//拿到xxx.1.tmp
      this.body = new ForwardingSink(cacheOut) {
        @Override public void close() throws IOException {
          synchronized (Cache.this) {
            if (done) {
              return;
            }
            done = true;
            writeSuccessCount++;
          }
          super.close();
          editor.commit();
        }
      };
    }

private Response cacheWritingResponse(final CacheRequest cacheRequest, Response response)
      throws IOException {
    // Some apps return a null body; for compatibility we treat that like a null cache request.
    if (cacheRequest == null) return response;
    Sink cacheBodyUnbuffered = cacheRequest.body();
    if (cacheBodyUnbuffered == null) return response;

    final BufferedSource source = response.body().source();//从Response中取出body
    final BufferedSink cacheBody = Okio.buffer(cacheBodyUnbuffered);//通过cacheBody将文件写入存储设备中

    Source cacheWritingSource = new Source() {
      boolean cacheRequestClosed;

      @Override public long read(Buffer sink, long byteCount) throws IOException {
        long bytesRead;
        try {
          bytesRead = source.read(sink, byteCount);
        } catch (IOException e) {
          if (!cacheRequestClosed) {
            cacheRequestClosed = true;
            cacheRequest.abort(); // Failed to write a complete cache response.
          }
          throw e;
        }

        if (bytesRead == -1) {
          if (!cacheRequestClosed) {
            cacheRequestClosed = true;
            cacheBody.close(); // The cache response is complete!
          }
          return -1;
        }

        sink.copyTo(cacheBody.buffer(), sink.size() - bytesRead, bytesRead);
        cacheBody.emitCompleteSegments();
        return bytesRead;
      }

      @Override public Timeout timeout() {
        return source.timeout();
      }

      @Override public void close() throws IOException {
        if (!cacheRequestClosed
            && !discard(this, HttpCodec.DISCARD_STREAM_TIMEOUT_MILLIS, MILLISECONDS)) {
          cacheRequestClosed = true;
          cacheRequest.abort();
        }
        source.close();
      }
    };

    String contentType = response.header("Content-Type");
    long contentLength = response.body().contentLength();
    return response.newBuilder()
        .body(new RealResponseBody(contentType, contentLength, Okio.buffer(cacheWritingSource)))
        .build();
  }

这一步涉及较多okio的知识，但主要意思是将ResponseBody通过写io缓存到磁盘中

提交更改

    public void commit() throws IOException {
      synchronized (DiskLruCache.this) {
        if (done) {
          throw new IllegalStateException();
        }
        if (entry.currentEditor == this) {
          completeEdit(this, true);
        }
        done = true;
      }
    }

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.
    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;
        }
      }
    }//保证两个Dirty File都“写完”才提交

    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;
        }//将写完的Dirty File（xxx.0.tmp）重命名为Clean File的名字（xxx.0），并删除Dirty File
      } else {
        fileSystem.delete(dirty);
      }
    }

    redundantOpCount++;
    entry.currentEditor = null;
    if (entry.readable | success) {//更新journar文件
      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);
    }
  }

commit成功代表写入缓存完成

缓存的清理

boolean journalRebuildRequired() {
    final int redundantOpCompactThreshold = 2000;
    return redundantOpCount >= redundantOpCompactThreshold
        && redundantOpCount >= lruEntries.size();
  }

journal文件的缓存条目数量同时超出阈值（2000）和Entry的数量，说明journal需要重建

private final Runnable cleanupRunnable = new Runnable() {
    public void run() {
      synchronized (DiskLruCache.this) {
        if (!initialized | closed) {
          return; // Nothing to do
        }

        try {
          trimToSize();
        } catch (IOException ignored) {
          mostRecentTrimFailed = true;
        }

        try {
          if (journalRebuildRequired()) {
            rebuildJournal();
            redundantOpCount = 0;
          }
        } catch (IOException e) {
          mostRecentRebuildFailed = true;
          journalWriter = Okio.buffer(Okio.blackhole());
        }
      }
    }
  };

void trimToSize() throws IOException {
    while (size > maxSize) {
      Entry toEvict = lruEntries.values().iterator().next();
      removeEntry(toEvict);
    }
    mostRecentTrimFailed = false;
  }

在循环中不断删除“旧”文件，直到剩余缓存文件的大小总和小于DiskLruCache初始化时传入的maxSize

boolean removeEntry(Entry entry) throws IOException {
    if (entry.currentEditor != null) {
      entry.currentEditor.detach(); // Prevent the edit from completing normally.
    }

    for (int i = 0; i < valueCount; i++) {
      fileSystem.delete(entry.cleanFiles[i]);//删除缓存文件
      size -= entry.lengths[i];
      entry.lengths[i] = 0;
    }

    redundantOpCount++;
    journalWriter.writeUtf8(REMOVE).writeByte(' ').writeUtf8(entry.key).writeByte('\n');//更新journal
    lruEntries.remove(entry.key);//移除entry

    if (journalRebuildRequired()) {
      executor.execute(cleanupRunnable);
    }

    return true;
  }

重建journal

synchronized void rebuildJournal() throws IOException {
    if (journalWriter != null) {
      journalWriter.close();
    }

    BufferedSink writer = Okio.buffer(fileSystem.sink(journalFileTmp));
    try {
      writer.writeUtf8(MAGIC).writeByte('\n');
      writer.writeUtf8(VERSION_1).writeByte('\n');
      writer.writeDecimalLong(appVersion).writeByte('\n');
      writer.writeDecimalLong(valueCount).writeByte('\n');
      writer.writeByte('\n');

      for (Entry entry : lruEntries.values()) {
        if (entry.currentEditor != null) {
          writer.writeUtf8(DIRTY).writeByte(' ');
          writer.writeUtf8(entry.key);
          writer.writeByte('\n');
        } else {
          writer.writeUtf8(CLEAN).writeByte(' ');
          writer.writeUtf8(entry.key);
          entry.writeLengths(writer);
          writer.writeByte('\n');
        }
      }
    } finally {
      writer.close();
    }

    if (fileSystem.exists(journalFile)) {
      fileSystem.rename(journalFile, journalFileBackup);
    }
    fileSystem.rename(journalFileTmp, journalFile);
    fileSystem.delete(journalFileBackup);

    journalWriter = newJournalWriter();
    hasJournalErrors = false;
    mostRecentRebuildFailed = false;
  }

• 在journalFileTmp中新建journal，且只保留 DIRTY和CLEAN的记录

• journalFileTmp新建完成后重命名为journalFile，并将旧的journalFile重命名为journalFileBackup

  DiskLruCache总结

• 通过LinkedHashMap实现LRU替换
• 通过journal保证Cache操作的原子性及可用性
• 每一条缓存对应两个状态副本：DIRTY，CLEAN。CLEAN表示当前可用的Cache。DIRTY为编辑状态的cache。由于更新和创建都只操作DIRTY状态的副本，实现了读和写的分离。
• 每一个url对应四个文件，两个状态(DIRY，CLEAN)，每个状态对应两个文件：0文件对应存储meta数据，1文件存储body数据。