okhttp-源码简析
1 Request req = new Request.Builder() 2 .cacheControl(new CacheControl.Builder().noCache().build()) 3 .url("http://publicobject.com/helloworld.txt") 4 .build();//GET 5 //同步执行 6 client.newCall(req).execute(); 7 //异步执行 8 client.newCall(req).enqueue(callback);
不管同步请求还是异步请求，都需要通过newCall创建一个Call实例
1 public Call newCall(Request request) { 2 //client、originalRequest、forWebSocket 3 return new RealCall(this, request, false /* for web socket */); 4 }
同步请求
1 //RealCall的execute方法 2 public Response execute() throws IOException { 3 synchronized (this) { 4 //RealCall只能执行一次 5 if (executed) throw new IllegalStateException("Already Executed"); 6 executed = true; 7 } 8 captureCallStackTrace(); 9 try { 10 //调用runningSyncCalls.add(call)，runningSyncCalls是一个queue， 11 //finally中的finished会调用runningSyncCalls.remove(call)， 12 //runningSyncCalls的意义在于client可以统一管理同步call（异步call同样有一个queue）， 13 //比如通过runningCalls()得到正在执行的http请求（包括同步、异步），还可以通过cancelAll()取消所有http请求（包括同步、异步） 14 client.dispatcher().executed(this); 15 //getResponseWithInterceptorChain是真正的执行逻辑，从方法名可以看出okhttp将执行逻辑分隔成了一个个拦截器，使用责任链来执行。 16 Response result = getResponseWithInterceptorChain(); 17 if (result == null) throw new IOException("Canceled"); 18 return result; 19 } finally { 20 //runningSyncCalls.remove(call) 21 client.dispatcher().finished(this); 22 } 23 }
1 //getResponseWithInterceptorChain 2 Response getResponseWithInterceptorChain() throws IOException { 3 // Build a full stack of interceptors. 4 List<Interceptor> interceptors = new ArrayList<>(); 5 //自定义的interceptor 6 interceptors.addAll(client.interceptors()); 7 interceptors.add(retryAndFollowUpInterceptor); 8 interceptors.add(new BridgeInterceptor(client.cookieJar())); 9 interceptors.add(new CacheInterceptor(client.internalCache())); 10 interceptors.add(new ConnectInterceptor(client)); 11 if (!forWebSocket) { 12 interceptors.addAll(client.networkInterceptors()); 13 } 14 //拦截链的最后一个拦截器，通常来说最后一个拦截器就是真正的执行逻辑，比如tomcat的filters中最后一个filter其实就是Servlet 15 interceptors.add(new CallServerInterceptor(forWebSocket)); 16 17 Interceptor.Chain chain = new RealInterceptorChain( 18 interceptors, null, null, null, 0, originalRequest); 19 return chain.proceed(originalRequest);//执行拦截链 20 }
CallServerInterceptor的intercept方法
1 //CallServerInterceptor的intercept，有删减 2 public Response intercept(Chain chain) throws IOException { 3 RealInterceptorChain realChain = (RealInterceptorChain) chain; 4 HttpCodec httpCodec = realChain.httpStream(); 5 StreamAllocation streamAllocation = realChain.streamAllocation(); 6 RealConnection connection = (RealConnection) realChain.connection(); 7 Request request = realChain.request(); 8 //请求时间 9 long sentRequestMillis = System.currentTimeMillis(); 10 //内部会调用writeRequest(request.headers(), requestLine);，writeRquest会先写请求行，再遍历请求头并写出。 11 //这里的“写”操作，只是将字符串写入一个Buffer（后面会提到Buffer），socket还没有write。 12 httpCodec.writeRequestHeaders(request); 13 /** 什么是sink(okio.Sink接口)？sink翻译为水槽，可以理解为水的最终汇聚地，相对的有okio.Source，即水的发源地。 14 * Receives a stream of bytes. Use this interface to write data wherever it's 15 * needed: to the network, storage, or a buffer in memory. Sinks may be layered 16 * to transform received data, such as to compress, encrypt, throttle, or add 17 * protocol framing. 18 */ 19 Sink requestBodyOut = httpCodec.createRequestBody(request, request.body().contentLength()); 20 //将sink包装一层缓冲区 21 BufferedSink bufferedRequestBody = Okio.buffer(requestBodyOut); 22 //将body字节序列全部写入bufferedRequestBody，注意是全部，这里就有可能出现OOM。 23 //此时body只是写入了buffer，还未写入socket。 24 request.body().writeTo(bufferedRequestBody); 25 //close时会将buffer中的数据写入sink（即requestBodyOut），这里仍旧没有写入socket， 26 //这里我不太理解，httpCodec.createRequestBody返回的sink其实已经是一个buffer了，为什么还要套一层？ 27 bufferedRequestBody.close(); 28 29 //这里才真正写出socket，socketOutputStream.write 30 /** Flush the request to the underlying socket and signal no more bytes will be transmitted. */ 31 httpCodec.finishRequest(); 32 33 //开始构造response 34 //读响应行、响应头 35 Response.Builder responseBuilder = httpCodec.readResponseHeaders(false); 36 37 Response response = responseBuilder 38 .request(request) 39 .handshake(streamAllocation.connection().handshake()) 40 .sentRequestAtMillis(sentRequestMillis) 41 .receivedResponseAtMillis(System.currentTimeMillis()) 42 .build(); 43 44 /** 45 * httpCodec.openResponseBody会返回一个输入流，类似socketInputStream，还未从流中读取数据。 46 * 需要手动关闭 47 * Response.close() 48 * Response.body().close() 49 * Response.body().source().close() 50 * Response.body().charStream().close() 51 * Response.body().byteString().close() 52 * Response.body().bytes() 53 * Response.body().string() 54 */ 55 response = response.newBuilder() 56 .body(httpCodec.openResponseBody(response)) 57 .build(); 58 return response; 59 }
异步请求
1 //RealCall的enqueue方法 2 public void enqueue(Callback responseCallback) { 3 synchronized (this) { 4 if (executed) throw new IllegalStateException("Already Executed"); 5 executed = true; 6 } 7 captureCallStackTrace(); 8 //入队 9 client.dispatcher().enqueue(new AsyncCall(responseCallback)); 10 }
dispatcher()会返回与该okHttpClient关联的Dispatcher实例，dispatcher的enqueue方法
1 //Dispatcher的enqueue方法 2 synchronized void enqueue(AsyncCall call) { 3 if (runningAsyncCalls.size() < maxRequests && runningCallsForHost(call) < maxRequestsPerHost) { 4 //放入异步队列，与之前提到的同步队列意义相同 5 runningAsyncCalls.add(call); 6 //异步执行，通常的做法是放入executorService维护的任务队列中，然后由线程池竞争执行队列中的任务。 7 //executorService的默认构造逻辑：executorService = new java.util.concurrent.ThreadPoolExecutor(0, Integer.MAX_VALUE, 60, TimeUnit.SECONDS, 8 //new SynchronousQueue<Runnable>(), Util.threadFactory("OkHttp Dispatcher", false)); 9 //参数对应：corePoolSize,maximumPoolSize,keepAliveTime,timeUnit,workQueue,threadFactory。 10 //也可以自定义executorService： 11 //OkHttpClient okHttpClient = new OkHttpClient.Builder().dispatcher(new Dispatcher(Executors.newSingleThreadExecutor())).build(); 12 13 executorService().execute(call); 14 } else { 15 readyAsyncCalls.add(call); 16 } 17 }
AsynCall在executorService.execute中的执行逻辑是AsynCall的execute逻辑
1 //AsyncCall的执行逻辑 2 protected void execute() { 3 boolean signalledCallback = false; 4 try { 5 //getResponseWithInterceptorChain与同步调用一样 6 Response response = getResponseWithInterceptorChain(); 7 if (retryAndFollowUpInterceptor.isCanceled()) { 8 signalledCallback = true; 9 //onFailure回调 10 responseCallback.onFailure(RealCall.this, new IOException("Canceled")); 11 } else { 12 signalledCallback = true; 13 //onResponse回调 14 responseCallback.onResponse(RealCall.this, response); 15 } 16 } catch (IOException e) { 17 if (signalledCallback) { 18 // Do not signal the callback twice! 19 Platform.get().log(INFO, "Callback failure for " + toLoggableString(), e); 20 } else { 21 responseCallback.onFailure(RealCall.this, e); 22 } 23 } finally { 24 //从异步队列中删除该call 25 client.dispatcher().finished(this); 26 } 27 }
Buffer
okhttp并没有使用jdk的ByteBuffer，而是自己对byte[]进行了池化，这也使得okhttp没有机会使用DirectByteBuffer了，同时由于池化的byte[]被限定为64kb，okhttp也不适合大数据量应用。
1 //Buffer即是sink，又是source 2 public final class Buffer implements BufferedSource, BufferedSink, Cloneable { 3 //双向链表头结点 4 Segment head; 5 //...... 6 7 public void write(Buffer source, long byteCount) { 8 // Move bytes from the head of the source buffer to the tail of this buffer 9 // while balancing two conflicting goals: don't waste CPU and don't waste 10 // memory. 11 // 12 // Don't waste CPU (ie. don't copy data around). 13 // 14 // Copying large amounts of data is expensive. Instead, we prefer to 15 // reassign entire segments from one buffer to the other. 16 //内存间的复制操作是需要cpu参与的，为了避免复制，就将source中的Segment链表接到本Buffer的Segment链表尾部. 17 // 18 // Don't waste memory. 19 // 20 // As an invariant, adjacent pairs of segments in a buffer should be at 21 // least 50% full, except for the head segment and the tail segment. 22 //当Segment的利用率低于50%时，依旧需要使用System.arrayCopy将next的数据复制到pre中，也就是进行compact. 23 } 24 25 public Buffer writeUtf8(String string, int beginIndex, int endIndex) { 26 //如果调用String.getBytes必然会在内部new byte[]，这里的writeUtf8是通过String的getBytes编码string的吗？ 27 //writeUtf8并没有使用getBytes，而是自己实现了utf8的编码规则，并将字节编码序列存放在Buffer持有的data字节数组中。 28 } 29 }
1 final class Segment { 2 /** The size of all segments in bytes. */ 3 static final int SIZE = 8192; 4 5 /** Segments will be shared when doing so avoids {@code arraycopy()} of this many bytes. */ 6 static final int SHARE_MINIMUM = 1024; 7 //字节数组，真正存放数据的地方 8 final byte[] data; 9 10 /** The next byte of application data byte to read in this segment. */ 11 int pos; 12 13 /** The first byte of available data ready to be written to. */ 14 int limit; 15 16 /** True if other segments or byte strings use the same byte array. */ 17 boolean shared; 18 19 /** True if this segment owns the byte array and can append to it, extending {@code limit}. */ 20 boolean owner; 21 22 /** Next segment in a linked or circularly-linked list. */ 23 //segment可以组成一个单链表（SegmentPool中）或循环链表（Buffer中） 24 Segment next; 25 26 /** Previous segment in a circularly-linked list. */ 27 //双向链表的前指针 28 Segment prev; 29 30 Segment() { 31 this.data = new byte[SIZE]; 32 this.owner = true; 33 this.shared = false; 34 } 35 36 Segment(Segment shareFrom) { 37 this(shareFrom.data, shareFrom.pos, shareFrom.limit); 38 shareFrom.shared = true; 39 } 40 41 Segment(byte[] data, int pos, int limit) { 42 this.data = data; 43 this.pos = pos; 44 this.limit = limit; 45 this.owner = false; 46 this.shared = true; 47 } 48 //...... 49 }
1 /** 2 * A collection of unused segments, necessary to avoid GC churn and zero-fill. 3 * This pool is a thread-safe static singleton. 4 */ 5 //SegmentPool是未使用的Segment的集合，即池化的byte[] 6 final class SegmentPool { 7 /** The maximum number of bytes to pool. */ 8 // TODO: Is 64 KiB a good maximum size? Do we ever have that many idle segments? 9 //client最大持有64kb的缓冲池，超出的部分可以申请，但回收时不会被放回池中，看来okio.SegmentPool只适合小数据量应用 10 static final long MAX_SIZE = 64 * 1024; // 64 KiB. 11 12 /** Singly-linked list of segments. */ 13 //单链表 14 static @Nullable Segment next; 15 16 /** Total bytes in this pool. */ 17 static long byteCount; 18 19 private SegmentPool() { 20 } 21 //从池中获取Segment 22 static Segment take() { 23 synchronized (SegmentPool.class) { 24 if (next != null) { 25 Segment result = next; 26 next = result.next; 27 result.next = null; 28 byteCount -= Segment.SIZE; 29 return result; 30 } 31 } 32 return new Segment(); // Pool is empty. Don't zero-fill while holding a lock. 33 } 34 //回收Segment置池中 35 static void recycle(Segment segment) { 36 if (segment.next != null || segment.prev != null) throw new IllegalArgumentException(); 37 if (segment.shared) return; // This segment cannot be recycled. 38 synchronized (SegmentPool.class) { 39 if (byteCount + Segment.SIZE > MAX_SIZE) return; // Pool is full. 40 byteCount += Segment.SIZE; 41 segment.next = next; 42 segment.pos = segment.limit = 0; 43 next = segment; 44 } 45 } 46 }
Cache
http缓存，首先server需要发送带缓存首部的响应（比如Cache-Control:max-age=3600），然后client需要开启缓存功能（浏览器自动会开启，chrome在debug时可以disable cache，okhttp需要在创建okHttpClient时设置cache）。
1 int cacheSize = 10 * 1024 * 1024; // 10 MiB 2 //最终会得到一个okhttp3.internal.cache.DiskLruCache 3 Cache cache = new Cache(cacheDirectory, cacheSize); 4 OkHttpClient client = new OkHttpClient.Builder() 5 .cache(cache) 6 .build();
client缓存响应时会记录一个缓存时间（不是Date，是本地时间），响应头的max-age表明在max-age时间内，该response都是新鲜的，即不需要去server验证；响应头的no-cache表示client可以对response缓存，但client请求时必须先向server验证该缓存的新鲜度。
请求头的max-age表示client只接收max-age之内的缓存，max-stale表示client能接受最大的缓存过期时间，no-cache也是要先向server验证新鲜度，if-only-cached表示只从缓存获取数据。
1 Request request = new Request.Builder() 2 //Cache-Control:no-cache 3 //Forces caches to submit the request to the origin server for validation before releasing a cached copy. 4 //准确的讲应该叫donot-serve-from-cache-without-revalidation 5 .cacheControl(new CacheControl.Builder().noCache().build()) 6 .url("http://publicobject.com/helloworld.txt") 7 .build(); 8 9 Request request = new Request.Builder() 10 //Cache-Control:max-age=<seconds> 11 //Specifies the maximum amount of time a resource will be considered fresh. 12 //Contrary to Expires, this directive is relative to the time of the request. 13 .cacheControl(new CacheControl.Builder() 14 .maxAge(0, TimeUnit.SECONDS) 15 .build()) 16 .url("http://publicobject.com/helloworld.txt") 17 .build(); 18 19 Request request = new Request.Builder() 20 //Cache-Control:only-if-cached 21 //Indicates to not retrieve new data. The client only wishes to obtain a cached response, 22 //and should not contact the origin-server to see if a newer copy exists. 23 .cacheControl(new CacheControl.Builder() 24 .onlyIfCached() 25 .build()) 26 .url("http://publicobject.com/helloworld.txt") 27 .build(); 28 Response forceCacheResponse = client.newCall(request).execute(); 29 if (forceCacheResponse.code() != 504) { 30 // The resource was cached! Show it. 31 } else { 32 // The resource was not cached. 33 } 34 35 Request request = new Request.Builder() 36 //Cache-Control:max-stale[=<seconds>] 37 //Indicates that the client is willing to accept a response that has exceeded its expiration time. 38 //Optionally, you can assign a value in seconds, indicating the time the response must not be expired by. 39 //okhttp中max-stale必须赋值？赋0表示max-stale无效？ 40 .cacheControl(new CacheControl.Builder() 41 .maxStale(365, TimeUnit.DAYS) 42 .build()) 43 .url("http://publicobject.com/helloworld.txt") 44 .build();
如下图，是否过期由响应头max-age决定，如果响应头没有ETag、Last-Modified则直接发送原始请求。
参考：https://developer.mozilla.org/en-US/docs/Web/HTTP/Headers/Cache-Control
https://segmentfault.com/a/1190000006741200
