一、前言
前面介绍了ServerCnxn，下面开始学习NIOServerCnxn。
二、NIOServerCnxn源码分析
2.1 类的继承关系
public class NIOServerCnxn extends ServerCnxn {}
说明：NIOServerCnxn继承了ServerCnxn抽象类，使用NIO来处理与客户端之间的通信，使用单线程处理。
2.2 类的内部类
1. SendBufferWriter类
private class SendBufferWriter extends Writer { private StringBuffer sb = new StringBuffer(); /** * Check if we are ready to send another chunk. * @param force force sending, even if not a full chunk */ // 是否准备好发送另一块 private void checkFlush(boolean force) { if ((force && sb.length() > 0) || sb.length() > 2048) { // 当强制发送并且sb大小大于0，或者sb大小大于2048即发送缓存 sendBufferSync(ByteBuffer.wrap(sb.toString().getBytes())); // clear our internal buffer sb.setLength(0); } } @Override public void close() throws IOException { if (sb == null) return; // 关闭之前需要强制性发送缓存 checkFlush(true); sb = null; // clear out the ref to ensure no reuse } @Override public void flush() throws IOException { checkFlush(true); } @Override public void write(char[] cbuf, int off, int len) throws IOException { sb.append(cbuf, off, len); checkFlush(false); } }
说明：该类用来将给客户端的响应进行分块，其核心方法是checkFlush方法，其源码如下
private void checkFlush(boolean force) { if ((force && sb.length() > 0) || sb.length() > 2048) { // 当强制发送并且sb大小大于0，或者sb大小大于2048即发送缓存 sendBufferSync(ByteBuffer.wrap(sb.toString().getBytes())); // clear our internal buffer sb.setLength(0); } }
说明：当需要强制发送时，sb缓冲中只要有内容就会同步发送，或者是当sb的大小超过2048（块）时就需要发送，其会调用NIOServerCnxn的sendBufferSync方法，该之后会进行分析，然后再清空sb缓冲。
2. CommandThread类
private abstract class CommandThread extends Thread { PrintWriter pw; CommandThread(PrintWriter pw) { this.pw = pw; } public void run() { try { commandRun(); } catch (IOException ie) { LOG.error("Error in running command ", ie); } finally { cleanupWriterSocket(pw); } } public abstract void commandRun() throws IOException; }
说明：该类用于处理ServerCnxn中的定义的命令，其主要逻辑定义在commandRun方法中，在子类中各自实现，这是一种典型的工厂方法，每个子类对应着一个命令，每个命令使用单独的线程进行处理。
2.3 类的属性
public class NIOServerCnxn extends ServerCnxn { // 日志 static final Logger LOG = LoggerFactory.getLogger(NIOServerCnxn.class); // ServerCnxn工厂 NIOServerCnxnFactory factory; // 针对面向流的连接套接字的可选择通道 final SocketChannel sock; // 表示 SelectableChannel 在 Selector 中注册的标记 private final SelectionKey sk; // 初始化标志 boolean initialized; // 分配四个字节缓冲区 ByteBuffer lenBuffer = ByteBuffer.allocate(4); // 赋值incomingBuffer ByteBuffer incomingBuffer = lenBuffer; // 缓冲队列 LinkedBlockingQueueoutgoingBuffers = new LinkedBlockingQueue (); // 会话超时时间 int sessionTimeout; // ZooKeeper服务器 private final ZooKeeperServer zkServer; /** * The number of requests that have been submitted but not yet responded to. */ // 已经被提交但还未响应的请求数量 int outstandingRequests; /** * This is the id that uniquely identifies the session of a client. Once * this session is no longer active, the ephemeral nodes will go away. */ // 会话ID long sessionId; // 下个会话ID static long nextSessionId = 1; int outstandingLimit = 1; private static final String ZK_NOT_SERVING = "This ZooKeeper instance is not currently serving requests"; private final static byte fourBytes[] = new byte[4]; }
说明：NIOServerCnxn维护了服务器与客户端之间的Socket通道、用于存储传输内容的缓冲区、会话ID、ZooKeeper服务器等。
2.4 类的构造函数
public NIOServerCnxn(ZooKeeperServer zk, SocketChannel sock, SelectionKey sk, NIOServerCnxnFactory factory) throws IOException { this.zkServer = zk; this.sock = sock; this.sk = sk; this.factory = factory; if (this.factory.login != null) { this.zooKeeperSaslServer = new ZooKeeperSaslServer(factory.login); } if (zk != null) { outstandingLimit = zk.getGlobalOutstandingLimit(); } sock.socket().setTcpNoDelay(true); /* set socket linger to false, so that socket close does not * block */ // 设置linger为false，以便在socket关闭时不会阻塞 sock.socket().setSoLinger(false, -1); // 获取IP地址 InetAddress addr = ((InetSocketAddress) sock.socket() .getRemoteSocketAddress()).getAddress(); // 认证信息中添加IP地址 authInfo.add(new Id("ip", addr.getHostAddress())); // 设置感兴趣的操作类型 sk.interestOps(SelectionKey.OP_READ); }
说明：在构造函数中会对Socket通道进行相应设置，如设置TCP连接无延迟、获取客户端的IP地址并将此信息进行记录，方便后续认证，最后设置SelectionKey感兴趣的操作类型为READ。
2.5 核心函数分析
1. sendBuffer函数
public void sendBuffer(ByteBuffer bb) { try { if (bb != ServerCnxnFactory.closeConn) { // 不关闭连接 // We check if write interest here because if it is NOT set, // nothing is queued, so we can try to send the buffer right // away without waking up the selector // 首先检查interestOps中是否存在WRITE操作，如果没有 // 则表示直接发送缓冲而不必先唤醒selector if ((sk.interestOps() & SelectionKey.OP_WRITE) == 0) { // 不为write操作 try { // 将缓冲写入socket sock.write(bb); } catch (IOException e) { // we are just doing best effort right now } } // if there is nothing left to send, we are done if (bb.remaining() == 0) { // bb中的内容已经被全部读取 // 统计发送包信息（调用ServerCnxn方法） packetSent(); return; } } synchronized(this.factory){ // 同步块 // Causes the first selection operation that has not yet returned to return immediately // 让第一个还没返回（阻塞）的selection操作马上返回结果 sk.selector().wakeup(); if (LOG.isTraceEnabled()) { LOG.trace("Add a buffer to outgoingBuffers, sk " + sk + " is valid: " + sk.isValid()); } // 将缓存添加至队列 outgoingBuffers.add(bb); if (sk.isValid()) { // key是否合法 // 将写操作添加至感兴趣的集合 sk.interestOps(sk.interestOps() | SelectionKey.OP_WRITE); } } } catch(Exception e) { LOG.error("Unexpected Exception: ", e); } }
说明：该函数将缓冲写入socket中，其大致处理可以分为两部分，首先会判断ByteBuffer是否为关闭连接的信号，并且当感兴趣的集合中没有写操作时，其会立刻将缓存写入socket，步骤如下
if (bb != ServerCnxnFactory.closeConn) { // 不关闭连接 // We check if write interest here because if it is NOT set, // nothing is queued, so we can try to send the buffer right // away without waking up the selector // 首先检查interestOps中是否存在WRITE操作，如果没有 // 则表示直接发送缓冲而不必先唤醒selector if ((sk.interestOps() & SelectionKey.OP_WRITE) == 0) { // 不为write操作 try { // 将缓冲写入socket sock.write(bb); } catch (IOException e) { // we are just doing best effort right now } } // if there is nothing left to send, we are done if (bb.remaining() == 0) { // bb中的内容已经被全部读取 // 统计发送包信息（调用ServerCnxn方法） packetSent(); return; } }
当缓冲区被正常的写入到socket后，会直接返回，然而，当原本就对写操作感兴趣时，其会走如下流程
synchronized(this.factory){ // 同步块 // Causes the first selection operation that has not yet returned to return immediately // 让第一个还没返回（阻塞）的selection操作马上返回结果 sk.selector().wakeup(); if (LOG.isTraceEnabled()) { LOG.trace("Add a buffer to outgoingBuffers, sk " + sk + " is valid: " + sk.isValid()); } // 将缓存添加至队列 outgoingBuffers.add(bb); if (sk.isValid()) { // key是否合法 // 将写操作添加至感兴趣的集合 sk.interestOps(sk.interestOps() | SelectionKey.OP_WRITE); } }
首先会唤醒上个被阻塞的selection操作，然后将缓冲添加至outgoingBuffers队列中，后续再进行发送。
2. doIO函数
void doIO(SelectionKey k) throws InterruptedException { try { if (isSocketOpen() == false) { // socket未开启 LOG.warn("trying to do i/o on a null socket for session:0x" + Long.toHexString(sessionId)); return; } if (k.isReadable()) { // key可读 // 将内容从socket写入incoming缓冲 int rc = sock.read(incomingBuffer); if (rc < 0) { // 流结束异常，无法从客户端读取数据 throw new EndOfStreamException( "Unable to read additional data from client sessionid 0x" + Long.toHexString(sessionId) + ", likely client has closed socket"); } if (incomingBuffer.remaining() == 0) { // 缓冲区已经写满 boolean isPayload; // 读取下个请求 if (incomingBuffer == lenBuffer) { // start of next request // 翻转缓冲区，可读 incomingBuffer.flip(); // 读取lenBuffer的前四个字节，当读取的是内容长度时则为true，否则为false isPayload = readLength(k); // 清除缓冲 incomingBuffer.clear(); } else { // 不等，因为在readLength中根据Len已经重新分配了incomingBuffer // continuation isPayload = true; } if (isPayload) { // 不为四个字母，为实际内容 // not the case for 4letterword // 读取内容 readPayload(); } else { // 四个字母，为四字母的命令 // four letter words take care // need not do anything else return; } } } if (k.isWritable()) { // key可写 // ZooLog.logTraceMessage(LOG, // ZooLog.CLIENT_DATA_PACKET_TRACE_MASK // "outgoingBuffers.size() = " + // outgoingBuffers.size()); if (outgoingBuffers.size() > 0) { // ZooLog.logTraceMessage(LOG, // ZooLog.CLIENT_DATA_PACKET_TRACE_MASK, // "sk " + k + " is valid: " + // k.isValid()); /* * This is going to reset the buffer position to 0 and the * limit to the size of the buffer, so that we can fill it * with data from the non-direct buffers that we need to * send. */ // 分配的直接缓冲 ByteBuffer directBuffer = factory.directBuffer; // 清除缓冲 directBuffer.clear(); for (ByteBuffer b : outgoingBuffers) { // 遍历 if (directBuffer.remaining() < b.remaining()) { // directBuffer的剩余空闲长度小于b的剩余空闲长度 /* * When we call put later, if the directBuffer is to * small to hold everything, nothing will be copied, * so we've got to slice the buffer if it's too big. */ // 缩小缓冲至directBuffer的大小 b = (ByteBuffer) b.slice().limit( directBuffer.remaining()); } /* * put() is going to modify the positions of both * buffers, put we don't want to change the position of * the source buffers (we'll do that after the send, if * needed), so we save and reset the position after the * copy */ // 记录b的当前position int p = b.position(); // 将b写入directBuffer directBuffer.put(b); // 设置回b的原来的position b.position(p); if (directBuffer.remaining() == 0) { // 已经写满 break; } } /* * Do the flip: limit becomes position, position gets set to * 0. This sets us up for the write. */ // 翻转缓冲区，可读 directBuffer.flip(); // 将directBuffer的内容写入socket int sent = sock.write(directBuffer); ByteBuffer bb; // Remove the buffers that we have sent while (outgoingBuffers.size() > 0) { // outgoingBuffers中还存在Buffer // 取队首元素，但并不移出 bb = outgoingBuffers.peek(); if (bb == ServerCnxnFactory.closeConn) { // 关闭连接，抛出异常 throw new CloseRequestException("close requested"); } // bb还剩余多少元素没有被发送 int left = bb.remaining() - sent; if (left > 0) { // 存在元素未被发送 /* * We only partially sent this buffer, so we update * the position and exit the loop. */ // 更新bb的position bb.position(bb.position() + sent); break; } // 发送包，调用ServerCnxn方法 packetSent(); /* We've sent the whole buffer, so drop the buffer */ // 已经发送完buffer的所有内容，移除buffer sent -= bb.remaining(); outgoingBuffers.remove(); } // ZooLog.logTraceMessage(LOG, // ZooLog.CLIENT_DATA_PACKET_TRACE_MASK, "after send, // outgoingBuffers.size() = " + outgoingBuffers.size()); } synchronized(this.factory){ // 同步块 if (outgoingBuffers.size() == 0) { // outgoingBuffers不存在buffer if (!initialized && (sk.interestOps() & SelectionKey.OP_READ) == 0) { // 未初始化并且无读请求 throw new CloseRequestException("responded to info probe"); } // 重置感兴趣的集合 sk.interestOps(sk.interestOps() & (~SelectionKey.OP_WRITE)); } else { // 重置感兴趣的集合 sk.interestOps(sk.interestOps() | SelectionKey.OP_WRITE); } } } } catch (CancelledKeyException e) { LOG.warn("Exception causing close of session 0x" + Long.toHexString(sessionId) + " due to " + e); if (LOG.isDebugEnabled()) { LOG.debug("CancelledKeyException stack trace", e); } close(); } catch (CloseRequestException e) { // expecting close to log session closure close(); } catch (EndOfStreamException e) { LOG.warn("caught end of stream exception",e); // tell user why // expecting close to log session closure close(); } catch (IOException e) { LOG.warn("Exception causing close of session 0x" + Long.toHexString(sessionId) + " due to " + e); if (LOG.isDebugEnabled()) { LOG.debug("IOException stack trace", e); } close(); } }
说明：该函数主要是进行IO处理，当传入的SelectionKey是可读时，其处理流程如下
if (k.isReadable()) { // key可读 // 将内容从socket写入incoming缓冲 int rc = sock.read(incomingBuffer); if (rc < 0) { // 流结束异常，无法从客户端读取数据 throw new EndOfStreamException( "Unable to read additional data from client sessionid 0x" + Long.toHexString(sessionId) + ", likely client has closed socket"); } if (incomingBuffer.remaining() == 0) { // 缓冲区已经写满 boolean isPayload; // 读取下个请求 if (incomingBuffer == lenBuffer) { // start of next request // 翻转缓冲区，可读 incomingBuffer.flip(); // 读取lenBuffer的前四个字节，当读取的是内容长度时则为true，否则为false isPayload = readLength(k); // 清除缓冲 incomingBuffer.clear(); } else { // 不等，因为在readLength中根据Len已经重新分配了incomingBuffer // continuation isPayload = true; } if (isPayload) { // 不为四个字母，为实际内容 // not the case for 4letterword // 读取内容 readPayload(); } else { // 四个字母，为四字母的命令 // four letter words take care // need not do anything else return; } } }
说明：首先从socket中将数据读入incomingBuffer中，再判断incomingBuffer是否与lenBuffer相等，若相等，则表示读取的是一个四个字母的命令，否则表示读取的是具体内容的长度，因为在readLength函数会根据socket中内容的长度重新分配incomingBuffer。其中，readLength函数的源码如下
private boolean readLength(SelectionKey k) throws IOException { // Read the length, now get the buffer // 读取position之后的四个字节 int len = lenBuffer.getInt(); if (!initialized && checkFourLetterWord(sk, len)) { // 未初始化并且是四个字母组成的命令 return false; } if (len < 0 || len > BinaryInputArchive.maxBuffer) { throw new IOException("Len error " + len); } if (zkServer == null) { throw new IOException("ZooKeeperServer not running"); } // 重新分配len长度的缓冲 incomingBuffer = ByteBuffer.allocate(len); return true; }
说明：首先会读取lenBuffer缓冲的position之后的四个字节，然后判断其是否是四字母的命令或者是长整形（具体内容的长度），之后再根据长度重新分配incomingBuffer大小。
同时，在调用完readLength后，会知道是否为内容，若为内容，则会调用readPayload函数来读取内容，其源码如下
private void readPayload() throws IOException, InterruptedException { // 表示还未读取完socket中内容 if (incomingBuffer.remaining() != 0) { // have we read length bytes? // 将socket的内容读入缓冲 int rc = sock.read(incomingBuffer); // sock is non-blocking, so ok if (rc < 0) { // 流结束异常，无法从客户端读取数据 throw new EndOfStreamException( "Unable to read additional data from client sessionid 0x" + Long.toHexString(sessionId) + ", likely client has closed socket"); } } // 表示已经读取完了Socket中内容 if (incomingBuffer.remaining() == 0) { // have we read length bytes? // 接收到packet packetReceived(); // 翻转缓冲区 incomingBuffer.flip(); if (!initialized) { // 未初始化 // 读取连接请求 readConnectRequest(); } else { // 读取请求 readRequest(); } // 清除缓冲 lenBuffer.clear(); // 赋值incomingBuffer，即清除incoming缓冲 incomingBuffer = lenBuffer; } }
说明：首先会将socket中的实际内容写入incomingBuffer中（已经重新分配大小），当读取完成后，则更新接收的包统计信息，之后再根据是否初始化了还确定读取连接请求还是直接请求，最后会清除缓存，并重新让incomingBuffer与lenBuffer相等，表示该读取过程结束。
而当doIO中的key为可写时，其处理流程如下
if (k.isWritable()) { // key可写 // ZooLog.logTraceMessage(LOG, // ZooLog.CLIENT_DATA_PACKET_TRACE_MASK // "outgoingBuffers.size() = " + // outgoingBuffers.size()); if (outgoingBuffers.size() > 0) { // ZooLog.logTraceMessage(LOG, // ZooLog.CLIENT_DATA_PACKET_TRACE_MASK, // "sk " + k + " is valid: " + // k.isValid()); /* * This is going to reset the buffer position to 0 and the * limit to the size of the buffer, so that we can fill it * with data from the non-direct buffers that we need to * send. */ // 分配的直接缓冲 ByteBuffer directBuffer = factory.directBuffer; // 清除缓冲 directBuffer.clear(); for (ByteBuffer b : outgoingBuffers) { // 遍历 if (directBuffer.remaining() < b.remaining()) { // directBuffer的剩余空闲长度小于b的剩余空闲长度 /* * When we call put later, if the directBuffer is to * small to hold everything, nothing will be copied, * so we've got to slice the buffer if it's too big. */ // 缩小缓冲至directBuffer的大小 b = (ByteBuffer) b.slice().limit( directBuffer.remaining()); } /* * put() is going to modify the positions of both * buffers, put we don't want to change the position of * the source buffers (we'll do that after the send, if * needed), so we save and reset the position after the * copy */ // 记录b的当前position int p = b.position(); // 将b写入directBuffer directBuffer.put(b); // 设置回b的原来的position b.position(p); if (directBuffer.remaining() == 0) { // 已经写满 break; } } /* * Do the flip: limit becomes position, position gets set to * 0. This sets us up for the write. */ // 翻转缓冲区，可读 directBuffer.flip(); // 将directBuffer的内容写入socket int sent = sock.write(directBuffer); ByteBuffer bb; // Remove the buffers that we have sent while (outgoingBuffers.size() > 0) { // outgoingBuffers中还存在Buffer // 取队首元素，但并不移出 bb = outgoingBuffers.peek(); if (bb == ServerCnxnFactory.closeConn) { // 关闭连接，抛出异常 throw new CloseRequestException("close requested"); } // bb还剩余多少元素没有被发送 int left = bb.remaining() - sent; if (left > 0) { // 存在元素未被发送 /* * We only partially sent this buffer, so we update * the position and exit the loop. */ // 更新bb的position bb.position(bb.position() + sent); break; } // 发送包，调用ServerCnxn方法 packetSent(); /* We've sent the whole buffer, so drop the buffer */ // 已经发送完buffer的所有内容，移除buffer sent -= bb.remaining(); outgoingBuffers.remove(); } // ZooLog.logTraceMessage(LOG, // ZooLog.CLIENT_DATA_PACKET_TRACE_MASK, "after send, // outgoingBuffers.size() = " + outgoingBuffers.size()); }
说明：其首先会判断outgoingBuffers中是否还有Buffer未发送，然后遍历Buffer，为提供IO效率，借助了directBuffer（64K大小），之后每次以directBuffer的大小(64K)来将缓冲的内容写入socket中发送，直至全部发送完成。
三、总结
本篇讲解了NIOServerCnxn的处理细节，其主要依托于Java的NIO相关接口来完成IO操作，也谢谢各位园友的观看~