问题描述：
最近在写一个通信相关的项目，服务器端和客户端通过socket进行通信。本来想利用read的阻塞特性，服务器端和客户端按照一定的流程进行文件读写。结果发现客户端或者服务器read方法一直都返回乱码。而且读到的一端可能是客户端，可能是服务器端，固定的读到前面有四个字节的乱码，后续读到的字节码都是正常的。
原因分析：
开始以为是流没有正常关闭。修改了代码确保正确关闭之后，发现即使重新启动服务器和客户端，还是会固定读到四个字节乱码。后面查资料分析才找出真正的原因：由于我实现的socket通信既有字符串通信，又有对象通信。所以我在传递字符串的时候，使用的是socket.getOutputStream得到的流。而在进行对象传输的时候，我在前面的输出流外面包裹了一层ObjectOutputStream。因为我是在一开始就对socket的输出流进行了包裹，而如果用ObjectOutputStream装饰输出流，默认的会自动在流前面带上四个字节的前缀。而因为开始我发消息只是发送字符串，所以我是直接使用socket的输出流。这就导致将前面的四个字节前缀发送出去，导致最终的乱码。具体参见下面相关代码：
1 /** 2 * Creates an ObjectOutputStream that writes to the specified OutputStream. 3 * This constructor writes the serialization stream header to the 4 * underlying stream; callers may wish to flush the stream immediately to 5 * ensure that constructors for receiving ObjectInputStreams will not block 6 * when reading the header. 7 * 8 * <p>If a security manager is installed, this constructor will check for 9 * the "enableSubclassImplementation" SerializablePermission when invoked 10 * directly or indirectly by the constructor of a subclass which overrides 11 * the ObjectOutputStream.putFields or ObjectOutputStream.writeUnshared 12 * methods. 13 * 14 * @param out output stream to write to 15 * @throws IOException if an I/O error occurs while writing stream header 16 * @throws SecurityException if untrusted subclass illegally overrides 17 * security-sensitive methods 18 * @throws NullPointerException if <code>out</code> is <code>null</code> 19 * @since 1.4 20 * @see ObjectOutputStream#ObjectOutputStream() 21 * @see ObjectOutputStream#putFields() 22 * @see ObjectInputStream#ObjectInputStream(InputStream) 23 */ 24 public ObjectOutputStream(OutputStream out) throws IOException { 25 verifySubclass(); 26 bout = new BlockDataOutputStream(out); 27 handles = new HandleTable(10, (float) 3.00); 28 subs = new ReplaceTable(10, (float) 3.00); 29 enableOverride = false; 30 writeStreamHeader(); 31 bout.setBlockDataMode(true); 32 if (extendedDebugInfo) { 33 debugInfoStack = new DebugTraceInfoStack(); 34 } else { 35 debugInfoStack = null; 36 } 37 } 38 39 40 41 /** 42 * The writeStreamHeader method is provided so subclasses can append or 43 * prepend their own header to the stream. It writes the magic number and 44 * version to the stream. 45 * 46 * @throws IOException if I/O errors occur while writing to the underlying 47 * stream 48 */ 49 protected void writeStreamHeader() throws IOException { 50 bout.writeShort(STREAM_MAGIC); 51 bout.writeShort(STREAM_VERSION); 52 }
解决办法：
既然直接用ObjectOutputStream将原来的socket的输出流进行包裹之后会出现固定四个字节的乱码，那么可以考虑用原来的socket输出流进行写数据的时候，接收方固定丢弃四个字节乱码。这样虽然可以实现，但是总感觉很别扭。最终我优化了相关的读写对象方法，只是用原来socket的输出流进行对象读写，具体代码实现如下：
1 public <T> void writeObj(T obj) throws ZSocketException { 2 if (obj == null) { 3 return; 4 } 5 try(ByteArrayOutputStream byteOut = new ByteArrayOutputStream(); 6 ObjectOutputStream objOut = new ObjectOutputStream(byteOut)) { // 这个只是为了计算出对象大小而用的中介输出流 7 objOut.writeObject(obj); 8 byte[] ObjByte = byteOut.toByteArray(); 9 Header header = new Header(StringMsgType.OBJECT, ObjByte.length); 10 HeaderAnalyser analyser = new HeaderAnalyser(); 11 12 // 先写消息头，再写消息内容 13 output.write(analyser.formatHeader(header), 0, Constants.HEADER_LEN); 14 output.write(ObjByte, 0, ObjByte.length); 15 output.flush(); 16 17 } catch (IOException e) { 18 throw new ZSocketException(e); 19 } 20 } 21 22 public <T> T readObj(long len, Class<T> clazz) throws ZSocketException { 23 if (len < 0 || clazz == null) { 24 throw new IllegalArgumentException("Negative read length or null object class!"); 25 } 26 27 try(ByteArrayOutputStream out = new ByteArrayOutputStream(Constants.BUFF_SIZE)) { 28 writeData(input, out, len); 29 try (ByteArrayInputStream byteIn = new ByteArrayInputStream(out.toByteArray()); 30 ObjectInputStream objIn = new ObjectInputStream(byteIn)) { 31 @SuppressWarnings("unchecked") 32 T result = (T) objIn.readObject(); 33 return result; 34 } 35 } catch (Exception e) { 36 throw new ZSocketException(e); 37 } 38 39 }
黎明前最黑暗，成功前最绝望！
