为什么DataOutputStream.writeUTF（）在开头添加额外的2个字节？ [英] Why does DataOutputStream.writeUTF() add additional 2 bytes at the beginning?
问题描述
当我尝试使用sax在套接字上解析xml时，我遇到了一个奇怪的现象。
分析后，我注意到DataOutputStream在我的数据前添加了2个字节。
When I was trying to parse xml using sax over sockets I came across a strange occurence. Upon analysing I noticed that DataOutputStream adds 2 bytes in front of my data.
DataOutputStream发送的消息：
Message send by DataOutputStream:
0020 50 18 00 20 0f df 00 00 00 9d 3c 3f 78 6d 6c 20 P.. .... ..<?xml
0030 76 65 72 73 69 6f 6e 3d 22 31 2e 30 22 3f 3e 3c version= "1.0"?><
0040 63 6f 6d 70 61 6e 79 3e 3c 73 74 61 66 66 3e 3c company> <staff><
0050 66 69 72 73 74 6e 61 6d 65 3e 79 6f 6e 67 3c 2f firstnam e>yong</
0060 66 69 72 73 74 6e 61 6d 65 3e 3c 6c 61 73 74 6e firstnam e><lastn
0070 61 6d 65 3e 6d 6f 6f 6b 20 6b 69 6d 3c 2f 6c 61 ame>mook kim</la
0080 73 74 6e 61 6d 65 3e 3c 6e 69 63 6b 6e 61 6d 65 stname>< nickname
0090 3e c2 a7 3c 2f 6e 69 63 6b 6e 61 6d 65 3e 3c 73 >..</nic kname><s
00a0 61 6c 61 72 79 3e 31 30 30 30 30 30 3c 2f 73 61 alary>10 0000</sa
00b0 6c 61 72 79 3e 3c 2f 73 74 61 66 66 3e 3c 2f 63 lary></s taff></c
00c0 6f 6d 70 61 6e 79 3e ompany>
使用Transformer发送消息：
Message send using Transformer:
0020 50 18 00 20 b6 b1 00 00 3c 3f 78 6d 6c 20 76 65 P.. .... <?xml ve
0030 72 73 69 6f 6e 3d 22 31 2e 30 22 20 65 6e 63 6f rsion="1 .0" enco
0040 64 69 6e 67 3d 22 75 74 66 2d 38 22 3f 3e 3c 63 ding="ut f-8"?><c
0050 6f 6d 70 61 6e 79 3e 3c 73 74 61 66 66 3e 3c 66 ompany>< staff><f
0060 69 72 73 74 6e 61 6d 65 3e 79 6f 6e 67 3c 2f 66 irstname >yong</f
0070 69 72 73 74 6e 61 6d 65 3e 3c 6c 61 73 74 6e 61 irstname ><lastna
0080 6d 65 3e 6d 6f 6f 6b 20 6b 69 6d 3c 2f 6c 61 73 me>mook kim</las
0090 74 6e 61 6d 65 3e 3c 6e 69 63 6b 6e 61 6d 65 3e tname><n ickname>
00a0 c2 a7 3c 2f 6e 69 63 6b 6e 61 6d 65 3e 3c 73 61 ..</nick name><sa
00b0 6c 61 72 79 3e 31 30 30 30 30 30 3c 2f 73 61 6c lary>100 000</sal
00c0 61 72 79 3e 3c 2f 73 74 61 66 66 3e 3c 2f 63 6f ary></st aff></co
00d0 6d 70 61 6e 79 3e mpany>
正如人们可能会注意到DataOutputStream在消息前添加了两个字节。因此，sax解析器抛出异常org.xml.sax.SAXParseException：prolog中不允许使用内容。但是，当我跳过这两个字节时，sax解析器工作得很好。
另外我注意到DataInputStream无法读取Transformer消息。
As one might notice DataOutputStream adds two bytes in front of the message. Thus the sax parser throws the exception "org.xml.sax.SAXParseException: Content is not allowed in prolog.". However when I skip over these 2 bytes the sax parser works just fine. Additional I noticed that DataInputStream is unable to read the Transformer message.
我的问题是：为什么DataOutputStream会添加这些字节，为什么不是Transformer？
My question is: Why does DataOutputStream adds these bytes and why doesn't the Transformer?
对于那些人有兴趣在这里复制问题的是一些代码：
For those who are interested in replicating the problem here is some code:
使用DataInputStream的服务器：
Server using DataInputStream:
String data = "<?xml version=\"1.0\"?><company><staff><firstname>yong</firstname><lastname>mook kim</lastname><nickname>§</nickname><salary>100000</salary></staff></company>";
ServerSocket server = new ServerSocket(60000);
Socket socket = server.accept();
DataOutputStream os = new DataOutputStream(socket.getOutputStream());
os.writeUTF(data);
os.close();
socket.close();
使用Transformer的服务器：
Server using Transformer:
ServerSocket server = new ServerSocket(60000);
Socket socket = server.accept();
Document doc = createDocument();
printXML(doc, os);
os.close();
socket.close();
public synchronized static void printXML(Document document, OutputStream stream) throws TransformerException
{
DOMSource domSource = new DOMSource(document);
StreamResult streamResult = new StreamResult(stream);
TransformerFactory tf = TransformerFactory.newInstance();
Transformer serializer = tf.newTransformer();
serializer.setOutputProperty(OutputKeys.ENCODING, "utf-8");
serializer.setOutputProperty(OutputKeys.INDENT, "no");
serializer.transform(domSource, streamResult);
}
private static Document createDocument() throws ParserConfigurationException
{
Document document = DocumentBuilderFactory.newInstance().newDocumentBuilder().newDocument();
Element company = document.createElement("company");
Element staff = document.createElement("staff");
Element firstname = document.createElement("firstname");
Element lastname = document.createElement("lastname");
Element nickname = document.createElement("nickname");
Element salary = document.createElement("salary");
Text firstnameText = document.createTextNode("yong");
Text lastnameText = document.createTextNode("mook kim");
Text nicknameText = document.createTextNode("§");
Text salaryText = document.createTextNode("100000");
document.appendChild(company);
company.appendChild(staff);
staff.appendChild(firstname);
staff.appendChild(lastname);
staff.appendChild(nickname);
staff.appendChild(salary);
firstname.appendChild(firstnameText);
lastname.appendChild(lastnameText);
nickname.appendChild(nicknameText);
salary.appendChild(salaryText);
return document;
}
使用SAX Parser的客户：
Client using SAX Parser:
SAXParserFactory factory = SAXParserFactory.newInstance();
SAXParser saxParser = factory.newSAXParser();
DefaultHandler handler = new MyHandler();
Socket socket = new Socket("localhost", 60000);
InputSource is = new InputSource(new InputStreamReader(socket.getInputStream()));
is.setEncoding("UTF-8");
//socket.getInputStream().skip(2); // skip over the 2 bytes from the DataInputStream
saxParser.parse(is, handler);
使用DataInputStream的客户：
Client using DataInputStream:
Socket socket = new Socket("localhost", 60000);
DataInputStream os = new DataInputStream(socket.getInputStream());
while(true) {
String data = os.readUTF();
System.out.println("Data: " + data);
}
推荐答案
<$ c的输出$ c> DataOutputStream.writeUTF（）是一种自定义格式，旨在由 DataInputStream.readUTF（）读取。
The output of DataOutputStream.writeUTF() is a custom format, intended to be read by DataInputStream.readUTF().
你打电话的 writeUTF 方法的javadoc说：
The javadocs of the writeUTF method you are calling say:
使用修改后的UTF-8编码以与机器无关的方式将字符串写入基础输出流。
Writes a string to the underlying output stream using modified UTF-8 encoding in a machine-independent manner.
首先，将两个字节写入输出流，就像通过 writeShort 方法一样给出数字要跟随的字节数。此值是实际写出的字节数，而不是字符串的长度。在该长度之后，使用针对该字符的修改的UTF-8编码依次输出该字符串的每个字符。如果没有抛出异常，则计数器写入将增加写入输出流的总字节数。这至少是两个加上 str 的长度，并且最多两次加上三倍的长度 str 。
First, two bytes are written to the output stream as if by the writeShort method giving the number of bytes to follow. This value is the number of bytes actually written out, not the length of the string. Following the length, each character of the string is output, in sequence, using the modified UTF-8 encoding for the character. If no exception is thrown, the counter written is incremented by the total number of bytes written to the output stream. This will be at least two plus the length of str, and at most two plus thrice the length of str.
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