XML concise tutorial (6)

Table of Contents

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Development History

##XMLComparison with HTML Extensible

XML Syntax details compared to HTML

XML validation DTD

XMLNamespace

XMLSyntax structure

DOM4JRead and write configuration file

About SLT

XML syntax structure

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<?xmlversion="1.0" encoding="gb2312"standalone="no"?>
<!DOCTYPEstudent SYSTEM "test.dtd">
<!--这是XML文档-->
<student>
         <name>张三</name>
         <age>24</age>
</student>

The above is A basic XML, we will use this as an example to explain the structure and syntax of the XML document in detail.

##XML

Declaration (line 1) XML

Documents always begin with a XML declaration, which indicates the XML version, document encoding and document independence information. The format is:

Version information[Encoding information][Document independence information]?>，Among them, brackets( The parts in []) represent optional information.

1.Version description:

It should be noted that there are question marks before and after. And there can be spaces between question marks and brackets, otherwise an error will occur. As of the time the author posted the blog, there were two versions of XML1.0 and 1.1 , the implementation details are different between different versions. When a low-version XML processor parses a high-version XML document, it may encounter some unrecognized symbols. or syntax, resulting in incorrect parsing results. XML is processed by writing the XML declaration at the beginning of the XML The processor can judge the XML specification that the document complies with based on the declaration. If it encounters an unsupported version, it will no longer parse and process it, and an error message will be given directly.

2.Document encoding declaration

in XML declaration You can also add document encoding information. The default is UTF-8. If you want to use Chinese, you can add encoding=# to the statement. ##”gb2312” What needs to be noted is the encoding of XML and encodingThe encoding of the attribute must correspond, otherwise the following error will be reported "Switching from the current encoding to the specified encoding is not supported. Processing resources...".

For example: Use Notepad to save XML as ANSIencoding The attribute of should be gb2312; when saved with Notepad as UTF-8encoding The attribute should be UTF-8, and the same applies to other encodings.

3.Independent document declaration

If our document does not depend on external documents, inXMLIn the declaration, you can declare that this document is independent by standalone="yes" document. If the document depends on an external document, it can be declared by standalone="no". The complete XML declaration looks like this:

Document type declaration (line 2)

Through the introduction in the previous article, we know that we can use DTD (Document Type Definition, document type definition) To standardize XML documents. XMLInherits the DTD mechanism for defining syntax rules from SGML,But DTD itself does not require compliance with XML rules,Almost all XML applications are defined using DTD .HTML has a standard DTD file,its organizational structure and all tags are It is fixed. The DTD file is also a text file, usually with ".dtd" as its extension.

And points out that the DTD used in the XML document is Declared via document type. There are two ways to declare a document type. One is to declare DTD in an external file, and the other is to declare it directly in XML The documentation gives DTD. (See the previous article for specific DTD content)

Element

In XML an element consists of a start tag, element content and an end tag. Empty elements are composed of empty element tags. Each element has a type marked by a name, and it can have a set of attribute descriptions, each of which has a name and a value.

When naming elements, please note that names starting with "xml" or any other matching characters are reserved for XML Standardization of the current or subsequent versions of the specification. In addition, you need to pay attention to the following points when naming elements:

1. The name can only use letters, underscores, or colons beginning.

2. The name cannot contain spaces.

3. Do not cause ambiguity

Comment (line 3)

Comments can help us read the document better. We can write almost any characters in the comments to help us read. However, in order to avoid ambiguity caused by the XML editor, when writing comments There are a few points that need to be noted when

1. cannot appear before the XML statement, XMLThe declaration must be the first part of the document.

2.Comments cannot appear within tags

3.Comments can surround and Hide the tags, but be aware that after commenting out the tags, ensure that the remaining text is still a well-structured XML document.

4.The string "--" (double connection characters) cannot appear in comments.

5. Comments ending with "--> are not allowed in XML "end.

Tips

In fact, the best way to judge whether the naming or comments are standardized is to use the browser to write the Open the XML file. If it can be opened, it means there is no problem with the naming. Otherwise, you need to modify the XML file according to the prompts until it can be opened normally with a browser.

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