XML

XML refers to extensible markup language (E X- tensible M arkup L anguage). It is designed to transmit and store data.

1. Features

xml is platform-independent and is an independent markup language
xml is self-descriptive

Second, the role

Network data transmission
data storage
Configuration file

Three, XML tree structure

The XML document forms a tree structure that starts from the "root" and then expands to the "branches and leaves".

<?xml version="1.0" encoding="UTF-8"?>
<note>
	<to>Qs</to>
	<from>Sky</from>
	<heading>Reminder</heading>
	<body>Don't forget to date this weekend!</body>
</note>

The first line is the xml declaration, which defines the version of XML (1.0) and the encoding used (UTF-8: Universal Code, which can display various languages).

Four, grammatical points

Case sensitive
The XML document must have a root element
All XML elements must have a closing tag
XML attribute values must be quoted

Entity reference

entity	character	description
`<`	<	less than
`>`	>	greater than
`&`	&	ampersand
`'`	’	apostrophe
`"`	"	quotation mark

In XML, only the characters "<" and "&" are indeed illegal. The greater than sign is legal, but it is a good practice to use entity references instead.

Comments in XML


In XML, spaces will be preserved, and HTML will cut (combine) multiple consecutive space characters into one
XML stores newlines in LF
1. In Windows applications, line breaks are usually stored as a pair of characters: carriage return (CR) and line feed (LF).
2. In Unix and Mac OSX, LF is used to store new lines.
3. In the old Mac system, CR was used to store new lines.
4. XML stores newlines in LF.
XML naming rules
1. The name can contain letters, numbers and other characters
2. The name cannot start with a number or punctuation
3. The name cannot start with the letters xml (or XML, Xml, etc.)
4. The name cannot contain spaces
5. Repeat tag names are allowed
XML elements are extensible. One of the advantages of XML is that it can be extended without interrupting the application

Five, there are XML parsing methods in Java

5.1 SAX analysis

The analysis method is an event-driven mechanism!
SAX parser, reads the XML file line by line and parses it, and triggers an event whenever the start/end/content/attribute of a tag is parsed
We can write programs to deal with these events accordingly

Disadvantages:

One-way parsing cannot locate the document level, and cannot access different parts of the same document at the same time (because of line-by-line parsing, when the nth line is parsed, the n-1th line has been released and cannot be operated on).
Cannot know the level of the element when the event occurs, and can only maintain the parent/child relationship of the node by itself
Read-only parsing mode, cannot modify the content of the XML document.

5.2 DOM analysis

It is the official W3C standard that expresses XML documents in a platform- and language-independent way. Analysis of this structure usually requires loading the entire document and building a document tree model in memory. Programmers can complete data acquisition, modification, and deletion by manipulating the document tree.

advantage:

Documents are loaded in memory, allowing changes to data and structure
Two-way access, data can be parsed in both directions in the tree at any time.

Disadvantages: all documents are loaded in memory, which consumes a lot of resources

5.3 JDOM analysis

The purpose is to become a Java-specific document model, which simplifies the interaction with XML and is faster than using DOM. Since it is the first Java specific model, JDOM has been vigorously promoted and promoted
The JDOM document states that its purpose is to "use 20% (or less) of effort to solve 80% (or more) Java/XML problems" (20% assumed based on the learning curve)

Disadvantages: no good flexibility; no good flexibility.

5.4 DOM4J analysis

It is an intelligent branch of JDOM. It incorporates many functions beyond basic XML document representation, including integrated XPath support, XML Schema support, and event-based processing for large or streaming documents. It also provides options for constructing document representation. DOM4J is an excellent Java XML API with excellent performance, powerful functions and extremely easy-to-use characteristics. It is also an open source software.

5.5 DOM4J parsing XML

step:

Import the jar file dom4j.jar
Create an input stream pointing to an XML file

FileInputStream fis = new FileInputStream("xml文件的地址");
Create an XML reading tool object

SAXReader sr = new SAXReader();
Use the reading tool object to read the input stream of the XML document and get the document object

Document doc = sr.read(fis);
Get the root element object in the XML document through the document object

Element root = doc.getRootElement();

5.5.1 Document Object

Refers to the entire XML document loaded into memory

Common methods:

Get the root element object in the XML document through the document object

Element root = doc.getRootElement();
Add root node

Element root = doc.addElement("根节点名称");

5.5.2 Element Object

Refers to a single node in the XML document

Common methods:

Get node name

String getName();
Get node content

String getText();
Set node content

String setText();
According to the name of the child node, get the first child node object matching the name

Element element(String 子节点名称);
Get all child node objects

List elements();
Get the attribute value of the node

String attributeValue(String 属性名称);
Get the content of the child node

String elementText(String 子节点名称);
Add child node

Element addElement(String 子节点名称);
Add attributes

void addAttribute(String 属性名,String 属性值);

5.5.3 DOM4J-XPATH parsing XML

Path expression, quickly find one or a group of elements through the path
1. /: Find from the root node
2. //: Find descendant nodes from the location of the node that initiated the search
3. .: Find the current node
4. …: Find the parent node
5. @: Select attribute
Attribute usage:

[@属性名='值']

[@属性名>'值']

[@属性名<'值']

[@属性名!='值']

Steps for usage

//通过Node类的两个方法, 来完成查找:
//(Node是 Document 与 Element 的父接口)
//方法1.
//根据路径表达式, 查找匹配的单个节点
Element e = selectSingleNode("路径表达式");
//方法2.
List<Element> es = selectNodes("路径表达式");

5.6 Java generates XML

step:

Create an empty document object through the DocumentHelper

Document doc = DocumentHelper.createDocument();
Add a root node to the document object

Element root = doc.addElement("根节点名称");
Enrich our child nodes through the root node object root

Element e = root.addElement("元素名称");
Create a file output stream for storing XML files

FileOutputStream fos = new FileOutputStream("要存储的位置");
Convert the file output stream to an XML document output stream

XMLWriter xw = new XMLWriter(fos);
Write out the document

xw.write(doc);
Release resources

xw.close();

//1. 通过文档帮助器, 创建空的文档对象
Document doc = DocumentHelper.createDocument();
//2. 向文档对象中, 加入根节点对象
Element books = doc.addElement("books");
//3. 向根节点中 丰富子节点
for(int i=0;i<1000;i++) {
    
    
//向根节点中加入1000个book节点.
Element book = books.addElement("book");
//向book节点, 加入id属性
book.addAttribute("id", 1+i+"");
//向book节点中加入name和info节点
Element name = book.addElement("name");
Element info = book.addElement("info");
name.setText("苹果"+i);
info.setText("哈哈哈"+i);
}
//4. 创建文件的输出流
FileOutputStream fos = new FileOutputStream("c:\\books.xml");
//5. 将文件输出流 , 转换为XML文档输出流
XMLWriter xw = new XMLWriter(fos);
//6. 写出XML文档
xw.write(doc);
//7. 释放资源
xw.close();
System.out.println("代码执行完毕");

5.7 Use of XStream

Quickly convert objects in Java into XML strings

step:

Create XStream object

XStream x = new XStream();
Modify the node name generated by the class (the default node name is package name.class name)

x.alias("节点名称",类名.class);
Pass in object, generate XML string

String xml字符串 = x.toXML(对象);

Person p = new Person(1001, "张三", "不详");
XStream x = new XStream();
x.alias("haha", Person.class);
String xml = x.toXML(p);
System.out.println(xml);

Java learning 4-8_XML learning

XML

1. Features

Second, the role

Three, XML tree structure

Four, grammatical points

Five, there are XML parsing methods in Java

5.1 SAX analysis

5.2 DOM analysis

5.3 JDOM analysis

5.4 DOM4J analysis

5.5 DOM4J parsing XML

5.5.1 Document Object

5.5.2 Element Object

5.5.3 DOM4J-XPATH parsing XML

5.6 Java generates XML

5.7 Use of XStream

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