java sax解析器

SAX Parser in java provides API to parse XML documents. SAX parser is different from DOM parser because it doesn’t load complete XML into memory and read xml document sequentially.

Java中的SAX Parser提供了用于解析XML文档的API。 SAX解析器与DOM解析器不同，因为它不会将完整的XML加载到内存中并不会顺序读取xml文档。

SAX解析器 (SAX Parser)

javax.xml.parsers.SAXParser provides method to parse XML document using event handlers. This class implements XMLReader interface and provides overloaded versions of parse() methods to read XML document from File, InputStream, SAX InputSource and String URI.

javax.xml.parsers.SAXParser提供了使用事件处理程序解析XML文档的方法。 此类实现XMLReader接口，并提供parse()方法的重载版本以从File，InputStream，SAX InputSource和String URI读取XML文档。

The actual parsing is done by the Handler class. We need to create our own handler class to parse the XML document. We need to implement org.xml.sax.ContentHandler interface to create our own handler classes. This interface contains callback methods that receive notification when an event occurs. For example StartDocument, EndDocument, StartElement, EndElement, CharacterData etc.

实际的解析由Handler类完成。 我们需要创建自己的处理程序类来解析XML文档。 我们需要实现org.xml.sax.ContentHandler接口来创建我们自己的处理程序类。 此接口包含在事件发生时接收通知的回调方法。 例如StartDocument，EndDocument，StartElement，EndElement，CharacterData等。

org.xml.sax.helpers.DefaultHandler provides default implementation of ContentHandler interface and we can extend this class to create our own handler. It’s advisable to extend this class because we might need only a few of the methods to implement. Extending this class will keep our code cleaner and maintainable.

org.xml.sax.helpers.DefaultHandler提供ContentHandler接口的默认实现，我们可以扩展此类以创建我们自己的处理程序。 建议扩展此类，因为我们可能只需要实现一些方法。 扩展此类将使我们的代码更干净和可维护。

SAX解析器示例 (SAX parser Example)

Let’s jump to the SAX parser example program now, I will explain different features in detail later on.

现在让我们跳到SAX解析器示例程序，稍后我将详细解释不同的功能。

employees.xml

employees.xml

<?xml version="1.0" encoding="UTF-8"?>
<Employees><Employee id="1"><age>29</age><name>Pankaj</name><gender>Male</gender><role>Java Developer</role></Employee><Employee id="2"><age>35</age><name>Lisa</name><gender>Female</gender><role>CEO</role></Employee><Employee id="3"><age>40</age><name>Tom</name><gender>Male</gender><role>Manager</role></Employee><Employee id="4"><age>25</age><name>Meghna</name><gender>Female</gender><role>Manager</role></Employee>
</Employees>

So we have a XML file stored somewhere in file system and by looking at it, we can conclude that it contains list of Employee. Every Employee has id attribute and fields age, name, gender and role.

因此，我们将XML文件存储在文件系统中的某个位置，通过查看它，我们可以得出结论，该文件包含Employee列表。 每个员工都有id属性，并填写age ， name ， gender和role 。

We will use SAX parser to parse this XML and create a list of Employee object.

我们将使用SAX解析器来解析此XML并创建Employee对象的列表。

Here is the Employee object representing Employee element from XML.

这是代表XML中的Employee元素的Employee对象。

package com.journaldev.xml;public class Employee {private int id;private String name;private String gender;private int age;private String role;public int getId() {return id;}public void setId(int id) {this.id = id;}public String getName() {return name;}public void setName(String name) {this.name = name;}public String getGender() {return gender;}public void setGender(String gender) {this.gender = gender;}public int getAge() {return age;}public void setAge(int age) {this.age = age;}public String getRole() {return role;}public void setRole(String role) {this.role = role;}@Overridepublic String toString() {return "Employee:: ID="+this.id+" Name=" + this.name + " Age=" + this.age + " Gender=" + this.gender +" Role=" + this.role;}}

Let’s create our own SAX Parser Handler class extending DefaultHandler class.

让我们创建扩展DefaultHandler类的自己的SAX Parser Handler类。

package com.journaldev.xml.sax;import java.util.ArrayList;
import java.util.List;import org.xml.sax.Attributes;
import org.xml.sax.SAXException;
import org.xml.sax.helpers.DefaultHandler;import com.journaldev.xml.Employee;public class MyHandler extends DefaultHandler {// List to hold Employees objectprivate List<Employee> empList = null;private Employee emp = null;private StringBuilder data = null;// getter method for employee listpublic List<Employee> getEmpList() {return empList;}boolean bAge = false;boolean bName = false;boolean bGender = false;boolean bRole = false;@Overridepublic void startElement(String uri, String localName, String qName, Attributes attributes) throws SAXException {if (qName.equalsIgnoreCase("Employee")) {// create a new Employee and put it in MapString id = attributes.getValue("id");// initialize Employee object and set id attributeemp = new Employee();emp.setId(Integer.parseInt(id));// initialize listif (empList == null)empList = new ArrayList<>();} else if (qName.equalsIgnoreCase("name")) {// set boolean values for fields, will be used in setting Employee variablesbName = true;} else if (qName.equalsIgnoreCase("age")) {bAge = true;} else if (qName.equalsIgnoreCase("gender")) {bGender = true;} else if (qName.equalsIgnoreCase("role")) {bRole = true;}// create the data containerdata = new StringBuilder();}@Overridepublic void endElement(String uri, String localName, String qName) throws SAXException {if (bAge) {// age element, set Employee ageemp.setAge(Integer.parseInt(data.toString()));bAge = false;} else if (bName) {emp.setName(data.toString());bName = false;} else if (bRole) {emp.setRole(data.toString());bRole = false;} else if (bGender) {emp.setGender(data.toString());bGender = false;}if (qName.equalsIgnoreCase("Employee")) {// add Employee object to listempList.add(emp);}}@Overridepublic void characters(char ch[], int start, int length) throws SAXException {data.append(new String(ch, start, length));}
}

MyHandler contains the list of the Employee object as a field with a getter method only. The Employee objects are getting added in the event handler methods. Also, we have an Employee field that will be used to create an Employee object and once all the fields are set, add it to the employee list.

MyHandler仅使用getter方法将Employee对象的列表作为字段包含在内。 Employee对象将添加到事件处理程序方法中。 另外，我们还有一个Employee字段，它将用于创建Employee对象，并且一旦所有字段都设置好，就将其添加到employee列表中。

SAX解析器方法重写 (SAX parser methods to override)

The important methods to override are startElement(), endElement() and characters().

覆盖的重要方法是startElement() ， endElement()和characters() 。

SAXParser starts parsing the document, when any start element is found, startElement() method is called. We are overriding this method to set boolean variables that will be used to identify the element.

SAXParser开始解析文档，当找到任何开始元素时，将调用startElement()方法。 我们重写此方法来设置将用于标识元素的布尔变量。

We are also using this method to create a new Employee object every time Employee start element is found. Check how id attribute is read here to set the Employee Object id field.

每当找到Employee start元素时，我们还使用此方法创建一个新的Employee对象。 检查此处如何读取id属性以设置Employee Object id字段。

characters() method is called when character data is found by SAXParser inside an element. Note that SAX parser may divide the data into multiple chunks and call characters() method multiple times (Read ContentHandler class characters() method documentation). That’s why we are using StringBuilder to keep this data using append() method.

SAXParser在元素内找到字符数据时，将调用characters()方法。 请注意，SAX解析器可能会将数据分为多个块，并多次调用characters()方法（请参阅ContentHandler类character（）方法文档）。 这就是为什么我们使用StringBuilder通过append（）方法保留此数据的原因。

The endElement() is the place where we use the StringBuilder data to set employee object properties and add Employee object to the list whenever we found Employee end element tag.

endElement()是我们使用StringBuilder数据设置员工对象属性并将Employee对象添加到列表的地方，只要我们找到Employee结束元素标签即可。

Below is the test program that uses MyHandler to parse above XML to list of Employee objects.

下面是使用MyHandler解析XML到Employee对象列表的测试程序。

package com.journaldev.xml.sax;import java.io.File;
import java.io.IOException;
import java.util.List;import javax.xml.parsers.ParserConfigurationException;
import javax.xml.parsers.SAXParser;
import javax.xml.parsers.SAXParserFactory;import org.xml.sax.SAXException;import com.journaldev.xml.Employee;public class XMLParserSAX {public static void main(String[] args) {SAXParserFactory saxParserFactory = SAXParserFactory.newInstance();try {SAXParser saxParser = saxParserFactory.newSAXParser();MyHandler handler = new MyHandler();saxParser.parse(new File("/Users/pankaj/employees.xml"), handler);//Get Employees listList<Employee> empList = handler.getEmpList();//print employee informationfor(Employee emp : empList)System.out.println(emp);} catch (ParserConfigurationException | SAXException | IOException e) {e.printStackTrace();}}}

Here is the output of the above program.

这是上面程序的输出。

Employee:: ID=1 Name=Pankaj Age=29 Gender=Male Role=Java Developer
Employee:: ID=2 Name=Lisa Age=35 Gender=Female Role=CEO
Employee:: ID=3 Name=Tom Age=40 Gender=Male Role=Manager
Employee:: ID=4 Name=Meghna Age=25 Gender=Female Role=Manager

SAXParserFactory provides factory methods to get the SAXParser instance. We are passing File object to the parse method along with MyHandler instance to handle the callback events.

SAXParserFactory提供了工厂方法来获取SAXParser实例。 我们将File对象与MyHandler实例一起传递给parse方法，以处理回调事件。

SAXParser is a little bit confusing in the start but if you are working on a large XML document, it provides a more efficient way to read XML than DOM Parser. That’s all for SAX Parser in Java.

首先，SAXParser有点令人困惑，但是如果您正在处理大型XML文档，则它比DOM Parser提供了一种更有效的XML读取方法。 Java的SAX解析器就这些了。

Reference: SAXParser, DefaultHandler

参考 ： SAXParser ， DefaultHandler

翻译自: https://www.journaldev.com/1198/java-sax-parser-example

java sax解析器