Java SAX(Simple API for XML) parser is an API in Java to parse XML documents. SAX parser is an event based parser and uses a Handler class to handle the events. The call back methods such as startElement(), characters(), endElement() etc., are implemented inside the Handler class to obtain the details of elements and their attributes. These call back methods are called when the parser identifies the respective events.
Parse XML Using Java SAX parser
Following are the steps we need to follow to parse an XML document in Java using SAX parser −
- Step 1: Implementing a Handler class
- Step 2: Creating a SAXParser Object
- Step 3: Reading the XML
- Step 4: Creating object for Handler class
- Step 5: Parsing the XML Document
- Step 6: Retrieving the Elements
Step 1: Implementing a Handler class
Application program must implement a handler class to handle the events inside the XML document. After implementing the Handler class, it must be registered with the SAX parser.
As discussed in the previous chapter, the DefaultHandler class implements ContentHandler interface. It has the methods, startDocument(), endDocument(), startElement(), endElement() and characters() functions that help us parse the XML documents. We write the code inside these methods according to our requirement.
classUserHandlerextendsDefaultHandler{publicvoidstartDocument(){...}publicvoidstartElement(String uri,String localName,String qName,Attributes attributes){...}publicvoidcharacters(char[] ch,int start,int length){...}publicvoidendElement(String uri,String localName,String qName){...}publicvoidendDocument(){...}}Step 2: Creating a SAXParser Object
The SAXParserFactory class is used to create a new factory instance which in turn is used to create the SAXParser object as follows −
SAXParserFactory factory =SAXParserFactory.newInstance();SAXParser saxParser = factory.newSAXParser();Step 3: Reading the XML
Read the XML file by specifying the proper file path as follows −
File xmlFile =newFile("input.xml");Instead of reading files, we can create an InputStream of the XML content as follows −
StringBuilder xmlBuilder =newStringBuilder();
xmlBuilder.append(""<?xml version="1.0"?><rootElement></rootElement>"");ByteArrayInputStream inputStream =newByteArrayInputStream( xmlBuilder.toString().getBytes("UTF-8"));Step 4: Creating object for Handler class
Create an object for the already implemented UserHandler class in first step as follows −
UserHandler userHandler =newUserHandler();Step 5: Parsing the XML Document
The SAXParser class has the parse() method that takes two arguments, one is the file and the other is the DefaultHandler object. This function parses the given file as XML document using the functions implemented inside the DefaultHandler class.
saxParser.parse(xmlFile, userHandler);The SAXParser class also has the function parse() that takes the content as InputStream −
saxParser.parse(inputStream, userHandler);Step 6: Retrieving the Elements
After following the above five steps, we can now easily retrieve the required information about the elements. We should write the required code inside the methods of our Handler class in first step. All the methods available inside the ContentHandler interface are discussed in the previous chapter and in this chapter, we will implement these methods to retrieve the basic information about elements such as element name, text content and attributes.
Retrieving Element Name
Element name can be obtained from the startElement() method of ContentHandler interface. The third argument of this method is the name of the Element and it is of String type. We can implement this method in our Handler class and get the name of an Element.
Example
In the following example, we have taken XML content in the form of a String using StringBuilder class and converted into bytes using ByteArrayInputStream.
In the UserHandler class, we have implemented the startElement() method and printed the name of the Element. Since, we have only single element in the XML content, that becomes the root element of the document.
Open Compiler
importjava.io.ByteArrayInputStream;importjavax.xml.parsers.SAXParser;importjavax.xml.parsers.SAXParserFactory;importorg.xml.sax.Attributes;importorg.xml.sax.SAXException;importorg.xml.sax.helpers.DefaultHandler;//Implementing UserHandler ClassclassUserHandlerextendsDefaultHandler{publicvoidstartElement(String uri,String localName,String qName,Attributes attributes)throwsSAXException{System.out.println("Root element is "+qName);}}publicclassRetrieveElementName{publicstaticvoidmain(String args[]){try{//Creating a SAXParser Object SAXParserFactory factory =SAXParserFactory.newInstance();SAXParser saxParser = factory.newSAXParser();//Reading the XMLStringBuilder xmlBuilder =newStringBuilder();
xmlBuilder.append("<college>XYZ College</college>");ByteArrayInputStream input =newByteArrayInputStream(xmlBuilder.toString().getBytes("UTF-8"));//Creating UserHandler objectUserHandler userhandler =newUserHandler();//Parsing the XML Document
saxParser.parse(input, userhandler);}catch(Exception e){
e.printStackTrace();}}}Root Element name, ‘college’ is printed on the output screen.
Root element is college
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Retrieving TextContent
To retrieve text content of an element, we have characters() method in ContentHandler interface. There is character array, start and length arguments in this method. As soon as the parser sees the content after “>” symbol, this method is called. The start argument carries the index of the first character after “>” symbol and length has the number of characters before it encounters “<” symbol.
Example
The following college.xml file has a single sub element, “department” with text content “Computer Science”. Let us write a Java program to retrieve this text content along with element names using SAX API.
<college><department>Computer Science</department></college>The UserHandler class inherits DefaultHandler and we have implemented startElement(), endElement() and characters() method. When the parser sees the text content inside department element, this method is called and we are printing it on the console.
importjava.io.File;importjavax.xml.parsers.SAXParser;importjavax.xml.parsers.SAXParserFactory;importorg.xml.sax.Attributes;importorg.xml.sax.SAXException;importorg.xml.sax.helpers.DefaultHandler;//Implementing UserHandler ClassclassUserHandlerextendsDefaultHandler{publicvoidstartElement(String uri,String localName,String qName,Attributes attributes)throwsSAXException{System.out.println("Start Element : "+ qName);}publicvoidendElement(String uri,String localName,String qName){System.out.println("End Element : "+ qName);}publicvoidcharacters(char[] ch,int start,int length)throwsSAXException{System.out.println("Text Content : "+newString(ch, start, length));}}publicclassRetrieveTextContent{publicstaticvoidmain(String args[]){try{//Creating a SAXParser Object SAXParserFactory factory =SAXParserFactory.newInstance();SAXParser saxParser = factory.newSAXParser();//Reading the XMLFile xmlFile =newFile("college.xml");//Creating UserHandler objectUserHandler userHandler =newUserHandler();//Parsing the XML Document
saxParser.parse(xmlFile, userHandler);}catch(Exception e){
e.printStackTrace();}}}The text content for department element is displayed. As there is no text content inside the “college” element, it is left blank.
Start Element : college
Text Content :
Start Element : department
Text Content : Computer Science
End Element : department
Text Content :
End Element : college
Retrieving Attributes
The method startElement() has Attributes as last argument which has the list of attributes inside the current Element. The getValue(“attr_name”) function inside the Attributes interface is used to get the value of the specified attribute.
Example
We have added few more department elements to our “college.xml” file and also added an attribute “deptcode” to each of the departments. Let us write a java program to retrieve all the elements along with their attributes.
<?xml version = "1.0"?><college><department deptcode = "DEP_CS23"><name>Computer Science</name><staffCount>20</staffCount></department><department deptcode = "DEP_EC34"><name>Electrical and Electronics</name><staffCount>23</staffCount></department><department deptcode = "DEP_MC89"><name>Mechanical</name><staffCount>15</staffCount></department></college>The following Java program implements startElement() and characters() methods in UserHandler class. We have initialised two boolean variables to let us notified about deptcode and staffCount attributes in department element, so that we can use these to print the attributes in characters() method.
importjava.io.File;importjavax.xml.parsers.SAXParser;importjavax.xml.parsers.SAXParserFactory;importorg.xml.sax.Attributes;importorg.xml.sax.SAXException;importorg.xml.sax.helpers.DefaultHandler;//Implementing UserHandler ClassclassUserHandlerextendsDefaultHandler{boolean hasDeptName=false;boolean hasStaffCount=false;publicvoidstartElement(String uri,String localName,String qName,Attributes attributes)throwsSAXException{if(qName.equals("college")){System.out.println("Root Element : "+qName +"\n");}if(qName.equals("department")){System.out.println("Current Element : "+qName);System.out.println("Department code : "+ attributes.getValue("deptcode"));}if(qName.equals("name")){
hasDeptName=true;}if(qName.equals("staffCount")){
hasStaffCount=true;}}publicvoidcharacters(char[] ch,int start,int length)throwsSAXException{if(hasDeptName){System.out.println("Department Name : "+newString(ch, start, length));
hasDeptName=false;}if(hasStaffCount){System.out.println("Staff Count : "+newString(ch, start, length)+"\n");
hasStaffCount=false;}}}publicclassParseAttributesSAX{publicstaticvoidmain(String args[]){try{//Creating a DocumentBuilder Object SAXParserFactory factory =SAXParserFactory.newInstance();SAXParser saxParser = factory.newSAXParser();//Reading the XMLFile xmlFile =newFile("college.xml");//Creating UserHandler objectUserHandler userHandler =newUserHandler();//Parsing the XML Document
saxParser.parse(xmlFile, userHandler);}catch(Exception e){
e.printStackTrace();}}}The ouput window displays names of each element along with the attributes.
Root Element : college
Current Element : department
Department code : DEP_CS23
Department Name : Computer Science
Staff Count : 20
Current Element : department
Department code : DEP_EC34
Department Name : Electrical and Electronics
Staff Count : 23
Current Element : department
Department code : DEP_MC89
Department Name : Mechanical
Staff Count : 15
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