ajax get request

Request acquiring step Ajax

Step 1: Create Ajax core objects: XMLHttpRequest object
new an XMLHttpRequest object
This object is a built-in object browser, can be used directly.
Step two: Register the callback function.
Call onreadystatechange callback function
program execution to here, behind the callback function does not perform, but the registration function to the XMLHttpRequest object.
So when readySate XMLHttpRequest object changes, followed by the callback function is executed.
XMLHttpRequest object in the request and response process, readyState state of the object from 0-4
0: Not initialized request
1: the server has been established
2: request accepted
3: Request Processing
4: request has completed
the third step: opening channel between the browser and the server.
Call open function.
Function has three parameters
first: embodiment POST request / GET
second: url addresses
Third: ture representative of asynchronous, false synchronization
Step Four: Ajax requests
calling the send method

Ajax's get seen cases

1. ajax request message sends the user name, and displays the server

<script type="text/javascript">
	function checkUsername(username){
		var xhr = new XMLHttpRequest();
		
		xhr.onreadystatechange = function(){
			if(xhr.readyState == 4){
				if(xhr.status == 200){
					var s = xhr.responseText;
					var name=document.getElementById("nameTipMsg");
					name.innerHTML= s;
				}else{
					alert(xhr.status);
				}
			}
		}
		xhr.open("GET","User.do?username="+username,true);
		xhr.send();
	}
</script>
	用户名：<input type="text" name="username" onblur="checkUsername(this.value)"/><span id="nameTipMsg"></span><br>
	密码:<input type="password" name="password"/><br>
</body>

2. Configure web.xml (configuration access path)

<servlet>
	<servlet-name>User</servlet-name>
	<servlet-class>com.ajax.User</servlet-class>
</servlet>

<servlet-mapping>
	<servlet-name>User</servlet-name>
	<url-pattern>/User.do</url-pattern>
</servlet-mapping>

3. Verify parameters ajax request, response sent to the client

import javax.servlet.http.HttpServlet;
import javax.servlet.http.HttpServletRequest;
import javax.servlet.http.HttpServletResponse;

public class User extends HttpServlet{
	@Override
	protected void doGet(HttpServletRequest request, HttpServletResponse response)
			throws ServletException, IOException {
		response.setContentType("text/html;charset=UTF-8");
		PrintWriter out = response.getWriter();
		String username  =request.getParameter("username");
		if("admin".equals(username)){
			
			out.print("用户名已存在");
		}else{
			out.print("用户名已注册");
		}
	}

}