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function callBack() {
if(xhr.readyState==4) {
if(xhr.status==200) {
message = "message="+xhr.responseText
alert(message);
}
}
}
package ajax;
import java.io.IOException;
import java.io.PrintWriter;
import javax.servlet.ServletException;
import javax.servlet.http.HttpServlet;
import javax.servlet.http.HttpServletRequest;
import javax.servlet.http.HttpServletResponse;
public class CheckUser extends HttpServlet {
protected void service(HttpServletRequest request, HttpServletResponse response)
throws ServletException, IOException {
request.setCharacterEncoding("utf-8");
response.setContentType("text/html;charset=utf-8");
PrintWriter out = response.getWriter();
String username = request.getParameter("username");
if(username==null || "".equals(username)) {
out.print("用户名不能为空");
} else if(username.length()>10 || username.length()<3) {
out.print("3到10个字符");
}
}
}