前端部分：

first:<input type="text" id="first">
first3:<input type="text" id="first3">
<button onclick="btn()">提交</button>
first1:<input type="text" name="first1" id="result">

js部分：

function btn(){
	$.ajax({
		type:"post",
		url:"servlet/indexservlet",
		data:{"first":$("#first").val(),"first3":$("#first3").val()},
		async:true,
		success:function(result){
			$("#result").val(result);
		}
	});
}

Servlet部分：

response.setContentType("text/html");
		response.setCharacterEncoding("utf-8");
		PrintWriter out = response.getWriter();
		String first = request.getParameter("first");
		String first3 = request.getParameter("first3");
		String result = first+first3;
		System.out.println(first3+first);
		out.print(result);

运行效果：