在开发的过程中，有时候我们需要设计一个数据接口。有时候呢，数据接口和Web服务器又不在一起，所以就有跨域访问的问题。

第一步：简单的设计一个数据接口。

数据接口，听起来高大上，其实呢就是一个简单的Serlvlet,在有get的请求的时候，返回我们要提供的数据就可以。现在JSON数据格式已经很普遍，因为很方便，所以我们做一个json数据的接口。直接看代码

先建立一个实体类，就是包装我们的数据的

bean/kapian.java

package bean;

public class kapian {

//头像路进

public String imgurl;

public String getImgurl() {

return imgurl;

}

public void setImgurl(String imgurl) {

this.imgurl = imgurl;

}

public String getName() {

return name;

}

public void setName(String name) {

this.name = name;

}

public String getMoney() {

return money;

}

public void setMoney(String money) {

this.money = money;

}

public String getStyle() {

return style;

}

public void setStyle(String style) {

this.style = style;

}

public String getNum() {

return num;

}

public void setNum(String num) {

this.num = num;

}

public String getRevenue() {

return revenue;

}

public void setRevenue(String revenue) {

this.revenue = revenue;

}

public String getId() {

return id;

}

public void setId(String id) {

this.id = id;

}

//名字

public String name;

//金额

public String money;

//交易风格

public String style;

//订阅人数

public String num;

//收益率

public String revenue;

//标记

public String id;

public kapian(String imgurl,String name,String money,String style,String num,String revenue,String id){

this.imgurl=imgurl;

this.name=name;

this.money=money;

this.style=style;

this.num=num;

this.revenue=revenue;

this.id=id;

}

}

Serlvet 类：

bean/message.java

package bean;

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;

import org.json.JSONObject;

import org.json.JSONArray;

import java.util.ArrayList;

import bean.kapian;

import java.util.List;

public class message extends HttpServlet {

/**

* Constructor of the object.

*/

public message() {

super();

}

/**

* Destruction of the servlet.

*/

public void destroy() {

super.destroy(); // Just puts "destroy" string in log

// Put your code here

}

/**

* The doGet method of the servlet.

*

* This method is called when a form has its tag value method equals to get.

*

* @param request the request send by the client to the server

* @param response the response send by the server to the client

* @throws ServletException if an error occurred

* @throws IOException if an error occurred

*/

public void doGet(HttpServletRequest request, HttpServletResponse response)

throws ServletException, IOException {

response.setHeader("Content-type", "text/html;charset=UTF-8");

response.setContentType("text/html;charset=utf-8");

List li = new ArrayList();

li.add(new kapian("1.jpg","陈永康","2345323","牛逼","10234323","200%","周"));

li.add(new kapian("1.jpg","唐露露","123143","稳健","1234234","190%","周"));

li.add(new kapian("1.jpg","宋豪杰","2345323","牛逼","10234323","200%","周"));

li.add(new kapian("1.jpg","小斌","2345323","牛逼","10234323","200%","周"));

li.add(new kapian("1.jpg","李研","2345323","厉害","234235","2000%","周"));

li.add(new kapian("1.jpg","陈永康","2345323","牛逼","10234323","200%","月"));

li.add(new kapian("1.jpg","唐露露","123143","稳健","1234234","190%","月"));

li.add(new kapian("1.jpg","宋豪杰","2345323","牛逼","10234323","200%","月"));

li.add(new kapian("1.jpg","小斌","2345323","牛逼","10234323","200%","月"));

li.add(new kapian("1.jpg","李研","2345323","厉害","234235","2000%","月"));

li.add(new kapian("1.jpg","陈永康","2345323","牛逼","10234323","200%","年"));

li.add(new kapian("1.jpg","唐露露","123143","稳健","1234234","190%","年"));

li.add(new kapian("1.jpg","宋豪杰","2345323","牛逼","10234323","200%","年"));

li.add(new kapian("1.jpg","小斌","2345323","牛逼","10234323","200%","年"));

li.add(new kapian("1.jpg","李研","2345323","厉害","234235","2000%","年"));

li.add(new kapian("1.jpg","陈永康","2345323","牛逼","10234323","200%","总"));

li.add(new kapian("1.jpg","唐露露","123143","稳健","1234234","190%","总"));

li.add(new kapian("1.jpg","宋豪杰","2345323","牛逼","10234323","200%","总"));

li.add(new kapian("1.jpg","小斌","2345323","牛逼","10234323","200%","总"));

li.add(new kapian("1.jpg","李研","2345323","厉害","234235","2000%","总"));

try{

JSONArray json = new JSONArray();

for(kapian a :li){

JSONObject obj = new JSONObject();

obj.put("imgurl",a.getImgurl());

obj.put("name",a.getName());

obj.put("money",a.getMoney());

obj.put("style",a.getStyle());

obj.put("num",a.getNum());

obj.put("revenue",a.getRevenue());

obj.put("id",a.getId());

json.put(obj);

}

JSONObject js =new JSONObject();

response.getWriter().print(json.toString());

}

catch(Exception e){

e.printStackTrace();

}

}

/**

* The doPost method of the servlet.

*

* This method is called when a form has its tag value method equals to post.

*

* @param request the request send by the client to the server

* @param response the response send by the server to the client

* @throws ServletException if an error occurred

* @throws IOException if an error occurred

*/

public void doPost(HttpServletRequest request, HttpServletResponse response)

throws ServletException, IOException {

response.setContentType("text/html");

PrintWriter out = response.getWriter();

out.println(""-//W3C//DTD HTML 4.01 Transitional//EN\">");

out.println("");

out.println("

A Servlet");

out.println("

");

out.print(" This is ");

out.print(this.getClass());

out.println(", using the POST method");

out.println(" ");

out.println("");

out.flush();

out.close();

}

/**

* Initialization of the servlet.

*

* @throws ServletException if an error occurs

*/

public void init() throws ServletException {

// Put your code here

}

}

使用Json的时候，我们要导入JSON包,org.json包，可以网上下载

然后用将设计好的List转换成json格式

我们用到两个json对象，一个是JSONObject,一个是JSONArray。顾名思义，前者就是一个json对象，后者是一个json数组。

这里扩展一下：

最后我们用response.getWrite()和print(),返回数据。也可以用response.getOututStream.write()

write和print的区别

write()：仅支持输出字符类型数据，字符、字符数组、字符串等

print()：可以将各种类型(包括Object)的数据通过默认编码转换成bytes字节形式，这些字节都通过write(int c)方法被输出

response.getWriter()和response.getOutputStream的区别：前者是字符流，后者是字节流

response.getOutputStream().write(data.getBytes("UTF-8"));

response.getWriter().write(data);

回到正题：

这是项目的文件接口，然后浏览器中输入

http://192.168.3.60:8089/Data/servlet/message

现在如果我们在另外一台电脑上使用ajax， 来获取数据

$.ajax({

url:'http://localhost:8089/Data/servlet/message',

method:'get',

dataType:'json',

success:function(data){

$.each(data,function(i,term){

alert(term.name);

});

},

error:function(XMLHttpRequest,textStatus,errorThrown){

alert(XMLHttpRequest.status);

alert(XMLHttpRequest.readyState);

alert(textStatus);

}

});

这个时候回到error的回调函数，XMLHtppRequset.status=0;readyState=0; 都是0。 0表示请求根本没有建立。这是因为，另一个电脑和我的数据接口

不在一个服务器上，当然，如果你在自己的电脑上写ajax也不可以，必须写到项目里，才可以访问。所以，跨域问题必须要解决。网上的办法很简单，

新建一个Filter类：

Filter是一个过滤器。对你设定的请求地址进行拦截，然后设置。

package Filter;

import java.io.IOException;

import javax.servlet.Filter;

import javax.servlet.FilterChain;

import javax.servlet.FilterConfig;

import javax.servlet.ServletException;

import javax.servlet.ServletRequest;

import javax.servlet.ServletResponse;

import javax.servlet.http.HttpServletResponse;

public class HeaderFilter implements Filter

{

public void doFilter(ServletRequest request, ServletResponse resp, FilterChain chain) throws IOException, ServletException

{

HttpServletResponse response = (HttpServletResponse) resp; response.setHeader("Access-Control-Allow-Origin", "*"); //解决跨域访问报错

response.setHeader("Access-Control-Allow-Methods", "POST, PUT, GET, OPTIONS, DELETE");

response.setHeader("Access-Control-Max-Age", "3600"); //设置过期时间

response.setHeader("Access-Control-Allow-Headers", "Origin, X-Requested-With, Content-Type, Accept, client_id, uuid, Authorization");

response.setHeader("Cache-Control", "no-cache, no-store, must-revalidate"); // 支持HTTP 1.1.

response.setHeader("Pragma", "no-cache"); // 支持HTTP 1.0. response.setHeader("Expires", "0");

chain.doFilter(request, resp);

}

public void init(FilterConfig filterConfig) {}

public void destroy() {}

}

在web.xml中配置：

HeaderFilter

Filter.HeaderFilter

HeaderFilter

/*

然后，ajax在任何地方就都可以访问了。

还有一点就是，ajax在处理json数据的时候：

有两种方式，一种数据格式不声明为json，直接是文本，然后传过来，需要解析一下，用eval() 或者JSON.parse()

还有一种，是声明为json,直接可以用

后端和前端，如果有一个声明为json,那么格式就是json.