java后台设计简单的json数据接口，设置可跨域访问，前端ajax获取json数据

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. <br>
   */
  public void destroy() {
    super.destroy(); // Just puts "destroy" string in log
    // Put your code here
  }

  /**
   * The doGet method of the servlet. <br>
   *
   * 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<kapian> li = new ArrayList<kapian>();
    
    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. <br>
   *
   * 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("<!DOCTYPE HTML PUBLIC \"-//W3C//DTD HTML 4.01 Transitional//EN\">");
    out.println("<HTML>");
    out.println("  <HEAD><TITLE>A Servlet</TITLE></HEAD>");
    out.println("  <BODY>");
    out.print("    This is ");
    out.print(this.getClass());
    out.println(", using the POST method");
    out.println("  </BODY>");
    out.println("</HTML>");
    out.flush();
    out.close();
  }

  /**
   * Initialization of the servlet. <br>
   *
   * @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-reval（"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：

<filter>
      <filter-name>HeaderFilter</filter-name>
      <filter-class>Filter.HeaderFilter</filter-class><!--你过滤器的包 -->
    </filter>
    <filter-mapping>
      <filter-name>HeaderFilter</filter-name>
      <url-pattern>/*</url-pattern><!-- 你开放的接口前缀  -->
    </filter-mapping>

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.