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-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：
<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.
