如何在 Servlet 中调用 RESTful API Web 服务

markkang

4.67/5 (3投票s)

2016年11月22日

CPOL

2分钟阅读

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在本文中，我将介绍如何在 servlet 中调用 RESTful API。

下载源码 - 251.5 KB

引言

在本文中，我将介绍如何在 servlet 中调用 RESTful API。

背景

假设我们需要搜索并显示指定城市的天气状况。互联网上有很多第三方 Web 服务。我将使用百度天气 Web 服务来演示这一点。现在，让我们开始执行以下步骤。

Using the Code

转到您选择使用的 Web 服务的网站，注册一个帐户并为您获取一个 apikey。

查找并理解 API 描述。以百度为例。

API 地址: http://apis.baidu.com/heweather/weather/free
请求方法: GET

请求参数在 Header 中

名称	类型	必需	location	描述	默认值
`apikey`	`字符串`	是	header	来自您的 Web 服务提供商的 APIkey	apikey

Url 参数

名称	类型	必需	location	描述	默认值
`city`	`字符串`	是	`urlParam`	`城市名称`	`北京`

Json 格式的结果

"HeWeather data service 3.0": [
	{
	    "status": "ok", //status,              
	    "basic": {  //basic info
	        "city": "Beijing",     //city name
	        "cnty": "China",       //country
	        "id": "CN101010100",   //city id
	        "lat": "39.904000",    //latitude of city
	        "lon": "116.391000",   //longitude of city
	        "update": {            //update time
	            "loc": "2015-07-02 14:44", //locate time
	            "utc": "2015-07-02 06:46"  //UTC time
	        }
	    },
	    "now": { // the weather at this time
	        "cond": { //weather condition
	            "code": "100", //weather condition code
	            "txt": "sunny day" //weather condiftion description
	         },
	,......  //Omitted
	    }
	]
	}

为了方便地测试 REST API，请在您的计算机上查找或安装一些工具，例如 restclient、fiddler 等。如果您使用的是 Chrome 或 Firefox 浏览器，您还可以安装 REST 客户端插件。我在我的 Chrome 浏览器上安装了 Advanced REST Client。
启动 rest 客户端。请输入 API 地址和 apikey，如下图所示，然后单击 Send 按钮。您将获得一个 json string 结果。
启动 http://www.jsonschema2pojo.org/ 并复制 json 字符串结果并将其粘贴到页面中以生成 javabean 类。然后，在 Eclipse 中创建一个动态 Web 项目后，下载 zip 文件并将其解压缩到您的项目文件夹中。
当您使用 Eclipse 打开您的项目时，您将看到以下图像

为服务创建一个新的包，例如 com.BaiduWeather.Services。这里是调用 RESTfull API 的代理所在的位置。添加一个名为BaiduWeatherService.java的类，并将下面的代码行添加到其中。

Notice:

在此之前，您应该从 Maven Central 的 Gson 下载下载 gson-2.8.0.jar，将其复制到文件夹 WebContent/WEB-INF/lib/gson-2.8.0.jar 并将其添加到 Java 构建路径。
请记住将 apiKey 更改为您自己的。

package com.BaiduWeather.Services;

import java.io.BufferedReader;
import java.io.IOException;
import java.io.InputStreamReader;
import java.io.UnsupportedEncodingException;
import java.net.HttpURLConnection;
import java.net.MalformedURLException;
import java.net.URL;
import java.net.URLEncoder;

import com.BaiduWeather.Entities.Root;
import com.google.gson.Gson;
import com.google.gson.GsonBuilder;
import com.google.gson.JsonSyntaxException;
/**
 * baidu weather service
 */
public class BaiduWeatherService {
    private static final String apiKey="0ae09eed4f3c024451ads12d1gsgsg1sg";//change this with yours
    private static final String baseBaiduUrl=
        "http://apis.baidu.com/heweather/weather/free?city=";//The constant url should be 
                                                             //stored in config file
    
    public static Root getWeatherInfo(String cityName)
    {
        String jsonResult=getWeatherJsonString(cityName);
        Root weatherInfoObject=toEntity(jsonResult);
        return weatherInfoObject;
    }
    
    //Covert json string to class object
    private static Root toEntity(String jsonString)
    {
        try{
            Gson gson = new GsonBuilder().create();
            Root weatherInfo = gson.fromJson(jsonString, Root.class);            
            return weatherInfo;
        }
        catch(JsonSyntaxException ex)
        {
            ex.printStackTrace();
            return null;
        }
    }
    
    //Get the weather of the specific city
    private static String getWeatherJsonString(String cityName) throws RuntimeException{  
        
        //define a variable to store the weather api url and set beijing as it's default value
        String baiduUrl = baseBaiduUrl+"beijing"; //default value hard-coded 'beijing' 
                                                  //should be stored in config file
  
        try {
            if(cityName!=null && cityName!="")
                baiduUrl = baseBaiduUrl+URLEncoder.encode(cityName, "utf-8");                    
        } catch (UnsupportedEncodingException e1) {               
            e1.printStackTrace();                     
        }  

        StringBuilder strBuf = new StringBuilder();  
        
        HttpURLConnection conn=null;
        BufferedReader reader=null;
        try{  
            //Declare the connection to weather api url
            URL url = new URL(baiduUrl);  
            conn = (HttpURLConnection)url.openConnection();  
            conn.setRequestMethod("GET");
            conn.setRequestProperty("Accept", "application/json");
            conn.setRequestProperty("apikey",apiKey);
            
            if (conn.getResponseCode() != 200) {
                throw new RuntimeException("HTTP GET Request Failed with Error code : "
                              + conn.getResponseCode());
            }
            
            //Read the content from the defined connection
            //Using IO Stream with Buffer raise highly the efficiency of IO
	        reader = new BufferedReader(new InputStreamReader(conn.getInputStream(),"utf-8"));
            String output = null;  
            while ((output = reader.readLine()) != null)  
                strBuf.append(output);  
        }catch(MalformedURLException e) {  
            e.printStackTrace();   
        }catch(IOException e){  
            e.printStackTrace();   
        }
        finally
        {
            if(reader!=null)
            {
                try {
                    reader.close();
                } catch (IOException e) {
                    e.printStackTrace();
                }
            }
            if(conn!=null)
            {
                conn.disconnect();
            }
        }

        return strBuf.toString();  
    }
}

创建一个新的包 com.BaiduWeather.Servlets 并在其下创建一个新的类 WeatherServlet.java。在此 Java 文件中添加以下代码行

package com.BaiduWeather.Servlets;

import java.io.IOException;
import java.util.List;

import javax.servlet.ServletException;
import javax.servlet.annotation.WebServlet;
import javax.servlet.http.HttpServlet;
import javax.servlet.http.HttpServletRequest;
import javax.servlet.http.HttpServletResponse;

import com.BaiduWeather.Entities.HeWeatherDataService30;
import com.BaiduWeather.Entities.Root;
import com.BaiduWeather.Services.BaiduWeatherService; 
/**
 * Servlet implementation class WeatherServlet
 * 
 *  We can use 。jsp instead of servlet
 */
@WebServlet("/weather")
public class WeatherServlet extends HttpServlet {
    private static final long serialVersionUID = 1L;    

    /**
     * @see HttpServlet#HttpServlet()
     */
    public WeatherServlet() {
        super();
        // TODO Auto-generated constructor stub
    }

    /**
     * @see HttpServlet#doGet(HttpServletRequest request, HttpServletResponse response)
     */
    protected void doGet(HttpServletRequest request, HttpServletResponse response) 
                      throws ServletException, IOException {
        //Get weather json info and convert it to object
        String city=request.getParameter("city");
        Root wheatherInfoObject=BaiduWeatherService.getWeatherInfo(city);
        if(wheatherInfoObject!=null){
            List<HeWeatherDataService30> weatherlist= wheatherInfoObject.getHeWeatherDataService30();            
            if(weatherlist!=null)
            {
                //output the weather content on web page.
                StringBuilder outputContent=new StringBuilder();//Unless the need of thread-safe,
                //Use StringBuilder not StringBuffer,because the latter takes more erformance overhead because of Synchronization
                //More cities can be store in a database or file，then load them into an array,then generators options of selection here
                outputContent.append("<!DOCTYPE html><html><head><meta charset=\"UTF-8\">
                 <title>Insert title here</title></head><body><form action=\"weather\" 
                 method=\"GET\"><select name=\"city\"><option value =\"beijing\">北京</option> 
                 <option value =\"shanghai\">上海</option><option value =\"xian\">西安</option>
                 </select><input type=\"submit\" value=\"Submit\"></form>");
                outputContent.append(weatherlist.get(0).getBasic().getCity());
                outputContent.append("<br/>");
                outputContent.append(weatherlist.get(0).getNow().getTmp());
                outputContent.append("℃");
                outputContent.append("<br/>");
                outputContent.append(weatherlist.get(0).getNow().getCond().getTxt());
                outputContent.append("<br/>");
                outputContent.append(weatherlist.get(0).getNow().getWind().getDir());
                outputContent.append("</body></html>");
                
                response.setContentType("text/html; charset=utf-8");
                response.getWriter().write(outputContent.toString());
            }
        }
    }

    /**
     * @see HttpServlet#doPost(HttpServletRequest request, HttpServletResponse response)
     */
    protected void doPost(HttpServletRequest request, HttpServletResponse response) 
                throws ServletException, IOException {
        // TODO Auto-generated method stub
        doGet(request, response);
    }
}

在 WebContent/WEB-INF/web.xml 中，添加以下配置项。

<?xml version="1.0" encoding="UTF-8"?>
<web-app xmlns:xsi="http://www.w3.org/2001/XMLSchema-instance" 
xmlns="http://xmlns.jcp.org/xml/ns/javaee" 
xsi:schemaLocation="http://xmlns.jcp.org/xml/ns/javaee http://xmlns.jcp.org/xml/ns/javaee/web-app_3_1.xsd" 
id="WebApp_ID" version="3.1">
  <display-name>BaiduWeather</display-name>
  <welcome-file-list>
    <!-- <welcome-file>index.html</welcome-file> -->
    <welcome-file>index.htm</welcome-file>
    <welcome-file>index.jsp</welcome-file>
    <welcome-file>default.html</welcome-file>
    <welcome-file>default.htm</welcome-file>
    <welcome-file>default.jsp</welcome-file>
    <welcome-file>weather</welcome-file>
  </welcome-file-list>
  <servlet>  
     <!-- class name -->  
    <servlet-name>WeatherServlet</servlet-name>  
    <!-- package-->  
    <servlet-class>com.BaiduWeather.Servlets.WeatherServlet</servlet-class>  
  </servlet>  
  <servlet-mapping>  
    <servlet-name>WeatherServlet</servlet-name>  
    <!-- address -->  
    <url-pattern>/BaiduWeather/weather</url-pattern>  
    </servlet-mapping> 
</web-app>

运行并调试此 servlet，您应该在 HTML 页面上获得天气信息。

感谢阅读！