1、创建实体类，模拟实体对象

创建一个类，模拟数据数据库来存储数据，这个类就叫Person。

其代码如下：

package com.restful;

public class Person {
	private String name;
	private String about;
	private int birthYear;
	
	public Person(String name, String about, int birthYear) {
		this.name = name;
		this.about = about;
		this.birthYear = birthYear;
	}
 
	public String getName() {
		return name;
	}
 
	public String getAbout() {
		return about;
	}
 
	public int getBirthYear() {
		return birthYear;
	}
}

2、创建数据类，模型数据库存储

再创建一个类，假设能够访问我们的数据库，类名字就叫DataStore。

编写如下代码：

package com.restful;

import java.util.HashMap;
import java.util.Map;

public class DataStore {
	//Map of names to Person instances.
	private Map<String, Person> personMap = new HashMap<>();
	
	//this class is a singleton and should not be instantiated directly!
	private static DataStore instance = new DataStore();
	public static DataStore getInstance(){
		return instance;
	}
 
	//private constructor so people know to use the getInstance() function instead
	private DataStore(){
		//dummy data
		personMap.put("Ada", new Person("Ada", "Ada Lovelace was the first programmer.", 1815));
		personMap.put("Kevin", new Person("Kevin", "Kevin is the author of HappyCoding.io.", 1986));
		personMap.put("Stanley", new Person("Stanley", "Stanley is Kevin's cat.", 2007));
	}
 
	public Person getPerson(String name) {
		return personMap.get(name);
	}
 
	public void putPerson(Person person) {
		personMap.put(person.getName(), person);
	}
}

3、创建Servlet，接收请求并访问数据对象

接下来，我们创建一个Servlet类，类名就叫做PersonServlet。

修改代码如下：

package com.servlet;

import java.io.IOException;
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 org.json.JSONObject;

import com.restful.DataStore;
import com.restful.Person;
/**
 * Servlet implementation class PersonServlet
 */
@WebServlet("/PersonServlet")
public class PersonServlet extends HttpServlet {
	private static final long serialVersionUID = 1L;
       
    /**
     * @see HttpServlet#HttpServlet()
     */
    public PersonServlet() {
        super();
        // TODO Auto-generated constructor stub
    }

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

		String requestUrl = request.getRequestURI();
		String name = requestUrl.substring(requestUrl.lastIndexOf("/")+1);
		System.out.println(name);
		Person person = DataStore.getInstance().getPerson(name);
		if(person != null){
			String json = "{\n";
			json += "\"name\": " +"\""+ person.getName() +"\""+ ",\n";
			json += "\"about\": " + "\""+ person.getAbout() + "\""+",\n";
			json += "\"birthYear\": " + "\""+ person.getBirthYear() + "\""+"\n";
			json += "}";
			response.getOutputStream().println(json);
		}
		else{
			//That person wasn't found, so return an empty JSON object. We could also return an error.
			response.getOutputStream().println("{}");
		}
	}

	/**
	 * @see HttpServlet#doPost(HttpServletRequest request, HttpServletResponse response)
	 */
	protected void doPost(HttpServletRequest request, HttpServletResponse response) throws ServletException, IOException {
		// TODO Auto-generated method stub
		String name = request.getParameter("name");
		String about = request.getParameter("about");
		int birthYear = Integer.parseInt(request.getParameter("birthYear"));
		
		DataStore.getInstance().putPerson(new Person(name, about, birthYear));

	}

}

上述创建的Servlet类包含了doGet函数和doPost函数。

4、配置一个Servlet映射

创建servlet后，我们修改一下我们的web.xml，做一个映射，如下所示：

<?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_4_0.xsd" id="WebApp_ID" version="4.0">
  <display-name>HelloJSP</display-name>
  <welcome-file-list>
    <welcome-file>index.html</welcome-file>
    <welcome-file>index.jsp</welcome-file>
    <welcome-file>index.htm</welcome-file>
    <welcome-file>default.html</welcome-file>
    <welcome-file>default.jsp</welcome-file>
    <welcome-file>default.htm</welcome-file>
  </welcome-file-list>
  
  	<servlet>
		<servlet-name>PersonServlet</servlet-name>
		<servlet-class>com.servlet.PersonServlet</servlet-class>
	</servlet>
 
	<servlet-mapping>
		<servlet-name>PersonServlet</servlet-name>
		<url-pattern>/people/*</url-pattern>
	</servlet-mapping>

</web-app>

doGet()函数从 URL 中获取一个人的姓名，然后使用DataStore类来获取那个人。接着它从该人的数据中创建一个 JSON 字符串，并将该 JSON 作为对请求的响应返回GET。请记住，JSON 使用用引号括起来的键和值，如下所示：

{
"name": "Ada",
"about": "Ada Lovelace was the first programmer.",
"birthYear": 1815
}

5、测试

接下来我们启动服务加载该servlet，在浏览器输入如下地址：localhost:8080/HelloJSP/people/Kevin

结果显示如下：

今天就先把怎么写一个简单的rest api介绍到这儿，后面说一下客户端怎么使用rest api访问数据。

参考资料：https://blog.csdn.net/allway2/article/details/123375541

https://happycoding.io/tutorials/java-server/rest-api