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理解Servlet
2018-08-12 21:57 更新
理解Servlet
本文依然使用00-03、從JSP開始中創(chuàng)建的項(xiàng)目HelloJSP。
本文主要有以下內(nèi)容：
- 如何使用Servlet編寫Hello Servlet
- 如何將Servlet與URL對(duì)應(yīng)起來(lái)
- Servlet如何調(diào)用JSP
- Servlet如何返回JSON數(shù)據(jù)
- 如何編寫一個(gè)Dispatcher
Hello Servlet
項(xiàng)目結(jié)構(gòu)如下：
HelloServlet.java內(nèi)容如下：
package me.letiantian.servlet;
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;
public class HelloServlet extends HttpServlet {
protected void processRequest(HttpServletRequest request, HttpServletResponse response)
throws ServletException, IOException {
response.setContentType("text/html;charset=UTF-8");
try (PrintWriter out = response.getWriter()) {
out.println("<!DOCTYPE html>");
out.println("<html>");
out.println("<head>");
out.println("<title>Servlet HelloServlet</title>");
out.println("</head>");
out.println("<body>");
out.println("<h1>Servlet HelloServlet at " + request.getContextPath() + "</h1>");
out.println("</body>");
out.println("</html>");
}
}
@Override
protected void doGet(HttpServletRequest request, HttpServletResponse response)
throws ServletException, IOException {
processRequest(request, response);
}
@Override
protected void doPost(HttpServletRequest request, HttpServletResponse response)
throws ServletException, IOException {
processRequest(request, response);
}
}
HTTP最常見(jiàn)的方法是GET和POST，在一個(gè)Servlet中對(duì)應(yīng)的處理方法分別是doGet()和doPost()。
response.setContentType("text/html;charset=UTF-8"); 用來(lái)設(shè)置HTTP響應(yīng)頭中的Content-Type。
PrintWriter對(duì)象out的輸出內(nèi)容則是響應(yīng)正文。
web.xml內(nèi)容如下：
<?xml version="1.0" encoding="UTF-8"?>
<web-app version="3.1" xmlns="http://xmlns.jcp.org/xml/ns/javaee" xmlns:xsi="http://www.w3.org/2001/XMLSchema-instance" xsi:schemaLocation="http://xmlns.jcp.org/xml/ns/javaee http://xmlns.jcp.org/xml/ns/javaee/web-app_3_1.xsd">
<servlet>
<servlet-name>HelloServlet</servlet-name>
<servlet-class>me.letiantian.servlet.HelloServlet</servlet-class>
</servlet>
<servlet-mapping>
<servlet-name>HelloServlet</servlet-name>
<url-pattern>/HelloServlet</url-pattern>
</servlet-mapping>
<session-config>
<session-timeout>
30
</session-timeout>
</session-config>
</web-app>在這個(gè)配置中，me.letiantian.servlet.HelloServlet與URL/HelloServlet對(duì)應(yīng)。
session-timeout設(shè)置了session的有效時(shí)間，單位是分鐘（不過(guò)目前的程序里還沒(méi)用過(guò)session）。
瀏覽器訪問(wèn)http://127.0.0.1:8084/HelloJSP會(huì)顯示404；訪問(wèn)http://127.0.0.1:8084/HelloJSP/HelloServlet會(huì)顯示Servlet HelloServlet at /HelloJSP，這也正是me.letiantian.servlet.HelloServlet輸出的HTML的渲染結(jié)果。
也可以使用注解將Servlet和URL對(duì)應(yīng)起來(lái)
首先清空web.xml中關(guān)于URL的配置，web.xml最終內(nèi)容如下：
<?xml version="1.0" encoding="UTF-8"?>
<web-app version="3.1" xmlns="http://xmlns.jcp.org/xml/ns/javaee" xmlns:xsi="http://www.w3.org/2001/XMLSchema-instance" xsi:schemaLocation="http://xmlns.jcp.org/xml/ns/javaee http://xmlns.jcp.org/xml/ns/javaee/web-app_3_1.xsd">
<session-config>
<session-timeout>
30
</session-timeout>
</session-config>
</web-app>
然后對(duì)me.letiantian.servlet.HelloServlet類略做修改：
package me.letiantian.servlet;
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 javax.servlet.annotation.WebServlet;
@WebServlet("/HelloServlet")
public class HelloServlet extends HttpServlet {
// ......
}重新啟動(dòng)項(xiàng)目，瀏覽器訪問(wèn)效果和之前是相同的。
Servlet調(diào)用JSP
改寫me.letiantian.servlet.HelloServlet類，內(nèi)容如下：
package me.letiantian.servlet;
import java.io.IOException;
import java.io.PrintWriter;
import javax.servlet.RequestDispatcher;
import javax.servlet.ServletException;
import javax.servlet.http.HttpServlet;
import javax.servlet.http.HttpServletRequest;
import javax.servlet.http.HttpServletResponse;
import javax.servlet.annotation.WebServlet;
@WebServlet("/HelloServlet")
public class HelloServlet extends HttpServlet {
protected void processRequest(HttpServletRequest request, HttpServletResponse response)
throws ServletException, IOException {
response.setContentType("text/html;charset=UTF-8");
request.setAttribute("title", "Hello Servlet");
request.setAttribute("content", "你好");
RequestDispatcher rd = request.getRequestDispatcher("/WEB-INF/jsp/hello.jsp");
rd.forward(request, response);
}
@Override
protected void doGet(HttpServletRequest request, HttpServletResponse response)
throws ServletException, IOException {
processRequest(request, response);
}
@Override
protected void doPost(HttpServletRequest request, HttpServletResponse response)
throws ServletException, IOException {
processRequest(request, response);
}
}在WEB-INF/下創(chuàng)建目錄jsp，然后在jsp目錄下新建hello.jsp，內(nèi)容如下：
<!DOCTYPE html>
<html>
<head>
<meta http-equiv="Content-Type" content="text/html; charset=UTF-8">
<title>${title}</title>
</head>
<body>
<h1>${content}</h1>
</body>
</html>重啟該項(xiàng)目，訪問(wèn)http://127.0.0.1:8084/HelloJSP/HelloServlet：
$ curl -i http://127.0.0.1:8084/HelloJSP/HelloServlet
HTTP/1.1 200 OK
Server: Apache-Coyote/1.1
Set-Cookie: JSESSIONID=7CCCFD5467F8330066F827623802FB23; Path=/HelloJSP/; HttpOnly
Content-Type: text/html;charset=UTF-8
Content-Length: 215
Date: Fri, 18 Sep 2015 08:09:58 GMT
<!DOCTYPE html>
<html>
<head>
<meta http-equiv="Content-Type" content="text/html; charset=UTF-8">
<title>Hello Servlet</title>
</head>
<body>
<h1>你好</h1>
</body>
</html>CSS等靜態(tài)文件放在什么地方
在項(xiàng)目下建立static目錄，再這個(gè)目錄下添加test.js，內(nèi)容如下：
console.log("hello world");在web.xml添加以下內(nèi)容：
<servlet-mapping>
<servlet-name>default</servlet-name>
<url-pattern>*.jpg</url-pattern>
</servlet-mapping>
<servlet-mapping>
<servlet-name>default</servlet-name>
<url-pattern>*.png</url-pattern>
</servlet-mapping>
<servlet-mapping>
<servlet-name>default</servlet-name>
<url-pattern>*.js</url-pattern>
</servlet-mapping>
<servlet-mapping>
<servlet-name>default</servlet-name>
<url-pattern>*.css</url-pattern>
</servlet-mapping>此時(shí)，項(xiàng)目結(jié)構(gòu)如下：
啟動(dòng)項(xiàng)目，訪問(wèn)
$ curl -i http://localhost:8084/HelloJSP/static/test.js
HTTP/1.1 200 OK
Server: Apache-Coyote/1.1
Accept-Ranges: bytes
ETag: W/"27-1442566151000"
Last-Modified: Fri, 18 Sep 2015 08:49:11 GMT
Content-Type: application/javascript
Content-Length: 27
Date: Fri, 18 Sep 2015 08:58:00 GMT
console.log("hello world");$ curl -i http://localhost:8084/HelloJSP/static/test.js?time=123
HTTP/1.1 200 OK
Server: Apache-Coyote/1.1
Accept-Ranges: bytes
ETag: W/"27-1442566151000"
Last-Modified: Fri, 18 Sep 2015 08:49:11 GMT
Content-Type: application/javascript
Content-Length: 27
Date: Fri, 18 Sep 2015 08:58:09 GMT
console.log("hello world");Servlet如何返回JSON數(shù)據(jù)
將
response.setContentType("text/html;charset=UTF-8");修改為
response.setContentType("application/json;charset=UTF-8");。
out.println輸出JSON格式的字符串即可。
編寫Dispatcher
基于以上的學(xué)習(xí)，已經(jīng)可以編寫一個(gè)分發(fā)器了。 將HelloServlet.java修改為DispatcherServlet.java，內(nèi)容修改為：
package me.letiantian.servlet;
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 javax.servlet.annotation.WebServlet;
@WebServlet("/")
public class HelloServlet extends HttpServlet {
protected void processRequest(HttpServletRequest request, HttpServletResponse response)
throws ServletException, IOException {
response.setContentType("text/plain;charset=UTF-8");
try (PrintWriter out = response.getWriter()) {
out.println("context: " + request.getContextPath());
out.println("request uri: " + request.getRequestURI());
out.println("params: " + request.getParameterMap());
}
}
@Override
protected void doGet(HttpServletRequest request, HttpServletResponse response)
throws ServletException, IOException {
processRequest(request, response);
}
@Override
protected void doPost(HttpServletRequest request, HttpServletResponse response)
throws ServletException, IOException {
processRequest(request, response);
}
}運(yùn)行項(xiàng)目，訪問(wèn)結(jié)果如下：
$ curl http://localhost:8084/HelloJSP/user
context: /HelloJSP
request uri: /HelloJSP/user
params: {}$ curl http://localhost:8084/HelloJSP/user?name=letian
context: /HelloJSP
request uri: /HelloJSP/user
params: {name=[Ljava.lang.String;@49ea47b4}$ curl http://localhost:8084/HelloJSP/static/test.js
console.log("hello world");（這個(gè)代碼并沒(méi)什么用～）
從這段代碼中可以看到，我們可以通過(guò)request對(duì)象得到HTTP請(qǐng)求信息，特別是request URI。在這個(gè)程序的基礎(chǔ)上，我們 可以繼續(xù)擴(kuò)充它，使得其遇到某個(gè)URI，就調(diào)用指定的處理函數(shù)。慢慢地補(bǔ)充，一個(gè)框架就出來(lái)了。
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