Day2 Java Web 框架的設計--開始

DAY 2

Java Web的設計與應用系列第 2 篇

鐵人賽

Milo Wang

2014-10-01 13:40:42

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準備技術:

Servlet
Servlet Container
HTTPServlet Request/Response
Sevlet Filter
Servlet Listener
Web是靠HTTP協定傳輸Server與Client之間的資料。
Servlet是一個Class，Container是一個抽象概念，管理一些東西，而Servlet Container是管理Servlet物件，負責HTTP資料處理。
Java Web Server的設計，是包含Container，Container種類眾多，其中是Servlet Container，多數是這種。
Servlet典型的處理HTTP程式。

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;

@WebServlet("/demoServlet")
public class demoServlet extends HttpServlet {
private static final long serialVersionUID = 1L;
```
 public demoServlet() {
     super();

 }

 protected void doGet(HttpServletRequest request, HttpServletResponse response) throws ServletException, IOException {
 //處理HTTP GET方式
 }


 protected void doPost(HttpServletRequest request, HttpServletResponse response) throws ServletException, IOException {
     //處理HTTP POST方式
 }
```
}

一個Servlet Container其實也是Servlet，只是提供管理功能，一般是Java Web Server來實做，框架本身如果要實做，就要先處理HTTP進來後轉到框架的Filter。

import javax.servlet.*;
import java.io.IOException;
import java.io.PrintWriter;

public class PrimitiveServlet implements Servlet {

 public void init(ServletConfig config) throws ServletException { 
     System.out.println("init"); 
 } 

 public void service(ServletRequest request, ServletResponse response) throws ServletException, IOException { 
     System.out.println("from service"); 
     PrintWriter out = response.getWriter(); 
     out.println("Hello. Roses are red."); 
     out.print("Violets are blue."); 
 } 

 public void destroy() { 
     System.out.println("destroy"); 
 } 

 public String getServletInfo() { return null; } 

 public ServletConfig getServletConfig() { return null; }

}

什麼是HTTPServletRequest/Response?Java提供介面給Java Servlet物件實做，包裝(Wrapper)了HTTP Request/Response，處理HTTP資料。
在第五點提到Filter，當Client傳送HTTP Request給Server時，如果Servlet想要過濾訊息，可設下機制，就是一個Filter，Filter是Servlet的一個功能。

基本的Filter舉例，攔截傳輸的 HTTP log資料。

import java.io.IOException;
import java.io.PrintWriter;
import java.util.Date;
import javax.servlet.;
import javax.servlet.annotation.WebFilter;
import javax.servlet.http.;

@WebFilter("/*")
public class LoggingFilter implements Filter {

 private FilterConfig filterConf = null;

 public void init(FilterConfig filterConf) {
     this.filterConf = filterConf;
 }

 //如果有多個Filter ，要設定FilterChain，並且設定順序，Filter會一個個順序執行。
 public void doFilter(ServletRequest request,ServletResponse response,FilterChain chain) throws IOException, ServletException {

     String userName = request.getRemoteHost();

     //攔截Log的使用者IP
     filterConf.getServletContext().log("User IP: " + userName);
     chain.doFilter(request, response);
 }

 @Override
 public void destroy() {
     throw new UnsupportedOperationException("Not supported yet.");
 }

}

Servlet提供Listener機制觀看Servlet運作，Listener種類眾多，這裡提供一個Listener程式來看Servlet 的Context狀況。

import java.util.Properties;
import javax.mail.Message;
import javax.mail.Session;
import javax.mail.Transport;
import javax.mail.internet.InternetAddress;
import javax.mail.internet.MimeMessage;
import javax.servlet.ServletContextListener;
import javax.servlet.ServletContextEvent;
import javax.servlet.annotation.WebListener;

@WebListener
public class StartupShutdownListener implements ServletContextListener {

 //Context初始，送一封信給系統。
 public void contextInitialized(ServletContextEvent event) {
     System.out.println("Servlet startup. . .");
     System.out.println(event.getServletContext().getServerInfo());
     System.out.println(System.currentTimeMillis());
     sendEmail("Servlet context has initialized");
 }

 //Context結束，送一封信給系統。
 public void contextDestroyed(ServletContextEvent event) {
     System.out.println("Servlet shutdown. . .");
     System.out.println(event.getServletContext().getServerInfo());
     System.out.println(System.currentTimeMillis());

     sendEmail("Servlet context has been destroyed. . .");

     private boolean sendEmail(String message) {
         boolean result = false;

         //借用Google Gmail發信
         String smtpHost = "smtp.gmail.com";
         String smtpUsername = "username";
         String smtpPassword = "password";
         String from = "fromaddress";
         String to = "toaddress";
         int smtpPort = 587;
         System.out.println("sending email. . .");
     try {
         //利用 java property存入一些資料
         Properties props = new Properties();
         props.put("mail.smtp.host", smtpHost);
         props.put("mail.smtp.auth", "true");
         props.put("mail.smtp.starttls.enable", "true");

         //信存入到Session暫存
         Session session = Session.getInstance(props);
         Message msg = new MimeMessage(session);

         InternetAddress addressFrom = new InternetAddress(from);
         msg.setFrom(addressFrom);
         InternetAddress[] address = new InternetAddress[1];
         address[0] = new InternetAddress(to);

         //信的內容
         msg.setRecipients(Message.RecipientType.TO, address);
         msg.setSubject("Servlet container shutting down");
         msg.setContent(message, "text/plain");

         //信件送出設定
         Transport transport = session.getTransport("smtp");
         transport.connect(smtpHost, smtpPort, smtpUsername, smtpPassword);
         Transport.send(msg);

         result = true;
     } catch (javax.mail.MessagingException ex) {
         ex.printStackTrace();
         result = false;
     }
     return result;
     }
 }

參考資料

Day2 結束