Servlet過濾器介紹之原理分析
本文主要介紹Servlet過濾器的基本原理
author: ZJ
2007-2-21
1．Servlet過濾器
1.1 什麼是過濾器
過濾器是一個程序，它先於與之相關的servlet或JSP頁面運行在服務器上。過濾器可附加到一個或多個servlet或JSP頁面上，而且能夠檢查進入這些資源的請求信息。在這以後，過濾器能夠做以下的選擇：
①以常規的方式調用資源（即，調用servlet或JSP頁面）。
②利用修改過的請求信息調用資源。
③調用資源，但在發送響應到客戶機前對其進行修改。
④阻止該資源調用，代之以轉到其餘的資源，返回一個特定的狀態代碼或生成替換輸出。
1.2 Servlet過濾器的基本原理
在Servlet做爲過濾器使用時，它能夠對客戶的請求進行處理。處理完成後，它會交給下一個過濾器處理，這樣，客戶的請求在過濾鏈裏逐個處理，直到請求發送到目標爲止。例如，某網站裏有提交「修改的註冊信息」的網頁，當用戶填寫完修改信息並提交後，服務器在進行處理時須要作兩項工做：判斷客戶端的會話是否有效；對提交的數據進行統一編碼。這兩項工做能夠在由兩個過濾器組成的過濾鏈裏進行處理。當過濾器處理成功後，把提交的數據發送到最終目標；若是過濾器處理不成功，將把視圖派發到指定的錯誤頁面。
2．Servlet過濾器開發步驟
開發Servlet過濾器的步驟以下：
①編寫實現Filter接口的Servlet類。
②在web.xml中配置Filter。
開發一個過濾器須要實現Filter接口，Filter接口定義瞭如下方法：
①destory（）由Web容器調用，初始化此Filter。
②init（FilterConfig filterConfig）由Web容器調用，初始化此Filter。
③doFilter（ServletRequest request,ServletResponse response,FilterChain chain）具體過濾處理代碼。
3．一個過濾器框架實例
SimpleFilter1.java
|
package com.zj.sample;
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;
public class SimpleFilter1 implements Filter {
@SuppressWarnings("unused")
private FilterConfig filterConfig;
public void init(FilterConfig config) throws ServletException {
this.filterConfig = config;
}
public void doFilter(ServletRequest request, ServletResponse response,
FilterChain chain) {
try {
System.out.println("Within SimpleFilter1:Filtering the Request...");
chain.doFilter(request, response);// 把處理髮送到下一個過濾器
System.out .println("Within SimpleFilter1:Filtering the Response...");
} catch (IOException ioe) {
ioe.printStackTrace();
} catch (ServletException se) {
se.printStackTrace();
}
}
public void destroy() {
this.filterConfig = null;
}
}
|
SimpleFilter2.java
|
package com.zj.sample;
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;
public class SimpleFilter2 implements Filter {
@SuppressWarnings("unused")
private FilterConfig filterConfig;
public void init(FilterConfig config) throws ServletException {
this.filterConfig = config;
}
public void doFilter(ServletRequest request, ServletResponse response,
FilterChain chain) {
try {
System.out.println("Within SimpleFilter2:Filtering the Request...");
chain.doFilter(request, response); // 把處理髮送到下一個過濾器
System.out.println("Within SimpleFilter2:Filtering the Response...");
} catch (IOException ioe) {
ioe.printStackTrace();
} catch (ServletException se) {
se.printStackTrace();
}
}
public void destroy() {
this.filterConfig = null;
}
}
|
web.xml
|
<filter>
<filter-name>filter1</filter-name>
<filter-class>com.zj.sample.SimpleFilter1</filter-class>
</filter>
<filter-mapping>
<filter-name>filter1</filter-name>
<url-pattern>/*</url-pattern>//爲全部的訪問作過濾
</filter-mapping>
<filter>
<filter-name>filter2</filter-name>
<filter-class>com.zj.sample.SimpleFilter2</filter-class>
</filter>
<filter-mapping>
<filter-name>filter2</filter-name>
<url-pattern>/*</url-pattern>//爲全部的訪問作過濾
</filter-mapping>
|
打開web容器中任意頁面輸出結果：（注意過濾器執行的請求/響應順序）
Within SimpleFilter1:Filtering the Request...
Within SimpleFilter2:Filtering the Request...
Within SimpleFilter2:Filtering the Response...
Within SimpleFilter1:Filtering the Response...
4．報告過濾器
咱們來試驗一個簡單的過濾器，只要調用相關的servlet或JSP頁面，它就打印一條消息到標準輸出。爲實現此功能，在doFilter方法中執行過濾行爲。每當調用與這個過濾器相關的servlet或JSP頁面時，doFilter方法就生成一個打印輸出，此輸出列出請求主機和調用的URL。由於getRequestURL方法位於HttpServletRequest而不是ServletRequest中，因此把ServletRequest對象構造爲HttpServletRequest類型。咱們改動一下章節3的SimpleFilter1.java。
SimpleFilter1.java
|
package com.zj.sample;
import java.io.IOException;
import java.util.Date;
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.HttpServletRequest;
public class SimpleFilter1 implements Filter {
@SuppressWarnings("unused")
private FilterConfig filterConfig;
public void init(FilterConfig config) throws ServletException {
this.filterConfig = config;
}
public void doFilter(ServletRequest request, ServletResponse response,
FilterChain chain) {
try {
System.out.println("Within SimpleFilter1:Filtering the Request...");
HttpServletRequest req = (HttpServletRequest) request;
System.out.println(req.getRemoteHost() + " tried to access "
+ req.getRequestURL() + " on " + new Date() + ".");
chain.doFilter(request, response);
System.out.println("Within SimpleFilter1:Filtering the Response...");
} catch (IOException ioe) {
ioe.printStackTrace();
} catch (ServletException se) {
se.printStackTrace();
}
}
public void destroy() {
this.filterConfig = null;
}
}
|
web.xml設置不變，同章節3。
測試：
輸入[url]http://localhost:8080/Test4Jsp/login.jsp[/url]
結果：
Within SimpleFilter1:Filtering the Request...
0:0:0:0:0:0:0:1 tried to access [url]http://localhost:8080/Test4Jsp/login.jsp[/url] on Sun Mar 04 17:01:37 CST 2007.
Within SimpleFilter2:Filtering the Request...
Within SimpleFilter2:Filtering the Response...
Within SimpleFilter1:Filtering the Response...
5．訪問時的過濾器（在過濾器中使用servlet初始化參數）
下面利用init設定一個正常訪問時間範圍，對那些不在此時間段的訪問做出記錄。咱們改動一下章節3的SimpleFilter2.java。
SimpleFilter2.java。
|
package com.zj.sample;
import java.io.IOException;
import java.text.DateFormat;
import java.util.Calendar;
import java.util.GregorianCalendar;
import javax.servlet.Filter;
import javax.servlet.FilterChain;
import javax.servlet.FilterConfig;
import javax.servlet.ServletContext;
import javax.servlet.ServletException;
import javax.servlet.ServletRequest;
import javax.servlet.ServletResponse;
import javax.servlet.http.HttpServletRequest;
public class SimpleFilter2 implements Filter {
@SuppressWarnings("unused")
private FilterConfig config;
private ServletContext context;
private int startTime, endTime;
private DateFormat formatter;
public void init(FilterConfig config) throws ServletException {
this.config = config;
context = config.getServletContext();
formatter = DateFormat.getDateTimeInstance(DateFormat.MEDIUM,
DateFormat.MEDIUM);
try {
startTime = Integer.parseInt(config.getInitParameter("startTime"));// web.xml
endTime = Integer.parseInt(config.getInitParameter("endTime"));// web.xml
} catch (NumberFormatException nfe) { // Malformed or null
// Default: access at or after 10 p.m. but before
// considered unusual.
startTime = 22; // 10:00 p.m.
endTime = 6; // 6:
}
}
public void doFilter(ServletRequest request, ServletResponse response,
FilterChain chain) {
try {
System.out.println("Within SimpleFilter2:Filtering the Request...");
HttpServletRequest req = (HttpServletRequest) request;
GregorianCalendar calendar = new GregorianCalendar();
int currentTime = calendar.get(Calendar.HOUR_OF_DAY);
if (isUnusualTime(currentTime, startTime, endTime)) {
context.log("WARNING: " + req.getRemoteHost() + " accessed "
+ req.getRequestURL() + " on "
+ formatter.format(calendar.getTime()));
// The log file is under <CATALINA_HOME>/logs.One log per day.
}
chain.doFilter(request, response);
System.out
.println("Within SimpleFilter2:Filtering the Response...");
} catch (IOException ioe) {
ioe.printStackTrace();
} catch (ServletException se) {
se.printStackTrace();
}
}
public void destroy() {}
// Is the current time between the start and end
// times that are marked as abnormal access times?
private boolean isUnusualTime(int currentTime, int startTime, int endTime) {
// If the start time is less than the end time (i.e.,
// they are two times on the same day), then the
// current time is considered unusual if it is
// between the start and end times.
if (startTime < endTime) {
return ((currentTime >= startTime) && (currentTime < endTime));
}
// If the start time is greater than or equal to the
// end time (i.e., the start time is on one day and
// the end time is on the next day), then the current
// time is considered unusual if it is NOT between
// the end and start times.
else {
return (!isUnusualTime(currentTime, endTime, startTime));
}
}
}
|
web.xml設置不變。
關於Tomcat日誌處理，這裏補充介紹一下。config.getServletContext().log（"log message"）會將日誌信息寫入<CATALINA_HOME>/logs文件夾下，文件名應該爲localhost_log.
2007-03-04
.txt這樣的形式（按日期天天產生一個，次日能夠看見）。要獲得這樣一個日誌文件，應該在server.xml中有：
|
<Logger className="org.apache.catalina.logger.FileLogger" prefix="catalina_log." suffix=".txt" timestamp="true"/>
|
參考資料
[1] Marty Halls ，Servlet與JSP權威指南，機械工業出版社
[2] 趙強，精通JSP編程，電子工業出版社
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