Servlet过滤器介绍之原理分析-白红宇

．

Servlet

过滤器

1.1

什么是过滤器

过滤器是一个程序，它先于与之相关的servlet

或JSP

页面运行在服务器上。过滤器可附加到一个或多个servlet

或JSP

页面上，并且可以检查进入这些资源的请求信息。在这之后，过滤器可以作如下的选择：

①以常规的方式调用资源（即，调用servlet

或JSP

页面）。

②利用修改过的请求信息调用资源。

③调用资源，但在发送响应到客户机前对其进行修改。

④阻止该资源调用，代之以转到其他的资源，返回一个特定的状态代码或生成替换输出。

1.2 Servlet

过滤器的基本原理

在Servlet

作为过滤器使用时，它可以对客户的请求进行处理。处理完成后，它会交给下一个过滤器处理，这样，客户的请求在过滤链里逐个处理，直到请求发送到目标为止。例如，某网站里有提交“修改的注册信息”的网页，当用户填写完修改信息并提交后，服务器在进行处理时需要做两项工作：判断客户端的会话是否有效；对提交的数据进行统一编码。这两项工作可以在由两个过滤器组成的过滤链里进行处理。当过滤器处理成功后，把提交的数据发送到最终目标；如果过滤器处理不成功，将把视图派发到指定的错误页面。

．

Servlet

过滤器开发步骤

开发Servlet

过滤器的步骤如下：

①编写实现Filter

接口的Servlet

类。

②在web.xml

中配置Filter

。

开发一个过滤器需要实现Filter

接口，Filter

接口定义了以下方法：

①destory

（）由Web

容器调用，初始化此Filter

。

②init

（FilterConfig filterConfig

）由Web

容器调用，初始化此Filter

。

③doFilter

（ServletRequest request,ServletResponse response,FilterChain chain

）具体过滤处理代码。

．一个过滤器框架实例

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);

把处理发送到下一个过滤器

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);

把处理发送到下一个过滤器

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

为所有的访问做过滤

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

为所有的访问做过滤

filter-mapping

打开web

容器中任意页面输出结果：

（注意过滤器执行的请求/

响应顺序）

Within SimpleFilter1:Filtering the Request...

Within SimpleFilter2:Filtering the Request...

Within SimpleFilter2:Filtering the Response...

Within SimpleFilter1:Filtering the Response...

．报告过滤器

我们来试验一个简单的过滤器，只要调用相关的servlet

或JSP

页面，它就打印一条消息到标准输出。为实现此功能，在doFilter

方法中执行过滤行为。每当调用与这个过滤器相关的servlet

或JSP

页面时，doFilter

方法就生成一个打印输出，此输出列出请求主机和调用的URL

。因为getRequestURL

方法位于HttpServletRequest

而不是ServletRequest

中，所以把ServletRequest

对象构造为HttpServletRequest

类型。我们改动一下章节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

设置不变，同章节3

。

测试：

输入[url]http://localhost:8080/Test4Jsp/login.jsp[/url]

结果：

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...

．访问时的过滤器

（在过滤器中使用servlet

初始化参数）

下面利用init

设定一个正常访问时间范围，对那些不在此时间段的访问作出记录。我们改动一下章节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 6 a.m. is

// considered unusual.

startTime

= 22;

// 10:00 p.m.

endTime

= 6;

// 6:00 a.m.

}

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

);

(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.

(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

设置不变。

关于Tomcat

日志处理，这里补充介绍一下。config.getServletContext().log

（"log message"

）会将日志信息写入<CATALINA_HOME>/logs

文件夹下，文件名应该为localhost_log.2007-03-04.txt

这样的形式（按日期每天产生一个，第二天可以看见）。要得到这样一个日志文件，应该在server.xml

中有：

本文转自zhangjunhd51CTO博客，原文链接：http://blog.51cto.com/zhangjunhd/20629，如需转载请自行联系原作者