java Filter__配置和讲解(二)

过滤器Filter也具有生命周期：init()->doFilter()->destroy()，由部署文件中的filter元素驱动。在servlet2.4中，过滤器同样可以用于请求分派器，但须在web.xml中声明，<dispatcher>INCLUDE或FORWARD或REQUEST或ERROR</dispatcher>该元素位于filter-mapping中。

一、批量设置请求编码

Java代码

public class EncodingFilter implements Filter {

private String encoding = null;

public void destroy() {

encoding = null;

}

public void doFilter(ServletRequest request, ServletResponse response,

FilterChain chain) throws IOException, ServletException {

String encoding = getEncoding();

if (encoding == null){

encoding = "gb2312";

}

request.setCharacterEncoding(encoding);// 在请求里设置上指定的编码

chain.doFilter(request, response);

}

public void init(FilterConfig filterConfig) throws ServletException {

this.encoding = filterConfig.getInitParameter("encoding");

}

private String getEncoding() {

return this.encoding;

}

}

public class EncodingFilter implements Filter {

private String encoding = null;

public void destroy() {
encoding = null;
}

public void doFilter(ServletRequest request, ServletResponse response,
FilterChain chain) throws IOException, ServletException {
String encoding = getEncoding();
if (encoding == null){
encoding = "gb2312";
}
request.setCharacterEncoding(encoding);// 在请求里设置上指定的编码
chain.doFilter(request, response);
}

public void init(FilterConfig filterConfig) throws ServletException {
this.encoding = filterConfig.getInitParameter("encoding");
}

private String getEncoding() {
return this.encoding;
}

}

Xml代码

<filter>

<filter-name>EncodingFilter</filter-name>

<filter-class>com.logcd.filter.EncodingFilter</filter-class>

<init-param>

<param-name>encoding</param-name>

<param-value>gb2312</param-value>

</init-param>

</filter>

<filter-mapping>

<filter-name>EncodingFilter</filter-name>

<url-pattern>/*</url-pattern>

</filter-mapping>

<filter>
<filter-name>EncodingFilter</filter-name>
<filter-class>com.logcd.filter.EncodingFilter</filter-class>
<init-param>
<param-name>encoding</param-name>
<param-value>gb2312</param-value>
</init-param>
</filter>

<filter-mapping>
<filter-name>EncodingFilter</filter-name>
<url-pattern>/*</url-pattern>
</filter-mapping>

二、用filter控制用户访问权限

Java代码

public void doFilter(ServletRequest request,

ServletResponse response,

FilterChain chain)

throws IOException, ServletException {

HttpServletRequest req = (HttpServletRequest) request;

HttpServletResponse res = (HttpServletResponse) response;

HttpSession session = req.getSession();

if (session.getAttribute("username") != null) {//登录后才能访问

chain.doFilter(request, response);

} else {

res.sendRedirect("../failure.jsp");

}

}

public void doFilter(ServletRequest request,
ServletResponse response,
FilterChain chain)
throws IOException, ServletException {

HttpServletRequest req = (HttpServletRequest) request;
HttpServletResponse res = (HttpServletResponse) response;

HttpSession session = req.getSession();
if (session.getAttribute("username") != null) {//登录后才能访问
chain.doFilter(request, response);
} else {
res.sendRedirect("../failure.jsp");
}
}

Xml代码

<filter>

<filter-name>SecurityFilter</filter-name>

<filter-class>com.logcd.filter.SecurityFilter</filter-class>

</filter>

<filter-mapping>

<filter-name>SecurityFilter</filter-name>

<url-pattern>/admin/*</url-pattern>

</filter-mapping>

<filter>
<filter-name>SecurityFilter</filter-name>
<filter-class>com.logcd.filter.SecurityFilter</filter-class>
</filter>
<filter-mapping>
<filter-name>SecurityFilter</filter-name>
<url-pattern>/admin/*</url-pattern>
</filter-mapping>

三、过滤链

两个过滤器，EncodingFilter负责设置编码，SecurityFilter负责控制权限，服务器会按照web.xml中过滤器定义的先后循序组装成一条链，然后一次执行其中的doFilter()方法。执行的顺序就如上图所示，执行第一个过滤器的chain.doFilter()之前的代码，第二个过滤器的chain.doFilter()之前的代码，请求的资源，第二个过滤器的chain.doFilter()之后的代码，第一个过滤器的chain.doFilter()之后的代码，最后返回响应。
执行的代码顺序是：

执行EncodingFilter.doFilter()中chain.doFilter()之前的部分：request.setCharacterEncoding("gb2312");

执行SecurityFilter.doFilter()中chain.doFilter()之前的部分：判断用户是否已登录。

如果用户已登录，则访问请求的资源：/admin/index.jsp。

如果用户未登录，则页面重定向到：/failure.jsp。

执行SecurityFilter.doFilter()中chain.doFilter()之后的部分：这里没有代码。

执行EncodingFilter.doFilter()中chain.doFilter()之后的部分：这里也没有代码。

过滤链的好处是，执行过程中任何时候都可以打断，只要不执行chain.doFilter()就不会再执行后面的过滤器和请求的内容。而在实际使用时，就要特别注意过滤链的执行顺序问题，像EncodingFilter就一定要放在所有Filter之前，这样才能确保在使用请求中的数据前设置正确的编码。