Servlet过滤器：FilterConfig用法

Web容器创建FilterConfig的对象。该对象可用于从web.xml文件获取配置信息。

FilterConfig接口的方法

FilterConfig界面中有以下4种方法。

1. public void init(FilterConfig config)：仅在将init()方法用于初始化过滤器后才调用它。
2. public String getInitParameter(String parameterName)：返回指定参数名称的参数值。
3. public java.util.Enumeration getInitParameterNames()：返回包含所有参数名称的枚举。
4. public ServletContext getServletContext()：返回ServletContext对象。

FilterConfig的示例

在此示例中, 如果将param-value更改为no, 则请求将被转发到servlet, 否则过滤器将创建带有以下消息的响应：该页面正在处理中。让我们看一下FilterConfig的简单示例。在这里, 我们创建了4个文件：

• index.html
• MyFilter.java
• HelloServlet.java
• web.xml

index.html

<a href="servlet1">click here</a>

MyFilter.java

import java.io.IOException;
import java.io.PrintWriter;

import javax.servlet.*;

public class MyFilter implements Filter{
FilterConfig config;

public void init(FilterConfig config) throws ServletException {
	this.config=config;
}

public void doFilter(ServletRequest req, ServletResponse resp, FilterChain chain) throws IOException, ServletException {
	
	PrintWriter out=resp.getWriter();
		
	String s=config.getInitParameter("construction");
		
	if(s.equals("yes")){
         out.print("This page is under construction");
	}
	else{
         chain.doFilter(req, resp);//sends request to next resource
	}
		
}
public void destroy() {}
}

HelloServlet.java

import java.io.IOException;
import java.io.PrintWriter;

import javax.servlet.ServletException;
import javax.servlet.http.*;

public class HelloServlet extends HttpServlet {
public void doGet(HttpServletRequest request, HttpServletResponse response)
		throws ServletException, IOException {

		response.setContentType("text/html");
		PrintWriter out = response.getWriter();
	
		out.print("<br>welcome to servlet<br>");
		
	}

}

web.xml

<web-app>

 <servlet>
    <servlet-name>HelloServlet</servlet-name>
    <servlet-class>HelloServlet</servlet-class>
  </servlet>

  <servlet-mapping>
    <servlet-name>HelloServlet</servlet-name>
    <url-pattern>/servlet1</url-pattern>
  </servlet-mapping>
  
  <filter>
  <filter-name>f1</filter-name>
  <filter-class>MyFilter</filter-class>
  <init-param>
  <param-name>construction</param-name>
  <param-value>no</param-value>
  </init-param>
  </filter>
  <filter-mapping>
  <filter-name>f1</filter-name>
  <url-pattern>/servlet1</url-pattern>
  </filter-mapping>
  
  
</web-app>

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