步骤1:Filter概念
步骤2:编写 FirstFilter
步骤3:配置web.xml
步骤4:访问页面
步骤5:init() 方法
步骤6:Filter启动失败
步骤 1 : Filter概念
Filter就像一个一个哨卡，用户的请求需要经过Filter
并且可以有多个过滤器
步骤 2 : 编写 FirstFilter
开发一个简单的FirstFilter，用来打印用户访问ip地址和访问的页面
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HttpServletRequest request = (HttpServletRequest) req;
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doFilter()方法中的req参数的类型是ServletRequest，需要转换为HttpServletRequest类型方便调用某些方法 (参考request常见方法)
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String ip = request.getRemoteAddr();
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获取来路用户的ip地址
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String url = request.getRequestURL().toString();
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获取用户访问的页面地址
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System.out.printf("%s %s 访问了 %s%n", date, ip, url);
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在控制台打印出来
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chain.doFilter(request, response);
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过滤器放行，表示继续运行下一个过滤器，或者最终访问的某个servlet,jsp,html等等
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package filter;
import java.io.IOException;
import java.text.SimpleDateFormat;
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;
import javax.servlet.http.HttpServletResponse;
public class FirstFilter implements Filter {
@Override
public void destroy() {
}
@Override
public void doFilter(ServletRequest req, ServletResponse res, FilterChain chain)
throws IOException, ServletException {
HttpServletRequest request = (HttpServletRequest) req;
HttpServletResponse response = (HttpServletResponse) res;
String ip = request.getRemoteAddr();
String url = request.getRequestURL().toString();
SimpleDateFormat sdf = new SimpleDateFormat("yyyy-MM-dd HH:mm:ss");
Date d = new Date();
String date = sdf.format(d);
System.out.printf("%s %s 访问了 %s%n", date, ip, url);
chain.doFilter(request, response);
}
@Override
public void init(FilterConfig arg0) throws ServletException {
}
}
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步骤 3 : 配置web.xml
在web.xml中进行filter的配置，和servlet的配置很类似
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<url-pattern>/*</url-pattern>
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表示所有的访问都会过滤
如果配置成
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<url-pattern>*.jsp</url-pattern>
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就表示只过滤jsp
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<filter>
<filter-name>FirstFilter</filter-name>
<filter-class>filter.FirstFilter</filter-class>
</filter>
<filter-mapping>
<filter-name>FirstFilter</filter-name>
<url-pattern>/*</url-pattern>
</filter-mapping>
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步骤 4 : 访问页面
重启tomcat,所有用户的访问信息，都可以打印出来
步骤 5 : init() 方法
与Servlet需要配置自启动才会随着tomcat的启动而执行init()方法不一样。
Filter一定会随着tomcat的启动自启动。
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package filter;
import java.io.IOException;
import java.text.SimpleDateFormat;
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;
import javax.servlet.http.HttpServletResponse;
public class FirstFilter implements Filter {
@Override
public void destroy() {
}
@Override
public void doFilter(ServletRequest req, ServletResponse res, FilterChain chain)
throws IOException, ServletException {
HttpServletRequest request = (HttpServletRequest) req;
HttpServletResponse response = (HttpServletResponse) res;
String ip = request.getRemoteAddr();
String url = request.getRequestURL().toString();
SimpleDateFormat sdf = new SimpleDateFormat("yyyy-MM-dd HH:mm:ss");
Date d = new Date();
String date = sdf.format(d);
System.out.printf("%s %s 访问了 %s%n", date, ip, url);
chain.doFilter(request, response);
}
@Override
public void init(FilterConfig arg0) throws ServletException {
System.out.println("First Filter init()");
}
}
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步骤 6 : Filter启动失败
Filter是web应用非常重要的一个环节，如果Filter启动失败，或者本身有编译错误，不仅这个Filter不能使用，整个web应用会启动失败，导致用户无法访问页面
在启动tomcat过程中，也会看到这样的字样：
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严重: Context [] startup failed due to previous errors
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这常常用于提示Filter启动失败了
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package filter;
import java.io.IOException;
import java.text.SimpleDateFormat;
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;
import javax.servlet.http.HttpServletResponse;
public class FirstFilter implements Filter {
@Override
public void destroy() {
}
@Override
public void doFilter(ServletRequest req, ServletResponse res, FilterChain chain)
throws IOException, ServletException {
HttpServletRequest request = (HttpServletRequest) req;
HttpServletResponse response = (HttpServletResponse) res;
String ip = request.getRemoteAddr();
String url = request.getRequestURL().toString();
SimpleDateFormat sdf = new SimpleDateFormat("yyyy-MM-dd HH:mm:ss");
Date d = new Date();
String date = sdf.format(d);
System.out.printf("%s %s 访问了 %s%n", date, ip, url);
chain.doFilter(request, response);
}
@Override
public void init(FilterConfig arg0) throws ServletException {
System.out.println("故意造成一个异常");
Object o = null;
o.toString();
}
}
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