过滤器（Filter)是 J2EE Servlet模块性的组件。Filter的作用是对URL进行统一的拦截处理。Filter通常用于应用程序层面进行全局处理。
1. 实现Filter
开发过滤器三要素
- 任何过滤器都要实现javax.servlet.Filter接口
- 在 Filter接口的 doFilter()方法中编写过滤器的功能代码。
- 在web.xml中对过滤器进行配置，说明拦截URL的范围
接下来具体实现：
- xml配置方式：
public class FirstFilter implements Filter {
public void destroy() {
//...
}
public void doFilter(ServletRequest req, ServletResponse resp, FilterChain chain) throws ServletException, IOException {
System.out.println("过滤器FirstFilter已生效");
chain.doFilter(req,resp);
}
public void init(FilterConfig config) throws ServletException {
//...
}
}
First
Filter.FirstFilter
First
/*
- 注解@WebFilter配置方式
| 属性名 | 类型 | 描述 |
|---|---|---|
| filterName | String | 指定过滤器的 name 属性，等价于 |
| value | String[] | 该属性等价于 urlPatterns 属性。但是两者不应该同时使用。 |
| urlPatterns | String[] | 指定一组过滤器的 URL 匹配模式。等价于 |
| servletNames | String[] | 指定过滤器将应用于哪些 Servlet。取值是 @WebServlet 中的 name 属性的取值，或者是 web.xml 中 |
| dispatcherTypes | DispatcherType | 指定过滤器的转发模式。具体取值包括：ASYNC、ERROR、FORWARD、INCLUDE、REQUEST。 |
| initParams | WebInitParam[] | 指定一组过滤器初始化参数，等价于 |
| asyncSupported | boolean | 声明过滤器是否支持异步操作模式，等价于 |
| description | String | 该过滤器的描述信息，等价于 |
| displayName | String | 该过滤器的显示名，通常配合工具使用，等价于 |
@WebFilter(filterName = "FirstFilter", urlPatterns = "/*")
public class FirstFilter implements Filter { //...}
2. 过滤器生命周期
- 初始化 ：
Filter.init()服务器启动后 - 提供服务 ：
Filter.doFilter() - 销毁 ：
Filter.destroy()服务器关闭前
过滤器对象在Web应用启动时被创建且全局唯一。唯一的过滤器 对象在并发环境中采用”多线程“提供服务。
3. 过滤器参数化
过滤器为了增强灵活性，允许配置信息放在web.xml。在web.xml中配置
- xml方式
First
Filter.FirstFilter
encoding
UTF-8
First
/*
public class FirstFilter implements Filter {
String encoding;
public void init(FilterConfig config) throws ServletException {
encoding = config.getInitParameter("encoding");
}
......
}
- 注解方式
@WebFilter(filterName = "FirstFilter", urlPatterns = "/*",
initParams = {
@WebInitParam(name = "encoding",value = "UTF-8"),
@WebInitParam(name = "P2",value = "V2")
})
public class FirstFilter implements Filter {
String encoding;
public void init(FilterConfig config) throws ServletException {
encoding = config.getInitParameter("encoding");
}
}
4. 过滤链
过滤链
过滤器的执行顺序以
chain.doFilter()将请求向后转递。
- xml方式
FilterA
Filter.FilterA
FilterA
/*
FilterB
Filter.FilterB
FilterB
/*
FilterC
Filter.FilterC
FilterC
/*
public class FilterA implements Filter {
public void doFilter(ServletRequest req, ServletResponse resp, FilterChain chain) throws ServletException, IOException {
System.out.println("A is printed")
chain.doFilter(req,resp);
}
}
public class FilterB implements Filter {
public void doFilter(ServletRequest req, ServletResponse resp, FilterChain chain) throws ServletException, IOException {
System.out.println("B is printed")
chain.doFilter(req,resp);
}
}
public class FilterC implements Filter {
public void doFilter(ServletRequest req, ServletResponse resp, FilterChain chain) throws ServletException, IOException {
System.out.println("C is printed")
chain.doFilter(req,resp);
}
}
public class FirstServlet extends HttpServlet {
protected void service(HttpServletRequest req, HttpServletResponse resp) throws ServletException, IOException {
System.out.println("Servlet is printed")
}
}
输出：
A is printed
B is printed
C is printed
Servlet is printed
如果将输出语句位置调换
public class FilterA implements Filter {
public void doFilter(ServletRequest req, ServletResponse resp, FilterChain chain) throws ServletException, IOException {
chain.doFilter(req,resp);
System.out.println("A is printed")
}
}
输出：
Servlet is printed
C is printed
B is printed
A is printed
- 注解方式
使用注解方式时，过滤链按照名称的字典顺序，很不合理。
eg:字符集过滤器
Web中文乱码的解决
- GET请求 ：server.xml增加URIEncoding="UTF-8"
- POST请求 ：使用request.setCharacterEncoding("UTF-8")
- 响应 response.setContentType("text/html;charset=UTF-8")
@WebFilter(filterName = "CharacterEncodingFilter")
public class CharacterEncodingFilter implements Filter {
public void destroy() {
}
public void doFilter(ServletRequest req, ServletResponse resp, FilterChain chain) throws ServletException, IOException {
//因为Tomcat中request的实现类继承自HttpServletRequest
//所以可以转化为HttpServletRequest，以防ServletRequest不含有某些方法
//详情见前面的 Java Web(3）HttpServletRequest和HttpServletResponse
HttpServletRequest request = (HttpServletRequest) req;
HttpServletResponse response = (HttpServletResponse) resp;
request.setCharacterEncoding("UTF-8");
response.setContentType("text/html;charset=UTF-8");
chain.doFilter(request, response);
}
public void init(FilterConfig config) throws ServletException {
}
}