Java 三大器之过滤器(Filter)工作原理

Filter 简介

filter称为过滤器，可以对web 服务器管理的所有资源: Jsp, Servlet，静态图片文件或静态html文件等进行拦截，从而实现一些特殊的功能。例如实现URL级别的权限访问控制，过滤敏感词汇，压缩响应信息等高级功能。

public class LogCostFilter implements Filter {
    public abstract void init(FilterConfig paramFilterConfig) throws ServletException;
    public abstract void doFilter(ServletRequest paramServletRequest, ServletResponse paramServletResponse, FilterChain 
        paramFilterChain) throws IOException, ServletException;
    public abstract void destroy();
}

工作原理

doFilter方法中，配置对哪个web 资源进行拦截后，WEB服务器每次在调用web资源service方法之前，都会先调用一下filter的doFilter方法，因此，在该方法内编写代码可以达到此目的：

• 调用目标资源之前，让一段代码执行
• 是否调用目标资源（即是否让用户访问web 资源）
• 调用目标资源之后，让一段代码执行(filterChian)

Demo

LogCostFilter

public class LogCostFilter implements Filter {
    @Override
    public void init(FilterConfig filterConfig) throws ServletException {
        System.out.println("---Filter初始化---");
    }

    @Override
    public void doFilter(ServletRequest servletRequest, ServletResponse servletResponse, FilterChain filterChain) throws IOException, ServletException {
        servletRequest.setCharacterEncoding("UTF-8");
        servletResponse.setContentType("text/html;charset=UTF-8");
        long start = System.currentTimeMillis();
        System.out.println("---调用service之前执行的一段代码---");
        filterChain.doFilter(servletRequest, servletResponse);
        System.out.println("Execute cost="+(System.currentTimeMillis()-start));
    }

    @Override
    public void destroy() {
        System.out.println("---Filter销毁---");
    }
}

FilterConfig

@Configuration
public class FilterConfig {
    @Bean
    public FilterRegistrationBean registFilter() {
        FilterRegistrationBean registrationBean = new FilterRegistrationBean();
        registrationBean.setFilter(new LogCostFilter());
        registrationBean.addUrlPatterns("/*");
        registrationBean.setName("LogCostFilter");
        registrationBean.setOrder(1);
        return registrationBean;
    }
}

过滤字符串标准代码

public class CharacterEncodingFilter extends OncePerRequestFilter{
    private static final boolean responseSetCharacterEncodingAvailable = ClassUtils.hasMethod(
            HttpServletResponse.class, "setCharacterEncoding", new Class[] { String.class });

    // 需要设置的编码方式，为了支持可配置，Spring把编码方式设置成了一个变量
    private String encoding;
    // 是否强制使用统一编码，也是为了支持可配置
    private boolean forceEncoding;
    // 构造器，在这里，Spring把forceEncoding的值默认设置成了false
    public CharacterEncodingFilter(){
        this.forceEncoding = false;
    }
    // encoding/forceEncoding的setter方法
    public void setEncoding(String encoding){
        this.encoding = encoding;
    }
    public void setForceEncoding(boolean forceEncoding){
        this.forceEncoding = forceEncoding;
    }
    // Spring通过GenericFilterBean抽象类，对Filter接口进行了整合，
    protected void doFilterInternal(HttpServletRequest request, HttpServletResponse response, FilterChain filterChain)
            throws ServletException, IOException{
        if ((this.encoding != null) && (((this.forceEncoding) || (request.getCharacterEncoding() == null)))) {
            request.setCharacterEncoding(this.encoding);
            if ((this.forceEncoding) && (responseSetCharacterEncodingAvailable)) {
                response.setCharacterEncoding(this.encoding);
            }
        }
        filterChain.doFilter(request, response);
    }
}