JavaWeb的过滤器（Filter）的两种配置

Filter接口

要成为一个Filter需要实现Filter接口

Filter生命周期

容器启动就创建 Filter 实例，先执行无参构造器，然后执行 init() 方法，以后每次要过滤的请求会执行 doFilter() 方法，容器关闭时，执行 destroy() 方法

一、xml配置

1.web.xml

在 web.xml 中配置 Filter，<filter> 指定对应的 Filter 类，<filter-mapping> 映射信息，<url-pattern> 指定过滤哪些请求，/hello.jsp ，请求 hello.jsp 时，先执行过滤器，<init-param> 设置初始化参数，<dispatcher> 告诉服务器都拦截哪些方式到达的资源

<url-pattern>：

1. 精确匹配：/路径名/资源名，要拦截资源的详细信息，/hello.jsp
2. 路径匹配：/路径名/，/拦截所有
3. 后缀匹配：*.后缀，所有一给定后缀结尾的都拦截

<dispatcher>：

1. forward：拦截转发过来的
2. include：拦截包含的（动态包含会被拦截），栗：hello.jsp 页面包含 test.jsp 页面，静态包含 <%@include file="test.jsp" %> ；动态包含 <jsp:include page="test.jsp"/>
3. request：（默认）拦截直接请求的
4. error：发生错误（全局配置的一个错误页面会被拦截，例如，发生错误404,500转到hello.jsp）

<error-page>
    <error-code>404</error-code>
    <location>/hello.jsp</location>
</error-page>
<error-page>
    <error-code>500</error-code>
    <location>/hello.jsp</location>
</error-page>

<filter>
    <filter-name>HelloFilter</filter-name>
    <filter-class>com.example.filter.HelloFilter</filter-class>
    <init-param>
        <param-name>user</param-name>
        <param-value>root</param-value>
    </init-param>
</filter>
<filter-mapping>
    <filter-name>HelloFilter</filter-name>
    <url-pattern>/hello.jsp</url-pattern>
    <dispatcher>REQUEST</dispatcher>
    <dispatcher>FORWARD</dispatcher>
</filter-mapping>

2.Filter类

当请求为 /hello.jsp?pwd=123 时通过过滤器，可以访问 hello.jsp，否则，不能够范围 /hello.jsp

chain.doFilter() 方法，请求传递到下一层

public class HelloFilter implements Filter {
    @Override
    public void init(FilterConfig filterConfig) throws ServletException {
        String user = filterConfig.getInitParameter("user");
        System.out.println(user);
    }

    @Override
    public void doFilter(ServletRequest request, ServletResponse response, FilterChain chain) throws IOException, ServletException {
        String pwd = request.getParameter("pwd");
        if (pwd != null && "123".equals(pwd)){
            chain.doFilter(request, response);
        }else {
            response.getWriter().write("failed");
        }
    }

    @Override
    public void destroy() {

    }
}

二、@WebFilter 注解配置

// WebFilter.java
@Target({ElementType.TYPE})
@Retention(RetentionPolicy.RUNTIME)
@Documented
public @interface WebFilter {
    String description() default "";

    String displayName() default "";

    WebInitParam[] initParams() default {};

    String filterName() default "";

    String smallIcon() default "";

    String largeIcon() default "";

    String[] servletNames() default {};

    String[] value() default {};

    String[] urlPatterns() default {};

    DispatcherType[] dispatcherTypes() default {DispatcherType.REQUEST};

    boolean asyncSupported() default false;
}

name 等同于 <filter-name>，urlPatterns 等同于 <url-pattern>，initParams 等同于 <init-param>

@WebFilter(filterName = "HelloFilter",
        urlPatterns = "/hello.jsp",
        initParams = {
                @WebInitParam(name = "user", value = "root")
        })

2.Filter类

@WebFilter(filterName = "HelloFilter",
        urlPatterns = "/hello.jsp",
        dispatcherTypes = {
            DispatcherType.FORWARD, DispatcherType.INCLUDE
        },
        initParams = {
                @WebInitParam(name = "user", value = "root"),
        })
public class HelloFilter implements Filter {
    @Override
    public void init(FilterConfig filterConfig) throws ServletException {
        String user = filterConfig.getInitParameter("user");
        System.out.println(user);
    }

    @Override
    public void doFilter(ServletRequest request, ServletResponse response, FilterChain chain) throws IOException, ServletException {
        String pwd = request.getParameter("pwd");
        if (pwd != null && "123".equals(pwd)){
            chain.doFilter(request, response);
        }else {
            response.getWriter().write("failed");
        }
    }

    @Override
    public void destroy() {

    }
}