Filter

Posted 2021-03-23 howlet

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Filter 过滤器

过滤器是实现了Filter接口的一个java类，可以处理request和response，它有下面三种方法

public void destroy() {
}

public void doFilter(ServletRequest request, ServletResponse response, FilterChain chain)
}

public void init(FilterConfig filterConfig) throws ServletException {
}

工作区间

操作流程

实现Filter接口

//注解配置（@WebFilter(filterName="FilterTest",urlPatterns="/*"))
//但我这里用xml配置
public class FilterTest implements Filter {

public void destroy() {
}

public void doFilter(ServletRequest request, ServletResponse response, FilterChain chain)
        throws IOException, ServletException {
    
    //需要执行的逻辑代码在这里
    System.out.println("我是首个Filter");
    
    //表示放行
    chain.doFilter(request, response);
}

public void init(FilterConfig filterConfig) throws ServletException {
}

web.xml配置

<filter>
    <filter-name>FilterTest</filter-name>
    <filter-class>filter.FilterTest</filter-class>
</filter>
<filter-mapping>
    <filter-name>FilterTest</filter-name>
    <url-pattern>/*</url-pattern>
</filter-mapping>

• xml配置的顺序就是Filter的执行顺序，而注解方式是通过匹配地址的自然顺序执行的，这是注解的一个遗憾

其中chain.doFilter表示放行，配置多个过滤器时会把这些过滤器放到FilterChain里，调用它就会带上request,和response，自动执行下一个过滤器

这样的调用属于链式调用，类似于递归，调用完后还会回到该函数继续执行下面的代码

chain.doFilter如果没有下一个过滤器就访问资源

作用

过滤POST乱码

@Override
public void doFilter(ServletRequest request, ServletResponse response, FilterChain chain)
        throws IOException, ServletException {
    
    //ServletRequest功能不够强大，需要强转成HttpSerlvet
    HttpServletRequest httpServletRequest = (HttpServletRequest) request;
    HttpServletResponse httpServletResponse = (HttpServletResponse) response;
    
    //设置编码
    httpServletRequest.setCharacterEncoding("UTF-8");
    httpServletResponse.setContentType("text/html;charset=UTF-8");
    
    //表示放行
    chain.doFilter(request, response);
}

过滤Get乱码（使用到了 装饰者模式)

装饰类

class MyRequest extends HttpServletRequestWrapper {

    private HttpServletRequest request;

    public MyRequest(HttpServletRequest request) {
        this.request = request;
    }

    public String getParameter(String name) {
        String value = this.request.getParameter(name);

        if (value == null) {
            return null;
        }

        //如果不是get方法的，直接返回就行了
        if (!this.request.getMethod().equalsIgnoreCase("get")) {
            return null;
        }

        try {

            //进来了就说明是get方法，把乱码的数据
            value = new String(value.getBytes("ISO8859-1"), this.request.getCharacterEncoding());
            return value ;

        } catch (UnsupportedEncodingException e) {
            e.printStackTrace();

            throw new RuntimeException("不支持该编码");
        }
    }
}

实现

MyRequest myRequest = new MyRequest(httpServletRequest);
//装饰增加新功能

参考Java3y