关于HiddenHttpMethodFilter

这个类的代码比较少，所以把整个类的代码都复制过来。在注释中添加上自己的理解。

public class HiddenHttpMethodFilter extends OncePerRequestFilter {

    /** Default method parameter: {@code _method} */
    public static final String DEFAULT_METHOD_PARAM = "_method";

    private String methodParam = DEFAULT_METHOD_PARAM;

    /**
     * Set the parameter name to look for HTTP methods.
     * @see #DEFAULT_METHOD_PARAM
     */
    public void setMethodParam(String methodParam) {
        Assert.hasText(methodParam, "'methodParam' must not be empty");
        this.methodParam = methodParam;
    }

    @Override
    protected void doFilterInternal(HttpServletRequest request, HttpServletResponse response, FilterChain filterChain)
            throws ServletException, IOException {

        HttpServletRequest requestToUse = request;
        //如果这是一个POST请求
        if ("POST".equals(request.getMethod()) && request.getAttribute(WebUtils.ERROR_EXCEPTION_ATTRIBUTE) == null) {
            //接着获取参数值比如：PUT、DELETE等
            String paramValue = request.getParameter(this.methodParam);
            //封装成的相应的请求
            if (StringUtils.hasLength(paramValue)) {
                requestToUse = new HttpMethodRequestWrapper(request, paramValue);
            }
        }
        //最后将请求发送给下一个filter
        filterChain.doFilter(requestToUse, response);
    }

    /**
     * Simple {@link HttpServletRequest} wrapper that returns the supplied method for
     * {@link HttpServletRequest#getMethod()}.
     */
    private static class HttpMethodRequestWrapper extends HttpServletRequestWrapper {

        private final String method;
　　　　
        public HttpMethodRequestWrapper(HttpServletRequest request, String method) {
            super(request);
            this.method = method.toUpperCase(Locale.ENGLISH);
        }

        @Override
        public String getMethod() {
            return this.method;
        }
    }