This method allows one servlet to do divliminary processing of a request and another resource to generate the response. 该方法允许请求和其他资源生成响应之前做一些预处理。
Errors generated during later stages of processing refer to the line numbers of the preprocessed file rather than the original source file. 在处理后期生成的错误引用的是预处理文件的行号而非原始源文件的行号。