API
HttpServletRequest and HttpServletResponse两个接口由容器实现，不需要码农实现，码农只需要知道两接口的方法使用。
请求参数的获取
getParameter("parameterName")返回String;
getParameterValues("parameterName")返回String[]
response:
设置响应内容类型
response.setContentType("MIME type");//必须首先设置
Common MIME types:
text/html application/pdf video/quicktime application/java image/jpeg
application/jar application/octet-stream application/x-zip
获取字符流
PrintWriter writer = response.getWriter();
writer.println("some text and html");
获取字节流
OutputStream out = response.getOutputStream();
out.write(aByteArray);
获取到流后，可以像普通I/O一样进行流操作
