web服務器接收到客户端http請求，針對這個請求，分別創建一個代表請求的HttpServletRequest對象，代表響應的一個HttpServletResponse；

• 如果要獲取客户端請求過來的參數：找HttpServletRequest
• 如果要給客户端響應一些信息：找HttpServletResponse

1、簡單分類

負責向瀏覽器發送數據的方法

 ServletOutputStream getOutputStream() throws IOException;

    PrintWriter getWriter() throws IOException;

負責向瀏覽器發送響應頭的方法

void setCharacterEncoding(String var1);

    void setContentLength(int var1);

    void setContentLengthLong(long var1);

    void setContentType(String var1);

     void setDateHeader(String var1, long var2);

    void addDateHeader(String var1, long var2);

    void setHeader(String var1, String var2);

    void addHeader(String var1, String var2);

    void setIntHeader(String var1, int var2);

    void addIntHeader(String var1, int var2);

響應的狀態碼

 int SC_CONTINUE = 100;
    int SC_SWITCHING_PROTOCOLS = 101;
    int SC_OK = 200;
    int SC_CREATED = 201;
    int SC_ACCEPTED = 202;
    int SC_NON_AUTHORITATIVE_INFORMATION = 203;
    int SC_NO_CONTENT = 204;
    int SC_RESET_CONTENT = 205;
    int SC_PARTIAL_CONTENT = 206;
    int SC_MULTIPLE_CHOICES = 300;
    int SC_MOVED_PERMANENTLY = 301;
    int SC_MOVED_TEMPORARILY = 302;
    int SC_FOUND = 302;
    int SC_SEE_OTHER = 303;
    int SC_NOT_MODIFIED = 304;
    int SC_USE_PROXY = 305;
    int SC_TEMPORARY_REDIRECT = 307;
    int SC_BAD_REQUEST = 400;
    int SC_UNAUTHORIZED = 401;
    int SC_PAYMENT_REQUIRED = 402;
    int SC_FORBIDDEN = 403;
    int SC_NOT_FOUND = 404;
    int SC_METHOD_NOT_ALLOWED = 405;
    int SC_NOT_ACCEPTABLE = 406;
    int SC_PROXY_AUTHENTICATION_REQUIRED = 407;
    int SC_REQUEST_TIMEOUT = 408;
    int SC_CONFLICT = 409;
    int SC_GONE = 410;
    int SC_LENGTH_REQUIRED = 411;
    int SC_PRECONDITION_FAILED = 412;
    int SC_REQUEST_ENTITY_TOO_LARGE = 413;
    int SC_REQUEST_URI_TOO_LONG = 414;
    int SC_UNSUPPORTED_MEDIA_TYPE = 415;
    int SC_REQUESTED_RANGE_NOT_SATISFIABLE = 416;
    int SC_EXPECTATION_FAILED = 417;
    int SC_INTERNAL_SERVER_ERROR = 500;
    int SC_NOT_IMPLEMENTED = 501;
    int SC_BAD_GATEWAY = 502;
    int SC_SERVICE_UNAVAILABLE = 503;
    int SC_GATEWAY_TIMEOUT = 504;
    int SC_HTTP_VERSION_NOT_SUPPORTED = 505;

1.1、常見應用

1、向瀏覽器輸出消息
2、下載文件
1)要獲取下載文件的路徑
2)下載的文件名是啥？
3)設置想辦法讓瀏覽器能夠支持下載我們需要的東西
4)獲取下載文件的輸入流
5)創建緩衝區
6)獲取輸出流對象
7)將FileOutputStream流寫入到buffer緩衝區
8)使用OutputStream將緩衝區中的數據輸出到客户端！

 @Override
    protected void doGet(HttpServletRequest req, HttpServletResponse resp) throws ServletException, IOException {
            //1)要獲取下載文件的路徑
        String realPath = this.getServletContext().getRealPath("/1.png");
        System.out.println("下載文件的路徑："+realPath);
            //2)下載的文件名是啥？
        String fileName = realPath.substring(realPath.lastIndexOf("\\") + 1);

//            3)設置想辦法讓瀏覽器能夠支持下載我們需要的東西      URLEncoder.encode(fileName,"UTF-8")中文文件名編碼，否則沒有亂碼
        resp.setHeader("Context-Disposition", "attachment;fileName"+ URLEncoder.encode(fileName,"UTF-8"));

//            4)獲取下載文件的輸入流
        FileInputStream fis = new FileInputStream(realPath);
//            5)創建緩衝區
        int len = 0;
        byte[] buffer = new byte[1024];

//            6)獲取輸出流對象
        ServletOutputStream os = resp.getOutputStream();

//            7)將FileOutputStream流寫入到buffer緩衝區   8)使用OutputStream將緩衝區中的數據輸出到客户端！
        while((len = fis.read(buffer))>0){
            os.write(buffer,0,len);
        }
        fis.close();
        os.close();
//
    }