HttpServletResponse对象代表服务器的响应。这个对象中封装了向客户端发送数据、发送响应头，发送响应状态码的方法。

1、负责向客户端(浏览器)发送数据的方法:

ServletOutputStream getOutputStream() throws IOException;

PrintWriter getWriter() throws IOException;

2、负责向客户端(浏览器)发送响应头的方法：

oid 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);

3、负责向客户端(浏览器)发送响应状态码的方法：

void setStatus(int var1);

4、响应的状态码（常量）：

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;

5、应用：

• 向浏览器发送数据：getWriter()、getOutputStream()；

import javax.servlet.ServletException;
import javax.servlet.http.HttpServlet;
import javax.servlet.http.HttpServletRequest;
import javax.servlet.http.HttpServletResponse;
import java.io.IOException;
import java.io.PrintWriter;

@WebServlet("/hello")
public class HelloServlet extends HttpServlet {
    @Override
    protected void doGet(HttpServletRequest req, HttpServletResponse resp) throws ServletException, IOException {
        resp.setContentType("text/html");
        resp.setCharacterEncoding("utf-8");
        PrintWriter writer = resp.getWriter();
        writer.print("<h1>HelloServlet</h1>");
    }

    @Override
    protected void doPost(HttpServletRequest req, HttpServletResponse resp) throws ServletException, IOException {
       doGet(req,resp);
    }
}

运行结果如下：

• 下载文件：

import javax.servlet.ServletException;
import javax.servlet.ServletOutputStream;
import javax.servlet.http.HttpServlet;
import javax.servlet.http.HttpServletRequest;
import javax.servlet.http.HttpServletResponse;
import java.io.FileInputStream;
import java.io.IOException;
import java.net.URLEncoder;

@WebServlet("/down")
public class FileServlet extends HttpServlet {
	@Override
	protected void doGet(HttpServletRequest req, HttpServletResponse resp) throws IOException {
		//1、获取下载文件的路径
		//String realpath = this.getServletContext().getRealPath("xxx"); //获取要下载的文件的绝对路径
		String realpath = "D:\\Users\\Java\\JavaWeb\\Servlet\\Servlet-03-Response\\target\\classes\\白豆五.png";
		System.out.println("下载文件的路径:" + realpath);
		//2、获取文件名
		String fileName = realpath.substring(realpath.lastIndexOf("\\") + 1);
		//3、设置content-disposition响应头控制浏览器以下载的形式打开文件,使用URLEncoder.encode解决文件名乱码问题
		resp.setHeader("Content-disposition", "attachment;filename=" + URLEncoder.encode(fileName, "UTF-8"));
		//4、获取下载文件的输入流；
		FileInputStream in = new FileInputStream(realpath);
		//5、创建缓冲区
		int len = 0;
		byte[] buffer = new byte[1024];
		//6、获取OutputStream对象；
		ServletOutputStream out = resp.getOutputStream();
		//7、将FileOutputStream流写入到buffer缓冲区,使用OutputStream将缓冲区中的数据输出到客户端。
		while ((len = in.read(buffer)) > 0) {
			out.write(buffer, 0, len);
		}
		in.close();
		out.close();
	}

	@Override
	protected void doPost(HttpServletRequest req, HttpServletResponse resp) throws ServletException, IOException {
		doGet(req, resp);
	}
}

运行结果如下：