HttpServletResponse
public interface HttpServletResponse extends ServletResponse {
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;
void addCookie(Cookie var1);
boolean containsHeader(String var1);
String encodeURL(String var1);
String encodeRedirectURL(String var1);
/** @deprecated */
String encodeUrl(String var1);
/** @deprecated */
String encodeRedirectUrl(String var1);
void sendError(int var1, String var2) throws IOException;
void sendError(int var1) throws IOException;
void sendRedirect(String var1) throws IOException;
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);
void setStatus(int var1);
/** @deprecated */
void setStatus(int var1, String var2);
int getStatus();
String getHeader(String var1);
Collection<String> getHeaders(String var1);
Collection<String> getHeaderNames();
}
可以使用HttpServletResponse来对浏览器进行响应，常见的操作如下：
1. 使用resp.setContentType()设置响应类型
2. 使用resp.getWriter()获取PrintWriter对象，来对页面打印信息
3. 使用resp.setHeader()、addHeader()等方法设置响应头信息
4. 使用resp.sendRedirect()、sendError()方法要求客户端重定向页面，或传输错误状态信息
5. 使用resp.getOutputStream()取得ServletOutputStream对象，对浏览器进行字节数据响应
设置响应头、缓冲区
/**
* 处理响应头信息的Servlet
*/
public class ResponseHeaderServlet extends HttpServlet {
protected void doPost(HttpServletRequest request, HttpServletResponse response) throws ServletException, IOException {
}
protected void doGet(HttpServletRequest request, HttpServletResponse response) throws ServletException, IOException {
// 设置响应头Content-Language来设置编码格式，
// 相当于response.setCharacterEncoding()或response.setContentType()
// 当通过setCharacterEncoding()或setContentType()设置了编码格式后，则会忽略setLocale()方法
response.setLocale(Locale.CHINA);
// 为某个头部信息设置值
response.setHeader("Set-Cookie", "username=niezhicisgod; age=23; ");
// 为某个头部信息添加值
response.addHeader("Allow", "POST");
// 如果头部的值为整数类型
response.setIntHeader("Age", 233);
response.addIntHeader("Retry-After", 233);
// 如果头部的值为日期类型
response.setDateHeader("Date", 233L);
response.addDateHeader("Last-Modified", 233L);
// 所有响应头的设置或添加，都必须在响应确认之前，否则都会被忽略；通过isConmmitted()判断响应是否确认
boolean flag = response.isCommitted();
PrintWriter out = response.getWriter();
for (String head : response.getHeaderNames()) {
out.print(head + ":" + response.getHeader(head));
}
// 清除所有对响应头的设置
response.reset();
}
}
/**
* 对响应进行缓冲操作的Servlet
*/
public class ResponseBufferServlet extends HttpServlet {
protected void doPost(HttpServletRequest request, HttpServletResponse response) throws ServletException, IOException {
}
protected void doGet(HttpServletRequest request, HttpServletResponse response) throws ServletException, IOException {
// 设置响应的缓冲区大小，
// 必须在response.getWriter()或response.getOutputStream()方法之前设置，
// 否则会抛出IllegalStateException
response.setBufferSize(233);
// 将缓冲区中的信息发送给浏览器，然后清空缓冲区
response.flushBuffer();
//清除所有缓冲
response.resetBuffer();
}
}
response.setContentType()
因为浏览器需要知道如何处理你的响应，所以必须告知内容类型，setContentType()方法在响应头中设置content-type的值，只要指定MIME类型就可以了。由于编码设置与内容类型通常都要设置，所以调用setContentType()设置类型时，同时指定charset属性是个方便且常见的做法。
常见的MIME类型有：text/html、application/pdf、application/jar、application/x-zip、image/jpeg等
可以在web部署描述文件中定义MIME的类型映射：
<!-- 在web.xml在配置mime类型的映射 -->
<mime-mapping>
<!-- extension设置文件的后缀名 -->
<extension>pdf</extension>
<!-- mime-type设置具体的mime类型 -->
<mime-type>application/pdf</mime-type>
</mime-mapping>
在Servlet中可以使用ServletContext中的getMimeType()方法获取到文件的MIME类型
response.getOutputStream()输出字节流
通过getWriter()对浏览器进行字符输出，但也有时候需要直接对浏览器进行字节输出，此时可以通过response.getOutputStream()获取ServletOutputStream对象
/**
* 下载文件的Servlet
*/
@WebServlet(name = "DownloadServlet", urlPatterns = "/downloadServlet")
public class DownloadServlet extends HttpServlet {
protected void doPost(HttpServletRequest request, HttpServletResponse response) throws ServletException, IOException {
}
protected void doGet(HttpServletRequest request, HttpServletResponse response) throws ServletException, IOException {
response.setContentType("application/pdf");
// 声明不直接在浏览器中打开/下载，让用户自定义下载文件的目录
response.setHeader("Content-Disposition", "attachment; filename=" + "dsaj.pdf");
//ServletContext对象就代表着当前整个web应用程序
InputStream in = getServletContext().getResourceAsStream("/WEB-INF/pdf/dsaj.pdf");
ServletOutputStream out = response.getOutputStream();
writeTo(in, out);
}
private void writeTo(InputStream in, OutputStream out) throws IOException {
byte[] buffer = new byte[1024];
int length = -1;
while((length = in.read(buffer)) != -1) {
out.write(buffer, 0, length);
}
in.close();
out.close();
}
}
response.sendRedirect()、response.sendError()
之前的forward()方法都是在同一个请求中，所以request.setAttribute()设置属性对象，都可以在转发过程中共享
而response.sendRedirect()是要求浏览器重新请求另一个的URL，又称为重定向，这个方法会在响应中设置HTTP状态码301以及Location头（就是需要跳转的URL），浏览器会使用GET方式请求指定的URL，因此浏览器地址栏会发现URL的变更
/**
* 演示重定向的Servlet
*/
@WebServlet(name = "RedirectServlet", urlPatterns = "/redirectServlet")
public class RedirectServlet extends HttpServlet {
protected void doPost(HttpServletRequest request, HttpServletResponse response) throws ServletException, IOException {
}
protected void doGet(HttpServletRequest request, HttpServletResponse response) throws ServletException, IOException {
// 重定向到下载的Servlet
response.sendRedirect("/downloadServlet");
}
}
如果在处理请求的过程中发生一些异常，可以使用response.sendError()方法，返回错误信息给浏览器；HttpServlet中的doGet()方法就默认使用了sendError()方法：
protected void doGet(HttpServletRequest req, HttpServletResponse resp) throws ServletException, IOException {
String protocol = req.getProtocol();
String msg = lStrings.getString("http.method_get_not_supported");
if(protocol.endsWith("1.1")) {
resp.sendError(405, msg); } else {
resp.sendError(400, msg);
}
}
