web服务器接收到客户端的http请求，针对这个请求分别创建一个代表请求的HttpServletResponse对象，一个代表响应的HttpServletResponse对象；
public ServletOutputStream getOutputStream() throws IOException;
public PrintWriter getWriter() throws IOException;
void setCharacterEncoding(String charset);
void setContentLength(int len);
void setContentLengthLong(long length);
void setContentType(String type);
void setDateHeader(String name, long date);
void addDateHeader(String name, long date);
void setHeader(String name, String value);
void addHeader(String name, String value);
void setIntHeader(String name, int value);
void addIntHeader(String name, int value);
/**
* Status code (100) indicating the client can continue.
*/
public static final int SC_CONTINUE = 100;
/**
* Status code (101) indicating the server is switching protocols according
* to Upgrade header.
*/
public static final int SC_SWITCHING_PROTOCOLS = 101;
/**
* Status code (200) indicating the request succeeded normally.
*/
public static final int SC_OK = 200;
/**
* Status code (201) indicating the request succeeded and created a new
* resource on the server.
*/
public static final int SC_CREATED = 201;
/**
* Status code (202) indicating that a request was accepted for processing,
* but was not completed.
*/
public static final int SC_ACCEPTED = 202;
/**
* Status code (203) indicating that the meta information presented by the
* client did not originate from the server.
*/
public static final int SC_NON_AUTHORITATIVE_INFORMATION = 203;
/**
* Status code (204) indicating that the request succeeded but that there
* was no new information to return.
*/
public static final int SC_NO_CONTENT = 204;
/**
* Status code (205) indicating that the agent <em>SHOULD</em> reset the
* document view which caused the request to be sent.
*/
public static final int SC_RESET_CONTENT = 205;
/**
* Status code (206) indicating that the server has fulfilled the partial
* GET request for the resource.
*/
public static final int SC_PARTIAL_CONTENT = 206;
/**
* Status code (300) indicating that the requested resource corresponds to
* any one of a set of representations, each with its own specific location.
*/
public static final int SC_MULTIPLE_CHOICES = 300;
/**
* Status code (301) indicating that the resource has permanently moved to a
* new location, and that future references should use a new URI with their
* requests.
*/
public static final int SC_MOVED_PERMANENTLY = 301;
/**
* Status code (302) indicating that the resource has temporarily moved to
* another location, but that future references should still use the
* original URI to access the resource. This definition is being retained
* for backwards compatibility. SC_FOUND is now the preferred definition.
*/
public static final int SC_MOVED_TEMPORARILY = 302;
/**
* Status code (302) indicating that the resource reside temporarily under a
* different URI. Since the redirection might be altered on occasion, the
* client should continue to use the Request-URI for future
* requests.(HTTP/1.1) To represent the status code (302), it is recommended
* to use this variable.
*/
public static final int SC_FOUND = 302;
/**
* Status code (303) indicating that the response to the request can be
* found under a different URI.
*/
public static final int SC_SEE_OTHER = 303;
/**
* Status code (304) indicating that a conditional GET operation found that
* the resource was available and not modified.
*/
public static final int SC_NOT_MODIFIED = 304;
/**
* Status code (305) indicating that the requested resource <em>MUST</em> be
* accessed through the proxy given by the <code><em>Location</em></code>
* field.
*/
public static final int SC_USE_PROXY = 305;
/**
* Status code (307) indicating that the requested resource resides
* temporarily under a different URI. The temporary URI <em>SHOULD</em> be
* given by the <code><em>Location</em></code> field in the response.
*/
public static final int SC_TEMPORARY_REDIRECT = 307;
/**
* Status code (400) indicating the request sent by the client was
* syntactically incorrect.
*/
public static final int SC_BAD_REQUEST = 400;
/**
* Status code (401) indicating that the request requires HTTP
* authentication.
*/
public static final int SC_UNAUTHORIZED = 401;
/**
* Status code (402) reserved for future use.
*/
public static final int SC_PAYMENT_REQUIRED = 402;
/**
* Status code (403) indicating the server understood the request but
* refused to fulfill it.
*/
public static final int SC_FORBIDDEN = 403;
/**
* Status code (404) indicating that the requested resource is not
* available.
*/
public static final int SC_NOT_FOUND = 404;
/**
* Status code (405) indicating that the method specified in the
* <code><em>Request-Line</em></code> is not allowed for the resource
* identified by the <code><em>Request-URI</em></code>.
*/
public static final int SC_METHOD_NOT_ALLOWED = 405;
/**
* Status code (406) indicating that the resource identified by the request
* is only capable of generating response entities which have content
* characteristics not acceptable according to the accept headers sent in
* the request.
*/
public static final int SC_NOT_ACCEPTABLE = 406;
/**
* Status code (407) indicating that the client <em>MUST</em> first
* authenticate itself with the proxy.
*/
public static final int SC_PROXY_AUTHENTICATION_REQUIRED = 407;
/**
* Status code (408) indicating that the client did not produce a request
* within the time that the server was prepared to wait.
*/
public static final int SC_REQUEST_TIMEOUT = 408;
/**
* Status code (409) indicating that the request could not be completed due
* to a conflict with the current state of the resource.
*/
public static final int SC_CONFLICT = 409;
/**
* Status code (410) indicating that the resource is no longer available at
* the server and no forwarding address is known. This condition
* <em>SHOULD</em> be considered permanent.
*/
public static final int SC_GONE = 410;
/**
* Status code (411) indicating that the request cannot be handled without a
* defined <code><em>Content-Length</em></code>.
*/
public static final int SC_LENGTH_REQUIRED = 411;
/**
* Status code (412) indicating that the precondition given in one or more
* of the request-header fields evaluated to false when it was tested on the
* server.
*/
public static final int SC_PRECONDITION_FAILED = 412;
/**
* Status code (413) indicating that the server is refusing to process the
* request because the request entity is larger than the server is willing
* or able to process.
*/
public static final int SC_REQUEST_ENTITY_TOO_LARGE = 413;
/**
* Status code (414) indicating that the server is refusing to service the
* request because the <code><em>Request-URI</em></code> is longer than the
* server is willing to interpret.
*/
public static final int SC_REQUEST_URI_TOO_LONG = 414;
/**
* Status code (415) indicating that the server is refusing to service the
* request because the entity of the request is in a format not supported by
* the requested resource for the requested method.
*/
public static final int SC_UNSUPPORTED_MEDIA_TYPE = 415;
/**
* Status code (416) indicating that the server cannot serve the requested
* byte range.
*/
public static final int SC_REQUESTED_RANGE_NOT_SATISFIABLE = 416;
/**
* Status code (417) indicating that the server could not meet the
* expectation given in the Expect request header.
*/
public static final int SC_EXPECTATION_FAILED = 417;
/**
* Status code (500) indicating an error inside the HTTP server which
* prevented it from fulfilling the request.
*/
public static final int SC_INTERNAL_SERVER_ERROR = 500;
/**
* Status code (501) indicating the HTTP server does not support the
* functionality needed to fulfill the request.
*/
public static final int SC_NOT_IMPLEMENTED = 501;
/**
* Status code (502) indicating that the HTTP server received an invalid
* response from a server it consulted when acting as a proxy or gateway.
*/
public static final int SC_BAD_GATEWAY = 502;
/**
* Status code (503) indicating that the HTTP server is temporarily
* overloaded, and unable to handle the request.
*/
public static final int SC_SERVICE_UNAVAILABLE = 503;
/**
* Status code (504) indicating that the server did not receive a timely
* response from the upstream server while acting as a gateway or proxy.
*/
public static final int SC_GATEWAY_TIMEOUT = 504;
/**
* Status code (505) indicating that the server does not support or refuses
* to support the HTTP protocol version that was used in the request
* message.
*/
public static final int SC_HTTP_VERSION_NOT_SUPPORTED = 505;
200：请求响应成功
3xx:请求重定向
重定向：你重新到我给你新位置去；
4xx:找不到资源 404
资源不存在
5xx：服务器代码错误 500
502： 网关错误
1.向浏览器输出消息
2.下载文件——
（1）要获取下载文件的 路径
（2）下载的文件名是什么？
（3）设置想办法让浏览器能支持下载我们需要的东西
（4）获取下载文件的输入流
（5）创建缓冲区
（6）获取OutputStream对象
（7）将FileOutputStream流写入到buffer缓冲区
（8）将OutputStream将缓冲区中的数据输出到客户端
<?xml version="1.0" encoding="UTF-8"?>
<web-app xmlns="http://xmlns.jcp.org/xml/ns/javaee"
xmlns:xsi="http://www.w3.org/2001/XMLSchema-instance"
xsi:schemaLocation="http://xmlns.jcp.org/xml/ns/javaee
http://xmlns.jcp.org/xml/ns/javaee/web-app_4_0.xsd"
version="4.0"
metadata-complete="true">
</web-app>
package com.kuang.servlet;
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;
public class FileServlet extends HttpServlet {
@Override
protected void doGet(HttpServletRequest req, HttpServletResponse resp) throws ServletException, IOException {
//
// （1）要获取下载文件的 路径
// String realPath = this.getServletContext().getRealPath("/1.png");
String realPath = "D:\\学习笔记\\狂神说Java\\javaweb-02-servlet\\response\\target\\classes\\1.png";
System.out.println("下载文件的路径："+realPath);
// （2）下载的文件名是什么？
String fileName = realPath.substring(realPath.lastIndexOf("\\")+1);
// （3）设置想办法让浏览器能支持(Content-Disposition)下载我们需要的东西.URLEncoder.encode(fileName,"UTF-8"))：用来处理中文编码,否则可能会乱码
// resp.setHeader("Content-Disposition","attachment;filename="+ fileName);
resp.setHeader("Content-Disposition","attachment;filename="+ URLEncoder.encode(fileName,"UTF-8"));
// （4）获取下载文件的输入流;FileInputStream将文件变成流
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，将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);
}
//（8）关闭流
in.close();
out.close();
}
@Override
protected void doPost(HttpServletRequest req, HttpServletResponse resp) throws ServletException, IOException {
super.doPost(req, resp);
}
}
<servlet>
<servlet-name>fileDown</servlet-name>
<servlet-class>com.kuang.servlet.FileServlet</servlet-class>
</servlet>
<servlet-mapping>
<servlet-name>fileDown</servlet-name>
<url-pattern>/down</url-pattern>
</servlet-mapping>
文件下载只显示页面，不提示文件下载
错误显示
原因：
正确显示：
正确代码：
package com.kuang.servlet;
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;
public class FileServlet extends HttpServlet {
@Override
protected void doGet(HttpServletRequest req, HttpServletResponse resp) throws ServletException, IOException {
//
// （1）要获取下载文件的 路径
// String realPath = this.getServletContext().getRealPath("/1.png");
String realPath = "D:\\学习笔记\\狂神说Java\\javaweb-02-servlet\\response\\target\\classes\\1.png";
System.out.println("下载文件的路径："+realPath);
// （2）下载的文件名是什么？
String fileName = realPath.substring(realPath.lastIndexOf("\\")+1);
// （3）设置想办法让浏览器能支持(Content-Disposition)下载我们需要的东西.URLEncoder.encode(fileName,"UTF-8"))：用来处理中文编码
// resp.setHeader("Content-Disposition","attachment;filename="+ fileName);
// resp.setHeader("Content-Disposition","attachment:filename="+ URLEncoder.encode(fileName,"UTF-8"));
resp.setHeader("Content-Disposition","attachment;filename="+ URLEncoder.encode(fileName,"UTF-8"));
// （4）获取下载文件的输入流;FileInputStream将文件变成流
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，将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);
}
//（8）关闭流
in.close();
out.close();
}
@Override
protected void doPost(HttpServletRequest req, HttpServletResponse resp) throws ServletException, IOException {
super.doPost(req, resp);
}
}