Servlet HTTP Status Codes Servlet Client HTTP Request

Servlet Server HTTP Response

As discussed in the previous chapters, when a web server responds to an HTTP request, the response typically includes a status line, some response headers, a blank line, and the document. A typical response is as follows:

HTTP/1.1　200　OK
Content-Type: ...text/html
Header2: ...
...
HeaderN: ...
　　(Blank Line)
<!doctype ...>
<html>
<head>.../head>
<body>
...
</body>
</html>

Status line includes the HTTP version (in this example, HTTP/1.1), a status code (in this example, 2(00) and a corresponding short message (in this example, OK).

The following table summarizes the most useful HTTP headers returned from the web server to the browser: 1.1 Response headers, which you will frequently use in web programming:

Header	Description
Allow	This header information specifies the request methods supported by the server (GET, POST, etc.).
Cache-Control	This header information specifies under what circumstances the response document can be safely cached. Possible values include:public, private or no-cache etc. Public means that the document is cacheable, Private means that the document is a private document for a single user and can only be stored in private (non-shared) cache, no-cache means that the document should not be cached.
Connection	This header information indicates whether the browser uses a persistent HTTP connection. Value close indicates that the browser does not use a persistent HTTP connection, value keep-alive means using a persistent connection.
Content-Disposition	This header information allows you to request the browser to save the response to the disk with the given file name.
Content-Encoding	During the transmission process, this header information specifies the encoding method of the page.
Content-Language	This header information indicates the language used to write the document. For example, en, en-us, ru, etc.
Content-Length	This header information indicates the number of bytes in the response. It is only used when the browser uses a persistent (keep)-These pieces of information are needed only when there is a persistent (keep alive) HTTP connection.
Content-Type	This header information provides the MIME (Multipurpose Internet Mail Extension) type of the response document.
Expires	This header information specifies the time when the content expires, after which the content will no longer be cached.
Last-Modified	This header information indicates the last modification time of the document. Then, the client can cache the file and use it in subsequent requests through If-Modified-Since The request header information provides a date.
Location	This header information should be included in all responses with status codes. In 3Within 00s, this will notify the browser of the document address. The browser will automatically reconnect to this location and retrieve the new document.
Refresh	This header information specifies how the browser should request the updated page as soon as possible. You can specify the number of seconds for page refresh.
Retry-After	This header information can be used with 503Used in conjunction with the (Service Unavailable service unavailable) response to tell the client how long it can repeat its request.
Set-Cookie	This header information specifies a cookie associated with the page.

Methods for setting HTTP response headers

The following methods can be used to set HTTP response headers in a Servlet program. These methods are set through HttpServletResponse Object available.

Serial Number	Method & Description
1	String encodeRedirectURL(String url) Encode the specified URL for use with the sendRedirect method, or return the URL unchanged if encoding is not necessary.
2	String encodeURL(String url) Encode the specified URL containing the session ID, or return the URL unchanged if encoding is not necessary.
3	boolean containsHeader(String name) Return a boolean value indicating whether a named response header has already been set.
4	boolean isCommitted() Return a boolean value indicating whether the response has already been committed.
5	void addCookie(Cookie cookie) Add the specified cookie to the response.
6	void addDateHeader(String name, long date) Add a response header with a given name and date value.
7	void addHeader(String name, String value) Add a response header with a given name and value.
8	void addIntHeader(String name, int value) Add a response header with a given name and integer value.
9	void flushBuffer() Force any content in the buffer to be written to the client.
10	void reset() Clear any existing data in the buffer, including the status code and headers.
11	void resetBuffer() Clear the content of the basic buffer in the response, but do not clear the status code and headers.
12	void sendError(int sc) Send an error response to the client with the specified status code, and clear the buffer.
13	void sendError(int sc, String msg) Send an error response to the client with the specified status.
14	void sendRedirect(String location) Send a temporary redirect response to the client using the specified redirect location URL.
15	void setBufferSize(int size) Set the preferred buffer size for the response body.
16	void setCharacterEncoding(String charset) Set the character encoding (MIME character set) of the response sent to the client, for example, UTF-8.
17	void setContentLength(int len) Set the length of the content body in the HTTP Servlet response, this method sets the HTTP Content-Length header.
18	void setContentType(String type) If the response has not been submitted yet, set the content type of the response sent to the client.
19	void setDateHeader(String name, long date) Set a response header with a given name and date value.
20	void setHeader(String name, String value) Set a response header with a given name and value.
21	void setIntHeader(String name, int value) Set a response header with a given name and integer value.
22	void setLocale(Locale loc) If the response has not been submitted yet, set the response locale.
23	void setStatus(int sc) Set the status code for the response.

HTTP Header Response Example

You have seen the setContentType() method in the previous example, the following example also uses the same method, in addition, we will use setIntHeader() Method to set Refresh Header.

//Import the necessary java libraries
import java.io.IOException;
import java.io.PrintWriter;
import java.text.SimpleDateFormat;
import java.util.Calendar;
import java.util.Date;
import　javax.servlet.ServletException;
import　javax.servlet.annotation.WebServlet;
import　javax.servlet.http.HttpServlet;
import　javax.servlet.http.HttpServletRequest;
import　javax.servlet.http.HttpServletResponse;
@WebServlet("/Refresh")
//Extend the HttpServlet class
public　class　Refresh　extends　HttpServlet　{
　　　　//　Method to handle GET method requests
　　　　　　public　void　doGet(HttpServletRequest　request,
　　　　　　　　　　　　　　　　　　　　　　　　HttpServletResponse response)
　　　　　　　　　　　　　　　　throws　ServletException,　IOException
　　　　　　{
　　　　　　　　　　//　Set the automatic reload time　5　seconds
　　　　　　　　　　response.setIntHeader("Refresh",　5);
　　　　　　　　　　//　Set the response content type
　　　　　　　　　　response.setContentType("text/html;charset=UTF-8");
　　　　　　　　　
　　　　　　　　　　//Get a calendar with the default time zone and language environment　　
　　　　　　　　　　Calendar　cale　=　Calendar.getInstance();　　
　　　　　　　　　　//Convert Calendar type to Date type　　
　　　　　　　　　　Date　tasktime=cale.getTime();　　
　　　　　　　　　　//Set the date output format　　
　　　　　　　　　　SimpleDateFormat　df=new　SimpleDateFormat("yyyy-MM-dd　HH:mm:ss");　　
　　　　　　　　　　//Formatted Output　　
　　　　　　　　　　String　nowTime　=　df.format(tasktime);
　　　　　　　　　　PrintWriter　out　=　response.getWriter();
　　　　　　　　　　String　title　=　"Automatic Refresh Header Setting"　-　Basic Tutorial Website Example";
　　　　　　　　　　String　docType　=
　　　　　　　　　　"<!DOCTYPE　html>\n";
　　　　　　　　　　out.println(docType　+
　　　　　　　　　　　　"<html>\n"　+
　　　　　　　　　　　　"<head><title>"　+　title　+　"</title></head>\n"+
　　　　　　　　　　　　"<body　bgcolor="#f0f0f0">\n"　+
　　　　　　　　　　　　"<h1　align="center">"　+　title　+　"</h1>\n"　+
　　　　　　　　　　　　"<p>The current time is:"　+　nowTime　+　"</p>\n");
　　　　　　}
　　　　　　//　method to handle POST method requests
　　　　　　public void doPost(HttpServletRequest request,
　　　　　　　　　　　　　　　　　　　　　　　　　HttpServletResponse response)
　　　　　　　　　　throws ServletException, IOException {
　　　　　　　　　doGet(request, response);
　　　　　　}
}

The above test example is located under the TomcatTest project, and the corresponding web.xml configuration is as follows:

<?xml version="1.0" encoding="UTF-8"?>　　
<web-app>　　
　　<servlet　　
　　　　　<!--　class name　-->　　
　　　　<servlet-name>Refresh</servlet-name>　　
　　　　<!--　the package　-->　　
　　　　<servlet-class>com.w3codebox.test.Refresh</servlet-class>　　
　　</servlet>　　
　　<servlet-mapping>　　
　　　　<servlet-name>Refresh</servlet-name>　　
　　　　<!--　visited URL　-->　　
　　　　<url-pattern>/TomcatTest/Refresh</url-pattern>　　
　　　　</servlet-mapping>　　
</web-app>

Now, call the above Servlet every 5 The seconds will display the current system time. Just run the Servlet and wait a moment, and you will see the following result:

Servlet HTTP Status Codes Servlet Client HTTP Request

Servlet Tutorial

Servlet Server HTTP Response

Methods for setting HTTP response headers

HTTP Header Response Example