How to use Response object in Java Servlet
Servlet Response
ServletResponse The interface provides a series of methods to set and obtain HTTP response information. A ServletResponse instance represents an HTTP response, and this response can contain some HTML and other content and some status information, such as HTTP status code and header information.
Response body
Using the ServletResponse interface, you can easily send the response body to the client. The response body is the main content of the HTTP response and can be HTML, CSS, JavaScript, text, or other data types. The following are some commonly used methods:
void setContentType(String type): Set the type of the response body. For example, if you want to return HTML content, you can usetext/htmlas the value oftype.void setCharacterEncoding(String encoding): Set the character encoding of the response body. For example, if you want to return UTF-8 encoded HTML content, you can useUTF-8as the value ofencoding.PrintWriter getWriter(): Get aPrintWriterinstance for sending character data to the client.ServletOutputStream getOutputStream(): Obtain aServletOutputStreaminstance for sending byte data to the client.
Here is a sample code that will send a response containing HTML content to the client:
@WebServlet("/example")
public class ExampleServlet extends HttpServlet {
protected void doGet(HttpServletRequest request, HttpServletResponse response) throws ServletException, IOException {
response.setContentType("text/html");
response.setCharacterEncoding("UTF-8");
PrintWriter out = response.getWriter();
out.println("<html>");
out.println("<head>");
out.println("<title>Example Servlet</title>");
out.println("</head>");
out.println("<body>");
out.println("<h2>Hello, world!</h2>");
out.println("</body>");
out.println("</html>");
}
}In the above code, we first use response.setContentType("text/html") Method sets the response body type to HTML. Then, we use the response.setCharacterEncoding("UTF-8") method to set the character encoding of the response body to UTF-8. Next, we use the response.getWriter() method to obtain a PrintWriter instance, and then use the out.println() method to send HTML content to the client.
Response header
In addition to the response body, the HTTP response can also contain some metadata, such as response header information. The response header information provides some additional information about the response, such as response body type, character encoding, expiration time, etc. Using the ServletResponse interface, you can also easily set and get response header information. The following are some commonly used methods:
void setHeader(String name, String value): Set response header information. For example, if you want to set theContent-Dispositionheader information, you can useContent-Dispositionas the value ofname,attachment; filename="example. txt"as the value ofvalue.void addHeader(String name, String value): Add response header information. For example, if you want to add theCache-Controlheader, you can useCache-Controlas the value ofnameandno-cacheas The value ofvalue.void setIntHeader(String name, int value): Set the response header information, the value is an integer. For example, if you want to set theContent-Lengthheader, you can useContent-Lengthas the value ofnameand the file size asvaluevalue.void addIntHeader(String name, int value): Add response header information, the value is an integer. For example, if you want to add theExpiresheader, you can useExpiresas the value ofnameand the timestamp as the value ofvalue.String getHeader(String name): Get the response header information of the specified name. For example, if you want to get theContent-Typeheader information, you can useContent-Typeas the value ofname.Collection<String> getHeaders(String name): Get all response header information of the specified name. For example, if you want to get theSet-Cookieheader, you can useSet-Cookieas the value ofname.int getIntHeader(String name): Get the integer response header information of the specified name. For example, if you want to get theContent-Lengthheader information, you can useContent-Lengthas the value ofname.
Response status
The response status refers to the status code of the HTTP response, indicating the server's processing result of the request. Using the ServletResponse interface, you can also set the response status code. The following are some commonly used status codes:
200: Indicates that the request was successful and the server has returned the requested data.302: Indicates that the requested resource has been moved to a new location, and the new location has been returned in the response header information.404: Indicates that the requested resource does not exist.#500: Indicates that an error occurred while the server was processing the request.
设置响应状态码的方法如下：
void setStatus(int sc)：设置响应状态码。例如，如果要设置状态码为 200，则可以使用 setStatus(200)。
示例代码
下面是一个示例代码，该代码会向客户端发送一个包含 HTML 内容的响应，并设置响应头信息和状态码：
@WebServlet("/example")
public class ExampleServlet extends HttpServlet {
protected void doGet(HttpServletRequest request, HttpServletResponse response) throws ServletException, IOException {
response.setContentType("text/html");
response.setCharacterEncoding("UTF-8");
response.setStatus(HttpServletResponse.SC_OK);
response.setHeader("Cache-Control", "no-cache");
PrintWriter out = response.getWriter();
out.println("<html>");
out.println("<head>");
out.println("<title>Example Servlet</title>");
out.println("</head>");
out.println("<body>");
out.println("<h2>Hello, world!</h2>");
out.println("</body>");
out.println("</html>");
}
}在上面的代码中，我们使用 HttpServletResponse.SC_OK 设置响应状态码为 200。然后，我们使用 response.setHeader("Cache-Control", "no-cache") 方法设置 Cache-Control 头信息为 no-cache。最后，我们使用 out.println() 方法向客户端发送 HTML 内容。
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