The response object encapsulates all information to be returned from the server to the client. In the HTTP protocol, this information is transmitted from the server to the client either by HTTP headers or the message body of the request.
A servlet container is allowed, but not required, to buffer output going to the client for efficiency purposes. Typically servers that do buffering make it the default, but allow servlets to specify buffering parameters.
The following methods in the ServletResponse interface allow a servlet to access and set buffering information:
• getBufferSize • setBufferSize • isCommitted • reset • resetBuffer • flushBuffer
These methods are provided on the ServletResponse interface to allow buffering operations to be performed whether the servlet is using a
ServletOutputStream or a Writer.
The getBufferSize method returns the size of the underlying buffer being used. If no buffering is being used, this method must return the int value of 0 (zero).
The servlet can request a preferred buffer size by using the setBufferSize
method. The buffer assigned is not required to be the size requested by the servlet, but must be at least as large as the size requested. This allows the container to reuse a set of fixed size buffers, providing a larger buffer than requested if appropriate. The method must be called before any content is written using a
ServletOutputStream or Writer. If any content has been written or the response object has been committed, this method must throw an IllegalStateException.
The isCommitted method returns a boolean value indicating whether any response bytes have been returned to the client. The flushBuffer method forces content in the buffer to be written to the client.
The reset method clears data in the buffer when the response is not
committed. Headers and status codes set by the servlet prior to the reset call must be cleared as well. The resetBuffer method clears content in the buffer if the response is not committed without clearing the headers and status code.
If the response is committed and the reset or resetBuffer method is called, an IllegalStateException must be thrown. The response and its associated buffer will be unchanged.
When using a buffer, the container must immediately flush the contents of a filled buffer to the client. If this is the first data that is sent to the client, the response is considered to be committed.
A servlet can set headers of an HTTP response via the following methods of the
HttpServletResponse interface: • setHeader
The setHeader method sets a header with a given name and value. A previous header is replaced by the new header. Where a set of header values exist for the name, the values are cleared and replaced with the new value.
The addHeader method adds a header value to the set with a given name. If there are no headers already associated with the name, a new set is created.
Headers may contain data that represents an int or a Date object. The following convenience methods of the HttpServletResponse interface allow a servlet to set a header using the correct formatting for the appropriate data type:
To be successfully transmitted back to the client, headers must be set before the response is committed. Headers set after the response is committed will be ignored by the servlet container.
Servlet programmers are responsible for ensuring that the Content-Type
header is appropriately set in the response object for the content the servlet is generating. The HTTP 1.1 specification does not require that this header be set in an HTTP response. Servlet containers must not set a default content type when the servlet programmer does not set the type.
It is recommended that containers use the X-Powered-By HTTP header to publish its implementation information. The field value should consist of one or more implementation types, such as "Servlet/2.4". Optionally, the
supplementary information of the container and the underlying Java platform can be added after the implementation type within parentheses. The container should be configurable to suppress this header.
Here’s the examples of this header.
X-Powered-By: Servlet/2.4 JSP/2.0 (Tomcat/5.0 JRE/1.4.1)
The following convenience methods exist in the HttpServletResponse interface: • sendRedirect
The sendRedirect method will set the appropriate headers and content body to redirect the client to a different URL. It is legal to call this method with a relative URL path, however the underlying container must translate the relative path to a fully qualified URL for transmission back to the client. If a partial URL is given and, for whatever reason, cannot be converted into a valid URL, then this method must throw an IllegalArgumentException.
The sendError method will set the appropriate headers and content body for an error message to return to the client. An optional String argument can be
provided to the sendError method which can be used in the content body of the error.
These methods will have the side effect of committing the response, if it has not already been committed, and terminating it. No further output to the client should be made by the servlet after these methods are called. If data is written to the response after these methods are called, the data is ignored.
If data has been written to the response buffer, but not returned to the client (i.e. the response is not committed), the data in the response buffer must be cleared and replaced with the data set by these methods. If the response is committed, these methods must throw an IllegalStateException.