The actions and template data in a JSP page is written using the
JspWriter object that is referenced by the implicit variable out which
is initialized automatically using methods in the PageContext object.
This abstract class emulates some of the functionality found in the
java.io.BufferedWriter and java.io.PrintWriter classes,
however it differs in that it throws java.io.IOException from the print
methods while PrintWriter does not.
The initial JspWriter object is associated with the PrintWriter object
of the ServletResponse in a way that depends on whether the page is or
is not buffered. If the page is not buffered, output written to this
JspWriter object will be written through to the PrintWriter directly,
which will be created if necessary by invoking the getWriter() method
on the response object. But if the page is buffered, the PrintWriter
object will not be created until the buffer is flushed and
operations like setContentType() are legal. Since this flexibility
simplifies programming substantially, buffering is the default for JSP
Buffering raises the issue of what to do when the buffer is
exceeded. Two approaches can be taken:
Both approaches are valid, and thus both are supported in the JSP
technology. The behavior of a page is controlled by the autoFlush
attribute, which defaults to true. In general, JSP pages that need to
be sure that correct and complete data has been sent to their client
may want to set autoFlush to false, with a typical case being that
where the client is an application itself. On the other hand, JSP
pages that send data that is meaningful even when partially
constructed may want to set autoFlush to true; such as when the
data is sent for immediate display through a browser. Each application
will need to consider their specific needs.
An alternative considered was to make the buffer size unbounded; but,
this had the disadvantage that runaway computations would consume an
unbounded amount of resources.
The "out" implicit variable of a JSP implementation class is of this type.
If the page directive selects autoflush="true" then all the I/O operations
on this class shall automatically flush the contents of the buffer if an
overflow condition would result if the current operation were performed
without a flush. If autoflush="false" then all the I/O operations on this
class shall throw an IOException if performing the current operation would
result in a buffer overflow condition.