Return the system identifier for the current document event.
<p>The return value is the system identifier of the document
entity or of the external parsed entity in which the markup
triggering the event appears.</p>
<p>If the system identifier is a URL, the parser must resolve it
fully before passing it to the application. For example, a file
name must always be provided as a <em>file:...</em> URL, and other
kinds of relative URI are also resolved against their bases.</p>
@return A string containing the system identifier, or null
if none is available.
@see #getPublicId
Return the system identifier for the current document event.
<p>The return value is the system identifier of the document entity or of the external parsed entity in which the markup triggering the event appears.</p>
<p>If the system identifier is a URL, the parser must resolve it fully before passing it to the application. For example, a file name must always be provided as a <em>file:...</em> URL, and other kinds of relative URI are also resolved against their bases.</p>
@return A string containing the system identifier, or null if none is available. @see #getPublicId