/**
 * Throws an <code>SWTException</code> if the receiver can not
 * be accessed by the caller. This may include both checks on
 * the state of the receiver and more generally on the entire
 * execution context. This method <em>should</em> be called by
 * widget implementors to enforce the standard SWT invariants.
 * <p>
 * Currently, it is an error to invoke any method (other than
 * <code>isDisposed()</code>) on a widget that has had its 
 * <code>dispose()</code> method called. It is also an error
 * to call widget methods from any thread that is different
 * from the thread that created the widget.
 * </p><p>
 * In future releases of SWT, there may be more or fewer error
 * checks and exceptions may be thrown for different reasons.
 * </p>
 *
 * @exception SWTException <ul>
 *    <li>ERROR_WIDGET_DISPOSED - if the receiver has been disposed</li>
 *    <li>ERROR_THREAD_INVALID_ACCESS - if not called from the thread that created the receiver</li>
 * </ul>
 */
protected void checkWidget () {
	Display display = this.display;
	if (display == null) error (SWT.ERROR_WIDGET_DISPOSED);
	if (display.thread != Thread.currentThread ()) {
		/*
		* Bug in IBM JVM 1.6.  For some reason, under
		* conditions that are yet to be full understood,
		* Thread.currentThread() is either returning null
		* or a different instance from the one that was
		* saved when the Display was created.  This is
		* possibly a JIT problem because modifying this
		* method to print logging information when the
		* error happens seems to fix the problem.  The
		* fix is to use operating system calls to verify
		* that the current thread is not the Display thread.
		* 
		* NOTE: Despite the fact that Thread.currentThread()
		* is used in other places, the failure has not been
		* observed in all places where it is called. 
		*/
		if (display.threadId != OS.GetCurrentThreadId ()) {
			error (SWT.ERROR_THREAD_INVALID_ACCESS);
		}
	}
	if ((state & DISPOSED) != 0) error (SWT.ERROR_WIDGET_DISPOSED);
}