+ * When a servlet or filter is permanently unavailable, something is wrong with + * it, and it cannot handle requests until some action is taken. For example, a + * servlet might be configured incorrectly, or a filter's state may be + * corrupted. The component should log both the error and the corrective action + * that is needed. + *
+ * A servlet or filter is temporarily unavailable if it cannot handle requests + * momentarily due to some system-wide problem. For example, a third-tier server + * might not be accessible, or there may be insufficient memory or disk storage + * to handle requests. A system administrator may need to take corrective + * action. + *
+ * Servlet containers can safely treat both types of unavailable exceptions in
+ * the same way. However, treating temporary unavailability effectively makes
+ * the servlet container more robust. Specifically, the servlet container might
+ * block requests to the servlet or filter for a period of time suggested by the
+ * exception, rather than rejecting them until the servlet container restarts.
+ */
+public class UnavailableException extends ServletException {
+
+ private static final long serialVersionUID = 1L;
+
+ private final Servlet servlet; // what's unavailable
+ private final boolean permanent; // needs admin action?
+ private final int seconds; // unavailability estimate
+
+ /**
+ * @param servlet
+ * the Servlet instance that is unavailable
+ * @param msg
+ * a String specifying the descriptive message
+ * @deprecated As of Java Servlet API 2.2, use
+ * {@link #UnavailableException(String)} instead.
+ */
+ @SuppressWarnings("dep-ann")
+ // Spec API does not use @Deprecated
+ public UnavailableException(Servlet servlet, String msg) {
+ super(msg);
+ this.servlet = servlet;
+ permanent = true;
+ this.seconds = 0;
+ }
+
+ /**
+ * @param seconds
+ * an integer specifying the number of seconds the servlet
+ * expects to be unavailable; if zero or negative, indicates that
+ * the servlet can't make an estimate
+ * @param servlet
+ * the Servlet that is unavailable
+ * @param msg
+ * a String specifying the descriptive message,
+ * which can be written to a log file or displayed for the user.
+ * @deprecated As of Java Servlet API 2.2, use
+ * {@link #UnavailableException(String, int)} instead.
+ */
+ @SuppressWarnings("dep-ann")
+ // Spec API does not use @Deprecated
+ public UnavailableException(int seconds, Servlet servlet, String msg) {
+ super(msg);
+ this.servlet = servlet;
+ if (seconds <= 0)
+ this.seconds = -1;
+ else
+ this.seconds = seconds;
+ permanent = false;
+ }
+
+ /**
+ * Constructs a new exception with a descriptive message indicating that the
+ * servlet is permanently unavailable.
+ *
+ * @param msg
+ * a String specifying the descriptive message
+ */
+ public UnavailableException(String msg) {
+ super(msg);
+ seconds = 0;
+ servlet = null;
+ permanent = true;
+ }
+
+ /**
+ * Constructs a new exception with a descriptive message indicating that the
+ * servlet is temporarily unavailable and giving an estimate of how long it
+ * will be unavailable.
+ *
+ * In some cases, the servlet cannot make an estimate. For example, the
+ * servlet might know that a server it needs is not running, but not be able
+ * to report how long it will take to be restored to functionality. This can
+ * be indicated with a negative or zero value for the seconds
+ * argument.
+ *
+ * @param msg
+ * a String specifying the descriptive message,
+ * which can be written to a log file or displayed for the user.
+ * @param seconds
+ * an integer specifying the number of seconds the servlet
+ * expects to be unavailable; if zero or negative, indicates that
+ * the servlet can't make an estimate
+ */
+ public UnavailableException(String msg, int seconds) {
+ super(msg);
+
+ if (seconds <= 0)
+ this.seconds = -1;
+ else
+ this.seconds = seconds;
+ servlet = null;
+ permanent = false;
+ }
+
+ /**
+ * Returns a boolean indicating whether the servlet is
+ * permanently unavailable. If so, something is wrong with the servlet, and
+ * the system administrator must take some corrective action.
+ *
+ * @return true if the servlet is permanently unavailable;
+ * false if the servlet is available or temporarily
+ * unavailable
+ */
+ public boolean isPermanent() {
+ return permanent;
+ }
+
+ /**
+ * Returns the servlet that is reporting its unavailability.
+ *
+ * @return the Servlet object that is throwing the
+ * UnavailableException
+ * @deprecated As of Java Servlet API 2.2, with no replacement.
+ */
+ @SuppressWarnings("dep-ann")
+ // Spec API does not use @Deprecated
+ public Servlet getServlet() {
+ return servlet;
+ }
+
+ /**
+ * Returns the number of seconds the servlet expects to be temporarily
+ * unavailable.
+ *
+ * If this method returns a negative number, the servlet is permanently + * unavailable or cannot provide an estimate of how long it will be + * unavailable. No effort is made to correct for the time elapsed since the + * exception was first reported. + * + * @return an integer specifying the number of seconds the servlet will be + * temporarily unavailable, or a negative number if the servlet is + * permanently unavailable or cannot make an estimate + */ + public int getUnavailableSeconds() { + return permanent ? -1 : seconds; + } +}