java/javax/servlet/UnavailableException.java

/*
 * Licensed to the Apache Software Foundation (ASF) under one or more
 * contributor license agreements.  See the NOTICE file distributed with
 * this work for additional information regarding copyright ownership.
 * The ASF licenses this file to You under the Apache License, Version 2.0
 * (the "License"); you may not use this file except in compliance with
 * the License.  You may obtain a copy of the License at
 *
 *     http://www.apache.org/licenses/LICENSE-2.0
 *
 * Unless required by applicable law or agreed to in writing, software
 * distributed under the License is distributed on an "AS IS" BASIS,
 * WITHOUT WARRANTIES OR CONDITIONS OF ANY KIND, either express or implied.
 * See the License for the specific language governing permissions and
 * limitations under the License.
 */

package javax.servlet;

/**
 * Defines an exception that a servlet or filter throws to indicate that it is
 * permanently or temporarily unavailable.
 * <p>
 * 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.
 * <p>
 * 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.
 * <p>
 * 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 <code>Servlet</code> instance that is unavailable
     * @param msg
     *            a <code>String</code> 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 <code>Servlet</code> that is unavailable
     * @param msg
     *            a <code>String</code> 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 <code>String</code> 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.
     * <p>
     * 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 <code>seconds</code>
     * argument.
     *
     * @param msg
     *            a <code>String</code> 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 <code>boolean</code> 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 <code>true</code> if the servlet is permanently unavailable;
     *         <code>false</code> if the servlet is available or temporarily
     *         unavailable
     */
    public boolean isPermanent() {
        return permanent;
    }

    /**
     * Returns the servlet that is reporting its unavailability.
     *
     * @return the <code>Servlet</code> object that is throwing the
     *         <code>UnavailableException</code>
     * @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.
     * <p>
     * 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;
    }
}