Tomcat(2): 生命周期Lifecycle

Tomcat中有很多组件，这些组件会随着Tomcat启动而一起启动，也会随着Tomcat关闭而一起关闭。通过实现org.apache.catalina.Lifecycle接口，可以达到统一启动/关闭这些组件的效果。

生命周期涉及的三个重要接口或类：Lifecycle、LifecycleEvenet和LifecycleListener。

Lifecycle接口

Tomcat在设计上允许一个组件包含其他组件，父组件负责启动/关闭它的子组件。这样，Tomcat的启动类只需要启动一个组件就可以将全部应用的组件都启动起来。这种单一启动/关闭机制是通过 Lifecycle 接口实现的。

Lifecycle接口中最重要的是start()方法和stop()方法。组件必须提供这两个方法的实现，供其父组件使用，以实现对其进行启动/关闭操作。

addLifecycleListener()、findLifecycleListeners和removeLifecycleListener是与事件监听器有关的。组件可以注册多个事件监听器来对发生在该组件上的某些事件进行监听。

public interface Lifecycle {


    // ----------------------------------------------------- Manifest Constants


    /**
     * The LifecycleEvent type for the "component before init" event.
     */
    public static final String BEFORE_INIT_EVENT = "before_init";


    /**
     * The LifecycleEvent type for the "component after init" event.
     */
    public static final String AFTER_INIT_EVENT = "after_init";


    /**
     * The LifecycleEvent type for the "component start" event.
     */
    public static final String START_EVENT = "start";


    /**
     * The LifecycleEvent type for the "component before start" event.
     */
    public static final String BEFORE_START_EVENT = "before_start";


    /**
     * The LifecycleEvent type for the "component after start" event.
     */
    public static final String AFTER_START_EVENT = "after_start";


    /**
     * The LifecycleEvent type for the "component stop" event.
     */
    public static final String STOP_EVENT = "stop";


    /**
     * The LifecycleEvent type for the "component before stop" event.
     */
    public static final String BEFORE_STOP_EVENT = "before_stop";


    /**
     * The LifecycleEvent type for the "component after stop" event.
     */
    public static final String AFTER_STOP_EVENT = "after_stop";


    /**
     * The LifecycleEvent type for the "component after destroy" event.
     */
    public static final String AFTER_DESTROY_EVENT = "after_destroy";


    /**
     * The LifecycleEvent type for the "component before destroy" event.
     */
    public static final String BEFORE_DESTROY_EVENT = "before_destroy";


    /**
     * The LifecycleEvent type for the "periodic" event.
     */
    public static final String PERIODIC_EVENT = "periodic";


    /**
     * The LifecycleEvent type for the "configure_start" event. Used by those
     * components that use a separate component to perform configuration and
     * need to signal when configuration should be performed - usually after
     * {@link #BEFORE_START_EVENT} and before {@link #START_EVENT}.
     */
    public static final String CONFIGURE_START_EVENT = "configure_start";


    /**
     * The LifecycleEvent type for the "configure_stop" event. Used by those
     * components that use a separate component to perform configuration and
     * need to signal when de-configuration should be performed - usually after
     * {@link #STOP_EVENT} and before {@link #AFTER_STOP_EVENT}.
     */
    public static final String CONFIGURE_STOP_EVENT = "configure_stop";


    // --------------------------------------------------------- Public Methods


    /**
     * Add a LifecycleEvent listener to this component.
     *
     * @param listener The listener to add
     */
    public void addLifecycleListener(LifecycleListener listener);


    /**
     * Get the life cycle listeners associated with this life cycle.
     *
     * @return An array containing the life cycle listeners associated with this
     *         life cycle. If this component has no listeners registered, a
     *         zero-length array is returned.
     */
    public LifecycleListener[] findLifecycleListeners();


    /**
     * Remove a LifecycleEvent listener from this component.
     *
     * @param listener The listener to remove
     */
    public void removeLifecycleListener(LifecycleListener listener);


    /**
     * Prepare the component for starting. This method should perform any
     * initialization required post object creation. The following
     * {@link LifecycleEvent}s will be fired in the following order:
     * <ol>
     *   <li>INIT_EVENT: On the successful completion of component
     *                   initialization.</li>
     * </ol>
     *
     * @exception LifecycleException if this component detects a fatal error
     *  that prevents this component from being used
     */
    public void init() throws LifecycleException;

    /**
     * Prepare for the beginning of active use of the public methods other than
     * property getters/setters and life cycle methods of this component. This
     * method should be called before any of the public methods other than
     * property getters/setters and life cycle methods of this component are
     * utilized. The following {@link LifecycleEvent}s will be fired in the
     * following order:
     * <ol>
     *   <li>BEFORE_START_EVENT: At the beginning of the method. It is as this
     *                           point the state transitions to
     *                           {@link LifecycleState#STARTING_PREP}.</li>
     *   <li>START_EVENT: During the method once it is safe to call start() for
     *                    any child components. It is at this point that the
     *                    state transitions to {@link LifecycleState#STARTING}
     *                    and that the public methods other than property
     *                    getters/setters and life cycle methods may be
     *                    used.</li>
     *   <li>AFTER_START_EVENT: At the end of the method, immediately before it
     *                          returns. It is at this point that the state
     *                          transitions to {@link LifecycleState#STARTED}.
     *                          </li>
     * </ol>
     *
     * @exception LifecycleException if this component detects a fatal error
     *  that prevents this component from being used
     */
    public void start() throws LifecycleException;


    /**
     * Gracefully terminate the active use of the public methods other than
     * property getters/setters and life cycle methods of this component. Once
     * the STOP_EVENT is fired, the public methods other than property
     * getters/setters and life cycle methods should not be used. The following
     * {@link LifecycleEvent}s will be fired in the following order:
     * <ol>
     *   <li>BEFORE_STOP_EVENT: At the beginning of the method. It is at this
     *                          point that the state transitions to
     *                          {@link LifecycleState#STOPPING_PREP}.</li>
     *   <li>STOP_EVENT: During the method once it is safe to call stop() for
     *                   any child components. It is at this point that the
     *                   state transitions to {@link LifecycleState#STOPPING}
     *                   and that the public methods other than property
     *                   getters/setters and life cycle methods may no longer be
     *                   used.</li>
     *   <li>AFTER_STOP_EVENT: At the end of the method, immediately before it
     *                         returns. It is at this point that the state
     *                         transitions to {@link LifecycleState#STOPPED}.
     *                         </li>
     * </ol>
     *
     * Note that if transitioning from {@link LifecycleState#FAILED} then the
     * three events above will be fired but the component will transition
     * directly from {@link LifecycleState#FAILED} to
     * {@link LifecycleState#STOPPING}, bypassing
     * {@link LifecycleState#STOPPING_PREP}
     *
     * @exception LifecycleException if this component detects a fatal error
     *  that needs to be reported
     */
    public void stop() throws LifecycleException;

    /**
     * Prepare to discard the object. The following {@link LifecycleEvent}s will
     * be fired in the following order:
     * <ol>
     *   <li>DESTROY_EVENT: On the successful completion of component
     *                      destruction.</li>
     * </ol>
     *
     * @exception LifecycleException if this component detects a fatal error
     *  that prevents this component from being used
     */
    public void destroy() throws LifecycleException;


    /**
     * Obtain the current state of the source component.
     *
     * @return The current state of the source component.
     */
    public LifecycleState getState();


    /**
     * Obtain a textual representation of the current component state. Useful
     * for JMX. The format of this string may vary between point releases and
     * should not be relied upon to determine component state. To determine
     * component state, use {@link #getState()}.
     *
     * @return The name of the current component state.
     */
    public String getStateName();


    /**
     * Marker interface used to indicate that the instance should only be used
     * once. Calling {@link #stop()} on an instance that supports this interface
     * will automatically call {@link #destroy()} after {@link #stop()}
     * completes.
     */
    public interface SingleUse {
    }
}

LifecycleEvent

生命周期事件是org.apache.catalina.LifecycleEvent的实例。

public final class LifecycleEvent extends EventObject {

    private static final long serialVersionUID = 1L;


    /**
     * Construct a new LifecycleEvent with the specified parameters.
     *
     * @param lifecycle Component on which this event occurred
     * @param type Event type (required)
     * @param data Event data (if any)
     */
    public LifecycleEvent(Lifecycle lifecycle, String type, Object data) {
        super(lifecycle);
        this.type = type;
        this.data = data;
    }


    /**
     * The event data associated with this event.
     */
    private final Object data;


    /**
     * The event type this instance represents.
     */
    private final String type;


    /**
     * @return the event data of this event.
     */
    public Object getData() {
        return data;
    }


    /**
     * @return the Lifecycle on which this event occurred.
     */
    public Lifecycle getLifecycle() {
        return (Lifecycle) getSource();
    }


    /**
     * @return the event type of this event.
     */
    public String getType() {
        return this.type;
    }
}

LifecycleListener

生命周期事件的监听器实现了org.apache.catalina.LifecycleListener接口。

public interface LifecycleListener {


    /**
     * Acknowledge the occurrence of the specified event.
     *
     * @param event LifecycleEvent that has occurred
     */
    public void lifecycleEvent(LifecycleEvent event);


}