本文接下来描述servlet容器是怎样管理其相关组件的生命周期的，首先本人描述一下事件监听模式，也可以称为观察者模式，该模式分为以下角色
即抽象主题角色 具体主题角色 抽象观察者角色及具体观察者角色，通常在主题角色里面维持一个观察者角色的聚集，并定义了对观察者角色的添加和删除方法；在主题角色事件发生时，通过迭代向这些观察者角色发布事件。
在tomcat容器中，管理其相关组件的生命周期即是基于事件监听模式实现的，相关接口和类为org.apache.catalina.Lifecycle接口、org.apache.catalina.LifecycleListener接口、org.apache.catalina.LifecycleEvent类
org.apache.catalina.Lifecycle接口定义如下
public interface Lifecycle { public static final String START_EVENT = "start"; public static final String BEFORE_START_EVENT = "before_start"; public static final String AFTER_START_EVENT = "after_start"; public static final String STOP_EVENT = "stop"; public static final String BEFORE_STOP_EVENT = "before_stop"; public static final String AFTER_STOP_EVENT = "after_stop"; public void addLifecycleListener(LifecycleListener listener); public LifecycleListener[] findLifecycleListeners(); public void removeLifecycleListener(LifecycleListener listener); public void start() throws LifecycleException; public void stop() throws LifecycleException; }
上面的六个静态字符串代表六种类型的事件，接着的三个方法addLifecycleListener()、 findLifecycleListeners()、removeLifecycleListener()为与事件监听器相关的，最后的两个方法start()和stop()为启动与关闭操作，组件必须提供这两种方法的实现，供其父组件调用。
org.apache.catalina.LifecycleEvent类用于传递事件及携带事件数据
public final class LifecycleEvent extends EventObject { public LifecycleEvent(Lifecycle lifecycle, String type) { this(lifecycle, type, null); } public LifecycleEvent(Lifecycle lifecycle, String type, Object data) { super(lifecycle); this.lifecycle = lifecycle; this.type = type; this.data = data; } private Object data = null; private Lifecycle lifecycle = null; private String type = null; public Object getData() { return (this.data); } public Lifecycle getLifecycle() { return (this.lifecycle); } public String getType() { return (this.type); } }
org.apache.catalina.LifecycleListener为监听器接口
public interface LifecycleListener { /** * Acknowledge the occurrence of the specified event. * * @param event LifecycleEvent that has occurred */ public void lifecycleEvent(LifecycleEvent event); }
通常实现了Lifecycle接口并且对某个事件注册了监听器的组件必须提供 Lifecycle接口中三个与监听器相关方法（分别为addLifecycleListener()、 findLifecycleListeners()、removeLifecycleListener()）的实现，然后该组件需要将所有注册的监听器存储到数组、ArrayList或者其他类似的对象中；tomcat容器提供了一个工具类org.apache.catalina.util.LifecycleSupport来帮助组件管理监听器，并触发相应的生命周期事件。
public final class LifecycleSupport { public LifecycleSupport(Lifecycle lifecycle) { super(); this.lifecycle = lifecycle; } private Lifecycle lifecycle = null; private LifecycleListener listeners[] = new LifecycleListener[0]; public void addLifecycleListener(LifecycleListener listener) { synchronized (listeners) { LifecycleListener results[] = new LifecycleListener[listeners.length + 1]; for (int i = 0; i < listeners.length; i++) results[i] = listeners[i]; results[listeners.length] = listener; listeners = results; } } public LifecycleListener[] findLifecycleListeners() { return listeners; } public void fireLifecycleEvent(String type, Object data) { LifecycleEvent event = new LifecycleEvent(lifecycle, type, data); LifecycleListener interested[] = null; synchronized (listeners) { interested = (LifecycleListener[]) listeners.clone(); } for (int i = 0; i < interested.length; i++) interested[i].lifecycleEvent(event); } public void removeLifecycleListener(LifecycleListener listener) { synchronized (listeners) { int n = -1; for (int i = 0; i < listeners.length; i++) { if (listeners[i] == listener) { n = i; break; } } if (n < 0) return; LifecycleListener results[] = new LifecycleListener[listeners.length - 1]; int j = 0; for (int i = 0; i < listeners.length; i++) { if (i != n) results[j++] = listeners[i]; } listeners = results; } } }
下面我们来看Context容器（SimpleContext实例）是怎么实现生命周期管理及触发监听器事件的，关键代码如下
public class SimpleContext implements Context, Pipeline, Lifecycle { protected LifecycleSupport lifecycle = new LifecycleSupport(this); // implementation of the Lifecycle interface's methods public void addLifecycleListener(LifecycleListener listener) { lifecycle.addLifecycleListener(listener); } public LifecycleListener[] findLifecycleListeners() { return null; } public void removeLifecycleListener(LifecycleListener listener) { lifecycle.removeLifecycleListener(listener); } public synchronized void start() throws LifecycleException { if (started) throw new LifecycleException("SimpleContext has already started"); // Notify our interested LifecycleListeners lifecycle.fireLifecycleEvent(BEFORE_START_EVENT, null); started = true; try { // Start our subordinate components, if any if ((loader != null) && (loader instanceof Lifecycle)) ((Lifecycle) loader).start(); // Start our child containers, if any Container children[] = findChildren(); for (int i = 0; i < children.length; i++) { if (children[i] instanceof Lifecycle) ((Lifecycle) children[i]).start(); } // Start the Valves in our pipeline (including the basic), // if any if (pipeline instanceof Lifecycle) ((Lifecycle) pipeline).start(); // Notify our interested LifecycleListeners lifecycle.fireLifecycleEvent(START_EVENT, null); } catch (Exception e) { e.printStackTrace(); } // Notify our interested LifecycleListeners lifecycle.fireLifecycleEvent(AFTER_START_EVENT, null); } public void stop() throws LifecycleException { if (!started) throw new LifecycleException("SimpleContext has not been started"); // Notify our interested LifecycleListeners lifecycle.fireLifecycleEvent(BEFORE_STOP_EVENT, null); lifecycle.fireLifecycleEvent(STOP_EVENT, null); started = false; try { // Stop the Valves in our pipeline (including the basic), if any if (pipeline instanceof Lifecycle) { ((Lifecycle) pipeline).stop(); } // Stop our child containers, if any Container children[] = findChildren(); for (int i = 0; i < children.length; i++) { if (children[i] instanceof Lifecycle) ((Lifecycle) children[i]).stop(); } if ((loader != null) && (loader instanceof Lifecycle)) { ((Lifecycle) loader).stop(); } } catch (Exception e) { e.printStackTrace(); } // Notify our interested LifecycleListeners lifecycle.fireLifecycleEvent(AFTER_STOP_EVENT, null); } }
Context容器实例持有对LifecycleSupport lifecycle = new LifecycleSupport(this)的引用，与事件监听器相关方法均为操作该实例的相应方法
在其与生命周期相关方法start()和stop()方法里面，分别调用相关组件的生命周期方法，并采用LifecycleSupport lifecycle引用发布事件
一个简单的监听器实现如下：
public class SimpleContextLifecycleListener implements LifecycleListener { public void lifecycleEvent(LifecycleEvent event) { Lifecycle lifecycle = event.getLifecycle(); System.out.println("SimpleContextLifecycleListener's event " + event.getType().toString()); if (Lifecycle.START_EVENT.equals(event.getType())) { System.out.println("Starting context."); } else if (Lifecycle.STOP_EVENT.equals(event.getType())) { System.out.println("Stopping context."); } } }
子容器wrapper实例（SimpleWrapper），同样实现了Lifecycle接口，其相关实现与Context容器类似
public class SimpleWrapper implements Wrapper, Pipeline, Lifecycle { protected LifecycleSupport lifecycle = new LifecycleSupport(this); // implementation of the Lifecycle interface's methods public void addLifecycleListener(LifecycleListener listener) { } public LifecycleListener[] findLifecycleListeners() { return null; } public void removeLifecycleListener(LifecycleListener listener) { } public synchronized void start() throws LifecycleException { System.out.println("Starting Wrapper " + name); if (started) throw new LifecycleException("Wrapper already started"); // Notify our interested LifecycleListeners lifecycle.fireLifecycleEvent(BEFORE_START_EVENT, null); started = true; // Start our subordinate components, if any if ((loader != null) && (loader instanceof Lifecycle)) ((Lifecycle) loader).start(); // Start the Valves in our pipeline (including the basic), if any if (pipeline instanceof Lifecycle) ((Lifecycle) pipeline).start(); // Notify our interested LifecycleListeners lifecycle.fireLifecycleEvent(START_EVENT, null); // Notify our interested LifecycleListeners lifecycle.fireLifecycleEvent(AFTER_START_EVENT, null); } public void stop() throws LifecycleException { System.out.println("Stopping wrapper " + name); // Shut down our servlet instance (if it has been initialized) try { instance.destroy(); } catch (Throwable t) { } instance = null; if (!started) throw new LifecycleException("Wrapper " + name + " not started"); // Notify our interested LifecycleListeners lifecycle.fireLifecycleEvent(BEFORE_STOP_EVENT, null); // Notify our interested LifecycleListeners lifecycle.fireLifecycleEvent(STOP_EVENT, null); started = false; // Stop the Valves in our pipeline (including the basic), if any if (pipeline instanceof Lifecycle) { ((Lifecycle) pipeline).stop(); } // Stop our subordinate components, if any if ((loader != null) && (loader instanceof Lifecycle)) { ((Lifecycle) loader).stop(); } // Notify our interested LifecycleListeners lifecycle.fireLifecycleEvent(AFTER_STOP_EVENT, null); } }
其他相关组件的相关实现不再赘述
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