Tomcat中设计模式-模板模式
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开篇
模板方法模式抽象出某个业务操作公共的流程，将流程分为几个步骤，其中有一些步骤是固定不变的，有一些步骤是变化的，固定不变的步骤通过一个基类来实现，而变化的部分通过钩子方法让子类去实现，这样就实现了对系统中流程的统一化规范化管理。
Tomcat中关于生命周期管理的地方很好应用了模板方法模式，在一个组件的生命周期中都会涉及到init(初始化)，start（启动），stop(停止)，destory（销毁），而对于每一个生命周期阶段其实都有固定一些事情要做，比如判断前置状态，设置后置状态，以及通知状态变更事件的监听者等，而这些工作其实是可以固化的，所以Tomcat中就将每个生命周期阶段公共的部分固化，然后通过initInternal,startInternal,stopInternal,destoryInternal这几个钩子方法开放给子类去实现具体的逻辑。
源码分析
说明：
- 1.tomcat的所有容器都实现了Lifecycle的生命周期管理接口，这里以Tomcat几个核心组件为例。
- 2.抽象基类LifecycleBase实现Lifecycle接口，在基类当中实现了模板接口start、stop、init等方法接口。
源码部分
public interface Lifecycle { public static final String BEFORE_INIT_EVENT = "before_init"; public static final String AFTER_INIT_EVENT = "after_init"; 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 static final String AFTER_DESTROY_EVENT = "after_destroy"; public static final String BEFORE_DESTROY_EVENT = "before_destroy"; public static final String PERIODIC_EVENT = "periodic"; public static final String CONFIGURE_START_EVENT = "configure_start"; public static final String CONFIGURE_STOP_EVENT = "configure_stop"; public void addLifecycleListener(LifecycleListener listener); public LifecycleListener[] findLifecycleListeners(); public void removeLifecycleListener(LifecycleListener listener); public void init() throws LifecycleException; public void start() throws LifecycleException; public void stop() throws LifecycleException; public void destroy() throws LifecycleException;} 说明：
- 1.基类LifecycleBase实现了Lifecycle接口并实现init、start、stop、destroy等方法。
- 2.init()方法内部模板化初始化流程，抽象具体的实现initInternal()方法。
- 3.start()方法内部模板化启动流程，抽象具体的实现startInternal()方法。
- 4.stop()方法内部模板化停止流程，抽象具体的实现stopInternal()方法。
- 5.destroy()方法内部模板化释放流程，抽象具体的实现destroyInternal()方法。
public abstract class LifecycleBase implements Lifecycle { private static final Log log = LogFactory.getLog(LifecycleBase.class); private static final StringManager sm = StringManager.getManager(LifecycleBase.class); private final List lifecycleListeners = new CopyOnWriteArrayList<>(); private volatile LifecycleState state = LifecycleState.NEW; public void addLifecycleListener(LifecycleListener listener) { lifecycleListeners.add(listener); } public LifecycleListener[] findLifecycleListeners() { return lifecycleListeners.toArray(new LifecycleListener[0]); } public void removeLifecycleListener(LifecycleListener listener) { lifecycleListeners.remove(listener); } protected void fireLifecycleEvent(String type, Object data) { LifecycleEvent event = new LifecycleEvent(this, type, data); for (LifecycleListener listener : lifecycleListeners) { listener.lifecycleEvent(event); } } public final synchronized void init() throws LifecycleException { if (!state.equals(LifecycleState.NEW)) { invalidTransition(Lifecycle.BEFORE_INIT_EVENT); } try { setStateInternal(LifecycleState.INITIALIZING, null, false); initInternal(); setStateInternal(LifecycleState.INITIALIZED, null, false); } catch (Throwable t) { ExceptionUtils.handleThrowable(t); setStateInternal(LifecycleState.FAILED, null, false); throw new LifecycleException( sm.getString("lifecycleBase.initFail",toString()), t); } } protected abstract void initInternal() throws LifecycleException; public final synchronized void start() throws LifecycleException { if (LifecycleState.STARTING_PREP.equals(state) || LifecycleState.STARTING.equals(state) || LifecycleState.STARTED.equals(state)) { if (log.isDebugEnabled()) { Exception e = new LifecycleException(); log.debug(sm.getString("lifecycleBase.alreadyStarted", toString()), e); } else if (log.isInfoEnabled()) { log.info(sm.getString("lifecycleBase.alreadyStarted", toString())); } return; } if (state.equals(LifecycleState.NEW)) { init(); } else if (state.equals(LifecycleState.FAILED)) { stop(); } else if (!state.equals(LifecycleState.INITIALIZED) && !state.equals(LifecycleState.STOPPED)) { invalidTransition(Lifecycle.BEFORE_START_EVENT); } try { setStateInternal(LifecycleState.STARTING_PREP, null, false); startInternal(); if (state.equals(LifecycleState.FAILED)) { stop(); } else if (!state.equals(LifecycleState.STARTING)) { invalidTransition(Lifecycle.AFTER_START_EVENT); } else { setStateInternal(LifecycleState.STARTED, null, false); } } catch (Throwable t) { ExceptionUtils.handleThrowable(t); setStateInternal(LifecycleState.FAILED, null, false); throw new LifecycleException(sm.getString("lifecycleBase.startFail", toString()), t); } } protected abstract void startInternal() throws LifecycleException; public final synchronized void stop() throws LifecycleException { if (LifecycleState.STOPPING_PREP.equals(state) || LifecycleState.STOPPING.equals(state) || LifecycleState.STOPPED.equals(state)) { if (log.isDebugEnabled()) { Exception e = new LifecycleException(); log.debug(sm.getString("lifecycleBase.alreadyStopped", toString()), e); } else if (log.isInfoEnabled()) { log.info(sm.getString("lifecycleBase.alreadyStopped", toString())); } return; } if (state.equals(LifecycleState.NEW)) { state = LifecycleState.STOPPED; return; } if (!state.equals(LifecycleState.STARTED) && !state.equals(LifecycleState.FAILED)) { invalidTransition(Lifecycle.BEFORE_STOP_EVENT); } try { if (state.equals(LifecycleState.FAILED)) { fireLifecycleEvent(BEFORE_STOP_EVENT, null); } else { setStateInternal(LifecycleState.STOPPING_PREP, null, false); } stopInternal(); if (!state.equals(LifecycleState.STOPPING) && !state.equals(LifecycleState.FAILED)) { invalidTransition(Lifecycle.AFTER_STOP_EVENT); } setStateInternal(LifecycleState.STOPPED, null, false); } catch (Throwable t) { ExceptionUtils.handleThrowable(t); setStateInternal(LifecycleState.FAILED, null, false); throw new LifecycleException(sm.getString("lifecycleBase.stopFail",toString()), t); } finally { if (this instanceof Lifecycle.SingleUse) { // Complete stop process first setStateInternal(LifecycleState.STOPPED, null, false); destroy(); } } } protected abstract void stopInternal() throws LifecycleException; @Override public final synchronized void destroy() throws LifecycleException { if (LifecycleState.FAILED.equals(state)) { try { // Triggers clean-up stop(); } catch (LifecycleException e) { // Just log. Still want to destroy. log.warn(sm.getString( "lifecycleBase.destroyStopFail", toString()), e); } } if (LifecycleState.DESTROYING.equals(state) || LifecycleState.DESTROYED.equals(state)) { if (log.isDebugEnabled()) { Exception e = new LifecycleException(); log.debug(sm.getString("lifecycleBase.alreadyDestroyed", toString()), e); } else if (log.isInfoEnabled() && !(this instanceof Lifecycle.SingleUse)) { log.info(sm.getString("lifecycleBase.alreadyDestroyed", toString())); } return; } if (!state.equals(LifecycleState.STOPPED) && !state.equals(LifecycleState.FAILED) && !state.equals(LifecycleState.NEW) && !state.equals(LifecycleState.INITIALIZED)) { invalidTransition(Lifecycle.BEFORE_DESTROY_EVENT); } try { setStateInternal(LifecycleState.DESTROYING, null, false); destroyInternal(); setStateInternal(LifecycleState.DESTROYED, null, false); } catch (Throwable t) { ExceptionUtils.handleThrowable(t); setStateInternal(LifecycleState.FAILED, null, false); throw new LifecycleException( sm.getString("lifecycleBase.destroyFail",toString()), t); } } protected abstract void destroyInternal() throws LifecycleException;} 招聘信息
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