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：
- 基类LifecycleBase实现了Lifecycle接口并实现init、start、stop、destroy等方法。
- init()方法内部模板化初始化流程，抽象具体的实现initInternal()方法。
- start()方法内部模板化启动流程，抽象具体的实现startInternal()方法。
- stop()方法内部模板化停止流程，抽象具体的实现stopInternal()方法。
- 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<LifecycleListener> 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;
}
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