在上篇文章分析Tomcat7的各组件的init、start方法时经常会看到有一个setStateInternal方法的调用,在查看LifecycleBase类及其它各组件的源码时会在多处看到这个方法的调用,这篇文章就来说说这方法,以及与这个方法相关的Tomcat的Lifecycle机制和实现原理。
上篇文章里谈到Tomcat7的各组件的父类LifecycleBase类,该类实现了接口org.apache.catalina.Lifecycle,下面是这个接口里定义的常量和方法:
细心的读者会发现,上篇文章里提到的init和start方法实际上是在这个接口里面定义好的,也正因为有各组件最终都会实现这个接口作为前提条件,所以才能支持组件内部的initInternal、startInternal方法内对于子组件(组件里面嵌套的子组件都是以接口的形式定义的,但这些接口都会以Lifecycle作为父接口)的init和start方法的调用。通过这种方式,只要调用了最外层的Server组件的init和start方法,就可以将Tomcat内部的各级子组件初始化和启动起来。我叫这种方式为链式调用。实际上关于Tomcat的关闭机制也是通过这种方式一步步调用各层组件的stop方法的。这里不再展开叙述,留待读者自己研究研究吧。
Lifecycle接口中的这些字符串常量定义主要用于事件类型的定义,先按下不表,文章后面会提到。
重点看下面三个方法:
- /**
- * Add a LifecycleEvent listener to this component.
- *
- * @param listener The listener to add
- */
- public void addLifecycleListener(LifecycleListener listener);//给该组将添加一个监听器
-
-
- /**
- * Get 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();//获取该组件所有已注册的监听器
-
-
- /**
- * Remove a LifecycleEvent listener from this component.
- *
- * @param listener The listener to remove
- */
- public void removeLifecycleListener(LifecycleListener listener);//删除该组件中的一个监听器
/** * Add a LifecycleEvent listener to this component. * * @param listener The listener to add */ public void addLifecycleListener(LifecycleListener listener);//给该组将添加一个监听器 /** * Get 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();//获取该组件所有已注册的监听器 /** * Remove a LifecycleEvent listener from this component. * * @param listener The listener to remove */ public void removeLifecycleListener(LifecycleListener listener);//删除该组件中的一个监听器
这三个方法的作用在代码的注释里简要说明了一下。这三个方法涉及org.apache.catalina.LifecycleListener接口,那么就看下这个接口的定义:
- public interface LifecycleListener {
-
-
- /**
- * Acknowledge the occurrence of the specified event.
- *
- * @param event LifecycleEvent that has occurred
- */
- public void lifecycleEvent(LifecycleEvent event);
-
-
- }
public interface LifecycleListener { /** * Acknowledge the occurrence of the specified event. * * @param event LifecycleEvent that has occurred */ public void lifecycleEvent(LifecycleEvent event);}
如此简单,只有一个方法,这个方法用作某个事件(org.apache.catalina.LifecycleEvent)产生时通知当前监听器的实现类,具体针对该事件如何处理由监听器实现类自己决定。
看下LifecycleEvent的实现:
- public final class LifecycleEvent extends EventObject {
-
- private static final long serialVersionUID = 1L;
-
-
- // ----------------------------------------------------------- Constructors
-
- /**
- * 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;
- }
-
-
- // ----------------------------------------------------- Instance Variables
-
-
- /**
- * The event data associated with this event.
- */
- private Object data = null;
-
-
- /**
- * The event type this instance represents.
- */
- private String type = null;
-
-
- // ------------------------------------------------------------- Properties
-
-
- /**
- * Return the event data of this event.
- */
- public Object getData() {
-
- return (this.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);
-
- }
-
-
- }
public final class LifecycleEvent extends EventObject { private static final long serialVersionUID = 1L; // ----------------------------------------------------------- Constructors /** * 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; } // ----------------------------------------------------- Instance Variables /** * The event data associated with this event. */ private Object data = null; /** * The event type this instance represents. */ private String type = null; // ------------------------------------------------------------- Properties /** * Return the event data of this event. */ public Object getData() { return (this.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); }}
这个类也很简单,data和type作为类的内置实例变量,唯一特别是使用了jdk内置的java.util.EventObject作为父类来支持事件定义,这里在事件构造函数中将org.apache.catalina.Lifecycle类的实例lifecycle作为事件源,保存lifecycle对象的引用,并提供了getLifecycle方法返回这个引用。
那么Tomcat中是如何实现关于这些事件的监听以及通知的呢?
在本文开头提到的LifecycleBase类中第47行定义了一个实例变量lifecycle,正是通过该变量来注册组件上定义的各类监听器的。留心一下lifecycle这个实例变量,它并不是org.apache.catalina.Lifecycle类的实例,而是org.apache.catalina.util.LifecycleSupport类的实例。正是这个工具类提供了事件监听和事件通知的功能。
先看下实际代码中是如何给组件发布时间通知的,看下前面文章中曾经提到过的org.apache.catalina.core.StandardServer类的startInternal方法:
- protected void startInternal() throws LifecycleException {
-
- fireLifecycleEvent(CONFIGURE_START_EVENT, null);
- setState(LifecycleState.STARTING);
-
- globalNamingResources.start();
-
- // Start our defined Services
- synchronized (services) {
- for (int i = 0; i < services.length; i ) {
- services[i].start();
- }
- }
- }
protected void startInternal() throws LifecycleException { fireLifecycleEvent(CONFIGURE_START_EVENT, null); setState(LifecycleState.STARTING); globalNamingResources.start(); // Start our defined Services synchronized (services) { for (int i = 0; i < services.length; i ) { services[i].start(); } } }
我们前面已经分析过第9到13行代码,这里看下第3行,它调用了父类org.apache.catalina.util.LifecycleBase里的fireLifecycleEvent方法,这里的CONFIGURE_START_EVENT就是本文最开始Lifecycle接口中定义的常量,这里表示发布了一个start配置事件。
org.apache.catalina.util.LifecycleBase类中的fireLifecycleEvent方法里调用的是org.apache.catalina.util.LifecycleSupport类fireLifecycleEvent方法,该方法代码如下:
- public void fireLifecycleEvent(String type, Object data) {
-
- LifecycleEvent event = new LifecycleEvent(lifecycle, type, data);
- LifecycleListener interested[] = listeners;
- for (int i = 0; i < interested.length; i )
- interested[i].lifecycleEvent(event);
-
- }
public void fireLifecycleEvent(String type, Object data) { LifecycleEvent event = new LifecycleEvent(lifecycle, type, data); LifecycleListener interested[] = listeners; for (int i = 0; i < interested.length; i ) interested[i].lifecycleEvent(event); }
这里通过传进来的两个参数构造一个LifecycleEvent对象,然后向注册到组件中的所有监听器发布这个新构造的事件对象。
这里有个疑问,到底什么时候向组件里注册监听器的呢?
还是以StandardServer举例,在前面讲Digester的使用时,org.apache.catalina.startup.Catalina类的createStartDigester方法有这么一段代码:
- // Configure the actions we will be using
- digester.addObjectCreate('Server',
- 'org.apache.catalina.core.StandardServer',
- 'className');
- digester.addSetProperties('Server');
- digester.addSetNext('Server',
- 'setServer',
- 'org.apache.catalina.Server');
-
- digester.addObjectCreate('Server/GlobalNamingResources',
- 'org.apache.catalina.deploy.NamingResources');
- digester.addSetProperties('Server/GlobalNamingResources');
- digester.addSetNext('Server/GlobalNamingResources',
- 'setGlobalNamingResources',
- 'org.apache.catalina.deploy.NamingResources');
-
- digester.addObjectCreate('Server/Listener',
- null, // MUST be specified in the element
- 'className');
- digester.addSetProperties('Server/Listener');
- digester.addSetNext('Server/Listener',
- 'addLifecycleListener',
- 'org.apache.catalina.LifecycleListener');
// Configure the actions we will be using digester.addObjectCreate('Server', 'org.apache.catalina.core.StandardServer', 'className'); digester.addSetProperties('Server'); digester.addSetNext('Server', 'setServer', 'org.apache.catalina.Server'); digester.addObjectCreate('Server/GlobalNamingResources', 'org.apache.catalina.deploy.NamingResources'); digester.addSetProperties('Server/GlobalNamingResources'); digester.addSetNext('Server/GlobalNamingResources', 'setGlobalNamingResources', 'org.apache.catalina.deploy.NamingResources'); digester.addObjectCreate('Server/Listener', null, // MUST be specified in the element 'className'); digester.addSetProperties('Server/Listener'); digester.addSetNext('Server/Listener', 'addLifecycleListener', 'org.apache.catalina.LifecycleListener');
第17到24行,将调用org.apache.catalina.core.StandardServer类的addLifecycleListener方法,将根据server.xml中配置的Server节点下的Listener节点所定义的className属性构造对象实例,并作为addLifecycleListener方法的入参。所有的监听器都会实现上面提到的org.apache.catalina.LifecycleListener接口。Server节点下的Listener节点有好几个,这里以org.apache.catalina.core.JasperListener举例。
在构造完org.apache.catalina.core.JasperListener类的对象之后,调用addLifecycleListener方法,这个方法并没有直接在org.apache.catalina.core.StandardServer类中定义,而是在它的父类org.apache.catalina.util.LifecycleBase中:
- @Override
- public void addLifecycleListener(LifecycleListener listener) {
- lifecycle.addLifecycleListener(listener);
- }
@Override public void addLifecycleListener(LifecycleListener listener) { lifecycle.addLifecycleListener(listener); }
这里调用的是前述的org.apache.catalina.util.LifecycleSupport类的addLifecycleListener方法:
- /**
- * Add a lifecycle event listener to this component.
- *
- * @param listener The listener to add
- */
- public void addLifecycleListener(LifecycleListener listener) {
-
- synchronized (listenersLock) {
- 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;
- }
-
- }
/** * Add a lifecycle event listener to this component. * * @param listener The listener to add */ public void addLifecycleListener(LifecycleListener listener) { synchronized (listenersLock) { 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; } }
LifecycleSupport作为一个工具类,内部保存了一个监听器对象实例数组,见该类的第68行:
- /**
- * The set of registered LifecycleListeners for event notifications.
- */
- private LifecycleListener listeners[] = new LifecycleListener[0];
/** * The set of registered LifecycleListeners for event notifications. */ private LifecycleListener listeners[] = new LifecycleListener[0];
上面的addLifecycleListener方法内部实现的是同步给该数组增加一个监听器对象。
看到这里应该大体明白Tomcat中的Lifecycle是怎么回事了,总的来说就是通过一个工具类LifecycleSupport,调用该类的addLifecycleListener方法增加监听器,需要发布事件时还是调用该工具类的fireLifecycleEvent方法,将事件发布给组件上注册的所有监听器,由监听器内部实现来决定是否处理该事件。
以前面看到的一个监听器org.apache.catalina.core.JasperListener举例:
- public class JasperListener
- implements LifecycleListener {
-
- private static final Log log = LogFactory.getLog(JasperListener.class);
-
- /**
- * The string manager for this package.
- */
- protected static final StringManager sm =
- StringManager.getManager(Constants.Package);
-
-
- // ---------------------------------------------- LifecycleListener Methods
-
-
- /**
- * Primary entry point for startup and shutdown events.
- *
- * @param event The event that has occurred
- */
- @Override
- public void lifecycleEvent(LifecycleEvent event) {
-
- if (Lifecycle.BEFORE_INIT_EVENT.equals(event.getType())) {
- try {
- // Set JSP factory
- Class.forName('org.apache.jasper.compiler.JspRuntimeContext',
- true,
- this.getClass().getClassLoader());
- } catch (Throwable t) {
- ExceptionUtils.handleThrowable(t);
- // Should not occur, obviously
- log.warn('Couldn't initialize Jasper', t);
- }
- // Another possibility is to do directly:
- // JspFactory.setDefaultFactory(new JspFactoryImpl());
- }
-
- }
-
-
- }
public class JasperListener implements LifecycleListener { private static final Log log = LogFactory.getLog(JasperListener.class); /** * The string manager for this package. */ protected static final StringManager sm = StringManager.getManager(Constants.Package); // ---------------------------------------------- LifecycleListener Methods /** * Primary entry point for startup and shutdown events. * * @param event The event that has occurred */ @Override public void lifecycleEvent(LifecycleEvent event) { if (Lifecycle.BEFORE_INIT_EVENT.equals(event.getType())) { try { // Set JSP factory Class.forName('org.apache.jasper.compiler.JspRuntimeContext', true, this.getClass().getClassLoader()); } catch (Throwable t) { ExceptionUtils.handleThrowable(t); // Should not occur, obviously log.warn('Couldn't initialize Jasper', t); } // Another possibility is to do directly: // JspFactory.setDefaultFactory(new JspFactoryImpl()); } }}
重点关注来自接口的lifecycleEvent方法的实现,可以看到这个监听器只关心事件类型为BEFORE_INIT_EVENT的事件,如果发布了该事件,才会做后续处理(这里会产生一个org.apache.jasper.compiler.JspRuntimeContext对象)。
Lifecycle相关类UML关系图:
如果对设计模式比较熟悉的话会发现Tomcat的Lifecycle使用的是观察者模式:LifecycleListener代表的是抽象观察者,它定义一个lifecycleEvent方法,而实现该接口的监听器是作为具体的观察者。Lifecycle 接口代表的是抽象主题,它定义了管理观察者的方法和它要所做的其它方法。而各组件代表的是具体主题,它实现了抽象主题的所有方法。通常会由具体主题保存对具体观察者对象有用的内部状态;在这种内部状态改变时给其观察者发出一个通知。Tomcat对这种模式做了改进,增加了另外两个工具类:LifecycleSupport、LifecycleEvent,它们作为辅助类扩展了观察者的功能。LifecycleEvent中定义了事件类别,不同的事件在具体观察者中可区别处理,更加灵活。LifecycleSupport 类代理了所有具体主题对观察者的管理,将这个管理抽出来统一实现,以后如果修改只要修改 LifecycleSupport 类就可以了,不需要去修改所有具体主题,因为所有具体主题的对观察者的操作都被代理给 LifecycleSupport 类了。
事件的发布使用的是推模式,即每发布一个事件都会通知主题的所有具体观察者,由各观察者再来决定是否需要对该事件进行后续处理。
下面再来看看本文一开头所说的setStateInternal方法,以org.apache.catalina.core.StandardServer类为例,上面看到的startInternal方法中第4行:setState(LifecycleState.STARTING);
它调用了父类org.apache.catalina.util.LifecycleBase中的setState方法:
- /**
- * Provides a mechanism for sub-classes to update the component state.
- * Calling this method will automatically fire any associated
- * {@link Lifecycle} event. It will also check that any attempted state
- * transition is valid for a sub-class.
- *
- * @param state The new state for this component
- */
- protected synchronized void setState(LifecycleState state)
- throws LifecycleException {
- setStateInternal(state, null, true);
- }
/** * Provides a mechanism for sub-classes to update the component state. * Calling this method will automatically fire any associated * {@link Lifecycle} event. It will also check that any attempted state * transition is valid for a sub-class. * * @param state The new state for this component */ protected synchronized void setState(LifecycleState state) throws LifecycleException { setStateInternal(state, null, true); }
在这个类里面调用本类的一个同步方法setStateInternal:
- private synchronized void setStateInternal(LifecycleState state,
- Object data, boolean check) throws LifecycleException {
-
- if (log.isDebugEnabled()) {
- log.debug(sm.getString('lifecycleBase.setState', this, state));
- }
-
- if (check) {
- // Must have been triggered by one of the abstract methods (assume
- // code in this class is correct)
- // null is never a valid state
- if (state == null) {
- invalidTransition('null');
- // Unreachable code - here to stop eclipse complaining about
- // a possible NPE further down the method
- return;
- }
-
- // Any method can transition to failed
- // startInternal() permits STARTING_PREP to STARTING
- // stopInternal() permits STOPPING_PREP to STOPPING and FAILED to
- // STOPPING
- if (!(state == LifecycleState.FAILED ||
- (this.state == LifecycleState.STARTING_PREP &&
- state == LifecycleState.STARTING) ||
- (this.state == LifecycleState.STOPPING_PREP &&
- state == LifecycleState.STOPPING) ||
- (this.state == LifecycleState.FAILED &&
- state == LifecycleState.STOPPING))) {
- // No other transition permitted
- invalidTransition(state.name());
- }
- }
-
- this.state = state;
- String lifecycleEvent = state.getLifecycleEvent();
- if (lifecycleEvent != null) {
- fireLifecycleEvent(lifecycleEvent, data);
- }
- }
private synchronized void setStateInternal(LifecycleState state, Object data, boolean check) throws LifecycleException { if (log.isDebugEnabled()) { log.debug(sm.getString('lifecycleBase.setState', this, state)); } if (check) { // Must have been triggered by one of the abstract methods (assume // code in this class is correct) // null is never a valid state if (state == null) { invalidTransition('null'); // Unreachable code - here to stop eclipse complaining about // a possible NPE further down the method return; } // Any method can transition to failed // startInternal() permits STARTING_PREP to STARTING // stopInternal() permits STOPPING_PREP to STOPPING and FAILED to // STOPPING if (!(state == LifecycleState.FAILED || (this.state == LifecycleState.STARTING_PREP && state == LifecycleState.STARTING) || (this.state == LifecycleState.STOPPING_PREP && state == LifecycleState.STOPPING) || (this.state == LifecycleState.FAILED && state == LifecycleState.STOPPING))) { // No other transition permitted invalidTransition(state.name()); } } this.state = state; String lifecycleEvent = state.getLifecycleEvent(); if (lifecycleEvent != null) { fireLifecycleEvent(lifecycleEvent, data); } }
重点关注第35到39行,第35行将入参LifecycleState实例赋值给本类中的实例变量保存起来,第36行取出LifecycleState实例的LifecycleEvent事件,如果该事件非空,则调用fireLifecycleEvent方法发布该事件。
既然看到了LifecycleState类,就看下LifecycleState类的定义:
- public enum LifecycleState {
- NEW(false, null),
- INITIALIZING(false, Lifecycle.BEFORE_INIT_EVENT),
- INITIALIZED(false, Lifecycle.AFTER_INIT_EVENT),
- STARTING_PREP(false, Lifecycle.BEFORE_START_EVENT),
- STARTING(true, Lifecycle.START_EVENT),
- STARTED(true, Lifecycle.AFTER_START_EVENT),
- STOPPING_PREP(true, Lifecycle.BEFORE_STOP_EVENT),
- STOPPING(false, Lifecycle.STOP_EVENT),
- STOPPED(false, Lifecycle.AFTER_STOP_EVENT),
- DESTROYING(false, Lifecycle.BEFORE_DESTROY_EVENT),
- DESTROYED(false, Lifecycle.AFTER_DESTROY_EVENT),
- FAILED(false, null),
- MUST_STOP(true, null),
- MUST_DESTROY(false, null);
-
- private final boolean available;
- private final String lifecycleEvent;
-
- private LifecycleState(boolean available, String lifecycleEvent) {
- this.available = available;
- this.lifecycleEvent = lifecycleEvent;
- }
-
- /**
- * May the public methods other than property getters/setters and lifecycle
- * methods be called for a component in this state? It returns
- * <code>true</code> for any component in any of the following states:
- * <ul>
- * <li>{@link #STARTING}</li>
- * <li>{@link #STARTED}</li>
- * <li>{@link #STOPPING_PREP}</li>
- * <li>{@link #MUST_STOP}</li>
- * </ul>
- */
- public boolean isAvailable() {
- return available;
- }
-
- /**
- *
- */
- public String getLifecycleEvent() {
- return lifecycleEvent;
- }
- }
public enum LifecycleState { NEW(false, null), INITIALIZING(false, Lifecycle.BEFORE_INIT_EVENT), INITIALIZED(false, Lifecycle.AFTER_INIT_EVENT), STARTING_PREP(false, Lifecycle.BEFORE_START_EVENT), STARTING(true, Lifecycle.START_EVENT), STARTED(true, Lifecycle.AFTER_START_EVENT), STOPPING_PREP(true, Lifecycle.BEFORE_STOP_EVENT), STOPPING(false, Lifecycle.STOP_EVENT), STOPPED(false, Lifecycle.AFTER_STOP_EVENT), DESTROYING(false, Lifecycle.BEFORE_DESTROY_EVENT), DESTROYED(false, Lifecycle.AFTER_DESTROY_EVENT), FAILED(false, null), MUST_STOP(true, null), MUST_DESTROY(false, null); private final boolean available; private final String lifecycleEvent; private LifecycleState(boolean available, String lifecycleEvent) { this.available = available; this.lifecycleEvent = lifecycleEvent; } /** * May the public methods other than property getters/setters and lifecycle * methods be called for a component in this state? It returns * <code>true</code> for any component in any of the following states: * <ul> * <li>{@link #STARTING}</li> * <li>{@link #STARTED}</li> * <li>{@link #STOPPING_PREP}</li> * <li>{@link #MUST_STOP}</li> * </ul> */ public boolean isAvailable() { return available; } /** * */ public String getLifecycleEvent() { return lifecycleEvent; }}
这个类在之前的Tomcat4和Tomcat5中都没有看到,可能是Tomcat7里面新定义的吧,就是一个枚举,内嵌了两个实例变量,一个布尔值表示是否可用,一个字符串表示是事件类型,看已经定义的枚举值里面发现这个字符串要么不设值,要么就是Lifecycle类中定义好的字符串常量。这个类实际上就是对Lifecycle类中定义好的字符串常量做了另外一层封装。
再说回开头在各组件代码中经常会看到的setStateInternal方法的调用,实际上就是向该组件中已注册的监听器发布一个事件。