Tomcat 8（九）解读Tomcat组件的生命周期(Lifecycle)

Tomcat 8（七）解读Bootstrap介绍过，执行startup.bat，将引发Tomcat一连串组件的启动。其实这一连串启动是通过组件的生命周期(Lifecycle)实现的

今天来看看Lifecycle设计。解读Lifecycle，涉及到如下几个类

Lifecycle、LifecycleState、LifecycleEvent、LifecycleListener(位于org.apache.catalina)

LifecycleBase、LifecycleSupport、LifecycleMBeanBase(位于org.apache.catalina.util)

Lifecycle接口

package org.apache.catalina;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;    public LifecycleState getState();    public String getStateName();}

Lifecycle接口的作用

定义了事件类型(静态常量字符串)

定义了init/start/stop/destroy方法，即组件初始化/启动/停止/销毁

定义了addLifecycleListener/findLifecycleListeners/removeLifecycleListener方法，即添加/获取/移除监听器(监听器主要监听启动、停止动作)

LifecycleState枚举

package org.apache.catalina;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;    }    public boolean isAvailable() {        return available;    }    public String getLifecycleEvent() {        return lifecycleEvent;    }}

LifecycleState定义了组件状态的枚举。LifecycleState包含两个属性：available、lifecycleEvent

available(boolean)－处于此状态的组件是否可以执行public方法(get/set方法除外)，是否可以获取lifecycleEvent值

lifecycleEvent(String)－处于此状态的组件正进行的事件

LifecycleEvent

LifecycleEvent是个实体类，包含三个属性lifecycle、type、data

lifecycle(Lifecycle)－将触发事件的组件

(注意：每个组件都可以向上转成Lifecycle类型，因为组件继承LifecycleMBeanBase，而LifecycleMBeanBase继承LifecycleBase，而LifecycleBase是Lifecycle的一种实现)

type(String)－事件类型

data(String)－事件数据

LifecycleSupport

LifecycleSupport将组件正在进行的LifecycleEvent通知给监听器(LifecycleListener)

LifecycleListener

监听特定事件，并做出相应处理

AprLifecycleListener(org.apache.catalina.core)就是LifecycleListener的一种实现

组件更新状态时，如果组件当前状态的lifecycleEvent不为空，LifecycleSupport会将 lifecycleEvent通知给LifecycleListener，LifecycleListener会做出相应处理 

LifecycleBase

LifecycleBase实现了Lifecycle接口，添加了几个新的方法如setStateInternal(更新组件状态)、fireLifecycleEvent(触发LifecycleEvent)

添加了四个抽象方法initInternal、startInternal、stopInternal、destroyInternal

LifecycleBase内初始化了LifecycleSupport、LifecycleState

private final LifecycleSupport lifecycle = new LifecycleSupport(this);private volatile LifecycleState state = LifecycleState.NEW;

下面来看看LifecycleBase的init方法的逻辑(start/stop/destroy的逻辑与init类似)

public final synchronized void init() throws LifecycleException {//查看组件状态是否为LifecycleState.NEWif (!state.equals(LifecycleState.NEW)) {invalidTransition(Lifecycle.BEFORE_INIT_EVENT);}//更新组件状态LifecycleState.INITIALIZINGsetStateInternal(LifecycleState.INITIALIZING, null, false);try {//调用initInternalinitInternal();} catch (Throwable t) {ExceptionUtils.handleThrowable(t);setStateInternal(LifecycleState.FAILED, null, false);throw new LifecycleException(sm.getString("lifecycleBase.initFail",toString()), t);}//更新组件状态为LifecycleState.INITIALIZED。到此初始化完成setStateInternal(LifecycleState.INITIALIZED, null, false);}

注意init方法内部又调用了initInternal方法

LifecycleMBeanBase

LifecycleMBeanBase继承LifecycleBase，LifecycleMBeanBase内又添加了一些新的方法

StandardServer、StandardService等组件继承LifecycleMBeanBase，并重写initInternal、startInternal、stopInternal、destroyInternal方法

下面来看看StandardServer的initInternal方法

protected void initInternal() throws LifecycleException {//调用父类(LifecycleMBeanBase)的initInternal方法super.initInternal();onameStringCache = register(new StringCache(), "type=StringCache");// Register the MBeanFactoryMBeanFactory factory = new MBeanFactory();factory.setContainer(this);onameMBeanFactory = register(factory, "type=MBeanFactory");// Register the naming resourcesglobalNamingResources.init();// Populate the extension validator with JARs from common and shared// class loadersif (getCatalina() != null) {ClassLoader cl = getCatalina().getParentClassLoader();// Walk the class loader hierarchy. Stop at the system class loader.// This will add the shared (if present) and common class loaderswhile (cl != null && cl != ClassLoader.getSystemClassLoader()) {if (cl instanceof URLClassLoader) {URL[] urls = ((URLClassLoader) cl).getURLs();for (URL url : urls) {if (url.getProtocol().equals("file")) {try {File f = new File (url.toURI());if (f.isFile() &&f.getName().endsWith(".jar")) {ExtensionValidator.addSystemResource(f);}} catch (URISyntaxException e) {// Ignore} catch (IOException e) {// Ignore}}}}cl = cl.getParent();}}//Server调用其子组件(Service)的init方法，Service在初始化过程中又会调用其子组件的init方法for (int i = 0; i < services.length; i++) {services[i].init();}}

当Catalina调用StandardServer的init方法时，由于StandardServer未重写init方法，因此将使用LifecycleBase的init方法，当执行到initInternal()时，由于StandardServer重写了initInternal方法，因此将使用StandardServer的initInternal方法(应用了多态。每个组件的initInternal方法包含其自定义逻辑)

通过Lifecycle，Tomcat启动最顶层组件(Server)，即可启动所有组件