Tomcat 8（九）解读Tomcat组件的生命周期(Lifecycle)

Tomcat 8（七）解读Bootstrap介绍过。运行startup.bat。将引发Tomcat一连串组件的启动。事实上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。涉及到例如以下几个类

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，即加入/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包括两个属性：available、lifecycleEvent

available(boolean)－处于此状态的组件能否够运行public方法(get/set方法除外)。能否够获取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，包括三个属性lifecycle、type、data

lifecycle(Lifecycle)－将触发事件的组件

(注意：每一个组件都能够向上转成Lifecycle类型。由于组件继承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。并做出对应处理

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

组件更新状态时，假设组件当前状态的lifecycleEvent不为空。LifecycleSupport会将 lifecycleEvent通知给LifecycleListener，LifecycleListener会做出对应处理 

LifecycleBase

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

加入了四个抽象方法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;

以下来看看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.NEW
if (!state.equals(LifecycleState.NEW)) {
invalidTransition(Lifecycle.BEFORE_INIT_EVENT);
}
//更新组件状态LifecycleState.INITIALIZING
setStateInternal(LifecycleState.INITIALIZING, null, false);

try {
//调用initInternal
initInternal();
} 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。
到此初始化完毕
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内又加入了一些新的方法

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

以下来看看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 MBeanFactory
MBeanFactory factory = new MBeanFactory();
factory.setContainer(this);
onameMBeanFactory = register(factory, "type=MBeanFactory");

// Register the naming resources
globalNamingResources.init();

// Populate the extension validator with JARs from common and shared
// class loaders
if (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 loaders
while (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，因为StandardServer未重写init方法，因此将使用LifecycleBase的init方法。当运行到initInternal()时，因为StandardServer重写了initInternal方法，因此将使用StandardServer的initInternal方法(应用了多态。每一个组件的initInternal方法包括其自己定义逻辑)

通过Lifecycle。Tomcat启动最顶层组件(Server)。就可以启动全部组件