Tomcat8源码分析系列-启动分析(一) Lifecycle

Lifecycle在其他框架中也很常见，比如spring，它常用于具有生命周期的组件，由Lifecycle控制组件的初始化、启动、销毁等动作，方便应用程序获取、释放某些资源，或者是触发某些特定的事件。Tomcat也是如此，在学习整个启动流程之前，我们先行了解下Lifecycle的实现机制，便于理解整个流程。

Lifecycle

Lifecycle接口是一个公用的接口，定义了组件生命周期的一些方法，用于启动、停止Catalina组件。它是一个非常重要的接口，组件的生命周期包括：init、start、stop、destory，以及各种事件的常量、操作LifecycleListener的API，典型的观察者模式

public interface Lifecycle {

    // ----------------------- 定义各种EVENT事件 -----------------------

    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";

    // 省略事件常量定义……

    /**
     * 注册一个LifecycleListener
     */
    public void addLifecycleListener(LifecycleListener listener);

    /**
     * 获取所有注册的LifecycleListener
     */
    public LifecycleListener[] findLifecycleListeners();

    /**
     * 移除指定的LifecycleListener
     */
    public void removeLifecycleListener(LifecycleListener listener);

    /**
     * 组件被实例化之后，调用该方法完成初始化工作，发会出以下事件
     * <ol>
     *   <li>INIT_EVENT: On the successful completion of component initialization.</li>
     * </ol>
     * @exception LifecycleException if this component detects a fatal error
     *  that prevents this component from being used
     */
    public void init() throws LifecycleException;

    /**
     * 在组件投入使用之前调用该方法，先后会发出以下事件：BEFORE_START_EVENT、START_EVENT、AFTER_START_EVENT
     * @exception LifecycleException if this component detects a fatal error
     *  that prevents this component from being used
     */
    public void start() throws LifecycleException;

    /**
     * 使组件停止工作
     */
    public void stop() throws LifecycleException;

    /**
     * 销毁组件时被调用
     */
    public void destroy() throws LifecycleException;

    /**
     * Obtain the current state of the source component.
     */
    public LifecycleState getState();

    /**
     * 获取state的文字说明
     */
    public String getStateName();

    /**
     * Marker interface used to indicate that the instance should only be used
     * once. Calling {@link #stop()} on an instance that supports this interface
     * will automatically call {@link #destroy()} after {@link #stop()}
     * completes.
     */
    public interface SingleUse {
    }
}

各大组件均实现了Lifecycle接口，类图如下所示(查看原图)：

• LifecycleBase：它实现了Lifecycle的init、start、stop等主要逻辑，向注册在LifecycleBase内部的LifecycleListener发出对应的事件，并且预留了initInternal、startInternal、stopInternal等模板方法，便于子类完成自己的逻辑
• MBeanRegistration：JmxEnabled 的父类， jmx框架提供的注册MBean的接口，引入此接口是为了便于使用JMX提供的管理功能
• LifecycleMBeanBase：JmxEnabled的子类，通过重写initInternal、destroyInternal方法，统一向jmx中注册/取消注册当前实例，方便利用jmx对实例对象进行管理，代码上特别强调要求子类先行调用super.initInternal
• ContainerBase、StandardServer、StandardService、WebappLoader、Connector、StandardContext、StandardEngine、StandardHost、StandardWrapper等容器都继承了LifecycleMBeanBase，因此这些容器都具有了同样的生命周期并可以通过JMX进行管理

tomcat允许我们使用jmx对tomcat进行监控、管理，可以使用jconsole工具，准备后续写一篇博客分析tomcat jmx

public abstract class LifecycleMBeanBase extends LifecycleBase
        implements JmxEnabled {

    /**
     * Sub-classes wishing to perform additional initialization should override
     * this method, ensuring that super.initInternal() is the first call in the
     * overriding method.
     */
    protected void initInternal() throws LifecycleException {
        if (oname == null) {
            mserver = Registry.getRegistry(null, null).getMBeanServer();
            oname = register(this, getObjectNameKeyProperties());
        }
    }
}

public class Connector extends LifecycleMBeanBase  {
    protected void initInternal() throws LifecycleException {
        super.initInternal();
        adapter = new CoyoteAdapter(this);
        protocolHandler.setAdapter(adapter);
        // other code......
    }
}

LifecycleState

LifecycleState是枚举类，定义了各种状态

public enum LifecycleState {

    // LifecycleBase实例化完成时的状态
    NEW(false, null),   

    // 容器正在初始化的状态，在INITIALIZED之前
    INITIALIZING(false, Lifecycle.BEFORE_INIT_EVENT),

    // 初始化完成的状态
    INITIALIZED(false, Lifecycle.AFTER_INIT_EVENT),

    // 启动前
    STARTING_PREP(false, Lifecycle.BEFORE_START_EVENT),

    // 启动过程中的状态
    STARTING(true, Lifecycle.START_EVENT),

    // 启动完成
    STARTED(true, Lifecycle.AFTER_START_EVENT),

    // 停止前的状态
    STOPPING_PREP(true, Lifecycle.BEFORE_STOP_EVENT),

    // 停止过程中
    STOPPING(false, Lifecycle.STOP_EVENT),

    // 停止完成
    STOPPED(false, Lifecycle.AFTER_STOP_EVENT),

    // 销毁中
    DESTROYING(false, Lifecycle.BEFORE_DESTROY_EVENT),

    // 完成销毁
    DESTROYED(false, Lifecycle.AFTER_DESTROY_EVENT),

    // 启动、停止过程中出现异常
    FAILED(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;
    }

}

LifecycleListener

要订阅事件的实体类需要实现LifecycleListener

public interface LifecycleListener {
    public void lifecycleEvent(LifecycleEvent event);
}

默认情况下，tomcat会内置一些LifecycleListener，配置在server.xml中，除了xml中的LifecycleListener，还有org.apache.catalina.core.NamingContextListener，而这个LifecycleListener是在StandardServer的构造器中添加的，各个LifecycleListener的作用在此不再细说。如果我们在tomcat启动、停止的时候增加额外的逻辑，比如发送邮件通知，则可以从这个地方入手

<Listener className="org.apache.catalina.startup.VersionLoggerListener" />
<!--APR library loader. Documentation at /docs/apr.html -->
<Listener className="org.apache.catalina.core.AprLifecycleListener" SSLEngine="on" />
<!-- Prevent memory leaks due to use of particular java/javax APIs-->
<Listener className="org.apache.catalina.core.JreMemoryLeakPreventionListener" />
<Listener className="org.apache.catalina.mbeans.GlobalResourcesLifecycleListener" />
<Listener className="org.apache.catalina.core.ThreadLocalLeakPreventionListener" />

public StandardServer() {
    // 忽略部分代码
    if (isUseNaming()) {
        namingContextListener = new NamingContextListener();
        addLifecycleListener(namingContextListener);
    } else {
        namingContextListener = null;
    }
}

LifecycleBase

LifecycleBase实现了Lifecycle接口，完成了核心逻辑
- StringManager：用来做日志信息参数化输出的，支持国际化
- 内部使用CopyOnWriteArrayList维护所有的LifecycleListener，因为在各个生命周期，内部的LifecycleListener是会变化的，并且存在并发操作问题，因此使用了并发的List。注意，不同的LifecycleBase子类，其内部的lifecycleListeners存放不同的LifecyListener，比如Server和Service，它们是不同的Lifecycle实例，内部的lifecycleListeners也是不同
- LifecycleBase的state初始值是LifecycleState.NEW，也存在并发修改的问题，用了volatile修饰
- addLifecycleListener、removeLifecycleListener允许添加、删除LifecycleListener，告诉LifecycleBase有哪些监听者需要进行事件通知
- fireLifecycleEvent：向内部注册的LifecycleListener发出事件通知，它是protected的方法，所以LifecycleBase的子类可以在适当的时机调用该方法发出事件通知。事件通知由LifecycleListener实现类决定要不要对特定的事件进行处理
- setState：更新state值，并发出对应的事件通知，同样是调用fireLifecycleEvent

public abstract class LifecycleBase implements Lifecycle {

    // 日志国际化输出使用
    private static final StringManager sm = StringManager.getManager(LifecycleBase.class);

    // 维护LifecycleListener
    private final List<LifecycleListener> lifecycleListeners = new CopyOnWriteArrayList<>();

    // 初始状态是NEW
    private volatile LifecycleState state = LifecycleState.NEW;

    /**
     * 注册LifecycleListener
     */
    @Override
    public void addLifecycleListener(LifecycleListener listener) {
        lifecycleListeners.add(listener);
    }

    @Override
    public LifecycleListener[] findLifecycleListeners() {
        return lifecycleListeners.toArray(new LifecycleListener[0]);
    }

    /**
     * 移除LifecycleListener
     */
    @Override
    public void removeLifecycleListener(LifecycleListener listener) {
        lifecycleListeners.remove(listener);
    }

    /**
     * 发出事件通知，遍历内部所有的LifecycleListener，并调用其lifecycleEvent
     */
    protected void fireLifecycleEvent(String type, Object data) {
        LifecycleEvent event = new LifecycleEvent(this, type, data);
        for (LifecycleListener listener : lifecycleListeners) {
            listener.lifecycleEvent(event);
        }
    }

    @Override
    public LifecycleState getState() {
        return state;
    }

    @Override
    public String getStateName() {
        return getState().toString();
    }

    protected synchronized void setState(LifecycleState state)
            throws LifecycleException {
        setStateInternal(state, null, true);
    }

    protected synchronized void setState(LifecycleState state, Object data)
            throws LifecycleException {
        setStateInternal(state, data, true);
    }

    /**
     * 设置state值，并发出事件通知
     */
    private synchronized void setStateInternal(LifecycleState state,
            Object data, boolean check) throws LifecycleException {

        // 校验state的正确性
        if (check) {
            if (state == null) {
                invalidTransition("null");
                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);
        }
    }

    // 省略其它代码......
}

Lifecycle组件的init、start、stop、destory的套路基本上一样，先由LifecycleBase完成前期的校验、事件通知动作，再调用子类的方法完成自己的逻辑

graph LR
校验state-->发出事件通知
发出事件通知-->子类doInternal

start分析

start过程会触发LifecycleState的STARTING_PREP、STARTED事件，如果出现启动失败还会触发FAILED事件，并且调用stop。因为会涉及多线程操作，因此对方法加了锁。如果start期间出现了异常，则会调用stop停止tomcat，或者state状态有误也会抛出异常

state状态变更时调用setStateInternal方法，遍历内部所有的LifecycleListener，并向其发起对应的事件通知，由LifecycleListener去完成某些动作。其子类可以直接调用fireLifecycleEvent，比如在StandardServer中，start过程会发出CONFIGURE_START_EVENT事件。注：所有事件的命名均定义在Lifecycle接口中

public abstract class LifecycleBase implements Lifecycle {
    @Override
    public final synchronized void start() throws LifecycleException {

        // 如果是start前、进行中、start完成，则直接return
        if (LifecycleState.STARTING_PREP.equals(state) || LifecycleState.STARTING.equals(state) ||
                LifecycleState.STARTED.equals(state)) {
            // 忽略logger日志
            return;
        }

        // 完成init初始化
        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 {
            // 发出STARTING_PREP事件
            setStateInternal(LifecycleState.STARTING_PREP, null, false);

            // 由子类实现
            startInternal();

            // 如果启动失败直接调用stop
            if (state.equals(LifecycleState.FAILED)) {
                stop();
            } 
            // 说明状态有误
            else if (!state.equals(LifecycleState.STARTING)) {
                invalidTransition(Lifecycle.AFTER_START_EVENT);
            } 
            // 成功完成start，发出STARTED事件
            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);
        }
    }

    /**
     * 由子类实现
     */
    protected abstract void startInternal() throws LifecycleException;

}

LifecycleMBeanBase

由前面的类图可知，LifecycleMBeanBase是LifecycleBase的直接子类，并且实现了JmxEnabled接口，很多组件都是直接继承它

LifecycleMBeanBase完成了jmx注册的主要逻辑，重写了LifecycleBase的initInternal、destroyInternal方法，用于完成jmx的注册、注销动作，这两个模板方法中特别说明：

Sub-classes wishing to perform additional initialization should override this method, 
ensuring that super.initInternal() is the first call in the overriding method.

为了保证jmx的正常注册和注销，要求子类在重写initInternal、destroyInternal方法时，必须先调用super.initInternal()。例如Connector：

public class Connector extends LifecycleMBeanBase  {
    @Override
    protected void initInternal() throws LifecycleException {

        // 先行调用LifecycleMBeanBase的initInternal
        super.initInternal();

        // Initialize adapter
        adapter = new CoyoteAdapter(this);
        protocolHandler.setAdapter(adapter);

        // other code......
    }

    // other code......
}

我们再来看看LifecycleMBeanBase的内部实现，在initInternal阶段初始化MBeanServer实例，并且把当前实例注册到jmx中；而destroyInternal阶段则是根据ObjectName注销MBean

public abstract class LifecycleMBeanBase extends LifecycleBase
        implements JmxEnabled {

    /**
     * jmx的域，默认使用Service的name，即"Catalina"
     */
    private String domain = null;

    /**
     * 用于标识一个MBean的对象名称，也可以根据这个name来查找MBean
     */
    private ObjectName oname = null;

    /**
     * jmx的核心组件，提供代理端操作MBean的接口，提供了创建、注册、删除MBean的接口，它由MBeanServerFactory创建
     */
    protected MBeanServer mserver = null;

    @Override
    protected void initInternal() throws LifecycleException {
        if (oname == null) {
            mserver = Registry.getRegistry(null, null).getMBeanServer();
            oname = register(this, getObjectNameKeyProperties());
        }
    }

    @Override
    protected void destroyInternal() throws LifecycleException {
        unregister(oname);
    }

    protected final void unregister(ObjectName on) {
        if (on == null) {
            return;
        }
        if (mserver == null) {
            log.warn(sm.getString("lifecycleMBeanBase.unregisterNoServer", on));
            return;
        }
        try {
            mserver.unregisterMBean(on);
        } catch (MBeanRegistrationException e) {
            log.warn(sm.getString("lifecycleMBeanBase.unregisterFail", on), e);
        } catch (InstanceNotFoundException e) {
            log.warn(sm.getString("lifecycleMBeanBase.unregisterFail", on), e);
        }

    }
}