【深入剖析Tomcat笔记】第六篇 Tomcat Lifecycle

简述

上一篇中介绍了Tomcat四大容器，Enginee、Host、Context和Wrapper以及如何通过TaskPipeline对四大容器进行功能性扩展。在 第五篇 Tomcat Container和PipeliningTasks 中我们介绍了如何通过Pipeline Task去为Enginee添加一些共用的方法，但这样还存在一些问题，我们很难对Container实例进行管理。Composite模式的一个明显的弊端就是存在大量的子容器，难以进行管理。为了解决这个问题，本篇将以四大容器为核心介绍Tomcat的生命周期。

Event

在之前的程序中，我们对容器进行管理是通过代码来实现的。
例如

...
context.start();
context.execute();
context.end();
...

Tomcat中非常核心的一个特性是向用户提供一个容器环境实现多线程，且用户透明（用户不需要关心多线程实现）。但用户在某些高级特性时候需要介入容器的管理，此时采用代码形式进行管理不足以实现这部分功能，用户若需要介入，则需要改动tomcat源码。
Event相当于Tomcat暴露给用户的一个接口去实现这部分功能，用户在需要感知Tomcat容器时，仅仅需要实现对应的event即可。

相信熟悉Spring的小伙伴们已经对于Event已经非常熟悉了，在此简单介绍一下Event。

Event是运行状态和暴露资源的封装

JDK1.1中提供了EventObject对象实现事件驱动，在没有EventObject的情况下我们实现对象运行状态管理（例如：运行，停止、自定义….），一种方式是硬编码，这种方式无法由外部模块控制，另外一种方式是轮询，这种方式外部模块不需要感知内部状态，明显的缺点是消耗大量计算资源。EventObject很好的解决了这个问题。
以下是JDK对于EventObject介绍

/**
 * <p>
 * The root class from which all event state objects shall be derived.
 * <p>
 * All Events are constructed with a reference to the object, the "source",
 * that is logically deemed to be the object upon which the Event in question
 * initially occurred upon.
 *
 * @since JDK1.1
 */
public class EventObject implements java.io.Serializable {
    protected transient Object  source;

    //构造方法
    public EventObject(Object source) {
        if (source == null)
            throw new IllegalArgumentException("null source");

        this.source = source;
    }

    //Get
    public Object getSource() {
        return source;
    }
}

不难理解，Event核心在于对于资源的暴露。在EventObject对应的核心资源是发布这个事件的对象，因此我们看到实际是对发布对象的持有 。
进一步考虑，我们对于Tomcat的Event需求是什么？回到最初的问题，我们主要需要对Tomcat进行管理，Tomcat对于容器运行阶段做了一个抽象叫做Lifecycle（生命周期）。生命周期其实就是对于常用执行状态的抽象，下一节会给大家介绍。因此我们对于Tomcat生命周期LifecycleEvent设计如下

public final class LifecycleEvent extends EventObject {

    private final Lifecycle lifecycle;
    //生命周期具体阶段参数
    private final String type;
    //可扩展资源
    private final Object data;

    public LifecycleEvent(Object source, Lifecycle lifecycle, String type) {
        this(lifecycle, type, null);
    }

    public LifecycleEvent(Lifecycle lifecycle, String type, Object data) {
        super(lifecycle);
        this.lifecycle = lifecycle;
        this.type = type;
        this.data = data;
    }


    public Lifecycle getLifecycle() {
        return lifecycle;
    }

    public String getType() {
        return type;
    }

    public Object getData() {
        return data;
    }
}

Lifecycle

上一节中，我们了解到Tomcat对于容器的运行状态做了抽象，抽象的内容被叫做Lifecycle
实际上Tomcat 对于容器运行状态抽象为六个状态，如示例所示。并且Lifecycle对象允许LifecycleListener，LifecycleListener下一节会说明。
对于需要对状态监测的对象，需要实现Lifecycle接口

public interface Lifecycle {


    String START_EVENT = "start";

    String BEFORE_START_EVENT = "before_start";

    String AFTER_START_EVENT = "after_start";

    String STOP_EVENT = "stop";

    String BEFORE_STOP_EVENT  = "before_stop";

    String AFTER_STOP_EVENT = "after_stop";

    //增加LifecycleListener
    void addLifecycleListener(LifecycleListener listener);
    //获取全部LifecycleListener
    LifecycleListener[] findLifecycleListeners();
    //移除对应LifecycleListener
    void removeLifecycleListener(LifecycleListener listener);
    //Lifecycle对象启动
    void start() throws LifecycleException;
    //Lifecycle对象停止
    void stop() throws LifecycleException;
}

LifecycleListener

LifecycleListener是对Lifecycle的接口的抽象
核心方法lifecycleEvent通过LifecycleEvent进行容器声明周期管理

public interface LifecycleListener {

    void lifecycleEvent(LifecycleEvent event);
}

在此示例SimpleContextLifecycleListener

public class SimpleContextLifecycleListener implements LifecycleListener {

  public void lifecycleEvent(LifecycleEvent event) {
    Lifecycle lifecycle = event.getLifecycle();
    System.out.println("SimpleContextLifecycleListener's event " +
      event.getType().toString());
    //声明周期控制 启动阶段
    if (Lifecycle.START_EVENT.equals(event.getType())) {
      System.out.println("Starting context.");
    }
    //声明周期控制 停止阶段
    else if (Lifecycle.STOP_EVENT.equals(event.getType())) {
      System.out.println("Stopping context.");
    }
  }
}

实际上，基于Lifecycle的6个阶段，我们可以进行各个阶段的控制。
这三者的结构如下。

DEMO实现

由于每种Container都需要实现Lifecycle接口，对于这部分抽象对象LifecycleSupport

public final class LifecycleSupport {

    private  Lifecycle lifecycle;

    private LifecycleListener listeners[] = new LifecycleListener[0];

    public LifecycleSupport(Lifecycle lifecycle) {
        super();
        this.lifecycle = lifecycle;
    }

    public void addLiffecycleListener(LifecycleListener listener) {
        synchronized (listeners) {

            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;
        }
    }

    public LifecycleListener[] findLifecycleListeners() {
        return listeners;
    }

    public void fireLifecycleEvent(String type, Object data) {
        LifecycleEvent event = new LifecycleEvent(lifecycle, type, data);
        LifecycleListener interested[] = null;

        synchronized (listeners) {
            interested = (LifecycleListener[]) listeners.clone();
        }

        for (int i = 0; i < interested.length; i++) {
            interested[i].lifecycleEvent(event);
        }

    }

    public void removeLifecycleListener(LifecycleListener listener) {
        synchronized (listeners) {
            int n = -1;
            for (int i = 0; i < listeners.length; i++) {
                if(listeners[i] == listener) {
                    n = i;
                    break;
                }
            }
            if(n < 0)
                return;

            LifecycleListener results[] = new LifecycleListener[listeners.length - 1];

            int j = 0;
            for (int i = 0; i < listeners.length; i++) {
                if(i != n) {
                    results[j++ ] = listeners[i];
                }
            }
            listeners = results;
        }
    }
}

这里只演示之前写的SimpleContext，其余容器类似

public class SimpleContext extends ContainerBase implements Context, Lifecycle {

    private LifecycleSupport lifecycleSupport = new LifecycleSupport(this);

    @Override
    public void addLifecycleListener(LifecycleListener listener) {
        lifecycleSupport.addLiffecycleListener(listener);
    }

    @Override
    public LifecycleListener[] findLifecycleListeners() {
        return lifecycleSupport.findLifecycleListeners();
    }

    @Override
    public void removeLifecycleListener(LifecycleListener listener) {
        lifecycleSupport.removeLifecycleListener(listener);
    }

    @Override
    public void start() throws LifecycleException {

        lifecycleSupport.fireLifecycleEvent(Lifecycle.BEFORE_START_EVENT, null);


        lifecycleSupport.fireLifecycleEvent(Lifecycle.START_EVENT, null);
        Container[] lifeChildren = findChildren();
        for (Container child :
                lifeChildren) {
            ((Lifecycle) child).start();
        }

        lifecycleSupport.fireLifecycleEvent(Lifecycle.AFTER_START_EVENT, null);

    }

    @Override
    public void stop() throws LifecycleException {
        lifecycleSupport.fireLifecycleEvent(Lifecycle.BEFORE_START_EVENT, null);

        lifecycleSupport.fireLifecycleEvent(Lifecycle.STOP_EVENT, null);
        Container[] lifeChildren = findChildren();
        for (Container child :
                lifeChildren) {
            ((Lifecycle) child).stop();
        }

        lifecycleSupport.fireLifecycleEvent(Lifecycle.AFTER_STOP_EVENT, null);
    }
}