【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);
}
}