理解思路

• 第一：抓住StandardEngine整体类依赖结构来理解

• 第二：结合server.xml中Engine配置来理解

见下文具体阐述。

• 第三：结合Engine Config官方配置文档

http://tomcat.apache.org/tomcat-9.0-doc/config/engine.html

Engine接口设计

这看Engine.java接口前，先要看下相关属性

• 支持设置的属性列表

• Engine的接口设计

这里你会发现，如下接口中包含上述defaultHost和jvmRoute属性设置；同时还有Service，因为Engine的上层是service。

/**
 * An <b>Engine</b> is a Container that represents the entire Catalina servlet
 * engine.  It is useful in the following types of scenarios:
 * <ul>
 * <li>You wish to use Interceptors that see every single request processed
 *     by the entire engine.
 * <li>You wish to run Catalina in with a standalone HTTP connector, but still
 *     want support for multiple virtual hosts.
 * </ul>
 * In general, you would not use an Engine when deploying Catalina connected
 * to a web server (such as Apache), because the Connector will have
 * utilized the web server's facilities to determine which Context (or
 * perhaps even which Wrapper) should be utilized to process this request.
 * <p>
 * The child containers attached to an Engine are generally implementations
 * of Host (representing a virtual host) or Context (representing individual
 * an individual servlet context), depending upon the Engine implementation.
 * <p>
 * If used, an Engine is always the top level Container in a Catalina
 * hierarchy. Therefore, the implementation's <code>setParent()</code> method
 * should throw <code>IllegalArgumentException</code>.
 *
 * @author Craig R. McClanahan
 */
public interface Engine extends Container {

    /**
     * @return the default host name for this Engine.
     */
    public String getDefaultHost();


    /**
     * Set the default hostname for this Engine.
     *
     * @param defaultHost The new default host
     */
    public void setDefaultHost(String defaultHost);


    /**
     * @return the JvmRouteId for this engine.
     */
    public String getJvmRoute();


    /**
     * Set the JvmRouteId for this engine.
     *
     * @param jvmRouteId the (new) JVM Route ID. Each Engine within a cluster
     *        must have a unique JVM Route ID.
     */
    public void setJvmRoute(String jvmRouteId);


    /**
     * @return the <code>Service</code> with which we are associated (if any).
     */
    public Service getService();


    /**
     * Set the <code>Service</code> with which we are associated (if any).
     *
     * @param service The service that owns this Engine
     */
    public void setService(Service service);
}

• 其它属性支持都包含在我们上文分析的ContainerBase中

/**
  * The processor delay for this component.
  */
protected int backgroundProcessorDelay = -1;
/**
  * The number of threads available to process start and stop events for any
  * children associated with this container.
  */
private int startStopThreads = 1;

...

Engine接口实现：StandardEngine

接口中简单方法实现

上述接口里面的defaultHost, JvmRoute, service 很简单

/**
  * Return the default host.
  */
@Override
public String getDefaultHost() {
    return defaultHost;
}


/**
  * Set the default host.
  *
  * @param host The new default host
  */
@Override
public void setDefaultHost(String host) {

    String oldDefaultHost = this.defaultHost;
    if (host == null) {
        this.defaultHost = null;
    } else {
        this.defaultHost = host.toLowerCase(Locale.ENGLISH);
    }
    if (getState().isAvailable()) {
        service.getMapper().setDefaultHostName(host);
    }
    support.firePropertyChange("defaultHost", oldDefaultHost,
                                this.defaultHost);

}


/**
  * Set the cluster-wide unique identifier for this Engine.
  * This value is only useful in a load-balancing scenario.
  * <p>
  * This property should not be changed once it is set.
  */
@Override
public void setJvmRoute(String routeId) {
    jvmRouteId = routeId;
}


/**
  * Retrieve the cluster-wide unique identifier for this Engine.
  * This value is only useful in a load-balancing scenario.
  */
@Override
public String getJvmRoute() {
    return jvmRouteId;
}

/**
  * Return the <code>Service</code> with which we are associated (if any).
  */
@Override
public Service getService() {
    return this.service;
}


/**
  * Set the <code>Service</code> with which we are associated (if any).
  *
  * @param service The service that owns this Engine
  */
@Override
public void setService(Service service) {
    this.service = service;
}

child, parent

addChild重载方法，限制只能添加Host作为子容器；

setParent直接抛出异常，因为Engine接口中已经包含了setService方法作为它的上层，而Engine的上层没有容器的概念。

/**
  * Add a child Container, only if the proposed child is an implementation
  * of Host.
  *
  * @param child Child container to be added
  */
@Override
public void addChild(Container child) {

    if (!(child instanceof Host))
        throw new IllegalArgumentException
            (sm.getString("standardEngine.notHost"));
    super.addChild(child);

}


/**
  * Disallow any attempt to set a parent for this Container, since an
  * Engine is supposed to be at the top of the Container hierarchy.
  *
  * @param container Proposed parent Container
  */
@Override
public void setParent(Container container) {

    throw new IllegalArgumentException
        (sm.getString("standardEngine.notParent"));

}

Lifecycle的模板方法

无非就是调用上文中我们介绍ContainerBase中的方法

@Override
protected void initInternal() throws LifecycleException {
    // Ensure that a Realm is present before any attempt is made to start
    // one. This will create the default NullRealm if necessary.
    getRealm();
    super.initInternal();
}


/**
  * Start this component and implement the requirements
  * of {@link org.apache.catalina.util.LifecycleBase#startInternal()}.
  *
  * @exception LifecycleException if this component detects a fatal error
  *  that prevents this component from being used
  */
@Override
protected synchronized void startInternal() throws LifecycleException {

    // Log our server identification information
    if (log.isInfoEnabled()) {
        log.info(sm.getString("standardEngine.start", ServerInfo.getServerInfo()));
    }

    // Standard container startup
    super.startInternal();
}

LogAccess

这里需要补充下之前没有介绍的日志访问，这里介绍下。

运行Web服务器时，正常生成的输出文件之一是访问日志，该访问日志以标准格式为服务器处理的每个请求生成一行信息。Catalina包括一个可选的Valve实现，该实现可以创建与Web服务器创建的标准格式相同的访问日志，也可以创建任意数量的自定义格式。

需要先看下xml配置; 您可以通过嵌套如下所示的Valve元素，要求Catalina为Engine， Host或Context处理的所有请求创建访问日志：

<Engine name="Standalone" ...>
  ...
  <Valve className="org.apache.catalina.valves.AccessLogValve"
         prefix="catalina_access_log" suffix=".txt"
         pattern="common"/>
  ...
</Engine>

好了看下具体的实现，使用适配器模式获取AccessLog类型的Valve：

适配器模式看这里：结构型 - 适配器(Adapter)

@Override
public AccessLog getAccessLog() {

    if (accessLogScanComplete) {
        return accessLog;
    }

    AccessLogAdapter adapter = null;
    Valve valves[] = getPipeline().getValves();
    for (Valve valve : valves) {
        if (valve instanceof AccessLog) { // 看这里
            if (adapter == null) {
                adapter = new AccessLogAdapter((AccessLog) valve);
            } else {
                adapter.add((AccessLog) valve);
            }
        }
    }
    if (adapter != null) {
        accessLog = adapter;
    }
    accessLogScanComplete = true;
    return accessLog;
}

AccessLog(日志记录器)主要的作用就是记录日志，这个记录的方法就是logAccess()方法

/**
  * Override the default implementation. If no access log is defined for the
  * Engine, look for one in the Engine's default host and then the default
  * host's ROOT context. If still none is found, return the default NoOp
  * access log.
  */
@Override
public void logAccess(Request request, Response response, long time,
        boolean useDefault) {

    boolean logged = false;

     // 如果有accessLog，则记录日志
    if (getAccessLog() != null) {
        accessLog.log(request, response, time);
        logged = true;
    }

    // 没找到且使用useDefault，表示从下层容器中获取accessLog
    if (!logged && useDefault) {
        AccessLog newDefaultAccessLog = defaultAccessLog.get();
        if (newDefaultAccessLog == null) {
            // If we reached this point, this Engine can't have an AccessLog
            // Look in the defaultHost
            Host host = (Host) findChild(getDefaultHost()); // 如果没有默认的accessLog，则获取默认Host的accessLog
            Context context = null;
            if (host != null && host.getState().isAvailable()) {
                newDefaultAccessLog = host.getAccessLog();

                if (newDefaultAccessLog != null) {
                    if (defaultAccessLog.compareAndSet(null,
                            newDefaultAccessLog)) {
                        AccessLogListener l = new AccessLogListener(this,
                                host, null);
                        l.install(); // 注册AccessLog监听器至当前Engine
                    }
                } else {
                    // Try the ROOT context of default host
                    context = (Context) host.findChild(""); // 如果仍然没有找到，则获取默认host的ROOT Context的accessLog
                    if (context != null &&
                            context.getState().isAvailable()) {
                        newDefaultAccessLog = context.getAccessLog();
                        if (newDefaultAccessLog != null) {
                            if (defaultAccessLog.compareAndSet(null,
                                    newDefaultAccessLog)) {
                                AccessLogListener l = new AccessLogListener(
                                        this, null, context);
                                l.install();
                            }
                        }
                    }
                }
            }

            if (newDefaultAccessLog == null) { 
                newDefaultAccessLog = new NoopAccessLog(); // 这个其实是一个空模式，以便采用统一方式调用（不用判空了）
                if (defaultAccessLog.compareAndSet(null,
                        newDefaultAccessLog)) {
                    AccessLogListener l = new AccessLogListener(this, host,
                            context);
                    l.install();
                }
            }
        }

        // 最后记录日志，（上面最后有空模式实现，所以可以直接调用，不用判空）
        newDefaultAccessLog.log(request, response, time);
    }
}

其中涉及的相关内部类如下：

protected static final class NoopAccessLog implements AccessLog {

    @Override
    public void log(Request request, Response response, long time) {
        // NOOP
    }

    @Override
    public void setRequestAttributesEnabled(
            boolean requestAttributesEnabled) {
        // NOOP

    }

    @Override
    public boolean getRequestAttributesEnabled() {
        // NOOP
        return false;
    }
}

protected static final class AccessLogListener
        implements PropertyChangeListener, LifecycleListener,
        ContainerListener {

    private final StandardEngine engine;
    private final Host host;
    private final Context context;
    private volatile boolean disabled = false;

    public AccessLogListener(StandardEngine engine, Host host,
            Context context) {
        this.engine = engine;
        this.host = host;
        this.context = context;
    }

    public void install() {
        engine.addPropertyChangeListener(this);
        if (host != null) { // 同时注册至host和context
            host.addContainerListener(this);
            host.addLifecycleListener(this);
        }
        if (context != null) {
            context.addLifecycleListener(this);
        }
    }

    private void uninstall() {
        disabled = true;
        if (context != null) {
            context.removeLifecycleListener(this);
        }
        if (host != null) {
            host.removeLifecycleListener(this);
            host.removeContainerListener(this);
        }
        engine.removePropertyChangeListener(this);
    }

    @Override
    public void lifecycleEvent(LifecycleEvent event) {
        if (disabled) return;

        String type = event.getType();
        if (Lifecycle.AFTER_START_EVENT.equals(type) ||
                Lifecycle.BEFORE_STOP_EVENT.equals(type) ||
                Lifecycle.BEFORE_DESTROY_EVENT.equals(type)) {
            // Container is being started/stopped/removed
            // Force re-calculation and disable listener since it won't
            // be re-used
            engine.defaultAccessLog.set(null);
            uninstall();
        }
    }

    @Override
    public void propertyChange(PropertyChangeEvent evt) {
        if (disabled) return;
        if ("defaultHost".equals(evt.getPropertyName())) {
            // Force re-calculation and disable listener since it won't
            // be re-used
            engine.defaultAccessLog.set(null);
            uninstall();
        }
    }

    @Override
    public void containerEvent(ContainerEvent event) {
        // Only useful for hosts
        if (disabled) return;
        if (Container.ADD_CHILD_EVENT.equals(event.getType())) {
            Context context = (Context) event.getData();
            if (context.getPath().isEmpty()) {
                // Force re-calculation and disable listener since it won't
                // be re-used
                engine.defaultAccessLog.set(null);
                uninstall();
            }
        }
    }
}

JMX相关

之前已经有过相关介绍，这里不再介绍相关方法，只列出相关方法：

@Override
protected String getObjectNameKeyProperties() {
    return "type=Engine";
}


@Override
protected String getDomainInternal() {
    return getName();
}