Tomcat Container

在Tomcart中Container是處理web應用中對servlet request和response對象的請求。有四種容器,Engine, Host, Context, and Wrapper.下面是類圖

1.Engine. Represents the entire Catalina servlet engine.
2.Host. Represents a virtual host with a number of contexts.
3.Context. Represents a web application. A context contains one or more.

4. Wrapper. Represents an individual servlet.

一般一個容器可以有多個子容器

 public void addChild(Container child);
public void removeChild(Container child);

但是wrapper不能有子容器，如果調用addChild會拋出IllegalStateException

  public void addChild(Container child) {

        throw new IllegalStateException
            (sm.getString("standardWrapper.notChild"));

    }

容器提供了查找子容器的方法

public Container findChild(String name);

public Container[] findChildren();

當我們調用servlet的時候,要經過Pipelining,那麼這個Pipelining有什麼用呢？其實它就想servlet的filter chain,那麼它肯定就有filter了,那就是valve，下面是pipe的類圖

public interface Valve {


    //-------------------------------------------------------------- Properties


    /**
     * Return descriptive information about this Valve implementation.
     */
    public String getInfo();


    /**
     * Return the next Valve in the pipeline containing this Valve, if any.
     */
    public Valve getNext();


    /**
     * Set the next Valve in the pipeline containing this Valve.
     *
     * @param valve The new next valve, or <code>null</code> if none
     */
    public void setNext(Valve valve);


    //---------------------------------------------------------- Public Methods


    /**
     * Execute a periodic task, such as reloading, etc. This method will be
     * invoked inside the classloading context of this container. Unexpected
     * throwables will be caught and logged.
     */
    public void backgroundProcess();


    public void invoke(Request request, Response response)
        throws IOException, ServletException;

    
    /**
     * Process a Comet event.
     *
     * @param request The servlet request to be processed
     * @param response The servlet response to be created
     *
     * @exception IOException if an input/output error occurs, or is thrown
     *  by a subsequently invoked Valve, Filter, or Servlet
     * @exception ServletException if a servlet error occurs, or is thrown
     *  by a subsequently invoked Valve, Filter, or Servlet
     */
    public void event(Request request, Response response, CometEvent event)
        throws IOException, ServletException;


}

當我們往pipeline添加valve時，只要調用

 public void addValve(Valve valve);

那麼container是怎麼調用valve的invoke呢

在ContainerBase實現Pipeline接口，

public void invoke(Request request, Response response)
        throws IOException, ServletException {

        pipeline.getFirst().invoke(request, response);

    }

在valve的實現類中，我們看到

public void invoke(Request request, Response response)
        throws IOException, ServletException {
        //do something you want.....
         getNext().invoke(request, response);
    }

這樣valve在pipe好像組成一個鏈條一樣，第一個調用完，再調用第二個，一直到完爲止。不過要調用一個默認的valve，之後的才能調用

其實tomcat爲四個容器都提供個默認的valve。StandardContextValve、StandardEngineValve、StandardHostValve、StandardWrapperValve。

這裏的Contained接口是保證你這個對象最多隻屬於一個容器

public interface Contained {


    //-------------------------------------------------------------- Properties


    /**
     * Return the <code>Container</code> with which this instance is associated
     * (if any); otherwise return <code>null</code>.
     */
    public Container getContainer();


    /**
     * Set the <code>Container</code> with which this instance is associated.
     *
     * @param container The Container instance with which this instance is to
     *  be associated, or <code>null</code> to disassociate this instance
     *  from any Container
     */
    public void setContainer(Container container);


}

其實你也可以自定製valve然後在server.xml配置。
一個container可以有一些附加的組件，已有的組件有

• Loader - Class loader to use for integrating new Java classes for this Container into the JVM in which Catalina is running.
• Logger - Implementation of the log() method signatures of the ServletContext interface.
• Manager - Manager for the pool of Sessions associated with this Container.
• Realm - Read-only interface to a security domain, for authenticating user identities and their corresponding roles.
• Resources - JNDI directory context enabling access to static resources, enabling custom linkages to existing server components when Catalina is embedded in a larger server.