How Tomcat Works（十六）

本文接下来会介绍Host容器和Engine容器，在tomcat的实际部署中，总是会使用一个Host容器；本文介绍Host接口和Engine接口及其相关类

Host容器是org.apache.catalina.Host接口的实例，Host接口继承自Container接口， 其定义如下

public interface Host extends Container {
   
    public static final String ADD_ALIAS_EVENT = "addAlias";
    
    public static final String REMOVE_ALIAS_EVENT = "removeAlias";
   
    public String getAppBase();
   
    public void setAppBase(String appBase);
   
    public boolean getAutoDeploy();
   
    public void setAutoDeploy(boolean autoDeploy);
    
    public void addDefaultContext(DefaultContext defaultContext);
   
    public DefaultContext getDefaultContext();
    
    public String getName();
    
    public void setName(String name);
   
    public void importDefaultContext(Context context);
    
    public void addAlias(String alias);
    
    public String[] findAliases();
    
    public Context map(String uri);
   
    public void removeAlias(String alias);

}

该接口中比较重要的方法是map()方法，该方法返回一个用来处理引入的HTTP请求的Context容器的实例，该方法的具体实现在StandardHost类中

在tomcat中org.apache.catalina.core.StandardHost类是Host接口的标准实现，该类继承自org.apache.catalina.core.ContainerBase类，实现了Host和Deployer接口

与StandardContext类和StandardWrapper类相似，StandardHost类的构造函数会将一个基础阀的实例添加到其管道对象中：

public StandardHost() {

        super();
        pipeline.setBasic(new StandardHostValve());

    }

基础阀是org.apache.catalina.core.StandardHostValve类的实例

当调用其start()方法时，StandardHost实例会添加两个阀，分别为ErrorReportValve类和ErrorDispatcherValve类的实例，这两个阀均位于org.apache.catalina.valves包下

public synchronized void start() throws LifecycleException {
        // Set error report valve
        if ((errorReportValveClass != null)
            && (!errorReportValveClass.equals(""))) {
            try {
                Valve valve = (Valve) Class.forName(errorReportValveClass)
                    .newInstance();
                addValve(valve);
            } catch (Throwable t) {
                log(sm.getString
                    ("standardHost.invalidErrorReportValveClass",
                     errorReportValveClass));
            }
        }

        // Set dispatcher valve
        addValve(new ErrorDispatcherValve());

        super.start();

    }

每当引入一个HTTP请求，都会调用Host实例的invoke()方法，这里是StandardHost的父类ContainerBase类的invoke()方法，而ContainerBase类的invoke()方法会调用StandardHost实例的基础阀StandardHostValve实例的invoke()方法；StandardHostValve类的invoke()方法会调用StandardHost类的map()方法来获取相应的Context实例来处理HTTP请求

public Context map(String uri) {

        if (debug > 0)
            log("Mapping request URI '" + uri + "'");
        if (uri == null)
            return (null);

        // Match on the longest possible context path prefix
        if (debug > 1)
            log("  Trying the longest context path prefix");
        Context context = null;
        String mapuri = uri;
        while (true) {
            context = (Context) findChild(mapuri);
            if (context != null)
                break;
            int slash = mapuri.lastIndexOf('/');
            if (slash < 0)
                break;
            mapuri = mapuri.substring(0, slash);
        }

        // If no Context matches, select the default Context
        if (context == null) {
            if (debug > 1)
                log("  Trying the default context");
            context = (Context) findChild("");
        }

        // Complain if no Context has been selected
        if (context == null) {
            log(sm.getString("standardHost.mappingError", uri));
            return (null);
        }

        // Return the mapped Context (if any)
        if (debug > 0)
            log(" Mapped to context '" + context.getPath() + "'");
        return (context);

    }

在tomcat4中， StandardHost的父类ContainerBase类会调用其addDefaultMapper()方法创建一个默认映射器，默认映射器的类型由mapperClass属性的值决定

protected void addDefaultMapper(String mapperClass) {

        // Do we need a default Mapper?
        if (mapperClass == null)
            return;
        if (mappers.size() >= 1)
            return;

        // Instantiate and add a default Mapper
        try {
            Class clazz = Class.forName(mapperClass);
            Mapper mapper = (Mapper) clazz.newInstance();
            mapper.setProtocol("http");
            addMapper(mapper);
        } catch (Exception e) {
            log(sm.getString("containerBase.addDefaultMapper", mapperClass),
                e);
        }

    }

变量mapperClass的值定义在StandardHst类中

private String mapperClass =
        "org.apache.catalina.core.StandardHostMapper";

在tomcat4中，StandardHost类的start()方法会在方法末尾调用addDefaultMapper()方法，确保默认映射器的创建完成

当然，StandardHostMapper类中最重要的方法是map()方法，下面是map()方法的实现

public Container map(Request request, boolean update) {
        // Has this request already been mapped?
        if (update && (request.getContext() != null))
            return (request.getContext());

        // Perform mapping on our request URI
        String uri = ((HttpRequest) request).getDecodedRequestURI();
        Context context = host.map(uri);

        // Update the request (if requested) and return the selected Context
        if (update) {
            request.setContext(context);
            if (context != null)
                ((HttpRequest) request).setContextPath(context.getPath());
            else
                ((HttpRequest) request).setContextPath(null);
        }
        return (context);

    }

注意，这里map()方法仅仅是简单地调用了Host实例的map()方法

org.apache.catalina.core.StandardHostValve类是StandardHost实例的基础阀，当有引入的HTTP请求时，会调用StandardHostValve类的invoke()方法对其进行处理

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

        // Validate the request and response object types
        if (!(request.getRequest() instanceof HttpServletRequest) ||
            !(response.getResponse() instanceof HttpServletResponse)) {
            return;     // NOTE - Not much else we can do generically
        }

        // Select the Context to be used for this Request
        StandardHost host = (StandardHost) getContainer();
        Context context = (Context) host.map(request, true);
        if (context == null) {
            ((HttpServletResponse) response.getResponse()).sendError
                (HttpServletResponse.SC_INTERNAL_SERVER_ERROR,
                 sm.getString("standardHost.noContext"));
            return;
        }

        // Bind the context CL to the current thread
        Thread.currentThread().setContextClassLoader
            (context.getLoader().getClassLoader());

        // Update the session last access time for our session (if any)
        HttpServletRequest hreq = (HttpServletRequest) request.getRequest();
        String sessionId = hreq.getRequestedSessionId();
        if (sessionId != null) {
            Manager manager = context.getManager();
            if (manager != null) {
                Session session = manager.findSession(sessionId);
                if ((session != null) && session.isValid())
                    session.access();
            }
        }

        // Ask this Context to process this request
        context.invoke(request, response);

    }

在tomcat4中，invoke()方法会调用StandardHost实例的map()方法获取一个相应的Context实例；然后获取与该request对象相关联的session对象，并调用其access()方法，access()方法会修改session对象的最后访问时间；最后调用Context实例的invoke()来处理HTTP请求

接下来描述Engine容器，Engine容器是org.apache.catalina.Engine接口的实例，Engine容器也就是Tomcat的servlet引擎

public interface Engine extends Container {
    
    public String getDefaultHost();
   
    public void setDefaultHost(String defaultHost);
    
    public String getJvmRoute();
  
    public void setJvmRoute(String jvmRouteId);
    
    public Service getService();
   
    public void setService(Service service);
   
    public void addDefaultContext(DefaultContext defaultContext);
    
    public DefaultContext getDefaultContext();
    
    public void importDefaultContext(Context context);

}

在Engine容器中，可以设置一个默认的Host容器或一个默认的Context容器，注意，Engine容器可以与一个服务实例相关联

org.apache.catalina.core.StandardEngine类是Engine接口的标准实现，在实例化的时候，StandardEngine类会添加一个基础阀

public StandardEngine() {

        super();
        pipeline.setBasic(new StandardEngineValve());

    }

org.apache.catalina.core.StandardEngineValve类是StandardEngine容器的基础阀，下面是它的invoke()方法的实现代码

public void invoke(Request request, Response response,
                       ValveContext valveContext)
        throws IOException, ServletException {
        // Validate the request and response object types
        if (!(request.getRequest() instanceof HttpServletRequest) ||
            !(response.getResponse() instanceof HttpServletResponse)) {
            return;     // NOTE - Not much else we can do generically
        }

        // Validate that any HTTP/1.1 request included a host header
        HttpServletRequest hrequest = (HttpServletRequest) request;
        if ("HTTP/1.1".equals(hrequest.getProtocol()) &&
            (hrequest.getServerName() == null)) {
            ((HttpServletResponse) response.getResponse()).sendError
                (HttpServletResponse.SC_BAD_REQUEST,
                 sm.getString("standardEngine.noHostHeader",
                              request.getRequest().getServerName()));
            return;
        }

        // Select the Host to be used for this Request
        StandardEngine engine = (StandardEngine) getContainer();
        Host host = (Host) engine.map(request, true);
        if (host == null) {
            ((HttpServletResponse) response.getResponse()).sendError
                (HttpServletResponse.SC_BAD_REQUEST,
                 sm.getString("standardEngine.noHost",
                              request.getRequest().getServerName()));
            return;
        }

        // Ask this Host to process this request
        host.invoke(request, response);

    }

在验证了request和response对象的类型后，invoke()方法会通过调用Engine实例的map()方法获取Host对象；得到Host对象以后，调用其invoke()方法处理请求

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