How Tomcat Works

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要使用一个web应用程序，必须要将表示该应用程序的Context实例部署到一个host实例中。在tomcat中，context实例可以用war文件的形式来部署，也可以将整个web应用拷贝到Tomcat安装目录下的webapp下。对于部署的每个web应用程序，可以在其中包含一个描述文件（该文件是可选的），该文件中包含了对context的配置选项，是xml格式的文件。
注意，tomcat4和tomcat5使用两个应用程序来管理tomcat及其应用的部署，分别是manager应用程序和admin应用程序。这里两个应用程序位于%CATALINA_HOME%/server/webapps目录下，各自有一个描述文件，分别是manager.xml和admin.xml。
本文将讨论使用一个部署器来部署web应用程序，部署器是org.apache.catalina.Deployer接口的实例。部署器需要与一个host实例相关联，用于部署context实例。部署一个context到host，即创建一个StandardContext实例，并将该context实例添加到host实例中。创建的context实例会随其父容器——host实例而启动（容器的实例在启动时总是会调用其子容器的start方法，除非该该container是一个wrapper实例）。
本文会先说明tomcat部署器如何部署一个web应用程序，然后描述Deployer接口及其标准实现org.apache.catalina.core.StandardHostDeployer类的工作原理。

tomcat中在StandardHost中使用了一个生命周期监听器（lifecycle listener）org.apache.catalina.startup.HostConfig来部署应用。
当调用StandardHost实例的start方法时，会触发START事件，HostConfig实例会响应该事件，调用其start方法，在该方法中会部署并安装指定目录中的所有的web应用程序。

在How Tomcat Works（十八）中，描述了如何使用Digester对象来解析XML文档的内容，但并没有涉及Digester对象中所有的规则，其中被忽略掉的一个主题就是部署器，也就是本文的主题

在Tomcat中，org.apache.catalina.startup.Catalina类是启动类，使用Digester对象来解析server.xml文件，将其中的xml元素转换为java对象。

Catalina类中定义了createStartDigester方法来添加规则到Digester中：

digester.addRuleSet(new HostRuleSet("Server/Service/Engine/"));

org.apache.catalina.startup.HostRuleSet类继承自org.apache.commons.digester.RuleSetBase类，作为RuleSetBase的子类，HostRuleSet提供了addRuleInstances方法实现，该方法定义了RuleSet中的规则（Rule）。

下面是HostRuleSet类的addRuleInstances方法的实现片段：

public void addRuleInstances(Digester digester) { 
   digester.addObjectCreate(prefix + "Host", "org.apache.catalina.core.StandardHost", "className"); 
   digester.addSetProperties(prefix + "Host"); 
   digester.addRule(prefix + "Host", new CopyParentClassLoaderRule(digester)); 
   digester.addRule(prefix + "Host", 
new LifecycleListenerRule (digester, "org.apache.catalina.startup.HostConfig", "hostConfigClass"));

正如代码中所示，当出现模式Server/Service/Engine/Host时，会创建一个org.apache.catalina.startup.HostConfig实例，并被添加到host，作为一个生命周期监听器。换句话说，HostConfig对象会处理StandardHost对象的start和stop方法触发的事件。

下面的代码是HostConfig的lifecycleEvent方法实现：

public void lifecycleEvent(LifecycleEvent event) { 
   // Identify the host we are associated with 
   try { 
     host = (Host) event.getLifecycle(); 
     if (host instanceof StandardHost) { 
       int hostDebug = ((StandardHost) host).getDebug(); 
       if (hostDebug > this.debug) { 
         this.debug = hostDebug; 
       } 
       setDeployXML(((StandardHost) host).isDeployXML()); 
       setLiveDeploy(((StandardHost) host).getLiveDeploy()); 
       setUnpackWARs(((StandardHost) host).isUnpackWARs()); 
     } 
   } catch (ClassCastException e) { 
     log(sm.getString("hostConfig.cce", event.getLifecycle()), e); 
     return; 
   } 
 
   // Process the event that has occurred 
   if (event.getType().equals(Lifecycle.START_EVENT)) 
     start (); 
   else if (event.getType().equals(Lifecycle.STOP_EVENT)) 
     stop(); 
}

如果变量host指向的对象是一个org.apache.catalina.core.StandardHost实例，会调用setDeployXML方法，setLiveDeploy方法和setUnpackWARs方法：

setDeployXML(((StandardHost) host).isDeployXML()); 
setLiveDeploy(((StandardHost) host).getLiveDeploy());  
setUnpackWARs(((StandardHost) host).isUnpackWARs());

StandardHost类的isDeployXML方法指明host是否要部署一个描述文件，默认为true。liveDeploy属性指明host是否要周期性的检查是否有新的应用部署。unpackWARs属性指明host是否要解压缩war文件。
接收到START事件后，HostConfig的lifecycleEvent方法会调用start方法来部署web应用：

protected void start() { 
   if (debug >= 1) 
     log(sm.getString("hostConfig.start")); 
   if (host.getAutoDeploy()) { 
     deployApps(); 
   } 
   if (isLiveDeploy ()) { 
     threadStart(); 
   } 
}

当autoDeploy属性值为true时（默认为true），则start方法会调用deployApps方法。此外，若liveDeploy属性为true（默认为true），则该方法会开一个新线程调用threadStart方法。
deployApps方法从host中获取appBase属性值（默认为webapps），该值定义于server.xml文件中。部署进程会将%CATALINE_HOME%/webapps目录下的所有目录看做为Web应用程序的目录来执行部署工作。此外，该目录下找到的war文件和描述文件也会被部署。

deployApps方法实现如下：

protected void deployApps() { 
   if (!(host instanceof Deployer))      
return; 
   if (debug >= 1) 
     log(sm.getString("hostConfig.deploying")); 
   File appBase = appBase(); 
   if (!appBase.exists() || !appBase.isDirectory()) 
     return; 
   String files[] = appBase.list(); 
   deployDescriptors(appBase, files); 
   deployWARs(appBase, files); 
   deployDirectories(appBase, files); 
}

deployApps方法会调用其他三个方法，deployDescriptors，deployWARs和deployDirectories。对于所有方法，deployApps方法会传入appBase对象和appBase下所有的文件名的数组形式。context实例是通过其路径来标识的，所有的context必须有其唯一路径。已经被部署的contex实例t会被添加到HostConfig对象中已经部署的ArrayList中。因此，在部署一个context实例之前，deployDescriptors，deployWARs和deployDirectories方法必须确保已部署ArrayList中的没有相同路径的context实例。
注意，deployDescriptors，deployWARs和deployDirectories三个方法的调用顺序是固定的

下面方法为部署描述符：

/**
     * Deploy XML context descriptors.
     */
    protected void deployDescriptors(File appBase, String[] files) {

        if (!deployXML)
           return;

        for (int i = 0; i < files.length; i++) {

            if (files[i].equalsIgnoreCase("META-INF"))
                continue;
            if (files[i].equalsIgnoreCase("WEB-INF"))
                continue;
            if (deployed.contains(files[i]))
                continue;
            File dir = new File(appBase, files[i]);
            if (files[i].toLowerCase().endsWith(".xml")) {

                deployed.add(files[i]);

                // Calculate the context path and make sure it is unique
                String file = files[i].substring(0, files[i].length() - 4);
                String contextPath = "/" + file;
                if (file.equals("ROOT")) {
                    contextPath = "";
                }
                if (host.findChild(contextPath) != null) {
                    continue;
                }

                // Assume this is a configuration descriptor and deploy it
                log(sm.getString("hostConfig.deployDescriptor", files[i]));
                try {
                    URL config =
                        new URL("file", null, dir.getCanonicalPath());
                    ((Deployer) host).install(config, null);
                } catch (Throwable t) {
                    log(sm.getString("hostConfig.deployDescriptor.error",
                                     files[i]), t);
                }

            }

        }

    }

部署WAR文件：

/**
     * Deploy WAR files.
     */
    protected void deployWARs(File appBase, String[] files) {

        for (int i = 0; i < files.length; i++) {

            if (files[i].equalsIgnoreCase("META-INF"))
                continue;
            if (files[i].equalsIgnoreCase("WEB-INF"))
                continue;
            if (deployed.contains(files[i]))
                continue;
            File dir = new File(appBase, files[i]);
            if (files[i].toLowerCase().endsWith(".war")) {

                deployed.add(files[i]);

                // Calculate the context path and make sure it is unique
                String contextPath = "/" + files[i];
                int period = contextPath.lastIndexOf(".");
                if (period >= 0)
                    contextPath = contextPath.substring(0, period);
                if (contextPath.equals("/ROOT"))
                    contextPath = "";
                if (host.findChild(contextPath) != null)
                    continue;

                if (isUnpackWARs()) {

                    // Expand and deploy this application as a directory
                    log(sm.getString("hostConfig.expand", files[i]));
                    try {
                        URL url = new URL("jar:file:" +
                                          dir.getCanonicalPath() + "!/");
                        String path = expand(url);
                        url = new URL("file:" + path);
                        ((Deployer) host).install(contextPath, url);
                    } catch (Throwable t) {
                        log(sm.getString("hostConfig.expand.error", files[i]),
                            t);
                    }

                } else {

                    // Deploy the application in this WAR file
                    log(sm.getString("hostConfig.deployJar", files[i]));
                    try {
                        URL url = new URL("file", null,
                                          dir.getCanonicalPath());
                        url = new URL("jar:" + url.toString() + "!/");
                        ((Deployer) host).install(contextPath, url);
                    } catch (Throwable t) {
                        log(sm.getString("hostConfig.deployJar.error",
                                         files[i]), t);
                    }

                }

            }

        }

    }

也可以直接将Web应用程序整个目录复制到%CATALINA_HOME%/webapps目录下，部署目录：

/**
     * Deploy directories.
     */
    protected void deployDirectories(File appBase, String[] files) {

        for (int i = 0; i < files.length; i++) {

            if (files[i].equalsIgnoreCase("META-INF"))
                continue;
            if (files[i].equalsIgnoreCase("WEB-INF"))
                continue;
            if (deployed.contains(files[i]))
                continue;
            File dir = new File(appBase, files[i]);
            if (dir.isDirectory()) {

                deployed.add(files[i]);

                // Make sure there is an application configuration directory
                // This is needed if the Context appBase is the same as the
                // web server document root to make sure only web applications
                // are deployed and not directories for web space.
                File webInf = new File(dir, "/WEB-INF");
                if (!webInf.exists() || !webInf.isDirectory() ||
                    !webInf.canRead())
                    continue;

                // Calculate the context path and make sure it is unique
                String contextPath = "/" + files[i];
                if (files[i].equals("ROOT"))
                    contextPath = "";
                if (host.findChild(contextPath) != null)
                    continue;

                // Deploy the application in this directory
                log(sm.getString("hostConfig.deployDir", files[i]));
                try {
                    URL url = new URL("file", null, dir.getCanonicalPath());
                    ((Deployer) host).install(contextPath, url);
                } catch (Throwable t) {
                    log(sm.getString("hostConfig.deployDir.error", files[i]),
                        t);
                }

            }

        }

    }

正如前面描述的，如果变量liveDeploy的值为true，start方法会调用threadStart()方法

if (isLiveDeploy()) {
            threadStart();
        }

threadStart()方法会派生一个新线程并调用run()方法，run()方法会定期检查是否有新应用要部署，或已部署的Web应用程序的web.xml是否有修改

下面的run()方法的实现（HostConfig类实现了java.lang.Runnable接口）

/**
     * The background thread that checks for web application autoDeploy
     * and changes to the web.xml config.
     */
    public void run() {

        if (debug >= 1)
            log("BACKGROUND THREAD Starting");

        // Loop until the termination semaphore is set
        while (!threadDone) {

            // Wait for our check interval
            threadSleep();

            // Deploy apps if the Host allows auto deploying
            deployApps();

            // Check for web.xml modification
            checkWebXmlLastModified();

        }

        if (debug >= 1)
            log("BACKGROUND THREAD Stopping");

    }

部署器用org.apache.catalina.Deployer接口表示，StandardHost实现了 Deployer接口，因此，StandardHost也是一个部署器，它是一个容器，Web应用可以部署到其中，或从中取消部署

下面是Deployer接口的定义：

/* public interface Deployer extends Container { */
public interface Deployer  {
  
    public static final String PRE_INSTALL_EVENT = "pre-install";
   
    public static final String INSTALL_EVENT = "install";
    
    public static final String REMOVE_EVENT = "remove";
    
    public String getName();
   
    public void install(String contextPath, URL war) throws IOException;
    
    public void install(URL config, URL war) throws IOException;
  
    public Context findDeployedApp(String contextPath);
  
    public String[] findDeployedApps();
    
    public void remove(String contextPath) throws IOException;
   
    public void start(String contextPath) throws IOException;
    
    public void stop(String contextPath) throws IOException;

}

StandardHost类使用一个辅助类（org.apache.catalina.core.StandardHostDeployer，与StandardHost类都实现了Deployer接口）来完成部署与安装Web应用程序的相关任务，下面的代码片段演示了StandardHost对象如何将部署任务委托给StandardHostDeployer实例来完成

/**
* The <code>Deployer</code> to whom we delegate application
* deployment requests.
*/
private Deployer deployer = new StandardHostDeployer(this);
public void install(String contextPath, URL war) throws IOException {
    deployer.install(contextPath, war);
}
public synchronized void install(URL config, URL war) throws
IOException {
    deployer.install(config, war);
}
public Context findDeployedApp(String contextPath) {
    return (deployer.findDeployedApp(contextPath));
}
public String[] findDeployedApps() {
    return (deployer.findDeployedApps());
}
public void remove(String contextPath) throws IOException {
    deployer.remove(contextPath);
}
public void start(String contextPath) throws IOException {
    deployer.start(contextPath);
}
public void stop(String contextPath) throws IOException {
    deployer.stop(contextPath);
}

org.apache.catalina.core.StandardHostDeployer类是一个辅助类，帮助完成将Web应用程序部署到StandardHost实例的工作。StandardHostDeployer实例由StandardHost对象调用，在其构造函数中，会传入StandardHost类的实例

public StandardHostDeployer(StandardHost host) {

        super();
        this.host = host;

    }

下面的install()方法用于安装描述符，当HostConfig对象的deployDescriptors方法调用StandardHost实例的install()方法后， StandardHost实例调用该方法

public synchronized void install(URL config, URL war) throws IOException {

        // Validate the format and state of our arguments
        if (config == null)
            throw new IllegalArgumentException
                (sm.getString("standardHost.configRequired"));

        if (!host.isDeployXML())
            throw new IllegalArgumentException
                (sm.getString("standardHost.configNotAllowed"));

        // Calculate the document base for the new web application (if needed)
        String docBase = null; // Optional override for value in config file
        if (war != null) {
            String url = war.toString();
            host.log(sm.getString("standardHost.installingWAR", url));
            // Calculate the WAR file absolute pathname
            if (url.startsWith("jar:")) {
                url = url.substring(4, url.length() - 2);
            }
            if (url.startsWith("file://"))
                docBase = url.substring(7);
            else if (url.startsWith("file:"))
                docBase = url.substring(5);
            else
                throw new IllegalArgumentException
                    (sm.getString("standardHost.warURL", url));

        }

        // Install the new web application
        this.context = null;
        this.overrideDocBase = docBase;
        InputStream stream = null;
        try {
            stream = config.openStream();
            Digester digester = createDigester();
            digester.setDebug(host.getDebug());
            digester.clear();
            digester.push(this);
            digester.parse(stream);
            stream.close();
            stream = null;
        } catch (Exception e) {
            host.log
                (sm.getString("standardHost.installError", docBase), e);
            throw new IOException(e.toString());
        } finally {
            if (stream != null) {
                try {
                    stream.close();
                } catch (Throwable t) {
                    ;
                }
            }
        }

    }

第二个install()方法用于安装WAR文件或目录

public synchronized void install(String contextPath, URL war)
        throws IOException {

        // Validate the format and state of our arguments
        if (contextPath == null)
            throw new IllegalArgumentException
                (sm.getString("standardHost.pathRequired"));
        if (!contextPath.equals("") && !contextPath.startsWith("/"))
            throw new IllegalArgumentException
                (sm.getString("standardHost.pathFormat", contextPath));
        if (findDeployedApp(contextPath) != null)
            throw new IllegalStateException
                (sm.getString("standardHost.pathUsed", contextPath));
        if (war == null)
            throw new IllegalArgumentException
                (sm.getString("standardHost.warRequired"));

        // Calculate the document base for the new web application
        host.log(sm.getString("standardHost.installing",
                              contextPath, war.toString()));
        String url = war.toString();
        String docBase = null;
        if (url.startsWith("jar:")) {
            url = url.substring(4, url.length() - 2);
        }
        if (url.startsWith("file://"))
            docBase = url.substring(7);
        else if (url.startsWith("file:"))
            docBase = url.substring(5);
        else
            throw new IllegalArgumentException
                (sm.getString("standardHost.warURL", url));

        // Install the new web application
        try {
            Class clazz = Class.forName(host.getContextClass());
            Context context = (Context) clazz.newInstance();
            context.setPath(contextPath);
            
            context.setDocBase(docBase);
            if (context instanceof Lifecycle) {
                clazz = Class.forName(host.getConfigClass());
                LifecycleListener listener =
                    (LifecycleListener) clazz.newInstance();
                ((Lifecycle) context).addLifecycleListener(listener);
            }
            host.fireContainerEvent(PRE_INSTALL_EVENT, context);
            host.addChild(context);
            host.fireContainerEvent(INSTALL_EVENT, context);
        } catch (Exception e) {
            host.log(sm.getString("standardHost.installError", contextPath),
                     e);
            throw new IOException(e.toString());
        }

    }

start()方法用于启动Context实例：

public void start(String contextPath) throws IOException {
        // Validate the format and state of our arguments
        if (contextPath == null)
            throw new IllegalArgumentException
                (sm.getString("standardHost.pathRequired"));
        if (!contextPath.equals("") && !contextPath.startsWith("/"))
            throw new IllegalArgumentException
                (sm.getString("standardHost.pathFormat", contextPath));
        Context context = findDeployedApp(contextPath);
        if (context == null)
            throw new IllegalArgumentException
                (sm.getString("standardHost.pathMissing", contextPath));
        host.log("standardHost.start " + contextPath);
        try {
            ((Lifecycle) context).start();
        } catch (LifecycleException e) {
            host.log("standardHost.start " + contextPath + ": ", e);
            throw new IllegalStateException
                ("standardHost.start " + contextPath + ": " + e);
        }
    }

stop()方法用于停止Context实例：

public void stop(String contextPath) throws IOException {

        // Validate the format and state of our arguments
        if (contextPath == null)
            throw new IllegalArgumentException
                (sm.getString("standardHost.pathRequired"));
        if (!contextPath.equals("") && !contextPath.startsWith("/"))
            throw new IllegalArgumentException
                (sm.getString("standardHost.pathFormat", contextPath));
        Context context = findDeployedApp(contextPath);
        if (context == null)
            throw new IllegalArgumentException
                (sm.getString("standardHost.pathMissing", contextPath));
        host.log("standardHost.stop " + contextPath);
        try {
            ((Lifecycle) context).stop();
        } catch (LifecycleException e) {
            host.log("standardHost.stop " + contextPath + ": ", e);
            throw new IllegalStateException
                ("standardHost.stop " + contextPath + ": " + e);
        }

    }