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);
}
}
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