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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