本文深入解析Tomcat服务器架构，包括组件结构、启动流程、请求处理机制等关键内容，并详细介绍了类加载器的工作原理。
文章目录
组件成员
server.xml
<?xml version='1.0' encoding='utf-8'?>
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-->
<!-- Note: A "Server" is not itself a "Container", so you may not
define subcomponents such as "Valves" at this level.
Documentation at /docs/config/server.html
-->
<Server port="8005" shutdown="SHUTDOWN">
<Listener className="org.apache.catalina.startup.VersionLoggerListener" />
<!-- Security listener. Documentation at /docs/config/listeners.html
<Listener className="org.apache.catalina.security.SecurityListener" />
-->
<!--APR library loader. Documentation at /docs/apr.html -->
<Listener className="org.apache.catalina.core.AprLifecycleListener" SSLEngine="on" />
<!-- Prevent memory leaks due to use of particular java/javax APIs-->
<Listener className="org.apache.catalina.core.JreMemoryLeakPreventionListener" />
<Listener className="org.apache.catalina.mbeans.GlobalResourcesLifecycleListener" />
<Listener className="org.apache.catalina.core.ThreadLocalLeakPreventionListener" />
<!-- Global JNDI resources
Documentation at /docs/jndi-resources-howto.html
-->
<GlobalNamingResources>
<!-- Editable user database that can also be used by
UserDatabaseRealm to authenticate users
-->
<Resource name="UserDatabase" auth="Container"
type="org.apache.catalina.UserDatabase"
description="User database that can be updated and saved"
factory="org.apache.catalina.users.MemoryUserDatabaseFactory"
pathname="conf/tomcat-users.xml" />
</GlobalNamingResources>
<!-- A "Service" is a collection of one or more "Connectors" that share
a single "Container" Note: A "Service" is not itself a "Container",
so you may not define subcomponents such as "Valves" at this level.
Documentation at /docs/config/service.html
-->
<Service name="Catalina">
<!--The connectors can use a shared executor, you can define one or more named thread pools-->
<!--
<Executor name="tomcatThreadPool" namePrefix="catalina-exec-"
maxThreads="150" minSpareThreads="4"/>
-->
<!-- A "Connector" represents an endpoint by which requests are received
and responses are returned. Documentation at :
Java HTTP Connector: /docs/config/http.html (blocking & non-blocking)
Java AJP Connector: /docs/config/ajp.html
APR (HTTP/AJP) Connector: /docs/apr.html
Define a non-SSL/TLS HTTP/1.1 Connector on port 8080
-->
<Connector port="8080" protocol="HTTP/1.1"
connectionTimeout="20000"
redirectPort="8443" />
<!-- A "Connector" using the shared thread pool-->
<!--
<Connector executor="tomcatThreadPool"
port="8080" protocol="HTTP/1.1"
connectionTimeout="20000"
redirectPort="8443" />
-->
<!-- Define a SSL/TLS HTTP/1.1 Connector on port 8443
This connector uses the NIO implementation that requires the JSSE
style configuration. When using the APR/native implementation, the
OpenSSL style configuration is required as described in the APR/native
documentation -->
<!--
<Connector port="8443" protocol="org.apache.coyote.http11.Http11NioProtocol"
maxThreads="150" SSLEnabled="true" scheme="https" secure="true"
clientAuth="false" sslProtocol="TLS" />
-->
<!-- Define an AJP 1.3 Connector on port 8009 -->
<Connector port="8009" protocol="AJP/1.3" redirectPort="8443" />
<!-- An Engine represents the entry point (within Catalina) that processes
every request. The Engine implementation for Tomcat stand alone
analyzes the HTTP headers included with the request, and passes them
on to the appropriate Host (virtual host).
Documentation at /docs/config/engine.html -->
<!-- You should set jvmRoute to support load-balancing via AJP ie :
<Engine name="Catalina" defaultHost="localhost" jvmRoute="jvm1">
-->
<Engine name="Catalina" defaultHost="localhost">
<!--For clustering, please take a look at documentation at:
/docs/cluster-howto.html (simple how to)
/docs/config/cluster.html (reference documentation) -->
<!--
<Cluster className="org.apache.catalina.ha.tcp.SimpleTcpCluster"/>
-->
<!-- Use the LockOutRealm to prevent attempts to guess user passwords
via a brute-force attack -->
<Realm className="org.apache.catalina.realm.LockOutRealm">
<!-- This Realm uses the UserDatabase configured in the global JNDI
resources under the key "UserDatabase". Any edits
that are performed against this UserDatabase are immediately
available for use by the Realm. -->
<Realm className="org.apache.catalina.realm.UserDatabaseRealm"
resourceName="UserDatabase"/>
</Realm>
<Host name="localhost" appBase="webapps"
unpackWARs="true" autoDeploy="true">
<!-- SingleSignOn valve, share authentication between web applications
Documentation at: /docs/config/valve.html -->
<!--
<Valve className="org.apache.catalina.authenticator.SingleSignOn" />
-->
<!-- Access log processes all example.
Documentation at: /docs/config/valve.html
Note: The pattern used is equivalent to using pattern="common" -->
<Valve className="org.apache.catalina.valves.AccessLogValve" directory="logs"
prefix="localhost_access_log" suffix=".txt"
pattern="%h %l %u %t "%r" %s %b" />
</Host>
</Engine>
</Service>
</Server>
server
代表一个tomcat实例，可包含多个service，管理所有的service生命周期
service
包含多个connector和一个container
container
容器:负责处理请求
engine
只有一个，可包含多个host
host
虚拟主机，虚拟主机允许Tomcat引擎在将配置在一台机器上的多个域名
context
每个虚拟主机又可以支持多个web应用部署在它下边，这就是我们所熟知的上下文对象
wrapper
在上下文中又可以部署多个servlet，并且每个servlet都会被一个包装组件（Wrapper）所包含（一个wrapper对应一个servlet）
connector
连接器：负责接收请求,处理与客户端的通信，传递给container
Tomcat源码中与connector相关的类位于org.apache.coyote包中，Connector分为以下几类：
Http Connector
基于HTTP协议，负责建立HTTP连接。它又分为BIO Http Connector与NIO Http Connector两种，后者提供非阻塞IO与长连接Comet支持。默认情况下，Tomcat使用的就是这个Connector。
AJP Connector
基于AJP协议，AJP是专门设计用来为tomcat与http服务器之间通信专门定制的协议，能提供较高的通信速度和效率。如与Apache服务器集成时，采用这个协议。
APR HTTP Connector
用C实现，通过JNI调用的。主要提升对静态资源(如HTML、图片、CSS、JS等)的访问性能。现在这个库已独立出来可用在任何项目中。Tomcat在配置APR之后性能非常强劲
Connector的三种运行模式: bio nio apr
Endpoint
实现了建立连接、处理连接、和关闭连接等操作
Pipeline & Value
在Catalina中，我们有4种容器，request请求在这4个容器中流转，容器间不是直接调用，而是通过pipeline组件，每个容器都有自己的Pipeline组件，每个Pipeline组件上至少会设定一个Valve(阀门)，这个Valve我们称之为BaseValve（基础阀）。可以说基础阀是两个容器之间的桥梁。
图中可以看到在同一个Pipeline上可以有多个Valve,每个Valve都可以做一些操作，无论是Pipeline还是Valve操作的都是Request和Response。而在容器之间Pipeline和Valve则起到了桥梁的作用
初始化
catalina组件初始化阶段调用createStartDigester创建Digester对象，解析server.xml规则，实例化组件对象并部分赋值。（反射）
bootstrap 创建catalina 使用反射
Catalina及它依赖的类比如 XML parser与应用级别的类隔离（以使用不同类加载器的方法），
Catalina是由catalinaLoader加载的，而Bootstrap是由AppClassLoader加载的，所以在Bootstrap中我们无法直接调用Catalina的方法
bootstrap 加载一些公共类、配置环境、实例化Catalina类，具体的业务代码还是在catalina类里面的。这样反射的好处是解耦，可以不依赖应用层的classpath独立加载
tomcat启动过程
url请求流程
mapper 请求路由实现
收到客户端请求后，容器根据请求 URL 的上下文名称匹配 Web 应用程序，然后根据去除上下文路径和路径参数的路径，按以下规则顺序匹配，并且只使用第一个匹配的 Servlet，后续不再尝试匹配：
- 精确匹配，查找一个与请求路径完全匹配的 Servlet
- 前缀路径匹配，递归的尝试匹配最长路径前缀的 Servlet，通过使用 “/” 作为路径分隔符，在路径树上一次一个目录的匹配，选择路径最长的
- 扩展名匹配，如果 URL 最后一部分包含扩展名，如 .jsp，则尝试匹配处理此扩展名请求的 Servlet
如果前三个规则没有匹配成功，那么容器要为请求提供一个默认 Servlet，容器在匹配时区分大小写。
举例：
实现请求映射的一般方法是，首先构建一个路由表，然后按照规范进行匹配，最后返回匹配结果。Tomcat 就是如此，与请求映射相关的类有三个，分别是：
- Mapper: 存储请求路由表并执行匹配
- MapperListener: 查询所有的 Host、Context、Wrapper 构建路由表
- MappingData: 请求映射结果
启动时解析过程
双亲委派模型
TOMCAT类加载器
Tomcat的自定义类加载器，WebAppClassLoader打破了双亲委托机制：
public Class<?> loadClass(String name, boolean resolve) throws ClassNotFoundException {
synchronized (JreCompat.isGraalAvailable() ? this : getClassLoadingLock(name)) {
if (log.isDebugEnabled()) {
log.debug("loadClass(" + name + ", " + resolve + ")");
}
Class<?> clazz = null;
// Log access to stopped class loader
checkStateForClassLoading(name);
// (0) Check our previously loaded local class cache
// 先在本地cache查找该类是否已经加载过
clazz = findLoadedClass0(name);
if (clazz != null) {
if (log.isDebugEnabled()) {
log.debug(" Returning class from cache");
}
if (resolve) {
resolveClass(clazz);
}
return clazz;
}
// (0.1) Check our previously loaded class cache
clazz = JreCompat.isGraalAvailable() ? null : findLoadedClass(name);
if (clazz != null) {
if (log.isDebugEnabled()) {
log.debug(" Returning class from cache");
}
if (resolve) {
resolveClass(clazz);
}
return clazz;
}
// (0.2) Try loading the class with the bootstrap class loader, to prevent
// the webapp from overriding Java SE classes. This implements
// SRV.10.7.2
String resourceName = binaryNameToPath(name, false);
ClassLoader javaseLoader = getJavaseClassLoader();
boolean tryLoadingFromJavaseLoader;
try {
// Use getResource as it won't trigger an expensive
// ClassNotFoundException if the resource is not available from
// the Java SE class loader. However (see
// https://bz.apache.org/bugzilla/show_bug.cgi?id=58125 for
// details) when running under a security manager in rare cases
// this call may trigger a ClassCircularityError.
// See https://bz.apache.org/bugzilla/show_bug.cgi?id=61424 for
// details of how this may trigger a StackOverflowError
// Given these reported errors, catch Throwable to ensure any
// other edge cases are also caught
URL url;
if (securityManager != null) {
PrivilegedAction<URL> dp = new PrivilegedJavaseGetResource(resourceName);
url = AccessController.doPrivileged(dp);
} else {
url = javaseLoader.getResource(resourceName);
}
tryLoadingFromJavaseLoader = (url != null);
} catch (Throwable t) {
// Swallow all exceptions apart from those that must be re-thrown
ExceptionUtils.handleThrowable(t);
// The getResource() trick won't work for this class. We have to
// try loading it directly and accept that we might get a
// ClassNotFoundException.
tryLoadingFromJavaseLoader = true;
}
if (tryLoadingFromJavaseLoader) {
try {
clazz = javaseLoader.loadClass(name);
if (clazz != null) {
if (resolve) {
resolveClass(clazz);
}
return clazz;
}
} catch (ClassNotFoundException e) {
// Ignore
}
}
// (0.5) Permission to access this class when using a SecurityManager
if (securityManager != null) {
int i = name.lastIndexOf('.');
if (i >= 0) {
try {
securityManager.checkPackageAccess(name.substring(0,i));
} catch (SecurityException se) {
String error = sm.getString("webappClassLoader.restrictedPackage", name);
log.info(error, se);
throw new ClassNotFoundException(error, se);
}
}
}
boolean delegateLoad = delegate || filter(name, true);
// (1) Delegate to our parent if requested
if (delegateLoad) {
if (log.isDebugEnabled()) {
log.debug(" Delegating to parent classloader1 " + parent);
}
try {
clazz = Class.forName(name, false, parent);
if (clazz != null) {
if (log.isDebugEnabled()) {
log.debug(" Loading class from parent");
}
if (resolve) {
resolveClass(clazz);
}
return clazz;
}
} catch (ClassNotFoundException e) {
// Ignore
}
}
// (2) Search local repositories
if (log.isDebugEnabled()) {
log.debug(" Searching local repositories");
}
try {
clazz = findClass(name);
if (clazz != null) {
if (log.isDebugEnabled()) {
log.debug(" Loading class from local repository");
}
if (resolve) {
resolveClass(clazz);
}
return clazz;
}
} catch (ClassNotFoundException e) {
// Ignore
}
// (3) Delegate to parent unconditionally
if (!delegateLoad) {
if (log.isDebugEnabled()) {
log.debug(" Delegating to parent classloader at end: " + parent);
}
try {
clazz = Class.forName(name, false, parent);
if (clazz != null) {
if (log.isDebugEnabled()) {
log.debug(" Loading class from parent");
}
if (resolve) {
resolveClass(clazz);
}
return clazz;
}
} catch (ClassNotFoundException e) {
// Ignore
}
}
}
throw new ClassNotFoundException(name);
}
- 0先在本地cache查找该类是否已经加载过
- 0.1 查找该类是否已经加载过
- 0.2 尝试使用bootstrap class loader加载
为防止Web应用自己的类覆盖JRE的核心类
因为Tomcat需打破双亲委托，假如Web应用里自定义了一个叫Object的类，若先加载该Object类，就会覆盖JRE的Object类，所以Tomcat类加载器优先尝试委托给BootstrapClassLoader去加载，BootstrapClassLoader发现自己已经加载了Object类，直接返回给Tomcat的类加载器，这样Tomcat的类加载器就不会去加载Web应用下的Object类了，避免覆盖JRE核心类
- 0.5 安全管理器权限判断
- 1 过滤排除一些指定的类名，这些类委托父类先加载
- 2 尝试在本地资源下搜索class并加载
- 3 尝试用系统类加载器(Class.forName的默认加载器就是系统类加载器，也就是AppClassLoader)来加载
Tomcat 类加载器打破了双亲委托，没有一上来就直接委托给父加载器，为避免本地目录类覆盖JRE核心类先使用了BootstrapClassLoader加载，之后先在本地目录搜索并加载
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