Tomcat类加载机制
1. 概述
Tomcat内置了一系列的类加载器，保证运行在容器中的不同Web应用程序使用不同的类路径和资源仓库。在Java环境中，类加载机制采用双亲委派机制；但是，在Tomcat中WebappClassLoader加载器并未采用这种机制（在后面将详细讨论）。 随着Tomcat版本的不断变化，Tomcat5，Tomcat6和Tomcat7,8在类加载的过程中存在一些不同之处，在后面章节将逐一讨论。
2. Tomcat中的类加载器
Tomcat中自定义的类加载器有4种，分别是Common，Catalina，Shared 和WebApp，具体定义如下：
| 名称 | 实现类 | 父加载器 | ClassPath |
| Common | org.apache.catalina.loader.StandardClassLoader | system | /common/* |
| Catalina | 同上 | common | /server/* |
| Shared | 同上 | common | /shared/* |
| WebApp | org.apache.catalina.loader.WebappClassLoader | shared | /WebApp/WEB-INF/* |
3. 类加载器初始化过程
Tomcat启动后首先启动初始化：Common,Catalina和Shared类加载器。具体调用如下：
org.apache.catalina.startup.Bootstrap#main
↓
org.apache.catalina.startup.Bootstrap#init
↓
org.apache.catalina.startup.Bootstrap#initClassLoaders
private void initClassLoaders() {
try {
// Common类加载器
commonLoader = createClassLoader("common", null);
if( commonLoader == null ) {
// no config file, default to this loader - we might be in a 'single' env.
commonLoader=this.getClass().getClassLoader();
}
// Catalina类加载器
catalinaLoader = createClassLoader("server", commonLoader);
// Shared类加载器
sharedLoader = createClassLoader("shared", commonLoader);
} catch (Throwable t) {
log.error("Class loader creation threw exception", t);
System.exit(1);
}
}
org.apache.catalina.startup.Bootstrap#createClassLoader代码如下：
private ClassLoader createClassLoader(String name, ClassLoader parent)
throws Exception {
// 根据${Tomcat_home}\conf\catalina.properties获取属性值
String value = CatalinaProperties.getProperty(name + ".loader");
if ((value == null) || (value.equals("")))
return parent;
-中略-
4. Tomcat各个版本类加载器区别
Tomcat5.5
Tomcat5的中默认catalina.properties属性文件如下：
common.loader=${catalina.home}/common/classes,${catalina.home}/common/i18n/*.jar,${catalina.home}/common/endorsed/*.jar,${catalina.home}/common/lib/*.jar
server.loader=${catalina.home}/server/classes,${catalina.home}/server/lib/*.jar
shared.loader=${catalina.base}/shared/classes,${catalina.base}/shared/lib/*.jar
因此，Tomcat5 会分别创建：Common,Catalina和Shared类加载器，相应关系如（pic2-1）
Tomcat6.x or later
默认catalina.properties属性文件如下：
common.loader=${catalina.base}/lib,${catalina.base}/lib/*.jar,${catalina.home}/lib,${catalina.home}/lib/*.jar
server.loader=
shared.loader=
因此，他们创建的Common,Catalina和Shared类加载器为同一个实例，即Common，相应的关系图如下(pic4-1)
5. Tomcat的类加载机制
Tomcat 的类加载器实现类有两种：StandardClassLoader和WebappClassLoader两种。其中StandardClassLoader未重载loadClass方法，因此采用的仍然是双亲委派原则；然而WebappClassLoader重载了loadClass方法，使用了另外一种类加载策略。然而，各个版本又有稍许差别。
Tomcat6
当delegate="false"时，默认值
- 检查缓存中是否存在要加载的类
- 若缓存中没有，则首先使用系统类加载器加载，防止Web应用程序中的类覆盖J2SE的类
- 从当前仓库中载入相关类
- 使用父类载入器。若父类载入器为null，使用系统的类加载器进行加载
-
若仍未找到需要的类，则抛出ClasNotFoundException异常。
类加载顺序:
- Bootstrap classes of your JVM
- System class loader classes (described above)
- /WEB-INF/classes of your web application
- /WEB-INF/lib/*.jar of your web application
- $CATALINA_HOME/common/classes
- $CATALINA_HOME/common/endorsed/*.jar
- $CATALINA_HOME/common/i18n/*.jar
- $CATALINA_HOME/common/lib/*.jar
- $CATALINA_BASE/shared/classes
- $CATALINA_BASE/shared/lib/*.jar
当delegate="true"时
- 检查缓存中是否存在要加载的类
- 若缓存中没有，则首先使用系统类加载器加载，防止Web应用程序中的类覆盖J2SE的类
- 使用父类载入器。若父类载入器为null，使用系统的类加载器进行加载
- 从当前仓库中载入相关类
-
若仍未找到需要的类，则抛出ClasNotFoundException异常。
类加载顺序:
- Bootstrap classes of your JVM
- System class loader classes (described above)
- $CATALINA_HOME/common/classes
- $CATALINA_HOME/common/endorsed/*.jar
- $CATALINA_HOME/common/i18n/*.jar
- $CATALINA_HOME/common/lib/*.jar
- $CATALINA_BASE/shared/classes
- $CATALINA_BASE/shared/lib/*.jar
- /WEB-INF/classes of your web application
- /WEB-INF/lib/*.jar of your web application
备注：默认情况下，Tomcat6 shared类加载器路径。
org.apache.catalina.loader.WebappClassLoader#loadClass
public synchronized Class loadClass(String name, boolean resolve)
throws ClassNotFoundException {
if (log.isDebugEnabled())
log.debug("loadClass(" + name + ", " + resolve + ")");
Class clazz = null;
// Log access to stopped classloader
if (!started) {
try {
throw new IllegalStateException();
} catch (IllegalStateException e) {
log.info(sm.getString("webappClassLoader.stopped", name), e);
}
}
// (0) Check our previously loaded local class cache
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 = 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 system class loader, to prevent
// the webapp from overriding J2SE classes
try {
// 使用系统类加载器
clazz = system.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 = "Security Violation, attempt to use " +
"Restricted Class: " + name;
log.info(error, se);
throw new ClassNotFoundException(error, se);
}
}
}
boolean delegateLoad = delegate || filter(name);
// (1) Delegate to our parent if requested
if (delegateLoad) {
if (log.isDebugEnabled())
log.debug(" Delegating to parent classloader1 " + parent);
ClassLoader loader = parent;
if (loader == null)
loader = system;
try {
clazz = loader.loadClass(name);
if (clazz != null) {
if (log.isDebugEnabled())
log.debug(" Loading class from parent");
if (resolve)
resolveClass(clazz);
return (clazz);
}
} catch (ClassNotFoundException e) {
;
}
}
// (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) {
;
}
// (3) Delegate to parent unconditionally
if (!delegateLoad) {
if (log.isDebugEnabled())
log.debug(" Delegating to parent classloader at end: " + parent);
ClassLoader loader = parent;
if (loader == null)
loader = system;
try {
clazz = loader.loadClass(name);
if (clazz != null) {
if (log.isDebugEnabled())
log.debug(" Loading class from parent");
if (resolve)
resolveClass(clazz);
return (clazz);
}
} catch (ClassNotFoundException e) {
;
}
}
throw new ClassNotFoundException(name);
}
Tomcat7 or later
当delegate="false"时，默认值
- 检查缓存中是否存在要加载的类
- 若缓存中没有，则首先使用引导类加载器加载，防止Web应用程序中的类覆盖J2SE的类
- 从当前仓库中载入相关类
- 使用父类载入器。若父类载入器为null，使用引导类加载器进行加载
- 若仍未找到需要的类，则抛出ClasNotFoundException异常。
类加载顺序:
- Bootstrap classes of your JVM
- /WEB-INF/classes of your web application
- /WEB-INF/lib/*.jar of your web application
- System class loader classes (described above)
- Common class loader classes (described above)
当delegate="true"时
- 检查缓存中是否存在要加载的类
- 若缓存中没有，则首先使用引导类加载器加载，防止Web应用程序中的类覆盖J2SE的类
- 使用父类载入器。若父类载入器为null，使用引导类加载器进行加载
- 从当前仓库中载入相关类
-
若仍未找到需要的类，则抛出ClasNotFoundException异常。
类加载顺序:
- Bootstrap classes of your JVM
- System class loader classes (described above)
- Common class loader classes (described above)
- /WEB-INF/classes of your web application
- /WEB-INF/lib/*.jar of your web application
org.apache.catalina.loader.WebappClassLoader#loadClass
public synchronized Class<?> loadClass(String name, boolean resolve)
throws ClassNotFoundException {
if (log.isDebugEnabled())
log.debug("loadClass(" + name + ", " + resolve + ")");
Class<?> clazz = null;
// Log access to stopped classloader
if (!started) {
try {
throw new IllegalStateException();
} catch (IllegalStateException e) {
log.info(sm.getString("webappClassLoader.stopped", name), e);
}
}
// (0) Check our previously loaded local class cache
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 = 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 system class loader, to prevent
// the webapp from overriding J2SE classes
try {
// j2seClassLoader为引导类加载器。
clazz = j2seClassLoader.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 = "Security Violation, attempt to use " +
"Restricted Class: " + name;
log.info(error, se);
throw new ClassNotFoundException(error, se);
}
}
}
boolean delegateLoad = delegate || filter(name);
// (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);
}
j2seClassLoader类的初始化
public WebappClassLoader() {
-中略-
ClassLoader j = String.class.getClassLoader();
if (j == null) {
j = getSystemClassLoader();
while (j.getParent() != null) {
j = j.getParent();
}
}
this.j2seClassLoader = j;
-中略-
}
参考链接
[1].Tomcat5.5 Class Loader HOW-TO
[2].Tomcat6.0 Class Loader HOW-TO
[3].Tomcat7.0 Class Loader HOW-TO
[4].Tomcat8.0 Class Loader HOW-TO
[5].Tomcat download
以上。