前言
Tomcat遵循J2EE规范，实现了Web容器。很多有关web的书籍和文章都离不开对Tomcat的分析，初学者可以从Tomcat的实现对J2EE有更深入的了解。此外，Tomcat还根据Java虚拟机规范实现了经典的双亲委派模式的类加载体系。本文基于Tomcat7.0的Java源码，对其类加载体系进行分析。概述
首先简单介绍下Java虚拟机规范中提到的主要类加载器；- Bootstrap Loader：加载lib目录下或者System.getProperty(“sun.boot.class.path”)、或者-XBootclasspath所指定的路径或jar。
- Extended Loader：加载lib\ext目录下或者System.getProperty(“java.ext.dirs”) 所指定的 路径或jar。在使用Java运行程序时，也可以指定其搜索路径，例如：java -Djava.ext.dirs=d:\projects\testproj\classes HelloWorld。
- AppClass Loader：加载System.getProperty("java.class.path")所指定的 路径或jar。在使用Java运行程序时，也可以加上-cp来覆盖原有的Classpath设置，例如： java -cp ./lavasoft/classes HelloWorld。
这里对上图所示类加载体系进行介绍：
ClassLoader：Java提供的类加载器抽象类，用户自定义的类加载器需要继承实现
commonLoader：Tomcat最基本的类加载器，加载路径中的class可以被Tomcat容器本身以及各个Webapp访问；
catalinaLoader：Tomcat容器私有的类加载器，加载路径中的class对于Webapp不可见；
sharedLoader：各个Webapp共享的类加载器，加载路径中的class对于所有Webapp可见，但是对于Tomcat容器不可见；
WebappClassLoader：各个Webapp私有的类加载器，加载路径中的class只对当前Webapp可见；
源码分析
commonLoader、catalinaLoader和sharedLoader在Tomcat容器初始化的一开始，即调用Bootstrap的init方法时创建。catalinaLoader会被设置为Tomcat主线程的线程上下文类加载器，并且使用catalinaLoader加载Tomcat容器自身容器下的class。Bootstrap的init方法的部分代码清单如下： /**
* Initialize daemon.
*/
public void init()
throws Exception
{
// Set Catalina path
setCatalinaHome();
setCatalinaBase();
initClassLoaders();
Thread.currentThread().setContextClassLoader(catalinaLoader);
SecurityClassLoad.securityClassLoad(catalinaLoader);
// 省略后边的代码我们接着来看initClassLoaders方法的实现：
private void initClassLoaders() {
try {
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();
}
catalinaLoader = createClassLoader("server", commonLoader);
sharedLoader = createClassLoader("shared", commonLoader);
} catch (Throwable t) {
log.error("Class loader creation threw exception", t);
System.exit(1);
}
}创建类加载器的createClassLoader方法的实现：
private ClassLoader createClassLoader(String name, ClassLoader parent)
throws Exception {
String value = CatalinaProperties.getProperty(name + ".loader");
if ((value == null) || (value.equals("")))
return parent;
ArrayList<String> repositoryLocations = new ArrayList<String>();
ArrayList<Integer> repositoryTypes = new ArrayList<Integer>();
int i;
StringTokenizer tokenizer = new StringTokenizer(value, ",");
while (tokenizer.hasMoreElements()) {
String repository = tokenizer.nextToken();
// Local repository
boolean replace = false;
String before = repository;
while ((i=repository.indexOf(CATALINA_HOME_TOKEN))>=0) {
replace=true;
if (i>0) {
repository = repository.substring(0,i) + getCatalinaHome()
+ repository.substring(i+CATALINA_HOME_TOKEN.length());
} else {
repository = getCatalinaHome()
+ repository.substring(CATALINA_HOME_TOKEN.length());
}
}
while ((i=repository.indexOf(CATALINA_BASE_TOKEN))>=0) {
replace=true;
if (i>0) {
repository = repository.substring(0,i) + getCatalinaBase()
+ repository.substring(i+CATALINA_BASE_TOKEN.length());
} else {
repository = getCatalinaBase()
+ repository.substring(CATALINA_BASE_TOKEN.length());
}
}
if (replace && log.isDebugEnabled())
log.debug("Expanded " + before + " to " + repository);
// Check for a JAR URL repository
try {
new URL(repository);
repositoryLocations.add(repository);
repositoryTypes.add(ClassLoaderFactory.IS_URL);
continue;
} catch (MalformedURLException e) {
// Ignore
}
if (repository.endsWith("*.jar")) {
repository = repository.substring
(0, repository.length() - "*.jar".length());
repositoryLocations.add(repository);
repositoryTypes.add(ClassLoaderFactory.IS_GLOB);
} else if (repository.endsWith(".jar")) {
repositoryLocations.add(repository);
repositoryTypes.add(ClassLoaderFactory.IS_JAR);
} else {
repositoryLocations.add(repository);
repositoryTypes.add(ClassLoaderFactory.IS_DIR);
}
}
String[] locations = repositoryLocations.toArray(new String[0]);
Integer[] types = repositoryTypes.toArray(new Integer[0]);
ClassLoader classLoader = ClassLoaderFactory.createClassLoader
(locations, types, parent);
// Retrieving MBean server
MBeanServer mBeanServer = null;
if (MBeanServerFactory.findMBeanServer(null).size() > 0) {
mBeanServer = MBeanServerFactory.findMBeanServer(null).get(0);
} else {
mBeanServer = ManagementFactory.getPlatformMBeanServer();
}
// Register the server classloader
ObjectName objectName =
new ObjectName("Catalina:type=ServerClassLoader,name=" + name);
mBeanServer.registerMBean(classLoader, objectName);
return classLoader;
}createClassLoader最终使用ClassLoaderFactory.createClassLoader(locations, types, parent)方法创建ClassLoader。我们回头看看SecurityClassLoad.securityClassLoad(catalinaLoader)的实现：
public static void securityClassLoad(ClassLoader loader)
throws Exception {
if( System.getSecurityManager() == null ){
return;
}
loadCorePackage(loader);
loadLoaderPackage(loader);
loadSessionPackage(loader);
loadUtilPackage(loader);
loadJavaxPackage(loader);
loadCoyotePackage(loader);
loadTomcatPackage(loader);
}securityClassLoad方法主要加载Tomcat容器所需的class，包括：
- Tomcat核心class，即org.apache.catalina.core路径下的class；
- org.apache.catalina.loader.WebappClassLoader$PrivilegedFindResourceByName；
- Tomcat有关session的class，即org.apache.catalina.session路径下的class；
- Tomcat工具类的class，即org.apache.catalina.util路径下的class；
- javax.servlet.http.Cookie；
- Tomcat处理请求的class，即org.apache.catalina.connector路径下的class；
- Tomcat其它工具类的class，也是org.apache.catalina.util路径下的class；
我们以加载Tomcat核心class的loadCorePackage方法为例，查看其实现：
private final static void loadCorePackage(ClassLoader loader)
throws Exception {
String basePackage = "org.apache.catalina.";
loader.loadClass
(basePackage +
"core.ApplicationContextFacade$1");
loader.loadClass
(basePackage +
"core.ApplicationDispatcher$PrivilegedForward");
loader.loadClass
(basePackage +
"core.ApplicationDispatcher$PrivilegedInclude");
loader.loadClass
(basePackage +
"core.AsyncContextImpl");
loader.loadClass
(basePackage +
"core.AsyncContextImpl$AsyncState");
loader.loadClass
(basePackage +
"core.AsyncContextImpl$DebugException");
loader.loadClass
(basePackage +
"core.AsyncContextImpl$1");
loader.loadClass
(basePackage +
"core.AsyncContextImpl$2");
loader.loadClass
(basePackage +
"core.AsyncListenerWrapper");
loader.loadClass
(basePackage +
"core.ContainerBase$PrivilegedAddChild");
loader.loadClass
(basePackage +
"core.DefaultInstanceManager$1");
loader.loadClass
(basePackage +
"core.DefaultInstanceManager$2");
loader.loadClass
(basePackage +
"core.DefaultInstanceManager$3");
loader.loadClass
(basePackage +
"core.DefaultInstanceManager$4");
loader.loadClass
(basePackage +
"core.DefaultInstanceManager$5");
loader.loadClass
(basePackage +
"core.ApplicationHttpRequest$AttributeNamesEnumerator");
}至此，我们还没有看到WebappClassLoader。启动StandardContext的时候会创建WebappLoader，StandardContext的方法startInternal的部分代码如下：
/**
* Start this component and implement the requirements
* of {@link LifecycleBase#startInternal()}.
*
* @exception LifecycleException if this component detects a fatal error
* that prevents this component from being used
*/
@Override
protected synchronized void startInternal() throws LifecycleException {
// 省略前边的代码
if (getLoader() == null) {
WebappLoader webappLoader = new WebappLoader(getParentClassLoader());
webappLoader.setDelegate(getDelegate());
setLoader(webappLoader);
}
// 省略中间的代码
// Start our subordinate components, if any
if ((loader != null) && (loader instanceof Lifecycle))
((Lifecycle) loader).start();
// 省略后边的代码
}从上面代码看到最后会调用WebappLoader的start方法，start又调用了startInternal方法，startInternal的实现如下：
/**
* Start associated {@link ClassLoader} and implement the requirements
* of {@link LifecycleBase#startInternal()}.
*
* @exception LifecycleException if this component detects a fatal error
* that prevents this component from being used
*/
@Override
protected void startInternal() throws LifecycleException {
// Register a stream handler factory for the JNDI protocol
URLStreamHandlerFactory streamHandlerFactory =
new DirContextURLStreamHandlerFactory();
if (first) {
first = false;
try {
URL.setURLStreamHandlerFactory(streamHandlerFactory);
} catch (Exception e) {
// Log and continue anyway, this is not critical
log.error("Error registering jndi stream handler", e);
} catch (Throwable t) {
// This is likely a dual registration
log.info("Dual registration of jndi stream handler: "
+ t.getMessage());
}
}
// Construct a class loader based on our current repositories list
try {
classLoader = createClassLoader();
classLoader.setResources(container.getResources());
classLoader.setDelegate(this.delegate);
classLoader.setSearchExternalFirst(searchExternalFirst);
if (container instanceof StandardContext) {
classLoader.setAntiJARLocking(
((StandardContext) container).getAntiJARLocking());
classLoader.setClearReferencesStatic(
((StandardContext) container).getClearReferencesStatic());
classLoader.setClearReferencesStopThreads(
((StandardContext) container).getClearReferencesStopThreads());
classLoader.setClearReferencesStopTimerThreads(
((StandardContext) container).getClearReferencesStopTimerThreads());
classLoader.setClearReferencesThreadLocals(
((StandardContext) container).getClearReferencesThreadLocals());
}
for (int i = 0; i < repositories.length; i++) {
classLoader.addRepository(repositories[i]);
}最后我们看看createClassLoader的实现：
/**
* Create associated classLoader.
*/
private WebappClassLoader createClassLoader()
throws Exception {
//loaderClass即字符串org.apache.catalina.loader.WebappClassLoader
Class<?> clazz = Class.forName(loaderClass);
WebappClassLoader classLoader = null;
if (parentClassLoader == null) {
parentClassLoader = container.getParentClassLoader();
}
Class<?>[] argTypes = { ClassLoader.class };
Object[] args = { parentClassLoader };
Constructor<?> constr = clazz.getConstructor(argTypes);
classLoader = (WebappClassLoader) constr.newInstance(args);
return classLoader;
}至此，整个Tomcat的类加载体系构建完毕。
