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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。
本节简单介绍Java虚拟机规范中提到的主要类加载器：
Tomcat实现了自身的AppClassLoader。为便于理解，图1展示了Tomcat的类加载体系，各个类加载器之间不是继承关系，而是一种委派关系。
图1 Tomcat的类加载体系
这里对图1所示的类加载体系进行介绍：
commonLoader、catalinaLoader和sharedLoader是在Tomcat容器初始化的的过程刚刚开始，即调用Bootstrap的init方法时创建的。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容器自身的class。Bootstrap的init方法的部分如代码清单1所示。
代码清单1
/** * 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
代码清单1中有关类加载器的执行步骤如下：
initClassLoaders方法的实现如代码清单2所示。
代码清单2
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); } }
从代码清单2可以看到initClassLoaders调用createClassLoader方法来创建commonLoader、catalinaLoader和sharedLoader，我们来看看createClassLoader的实现，见代码清单3。
代码清单3
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); // 省略无关代码 return classLoader; }
createClassLoader的处理步骤如下：
需要注意的是，Tomcat默认只会指定commonLoader（通过common属性，默认值为${catalina.base}/lib,${catalina.base}/lib/*.jar,${catalina.home}/lib,${catalina.home}/lib/*.jar），catalinaLoader和sharedLoader实际也是commonLoader。属性catalina.home默认为Tomcat的根目录。
首先回头看看SecurityClassLoad.securityClassLoad(catalinaLoader)的实现，见代码清单4。
代码清单4
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方法主要负责加载Tomcat容器所需的class，包括：
以加载Tomcat核心class的loadCorePackage方法为例（见代码清单5），查看其实现。
代码清单5
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，启动StandardContext的方法startInternal的实现见代码清单6。
代码清单6
/** * 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 { // 省略前边无关的代码 if (getLoader() == null) { WebappLoader webappLoader = new WebappLoader(getParentClassLoader()); webappLoader.setDelegate(getDelegate()); setLoader(webappLoader); } // 省略中间无关的代码 // Start our subordinate components, if any if ((loader != null) && (loader instanceof Lifecycle)) ((Lifecycle) loader).start(); // 省略后边无关的代码 }
代码清单6的最后会调用WebappLoader的start方法，start又调用了startInternal方法，WebappLoader的startInternal的实现见代码清单7。
代码清单7
/** * 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 { // 省略无关代码// 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WebappLoader的createClassLoader方法的实现，见代码清单8。
代码清单8
/** * 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; }
代码清单8中的parentClassLoader实际就是sharedLoader，即org.apache.catalina.loader.StandardClassLoader。由此也证实了图1中的WebappClassLoader的父类加载器是sharedLoader。至此，整个Tomcat的类加载体系构建完毕。最后我们看看WebappClassLoader（见代码清单9）是如何实现以及部署在tomcat中的各个webapp的资源是如何隔离的？
代码清单9
@Override 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 { 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 = Class.forName(name, false, loader); 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); ClassLoader loader = parent; if (loader == null) loader = system; try { clazz = Class.forName(name, false, loader); 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); }
从代码清单9，可以看到WebappClassLoader加载class的步骤如下：
有关WebappClassLoader的findClass方法的实现很简单，其中主要调用findClassInternal方法来加载webapp自身路径下的class，有兴趣的读者可自行阅读源码。
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原文地址：http://www.cnblogs.com/jiaan-geng/p/4860432.html
