TOMCAT---类加载器
一、WebappClassLoader覆盖了classloader的loadclass方法
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) 检查自身缓存：WebappClassLoader之前是否已经load过这个资源 clazz = findLoadedClass0(name); if (clazz != null) { if (log.isDebugEnabled()) log.debug(" Returning class from cache"); if (resolve) resolveClass(clazz); return (clazz); } // (1) 检查上一级缓存：ClassLoader之前是否已经load过 clazz = findLoadedClass(name); if (clazz != null) { if (log.isDebugEnabled()) log.debug(" Returning class from cache"); if (resolve) resolveClass(clazz); return (clazz); } // (2) 先交由系统ClassLoader，因此WEB-INF/lib和WEB-INF/classes或{tomcat}/libs下的类定义不能覆盖JVM 底层能够查找到的定义(譬如不能通过定义java.lang.Integer替代底层的实现 try { clazz = system.loadClass(name); if (clazz != null) { if (resolve) resolveClass(clazz); return (clazz); } } catch (ClassNotFoundException e) { // Ignore } // (5) Permission to access this class when using a SecurityManager，检查是否允许载入该类，如果该类禁止载入则抛异常 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); } } } //这是一个很奇怪的定义，JVM的ClassLoader建议先由parent去load，load不到自己再去load(见如上 ClassLoader部分)，而Servelet规范的建议则恰好相反，Tomcat的实现则做个折中，由用户去决定(context的 delegate定义)，默认使用Servlet规范的建议，即delegate=false boolean delegateLoad = delegate || filter(name); // (1) 先由parent去尝试加载，此处的parent是SharedClassLoader，见如上说明，如上说明，除非设置了delegate，否则这里不执行 if (delegateLoad) { if (log.isDebugEnabled()) log.debug(" Delegating to parent classloader1 " + parent); ClassLoader loader = parent; //此处parent是否为空取决于context 的privileged属性配置，默认privileged=true，即parent为SharedClassLoader 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) 到WEB-INF/lib和WEB-INF/classes目录去搜索，细节部分可以再看一下findClass，会发现默认是先搜索WEB-INF/classes后搜索WEB-INF/lib 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) 由parent再去尝试加载一下 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); } ?
二、WebappClassLoader定义了自己的findclass方法
public Class findClass(String name) //调用 protected Class findClassInternal(String name)//调用 protected ResourceEntry findResourceInternal(String name, String path) {.... for (i = 0; (entry == null) && (i < repositoriesLength); i++) { try { String fullPath = repositories[i] + path;//repositories里面只有一个值，即：WEB-INF/classes Object lookupResult = resources.lookup(fullPath); if (lookupResult instanceof Resource) { resource = (Resource) lookupResult; } // Note : Not getting an exception here means the resource was // found entry = findResourceInternal(files[i], path); ResourceAttributes attributes = (ResourceAttributes) resources.getAttributes(fullPath); contentLength = (int) attributes.getContentLength(); entry.lastModified = attributes.getLastModified(); if (resource != null) { try { binaryStream = resource.streamContent(); } catch (IOException e) { return null; } if (needConvert) { if (path.endsWith(".properties")) { fileNeedConvert = true; } }.... } } catch (NamingException e) { } } if ((entry == null) && (notFoundResources.containsKey(name)))//这里已经缓存了加载失败的类名字以提高性能。 return null; JarEntry jarEntry = null; synchronized (jarFiles) {//如果在WEB-INF/classes中没找到则在CLASSPATH中JAR里找 try { if (!openJARs()) { return null; } for (i = 0; (entry == null) && (i < jarFilesLength); i++) { jarEntry = jarFiles[i].getJarEntry(path); }.... }?