Tomcat的WebappClassLoader(web应用类加载器)详解
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Tomcat负责Web应用的类加载的是org.apache.catalina.loader.WebappClassLoader，它有几个比较重要的方法：findClass(),loadClass(),findClassInternal(),findResourceInternal().???WebappClassLoader类加载器被用来加载一个类的时候，loadClass()会被调用，loadClass()则调用findClass()。后两个方法是WebappClassLoader的私有方法，findClass()调用findClassInternal()来创建class对象，而findClassInternal()则需要findResourceInternal()来查找.class文件。?通常自己实现类记载器只要实现findclass即可，这里为了实现特殊目的而override了loadClass().?下面是精简过的代码（去除了几乎全部关于log、异常和安全控制的代码）：
public Class loadClass(String name, boolean resolve)throws ClassNotFoundException {Class clazz = null;// (0) 先从自己的缓存中查找，有则返回，无则继续clazz = findLoadedClass0(name);if (clazz != null) {if (resolve)resolveClass(clazz);return (clazz);}// (0.1) 再从parent的缓存中查找clazz = findLoadedClass(name);if (clazz != null) {if (resolve)resolveClass(clazz);return (clazz);}// (0.2) 缓存中没有，则首先使用system类加载器来加载clazz = system.loadClass(name);if (clazz != null) {if (resolve)resolveClass(clazz);return (clazz);}// 判断是否需要先让parent代理boolean delegateLoad = delegate || filter(name);// (1) 先让parent加载，通常delegateLoad == false，即这一步不会执行if (delegateLoad) {ClassLoader loader = parent;if (loader == null)loader = system;clazz = loader.loadClass(name);if (clazz == null) {if (resolve)resolveClass(clazz);return (clazz);}}// (2) delegateLoad == false 或者 parent加载失败，调用自身的加载机制clazz = findClass(name);if (clazz == null) {if (resolve)resolveClass(clazz);return (clazz);}// (3) 自己加载失败，则请求parent代理加载if (!delegateLoad) {ClassLoader loader = parent;if (loader == null)loader = system;clazz = loader.loadClass(name);if (clazz == null) {return (clazz);}}throw new ClassNotFoundException(name);}?
public Class findClass(String name) throws ClassNotFoundException {// 先试图自己加载类，找不到则请求parent来加载// 注意这点和java默认的双亲委托模式不同Class clazz = null;clazz = findClassInternal(name);if ((clazz == null) && hasExternalRepositories) {synchronized (this) {clazz = super.findClass(name);}}if (clazz == null) {throw new ClassNotFoundException(name);}return (clazz);}?
protected Class findClassInternal(String name) throws ClassNotFoundException {if (!validate(name))throw new ClassNotFoundException(name);// 根据类名查找资源String tempPath = name.replace('.', '/');String classPath = tempPath + ".class";ResourceEntry entry = null;entry = findResourceInternal(name, classPath);if (entry == null)throw new ClassNotFoundException(name);// 如果以前已经加载成功过这个类，直接返回Class clazz = entry.loadedClass;if (clazz != null)return clazz;// 以下根据找到的资源(.class文件)进行:1、定义package;2、对package安全检查;3、定义class，即创建class对象synchronized (this) {if (entry.binaryContent == null && entry.loadedClass == null)throw new ClassNotFoundException(name);// Looking up the packageString packageName = null;int pos = name.lastIndexOf('.');if (pos != -1)packageName = name.substring(0, pos);Package pkg = null;if (packageName != null) {pkg = getPackage(packageName);// Define the package (if null)if (pkg == null) {// 定义package的操作，此处省略，具体参看源码pkg = getPackage(packageName);}}if (securityManager != null) {// 安全检查操作，此处省略，具体参看源码}// 创建class对象并返回if (entry.loadedClass == null) {try {clazz = defineClass(name, entry.binaryContent, 0, entry.binaryContent.length, new CodeSource(entry.codeBase, entry.certificates));} catch (UnsupportedClassVersionError ucve) {throw new UnsupportedClassVersionError(ucve.getLocalizedMessage() + " " + sm.getString("webappClassLoader.wrongVersion", name));}entry.loadedClass = clazz;entry.binaryContent = null;entry.source = null;entry.codeBase = null;entry.manifest = null;entry.certificates = null;} else {clazz = entry.loadedClass;}}return clazz;}?
protected ResourceEntry findResourceInternal(String name, String path) {// 先根据类名从缓存中查找对应资源 ，有则直接返回ResourceEntry entry = (ResourceEntry) resourceEntries.get(name);if (entry != null)return entry;int contentLength = -1;// 资源二进制数据长度InputStream binaryStream = null;// 资源二进制输入流int jarFilesLength = jarFiles.length;// classpath中的jar包个数int repositoriesLength = repositories.length;// 仓库数(classpath每一段称为repository仓库)int i;Resource resource = null;// 加载的资源实体boolean fileNeedConvert = false;// 对每个仓库迭代，直接找到相应的entry，如果查找的资源是一个独立的文件，在这个代码块可以查找到相应资源，// 如果是包含在jar包中的类，这段代码并不能找出其对应的资源for (i = 0; (entry == null) && (i < repositoriesLength); i++) {try {String fullPath = repositories[i] + path;// 仓库路径 加资源路径得到全路径Object lookupResult = resources.lookup(fullPath);// 从资源库中查找资源if (lookupResult instanceof Resource) {resource = (Resource) lookupResult;}// 到这里没有抛出异常，说明资源已经找到，现在构造entry对象if (securityManager != null) {PrivilegedAction dp = new PrivilegedFindResource(files[i], path);entry = (ResourceEntry) AccessController.doPrivileged(dp);} else {entry = findResourceInternal(files[i], path);// 这个方式只是构造entry并给其codebase和source赋值}// 获取资源长度和最后修改时间ResourceAttributes attributes = (ResourceAttributes) resources.getAttributes(fullPath);contentLength = (int) attributes.getContentLength();entry.lastModified = attributes.getLastModified();// 资源找到，将二进制输入流赋给binaryStreamif (resource != null) {try {binaryStream = resource.streamContent();} catch (IOException e) {return null;}// 将资源的最后修改时间加到列表中去，代码略去，参加源码}} catch (NamingException e) {}}if ((entry == null) && (notFoundResources.containsKey(name)))return null;// 开始从jar包中查找JarEntry jarEntry = null;synchronized (jarFiles) {if (!openJARs()) {return null;}for (i = 0; (entry == null) && (i < jarFilesLength); i++) {jarEntry = jarFiles[i].getJarEntry(path);// 根据路径从jar包中查找资源// 如果jar包中找到资源，则定义entry并将二进制流等数据赋给entry，同时将jar包解压到workdir.if (jarEntry != null) {entry = new ResourceEntry();try {entry.codeBase = getURL(jarRealFiles[i], false);String jarFakeUrl = getURI(jarRealFiles[i]).toString();jarFakeUrl = "jar:" + jarFakeUrl + "!/" + path;entry.source = new URL(jarFakeUrl);entry.lastModified = jarRealFiles[i].lastModified();} catch (MalformedURLException e) {return null;}contentLength = (int) jarEntry.getSize();try {entry.manifest = jarFiles[i].getManifest();binaryStream = jarFiles[i].getInputStream(jarEntry);} catch (IOException e) {return null;}if (antiJARLocking && !(path.endsWith(".class"))) {// 解压jar包代码，参见源码}}if (entry == null) {synchronized (notFoundResources) {notFoundResources.put(name, name);}return null;}// 从二进制流将资源内容读出if (binaryStream != null) {byte[] binaryContent = new byte[contentLength];// 读二进制流的代码，这里省去，参见源码entry.binaryContent = binaryContent;}}// 将资源加到缓存中synchronized (resourceEntries) {ResourceEntry entry2 = (ResourceEntry) resourceEntries.get(name);if (entry2 == null) {resourceEntries.put(name, entry);} else {entry = entry2;}}return entry;}}?