我目前的系统可能需要自己实现类加载器，想要参考Tomcat的实现。关于Tomcat的类加载机制，网上文章很多，当然大多都是互相copy，有价值的信息并不多，不得已我开始看Tomcat代码，略有所得，记录起来。主要针对WebappClassLoader。
负责Web应用的类加载的是org.apache.catalina.loader.WebappClassLoader，它几个比较重要的方法：findClass(),loadClass(),findClassInternal(),findResourceInternal().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）：
findClass:
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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);
}
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loadClass:
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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);
}
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findClassInternal:
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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 package
String 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;
}
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findResouceInternal():
下几篇介绍WebappLoader,StandardContext,StandardWrapper,ApplicationDispatcher在类加载中的作用。其中ApplicationDispatcher是核心。
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//要先加载相关实体资源(.jar) 再加载查找的资源本身
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binaryStream
if (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;
}
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