TOMCAT---类加载器

coverl

　　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 + 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, 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.getJarEntry(path);
}
....
}