1. jvm default class loading mechanism
When loading a class, it will first entrust the parent loader to find the target class. If it cannot find it, it will entrust the upper-level parent loader to load it. If all parent loaders cannot find the target class in their own loading class path, they will find the target class in their own class path. Find and load the target class in the loading path. This is the parental delegation mechanism.
2. tomcat breaks parent delegation
ClassLoader of tomcat
CommonClassLoader: Tomcat common class loader, the loaded resources can be obtained by Tomcat and all web applications.
CatalinaClassLoader: Tomcat class loader, the loaded resources can only be obtained by Tomcat (but all WebappClassLoader cannot obtain the classes loaded by catalinaLoader)
SharedClassLoader: The parent loader of each Tomcat Context. This class is the parent class of all WebappClassLoaders. The classes loaded by sharedLoader will be shared by all WebappClassLoaders.
WebappClassLoader: Each Context corresponds to a WebappClassloader, mainly used to load resources under WEB-INF/lib and WEB-INF/classes
Source code:
@Override
public Class<?> loadClass(String name, boolean resolve) throws ClassNotFoundException {
synchronized (JreCompat.isGraalAvailable() ? this : getClassLoadingLock(name)) {
if (log.isDebugEnabled()) {
log.debug("loadClass(" + name + ", " + resolve + ")");
}
Class<?> clazz = null;
// Log access to stopped class loader
checkStateForClassLoading(name);
// (0) Check our previously loaded local class cache
//1. 先在本地 cache 查找该类是否已经加载过
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
//2. 从系统类加载器的 cache 中查找是否加载过
clazz = JreCompat.isGraalAvailable() ? null : 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 bootstrap class loader, to prevent
// the webapp from overriding Java SE classes. This implements
// SRV.10.7.2
String resourceName = binaryNameToPath(name, false);
ClassLoader javaseLoader = getJavaseClassLoader();
boolean tryLoadingFromJavaseLoader;
try {
// Use getResource as it won't trigger an expensive
// ClassNotFoundException if the resource is not available from
// the Java SE class loader. However (see
// https://bz.apache.org/bugzilla/show_bug.cgi?id=58125 for
// details) when running under a security manager in rare cases
// this call may trigger a ClassCircularityError.
// See https://bz.apache.org/bugzilla/show_bug.cgi?id=61424 for
// details of how this may trigger a StackOverflowError
// Given these reported errors, catch Throwable to ensure any
// other edge cases are also caught
URL url;
if (securityManager != null) {
PrivilegedAction<URL> dp = new PrivilegedJavaseGetResource(resourceName);
url = AccessController.doPrivileged(dp);
} else {
url = javaseLoader.getResource(resourceName);
}
tryLoadingFromJavaseLoader = (url != null);
} catch (Throwable t) {
// Swallow all exceptions apart from those that must be re-thrown
ExceptionUtils.handleThrowable(t);
// The getResource() trick won't work for this class. We have to
// try loading it directly and accept that we might get a
// ClassNotFoundException.
tryLoadingFromJavaseLoader = true;
}
if (tryLoadingFromJavaseLoader) {
try {
// 3. 尝试用 ExtClassLoader 类加载器类加载
// 目的:防止 Web 应用自己的类覆盖 JRE 的核心类
clazz = javaseLoader.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 = sm.getString("webappClassLoader.restrictedPackage", name);
log.info(error, se);
throw new ClassNotFoundException(error, se);
}
}
}
boolean delegateLoad = delegate || filter(name, true);
// (1) Delegate to our parent if requested
if (delegateLoad) {
if (log.isDebugEnabled()) {
log.debug(" Delegating to parent classloader1 " + parent);
}
try {
clazz = Class.forName(name, false, parent);
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 {
// 4. 尝试在本地目录搜索 class 并加载
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
//默认为非委托
if (!delegateLoad) {
if (log.isDebugEnabled()) {
log.debug(" Delegating to parent classloader at end: " + parent);
}
try {
// 5. 尝试用系统类加载器 (也就是 AppClassLoader) 来加载
clazz = Class.forName(name, false, parent);
if (clazz != null) {
if (log.isDebugEnabled()) {
log.debug(" Loading class from parent");
}
if (resolve) {
resolveClass(clazz);
}
return clazz;
}
} catch (ClassNotFoundException e) {
// Ignore
}
}
}
//6. 上述过程都加载失败，抛出异常
throw new ClassNotFoundException(name);
}Summarize:
1. findLoadedClass0(name); Finds the class with the given name if it has previously been loaded and cached by this class loader 2. findLoadedClass(name); Returns the class with the given binary name if this loader has been recorded by the Java virtual machine as an initiating loader of a class with that binary name. 3.javaseLoader.loadClass(name). --> Try to use ExtClassLoader class loader class loader 4.findClass(name) --> Try to search for class in the local directory and Load/Find the specified class in our local repositories 5. Class.forName(name, false, parent); -->Try to load with the system class loader (that is, AppClassLoader)