细说Tomcat如何打破双亲委派(有源码和图)
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一句话概况: 不考虑缓存, 那么装载路径就是,先Classpath, 然后War包,当前工程, 最后才是Tomcat相关目录. 不考虑缓存,是因为缓存只是用来加快速度,对外不调用展现出的逻辑效果是可以忽略的.
下图是一个概况的流程图, 简介起见,绘图过程中移除了缓存功能. (个了感觉还是有些逻辑还是有些复杂的,图画出来了,自己看着都觉得还是不清楚,但是也没发现可以改进的地方)
下面是关键的代码,已经添加了注释:
@Override
public synchronized Class<?> loadClass(String name, boolean resolve)
throws ClassNotFoundException {
if (log.isDebugEnabled())
log.debug("loadClass(" + name + ", " + resolve + ")");
Class<?> clazz = null;
//检查当前ClassLoad是否已经停止了
// Log access to stopped classloader
if (!started) {
try {
throw new IllegalStateException();
} catch (IllegalStateException e) {
log.info(sm.getString("webappClassLoader.stopped", name), e);
}
}
//检查缓存1,Class是否已经被当前Class"实例"装载过
// (0) Check our previously loaded local class cache
clazz = findLoadedClass0(name);
if (clazz != null) {
if (log.isDebugEnabled())
log.debug(" Returning class from cache");
if (resolve)
resolveClass(clazz);
return (clazz);
}
//ClassLoader检查缓存2,Class是否已经被父类"实例"装载过
// (0.1) Check our previously loaded class cache
clazz = findLoadedClass(name);
if (clazz != null) {
if (log.isDebugEnabled())
log.debug(" Returning class from cache");
if (resolve)
resolveClass(clazz);
return (clazz);
}
//ClassLoader装载系统classpath下面的类, 阻止webapp覆盖了J2SE的类
// (0.2) Try loading the class with the system class loader, to prevent
// the webapp from overriding J2SE classes
try {
clazz = system.loadClass(name);
if (clazz != null) {
if (resolve)
resolveClass(clazz);
return (clazz);
}
} catch (ClassNotFoundException e) {
// Ignore
}
//安全检查
// (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 = "Security Violation, attempt to use " +
"Restricted Class: " + name;
log.info(error, se);
throw new ClassNotFoundException(error, se);
}
}
}
//默认的返回值为false,
//delegate = false
//filter(name) 通过一个List指定哪些package里面的Class使用双亲委派模式,但是默认这个List是空
//比如包名以Java开头的,就需要使用双亲委派模式,不过这一点"部分"由 (0.2) 在上面已经实现了
boolean delegateLoad = delegate || filter(name);
//针对一些明确指定的package,使用双亲委派模式加载
// (1) Delegate to our parent if requested
if (delegateLoad) {
if (log.isDebugEnabled())
log.debug(" Delegating to parent classloader1 " + parent);
ClassLoader loader = parent;
if (loader == null)
loader = system;
try {
clazz = Class.forName(name, false, loader);
if (clazz != null) {
if (log.isDebugEnabled())
log.debug(" Loading class from parent");
if (resolve)
resolveClass(clazz);
return (clazz);
}
} catch (ClassNotFoundException e) {
// Ignore
//如果(1)通过双亲委派加载发生异常, 忽略异常,
}
}
//尝试从本地加载Class( 如果(1)通过双亲委派加载发生异常, 忽略异常,也会走到这一步)
// (2) Search local repositories
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) {
// Ignore
}
//最后依旧没有成功, 忽略所有配置,再次尝试一下双亲加载模式加载
// (3) Delegate to parent unconditionally
if (!delegateLoad) {
if (log.isDebugEnabled())
log.debug(" Delegating to parent classloader at end: " + parent);
ClassLoader loader = parent;
if (loader == null)
loader = system;
try {
clazz = Class.forName(name, false, loader);
if (clazz != null) {
if (log.isDebugEnabled())
log.debug(" Loading class from parent");
if (resolve)
resolveClass(clazz);
return (clazz);
}
} catch (ClassNotFoundException e) {
// Ignore
}
}
//最后如果依旧没有成功,则抛出异常
throw new ClassNotFoundException(name);
}
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