Java ClassLoader授权模型？

当在 ClassLoader上调用loadClass()时,ClassLoader是否首先检查该类是否被加载,或者立即将此检查委托给其父级ClassLoader？

Java API说：

When requested to find a class or resource,a ClassLoader instance will delegate the search for the class or resource to its parent class loader before attempting to find the class or resource itself.

但是在Java反思行动书中有一个关于类加载器的具体章节：

Class loader calls findLoadedClass to check if the class has been loaded already.If a class loader does not find a loaded class,calls loadClass on the parent class loader.

哪个是对的？

解决方法

正确的类加载器实现将：

>检查课程是否已经加载.
>通常要求父类加载器加载类
>尝试在自己的类路径中找到该类.

ClassLoader.loadClass的默认实现类似于：

protected synchronized Class<?> loadClass(String name,boolean resolve) {
  // First,check if this class loader has directly defined the class or if the
  // JVM has initiated the class load with this class loader.
  Class<?> result = findLoadedClass(name);
  if (result == null) {
    try {
      // Next,delegate to the parent.
      result = getParent().loadClass(name);
    } catch (ClassNotFoundException ex) {
      // Finally,search locally if the parent Could not find the class.
      result = findClass(ex);
    }
  }
  // As a remnant of J2SE 1.0.2,link the class if a subclass of the class
  // loader class requested it (the JVM never calls the method,// loadClass(String) passes false,and the protected access modifier prevents
  // callers from passing true).
  if (resolve) {
    resolveClass(result);
  }
  return result;
}

一些类加载器实现将委派给其他非父类装载器(例如,Osgi,根据包代表委派给类加载器的图),一些类加载器实现将在委派之前在本地类路径中查找类.

原文地址：https://www.jb51.cc/java/125369.html