Class中isAssignableFrom() 方法

1     protected WebApplicationContext createWebApplicationContext(ServletContext sc) {
2         Class<?> contextClass = determineContextClass(sc);
3         if (!ConfigurableWebApplicationContext.class.isAssignableFrom(contextClass)) {
4             throw new ApplicationContextException("Custom context class [" + contextClass.getName() +
5                     "] is not of type [" + ConfigurableWebApplicationContext.class.getName() + "]");
6         }
7         return (ConfigurableWebApplicationContext) BeanUtils.instantiateClass(contextClass);
8     }

Determines if the class or interface represented by this Class object is either the same as, or is a superclass or superinterface of, the class or interface represented by the specified Class parameter. 
It returns true if so; otherwise it returns false. 
If this Class object represents a primitive type, this method returns true if the specified Class parameter is exactly this Class object; otherwise it returns false. 

Specifically, this method tests whether the type represented by the specified Class parameter can be converted to the type represented by this Class object via an identity conversion or via a widening reference conversion. 
See The Java Language Specification, sections 5.1.1 and 5.1.4 , for details.

A.class.isAssignableFrom(B.class)