1、概述
The Throwable class is the superclass of all errors and exceptions in the Java language.
Only objects that are instances of this class (or one of its subclasses) are thrown by the Java Virtual Machine or can be thrown by the Java throw statement.
Only this class or one of its subclasses can be the argument type in a catch clause.
Throwable类是Java语言中所有 Error 和 Exception 的超类。
只有Throwable 的实例 或者 Throwable 子类的实例 才能被JVM 抛出，或者通过code throw 抛出。
只有这个类或它的一个子类可以是catch子句中的参数类型。