一、Error和Exception什么关系

Error和Exception都继承自Throwable。继承关系如下图所示：

其实Error和Exception就是java语言设计Throwable时的两个大的分类：Error一般是比较严重的问题，不应该被catch，比如出现了OutOfMemoryError，StackOverFlowError这些错误，程序员是无能为力的；而Exception是程序员可以干预的，可以被catch。看下java源码的注释。

1. Error

/**

* An {@code Error} is a subclass of {@code Throwable}

* that indicates serious problems that a reasonable application

* should not try to catch. Most such errors are abnormal conditions.

* The {@code ThreadDeath} error, though a "normal" condition,

* is also a subclass of {@code Error} because most applications

* should not try to catch it.

*

* A method is not required to declare in its {@code throws}

* clause any subclasses of {@code Error} that might be thrown

* during the execution of the method but not caught, since these

* errors are abnormal conditions that should never occur.

*

* That is, {@code Error} and its subclasses are regarded as unchecked

* exceptions for the purposes of compile-time checking of exceptions.

*

* @author Frank Yellin

* @see java.lang.ThreadDeath

* @jls 11.2 Compile-Time Checking of Exceptions

* @since 1.0

*/

public class Error extends Throwable

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Error就是一个比较严重的问题，不应该被catch；它是一个unchecked exception，也就是说，不需要在方法上声明throws，它不是一个编译器可以发现的checked exception。

2. Exception

/**

* The class {@code Exception} and its subclasses are a form of

* {@code Throwable} that indicates conditions that a reasonable

* application might want to catch.

*

*

The class {@code Exception} and any subclasses that are not also

* subclasses of {@link RuntimeException} are checked

* exceptions. Checked exceptions need to be declared in a

* method or constructor's {@code throws} clause if they can be thrown

* by the execution of the method or constructor and propagate outside

* the method or constructor boundary.

*

* @author Frank Yellin

* @see java.lang.Error

* @jls 11.2 Compile-Time Checking of Exceptions

* @since 1.0

*/

public class Exception extends Throwable

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Exception及其子类是可以被catch处理的。Exception及其子类(不包括RuntimeException及其子类)都是checked exception，可以被编译器发现。也就是说，如果不显示处理这些checked exception，编译无法通过。怎么处理呢？可以在方法或者构造方法上声明throws，或者增加try catch语句块。

写到这里，除了checked exception，其他都是unchecked exception。所谓checked是针对编译器而言的，只有编译器会“检查”；运行时异常和Error显然是编译器无法“检查”到的。

二、NoClassDefFoundError 和 ClassNotFoundException

这两个有什么区别。从字面上就可以看到，一个是Error，一个是Exception。看下java定义：

1. ClassNotFoundException

/**

* Thrown when an application tries to load in a class through its

* string name using:

*

*

The forName method in class Class.

*

The findSystemClass method in class

* ClassLoader .

*

The loadClass method in class ClassLoader.

*

*

* but no definition for the class with the specified name could be found.

*

*

* @author unascribed

* @see java.lang.Class#forName(java.lang.String)

* @see java.lang.ClassLoader#findSystemClass(java.lang.String)

* @see java.lang.ClassLoader#loadClass(java.lang.String, boolean)

* @since 1.0

*/

public class ClassNotFoundException extends ReflectiveOperationException

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可以看到，它是个反射操作异常，并不是一个RuntimeException，也就是它是一个checked exception，必须显示的throws或者try catch，否则无法编译通过。

2. NoClassDefFoundError

/**

* Thrown if the Java Virtual Machine or a ClassLoader instance

* tries to load in the definition of a class (as part of a normal method call

* or as part of creating a new instance using the new expression)

* and no definition of the class could be found.

*

* The searched-for class definition existed when the currently

* executing class was compiled, but the definition can no longer be

* found.

*

* @author unascribed

* @since 1.0

*/

public

class NoClassDefFoundError extends LinkageError

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NoClassDefFoundError是当new这个类的实例的时候，在JVM中找不到这个类的定义。也就是，这个类在编译的时候还是在的，都已经装载到JVM中了，但是在new实例的时候却不存在了，真是匪夷所思。

三、参考链接

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