ClassNotFoundException和NoClassDefFoundError

Stella981
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定义:

对于术语的定义,最原汁原味的还是官方文档。

关于ClassNotFoundExceptionJavaDoc里这么描述:

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.

关于NoClassDefFoundErrorJavaDoc里这么描述:

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.


样例:

如果只看官方定义,不容易明白(我就是这样的),看例子:

public class A {
    public static void main(String[] args) {
        try {
            B b = new B(); //NoClassDefFoundError
        } catch(Throwable e) {
            e.printStackTrace();
        }
        System.out.println("----------------------------");
        try {
            Class b = Class.forName("B");//ClassNotFoundException
        } catch (Throwable e) {
            e.printStackTrace();
        }
        System.out.println("----------------------------");
        try {
            ClassLoader loader = A.class.getClassLoader();
            loader.loadClass("B");//ClassNotFoundException
        } catch(Throwable e) {
            e.printStackTrace();
        }
    }
}

class B {
    
}

这里定义了两个类A和B,其中在A中的main方法中使用了B。

如果使用Eclipse工具编写这个类,编写完按下保存后,Eclipse会自动编译这两个类生成A.class和B.class。

如果使用cmd命令行,则需要手动编译这两个类。

接下来,我们把编译好的B.class手动删掉,之后再执行A的main方法:

ClassNotFoundException和NoClassDefFoundError

可以看到,new B()这一句抛出了NoClassDefFoundError,而Class.forName("B")和ClassLoader.loadClass("B")这两句则抛出了ClassNotFoundException。

原因就是JavaDoc里提到的:当使用类名字符串去调用Class.forName等方法来加载一个类时,如果找不到这个类,则抛出ClassNotFoundException。如果JVM尝试去加载一个类(普通方法调用或者使用new关键字创建对象),而这个类又不存在时,则抛出NoClassDefFoundError。


区别:

一个是Error,一个是Exception。

通过类名,手动调用方法去加载不存在的类,抛出ClassNotFoundException;而如果是JVM调用方法加载不存在的类,抛出NoClassDefFoundError。


总结:

ClassNotFoundException :
Thrown when an application tries to load in a class through its name, but no definition for the class with the specified name could be found

NoClassDefFoundError :
Thrown if the Java Virtual Machine tries to load in the definition of a class and no definition of the class could be found.


参考:

http://stackoverflow.com/questions/1457863/what-causes-and-what-are-the-differences-between-noclassdeffounderror-and-classn

http://www.javaroots.com/2013/02/classnotfoundexception-vs.html

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