ClassNotFoundException

ClassNotFoundException一个异常，该异常为已检查异常（Checked Exception），可以在编译期检查到，需要用户手工处理。

在Java API中这样描述：

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.

大致的意思是，当应用调用Class.forName()、ClassLoader中的findSystemClass ，loadClass方法时，对应名称的类找不到时，会抛出此异常。

例如：

public class ClassNotFoundExceptionDemo {
    public static void main(String[] args) throws ClassNotFoundException {
        Class<?> c = Class.forName("com.mysql.jdbc.Driver");
    }
}

/*
Exception in thread "main" java.lang.ClassNotFoundException: com.mysql.jdbc.Driver
    at java.net.URLClassLoader.findClass(URLClassLoader.java:382)
    at java.lang.ClassLoader.loadClass(ClassLoader.java:424)
    at sun.misc.Launcher$AppClassLoader.loadClass(Launcher.java:349)
    at java.lang.ClassLoader.loadClass(ClassLoader.java:357)
    at java.lang.Class.forName0(Native Method)
    at java.lang.Class.forName(Class.java:264)
    at liheng.exception.ClassNotFoundExceptionDemo.main(ClassNotFoundExceptionDemo.java:5)
 */

当类路径中没有名为com.mysql.jdbc.Driver的类，而我们由通过方法反射加载这个类的时候，系统抛出ClassNotFoundException异常。

NoClassDefFoundError

NoClassDefFoundError是一个错误，无法在编译阶段检查到 。

在Java API中这样描述：

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.

大致意思未，执行过程中，JVM或ClassLoader尝试加载一个类的时候（调用类的方法或者创建类的对象），找不到某个在编译器存在且成功编译的类，此时会抛出NoClassDefFoundError错误。

例如：

public class NoClassDefFoundErrorDemo {
    public static void main(String[] args) {
        A a = new A();
        a.print();
    }
}

class A {
    public void print() {
        System.out.println("A print");
    }
}

将.java文件编译后会产生NoClassDefFoundErrorDemo.class和A.class两个类文件，当我们删除A.class文件，并且运行NoClassDefFoundErrorDemo类时：

/*
Exception in thread "main" java.lang.NoClassDefFoundError: liheng/exception/A
        at liheng.exception.NoClassDefFoundErrorDemo.main(NoClassDefFoundErrorDemo.java:5)
Caused by: java.lang.ClassNotFoundException: liheng.exception.A
        at java.net.URLClassLoader.findClass(URLClassLoader.java:382)
        at java.lang.ClassLoader.loadClass(ClassLoader.java:424)
        at sun.misc.Launcher$AppClassLoader.loadClass(Launcher.java:349)
        at java.lang.ClassLoader.loadClass(ClassLoader.java:357)
        ... 1 more
*/

会抛出NoClassDefFoundError错误，提示找不到liheng.exception.A类的定义。