ClassNotFoundException

ClassNotFoundException一個異常，該異常為已檢查異常（Checked Exception），可以在編譯期檢查到，需要用戶手工處理。

在Java API中這樣描述：

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，當應用調(diào)用Class.forName()、ClassLoader中的findSystemClass ，loadClass方法時，對應名稱的類找不到時，會拋出此異常。

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)
 */

當類路徑中沒有名為com.mysql.jdbc.Driver的類，而我們由通過方法反射加載這個類的時候，系統(tǒng)拋出ClassNotFoundException異常。

NoClassDefFoundError

NoClassDefFoundError是一個錯誤，無法在編譯階段檢查到 。

在Java API中這樣描述：

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，執(zhí)行過程中，JVM或ClassLoader嘗試加載一個類的時候（調(diào)用類的方法或者創(chuàng)建類的對象），找不到某個在編譯器存在且成功編譯的類，此時會拋出NoClassDefFoundError錯誤。

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");
    }
}

將.java文件編譯后會產(chǎn)生NoClassDefFoundErrorDemo.class和A.class兩個類文件，當我們刪除A.class文件，并且運行NoClassDefFoundErrorDemo類時：

/*
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
*/

會拋出NoClassDefFoundError錯誤，提示找不到liheng.exception.A類的定義。