ClassNotFoundException和 NoClassDefFoundError

ClassNotFoundException和 NoClassDefFoundError

1. 概述

java.lang.NoClassDefFoundError 和 java.lang.ClassNotFoundException 都是 Java 语言层面的异常

NoClassDefFoundError ：Thrown if the Java Virtual Machine or a ClassLoader instance tries to load in the definition of a class and no definition of the class could be found. 在ClassLoader.loadClass()方法中抛出
ClassNotFoundException ： Thrown when an application tries to load in a class through its string name using:forName、findSystemClass、loadClas but no definition for the class with the specified name could be found.；在ClassLoader.defineClass()中抛出

本质性区别：
NoClassDefFoundError 是程序在编译时候可以顺利找到所需要依赖的类的，但是在运行时依赖的类找不到或者可以找到多个，就会抛出这个Error。ClassNotFoundException在类路径错误，或者类名称发生更改，都会导致这个Exception，这两个异常的出现在生产环境中，通常是由于依赖jar包多版本存在，jar包升级中类或者方法不再提供引起的

2.测试代码

package com.vip.vcp.ryan.zhaunzheng.ToLeExcep.ClassNotFoundAndNoClassDef;import com.vip.vcp.ryan.zhaunzheng.map.PHashMap426.PLinkedHashMap;import java.util.LinkedHashMap;import java.util.Scanner;/** * 模拟ClassNotFoundException和 NoClassDefFoundError两个异常 * Created by ryan01.peng on 2017/5/8. */public class TestClassAboutException {    public static void main(String[] args) {        System.out.println("---》》》TestClassAboutException编译已经通过啦！！！");        toShowClassNotFoundException();        toShowNoClassDefErr();        toShowNoClassDefErr2();    }    public static  void  toShowClassNotFoundException(){        try {            //"com.vip.vcp.ryan.zhaunzheng.ToLeExcep.ClassNotFoundAndNoClassDef.TestClassAboutException"            // 模拟jar包路劲改变或者，类名称改变引起的ClassNotFoundException            Class cl = Class.forName("vip.vcp.ryan.zhaunzheng.ToLeExcep.ClassNotFoundAndNoClassDef.TestClassAboutException");// TestClassAboutException.class.getName()            System.out.println("---》》》toShowClassNotFoundException ::: "+cl.getName());        } catch (ClassNotFoundException e) {            System.out.println("---》》》ClassNotFoundException 被捕捉到啦！！！");            e.printStackTrace();        }    }    public static  void toShowNoClassDefErr(){        Scanner scanner = new Scanner(System.in);        System.out.println("/n"+"是否进入NoClassDefErr复现环节： 输入1/0   ");        //阻塞，删去类文件        if (scanner.nextInt() == 1)        {            try {                //com.vip.vcp.ryan.zhaunzheng.map.PHashMap426.PLinkedHashMap                // 模拟在运行时，类文件丢失，则会产生NoClassDefFoundError                Class c = Class.forName(PLinkedHashMap.class.getName());                System.out.println("---》》》toShowNoClassDefErr :::" +c.getName());            } catch (ClassNotFoundException e) {                System.out.println("---》》》NoClassDefErr 被捕捉啦！！！");                e.printStackTrace();            }        }        else            return;    }    public static  void toShowNoClassDefErr2(){        Scanner scanner = new Scanner(System.in);        System.out.println("/n"+"是否进入NoClassDefErr复现环节： 输入1/0   ");        if (scanner.nextInt() == 1)        {            try {                B bS = new B();            } catch (Throwable e) {                e.printStackTrace();            }            B b = new B();        }    }}

初始化失败类B

import java.util.HashMap;import java.util.Map;/** * Created by ryan01.peng on 2017/5/8. */public class B {    static {           init();    }    public B() {    }    static void init(){        System.out.println("B initializing and then Exception occurred");        int re = 5/0;        System.out.println(re);    }}

测试输出

"C:\Program Filcom.vip.vcp.ryan.zhaunzheng.ToLeExcep.ClassNotFoundAndNoClassDef.TestClassAboutExceptionjava.lang.ClassNotFoundException: vip.vcp.ryan.zhaunzheng.ToLeExcep.ClassNotFoundAndNoClassDef.TestClassAboutException    at java.net.URLClassLoader$1.run(URLClassLoader.java:366)    at java.net.URLClassLoader$1.run(URLClassLoader.java:355)    at java.security.AccessController.doPrivileged(Native Method)    at java.net.URLClassLoader.findClass(URLClassLoader.java:354)    at java.lang.ClassLoader.loadClass(ClassLoader.java:425)    at sun.misc.Launcher$AppClassLoader.loadClass(Launcher.java:308)    at java.lang.ClassLoader.loadClass(ClassLoader.java:358)    at java.lang.Class.forName0(Native Method)    at java.lang.Class.forName(Class.java:191)    at com.vip.vcp.ryan.zhaunzheng.ToLeExcep.ClassNotFoundAndNoClassDef.TestClassAboutException.toShowClassNotFoundException(TestClassAboutException.java:23)    at com.vip.vcp.ryan.zhaunzheng.ToLeExcep.ClassNotFoundAndNoClassDef.TestClassAboutException.main(TestClassAboutException.java:15)    at sun.reflect.NativeMethodAccessorImpl.invoke0(Native Method)    at sun.reflect.NativeMethodAccessorImpl.invoke(NativeMethodAccessorImpl.java:57)    at sun.reflect.DelegatingMethodAccessorImpl.invoke(DelegatingMethodAccessorImpl.java:43)    at java.lang.reflect.Method.invoke(Method.java:606)    at com.intellij.rt.execution.application.AppMain.main(AppMain.java:144)---》》》TestClassAboutException编译已经通过啦！！！---》》》ClassNotFoundException 被捕捉到啦！！！null/n是否进入NoClassDefErr复现环节： 输入1/0   1Exception in thread "main" java.lang.NoClassDefFoundError: com/vip/vcp/ryan/zhaunzheng/map/PHashMap426/PLinkedHashMap    at com.vip.vcp.ryan.zhaunzheng.ToLeExcep.ClassNotFoundAndNoClassDef.TestClassAboutException.toShowNoClassDefErr(TestClassAboutException.java:41)    at com.vip.vcp.ryan.zhaunzheng.ToLeExcep.ClassNotFoundAndNoClassDef.TestClassAboutException.main(TestClassAboutException.java:16)    at sun.reflect.NativeMethodAccessorImpl.invoke0(Native Method)    at sun.reflect.NativeMethodAccessorImpl.invoke(NativeMethodAccessorImpl.java:57)    at sun.reflect.DelegatingMethodAccessorImpl.invoke(DelegatingMethodAccessorImpl.java:43)    at java.lang.reflect.Method.invoke(Method.java:606)    at com.intellij.rt.execution.application.AppMain.main(AppMain.java:144)Caused by: java.lang.ClassNotFoundException: com.vip.vcp.ryan.zhaunzheng.map.PHashMap426.PLinkedHashMap    at java.net.URLClassLoader$1.run(URLClassLoader.java:366)    at java.net.URLClassLoader$1.run(URLClassLoader.java:355)    at java.security.AccessController.doPrivileged(Native Method)    at java.net.URLClassLoader.findClass(URLClassLoader.java:354)    at java.lang.ClassLoader.loadClass(ClassLoader.java:425)    at sun.misc.Launcher$AppClassLoader.loadClass(Launcher.java:308)    at java.lang.ClassLoader.loadClass(ClassLoader.java:358)    ... 7 moreProcess finished with exit code 1

由于静态代码块初始化失败引起的NoClassDefFoundError

---》》》TestClassAboutException编译已经通过啦！！！/n是否进入NoClassDefErr复现环节： 输入1/0   1B initializing and then Exception occurredjava.lang.ExceptionInInitializerError    at com.vip.vcp.ryan.zhaunzheng.ToLeExcep.ClassNotFoundAndNoClassDef.TestClassAboutException.toShowNoClassDefErr2(TestClassAboutException.java:63)    at com.vip.vcp.ryan.zhaunzheng.ToLeExcep.ClassNotFoundAndNoClassDef.TestClassAboutException.main(TestClassAboutException.java:18)    at sun.reflect.NativeMethodAccessorImpl.invoke0(Native Method)    at sun.reflect.NativeMethodAccessorImpl.invoke(NativeMethodAccessorImpl.java:57)    at sun.reflect.DelegatingMethodAccessorImpl.invoke(DelegatingMethodAccessorImpl.java:43)    at java.lang.reflect.Method.invoke(Method.java:606)    at com.intellij.rt.execution.application.AppMain.main(AppMain.java:144)Caused by: java.lang.ArithmeticException: / by zero    at com.vip.vcp.ryan.zhaunzheng.ToLeExcep.ClassNotFoundAndNoClassDef.B.init(B.java:24)    at com.vip.vcp.ryan.zhaunzheng.ToLeExcep.ClassNotFoundAndNoClassDef.B.<clinit>(B.java:16)    ... 7 moreException in thread "main" java.lang.NoClassDefFoundError: Could not initialize class com.vip.vcp.ryan.zhaunzheng.ToLeExcep.ClassNotFoundAndNoClassDef.B    at com.vip.vcp.ryan.zhaunzheng.ToLeExcep.ClassNotFoundAndNoClassDef.TestClassAboutException.toShowNoClassDefErr2(TestClassAboutException.java:67)    at com.vip.vcp.ryan.zhaunzheng.ToLeExcep.ClassNotFoundAndNoClassDef.TestClassAboutException.main(TestClassAboutException.java:18)    at sun.reflect.NativeMethodAccessorImpl.invoke0(Native Method)    at sun.reflect.NativeMethodAccessorImpl.invoke(NativeMethodAccessorImpl.java:57)    at sun.reflect.DelegatingMethodAccessorImpl.invoke(DelegatingMethodAccessorImpl.java:43)    at java.lang.reflect.Method.invoke(Method.java:606)    at com.intellij.rt.execution.application.AppMain.main(AppMain.java:144)Process finished with exit code 1