JAVA類的靜態載入和動態載入以及NoClassDefFoundError和ClassNotFoundException

我們都知道JAVA初始化一個類的時候可以用new 操作符來初始化,也可通過Class.forName的方式來得到一個Class類型的執行個體,然後通過這個Class類型的執行個體的newInstance來初始化.我們把前者叫做JAVA的靜態載入,把後者叫做動態載入.後者在很多架構中經常使用,通過用property檔案的方式指定類名稱.如我們熟悉的在寫JDBC代碼的時候載入不同資料庫的驅動.

靜態載入的時候如果在運行環境中找不到要初始化的類,拋出的是NoClassDefFoundError,它在JAVA的異常體系中是一個Error,錯誤.

動態態載入的時候如果在運行環境中找不到要初始化的類,拋出的是ClassNotFoundException,它在JAVA的異常體系中是一個checked異常,在寫代碼的時候就需要catch.

下面用執行個體來示範:ReferencedCls是需要初始化的類,它在一個單獨的jar中,它被StaticReferencingCls和DynamicReferencingCls分別以靜態和動態方式載入.

將prjT1做成一個jar包,t1.jar.將prjT2做成一個jar包,t2.jar.放在d:/temp檔案夾.

JAVA代碼如下:

package com.test1;public class ReferencedCls {private String str = "test value";public String getStr() {return str;}public void setStr(String str) {this.str = str;}public ReferencedCls() {System.out.println("Constructor: ReferencedCls");}}package com.test2;import com.test1.ReferencedCls;public class StaticReferencingCls {public static void main(String[] args) {System.out.println("Started testing static loading...............");ReferencedCls cls = new ReferencedCls();System.out.println(cls.getStr());}}package com.test2;import com.test1.ReferencedCls;public class DynamicReferencingCls {public static void main(String[] args) throws ClassNotFoundException, InstantiationException, IllegalAccessException {System.out.println("Started testing dynamic loading...............");Class cls = Class.forName("com.test1.ReferencedCls");ReferencedCls obj = (ReferencedCls)cls.newInstance();System.out.println(obj.getStr());}}

以不同的方式運行:
1,在運行環境中能找到需要的類時,都正常運行

D:\>java -cp d:/temp/t1.jar;d:/temp/t2.jar com.test2.StaticReferencingClsStarted testing static loading...............Constructor: ReferencedClstest valueD:\>java -cp d:/temp/t1.jar;d:/temp/t2.jar com.test2.DynamicReferencingClsStarted testing dynamic loading...............Constructor: ReferencedClstest value

2,以靜態方式載入,運行環境缺失需要的類,拋出NoClassDefFoundError

D:\>java -cp d:/temp/t2.jar com.test2.StaticReferencingClsStarted testing static loading...............Exception in thread "main" java.lang.NoClassDefFoundError: com/test1/ReferencedCls        at com.test2.StaticReferencingCls.main(StaticReferencingCls.java:9)Caused by: java.lang.ClassNotFoundException: com.test1.ReferencedCls        at java.net.URLClassLoader$1.run(Unknown Source)        at java.security.AccessController.doPrivileged(Native Method)        at java.net.URLClassLoader.findClass(Unknown Source)        at java.lang.ClassLoader.loadClass(Unknown Source)        at sun.misc.Launcher$AppClassLoader.loadClass(Unknown Source)        at java.lang.ClassLoader.loadClass(Unknown Source)        at java.lang.ClassLoader.loadClassInternal(Unknown Source)        ... 1 more

3,以動態方式載入,運行環境缺失需要的類,拋出ClassNotFoundException,同時在寫代碼的時候也需要catch這個異常或者聲明這個異常.

D:\>java -cp d:/temp/t2.jar com.test2.DynamicReferencingClsStarted testing dynamic loading...............Exception in thread "main" java.lang.ClassNotFoundException: com.test1.ReferencedCls        at java.net.URLClassLoader$1.run(Unknown Source)        at java.security.AccessController.doPrivileged(Native Method)        at java.net.URLClassLoader.findClass(Unknown Source)        at java.lang.ClassLoader.loadClass(Unknown Source)        at sun.misc.Launcher$AppClassLoader.loadClass(Unknown Source)        at java.lang.ClassLoader.loadClass(Unknown Source)        at java.lang.ClassLoader.loadClassInternal(Unknown Source)        at java.lang.Class.forName0(Native Method)        at java.lang.Class.forName(Unknown Source)        at com.test2.DynamicReferencingCls.main(DynamicReferencingCls.java:11)

4,JVM本身使用的方式也是靜態載入,如用JAVA命令運行一個帶main方法的類,這個類在運行環境缺失.

D:\>java com.test2.XXXClsException in thread "main" java.lang.NoClassDefFoundError: com/test2/XXXClsCaused by: java.lang.ClassNotFoundException: com.test2.XXXCls        at java.net.URLClassLoader$1.run(Unknown Source)        at java.security.AccessController.doPrivileged(Native Method)        at java.net.URLClassLoader.findClass(Unknown Source)        at java.lang.ClassLoader.loadClass(Unknown Source)        at sun.misc.Launcher$AppClassLoader.loadClass(Unknown Source)        at java.lang.ClassLoader.loadClass(Unknown Source)        at java.lang.ClassLoader.loadClassInternal(Unknown Source)Could not find the main class: com.test2.XXXCls.  Program will exit.