http://blog.csdn.net/irelandken/article/details/7048817

分析BootstrapClassLoader/ExtClassLoader/AppClassLoader的加載路徑及"父委托機制"
[java] view plaincopyprint?

import?java.net.URL;??

import?java.net.URLClassLoader;??

??

/*?

分析BootstrapClassLoader/ExtClassLoader/AppClassLoader的加載路徑?

*?

*/??

??

public?class?ClassPath_of_Bootstrap_Ext_AppClassLoader??

{??

????public?static?void?main(String[]?args)??

????{??

????????System.out.println("BootstrapClassLoader?的加載路徑:?");??

??????????

????????URL[]?urls?=?sun.misc.Launcher.getBootstrapClassPath().getURLs();??

????????for(URL?url?:?urls)??

????????????System.out.println(url);??

????????System.out.println("----------------------------");??

??????????????????

????????//取得擴展類加載器 ??

????????URLClassLoader?extClassLoader?=?(URLClassLoader)ClassLoader.getSystemClassLoader().getParent();??

??

????????System.out.println(extClassLoader);??

????????System.out.println("擴展類加載器?的加載路徑:?");??

??????????

????????urls?=?extClassLoader.getURLs();??

????????for(URL?url?:?urls)??

????????????System.out.println(url);??

??????????

????????System.out.println("----------------------------");??

??????????????????

??????????

????????//取得應用(系統)類加載器 ??

????????URLClassLoader?appClassLoader?=?(URLClassLoader)ClassLoader.getSystemClassLoader();??

??????????

????????System.out.println(appClassLoader);??

????????System.out.println("應用(系統)類加載器?的加載路徑:?");??

??????????

????????urls?=?appClassLoader.getURLs();??

????????for(URL?url?:?urls)??

????????????System.out.println(url);??

??????????????????

????????System.out.println("----------------------------");???

????}??

}??

import java.net.URL; import java.net.URLClassLoader;/* 分析BootstrapClassLoader/ExtClassLoader/AppClassLoader的加載路徑 * */public class ClassPath_of_Bootstrap_Ext_AppClassLoader {public static void main(String[] args){System.out.println("BootstrapClassLoader 的加載路徑: ");URL[] urls = sun.misc.Launcher.getBootstrapClassPath().getURLs();for(URL url : urls)System.out.println(url);System.out.println("----------------------------");//取得擴展類加載器URLClassLoader extClassLoader = (URLClassLoader)ClassLoader.getSystemClassLoader().getParent();System.out.println(extClassLoader);System.out.println("擴展類加載器 的加載路徑: ");urls = extClassLoader.getURLs();for(URL url : urls)System.out.println(url);System.out.println("----------------------------");//取得應用(系統)類加載器URLClassLoader appClassLoader = (URLClassLoader)ClassLoader.getSystemClassLoader();System.out.println(appClassLoader);System.out.println("應用(系統)類加載器 的加載路徑: ");urls = appClassLoader.getURLs();for(URL url : urls)System.out.println(url);System.out.println("----------------------------"); } }
運行時輸出如下:

BootstrapClassLoader 的加載路徑:?
file:/C:/jdk1.7.0_01/jre/lib/resources.jar
file:/C:/jdk1.7.0_01/jre/lib/rt.jar
file:/C:/jdk1.7.0_01/jre/lib/sunrsasign.jar
file:/C:/jdk1.7.0_01/jre/lib/jsse.jar
file:/C:/jdk1.7.0_01/jre/lib/jce.jar
file:/C:/jdk1.7.0_01/jre/lib/charsets.jar
file:/C:/jdk1.7.0_01/jre/classes
----------------------------
sun.misc.Launcher$ExtClassLoader@3e389405
擴展類加載器 的加載路徑:?
file:/C:/jdk1.7.0_01/jre/lib/ext/dnsns.jar
file:/C:/jdk1.7.0_01/jre/lib/ext/localedata.jar
file:/C:/jdk1.7.0_01/jre/lib/ext/sunec.jar
file:/C:/jdk1.7.0_01/jre/lib/ext/sunjce_provider.jar
file:/C:/jdk1.7.0_01/jre/lib/ext/sunmscapi.jar
file:/C:/jdk1.7.0_01/jre/lib/ext/zipfs.jar
----------------------------
sun.misc.Launcher$AppClassLoader@a200d0c
應用(系統)類加載器 的加載路徑:?
file:/E:/JAVA/JVM_Class_Reflect_Thread/ClassPath_of_Bootstrap_Ext_AppClassLoader/bin/
----------------------------

現在我們就知道了這三個類加載器的加載路徑了:

• BootstrapClassLoader ? ? ? ? ? ? ? ? ?---加載jre/目錄下的核心庫

• ExtClassLoader擴展類加載器 ? ? ? ? ? ? ?---加載/jre/lib/ext/目錄下的擴展包

• AppClassLoader應用(系統)類加載器 ? ? ? ?---加載CLASSPATH路徑下的包

JDK源代碼中,protected Class<?> loadClass(String name, boolean resolve)?方法?

已經很好地體現出"父親委托機制":

? ??

1:先調用parent.loadClass(name, false)

? ??

2:如果父親加載失敗,則調用?loadClass(name, false)?方法 查找"當前類加載器"對應的"目錄"來加載

? ? ?

public Class<?>loadClass(String?name,?boolean resolve)

此方法的默認實現將按以下順序搜索類：

? ??

1.調用?findLoadedClass(name)?來檢查是否已經加載類。

? ?

2.在父類加載器上調用 loadClass方法。如果父類加載器為 null，則使用虛擬機的內置類加載器。

? ?

3.調用?findClass(name)?方法查找類。

/-----------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------
PS: 如果用戶想從自定義的目錄下加載類(.class文件),則可以自定義類加載器:

?

總結

以上是生活随笔為你收集整理的分析BootstrapClassLoader/ExtClassLoader/AppClassLoader的加载路径 及父委托机制的全部內容，希望文章能夠幫你解決所遇到的問題。

如果覺得生活随笔網站內容還不錯，歡迎將生活随笔推薦給好友。