ClassLoader的基础详解

java应用环境中不同的class分别由不同的ClassLoader负责加载。
一个jvm中默认的classloader有Bootstrap ClassLoader、Extension ClassLoader、App ClassLoader，分别各司其职：
Bootstrap ClassLoader   
负责加载java基础类，主要是 %JRE_HOME/lib/ 目录下的rt.jar、resources.jar、charsets.jar和class等

Extension ClassLoader    
负责加载java扩展类，主要是 %JRE_HOME/lib/ext 目录下的jar和class

App ClassLoader          
负责加载当前java应用的classpath中的所有类。

其中Bootstrap ClassLoader是JVM级别的，由C++撰写；
Extension ClassLoader、App ClassLoader都是java类，都继承自URLClassLoader超类。
Bootstrap ClassLoader由JVM启动，然后初始化sun.misc.Launcher ，sun.misc.Launcher初始化Extension ClassLoader、App ClassLoader。
在jdk1.6.0_10\jre\lib下面有Launcher。看jdk包自带的源码 src中有Launcher的文件夹，里面是些c文件！
下面是维基的原语

引用

The bootstrap class loader loads the core Java libraries[5] (<JAVA_HOME>/lib directory). This class loader, which is part of the core JVM, is written in native code.
（上面的注释[5]： These libraries are stored in Jar files called rt.jar, core.jar, server.jar, etc.）

The extensions(扩展) class loader loads the code in the extensions directories (<JAVA_HOME>/lib/ext or any other directory specified by the java.ext.dirs system property). It is implemented by the sun.misc.Launcher$ExtClassLoader class.
（如果要引用ext下面的类 会提示你这些类是限制（restriction）访问的....
java.ext.dirs=C:\Program Files\Java\jdk1.6.0_10\jre\lib\ext;C:\WINDOWS\Sun\Java\lib\ext)

The system class loader loads code found on java.class.path, which maps to the system CLASSPATH variable. This is implemented by the sun.misc.Launcher$AppClassLoader class.

sun.misc.Launcher$AppClassLoader class.和sun.misc.Launcher$ExtClassLoader class.都在jre/lib/rt.jar下的包内

这些都是 Lantcher的内部类

看Launcher的一段代码 反编译过来的

ExtClassLoader extclassloader;
        try
        {
            extclassloader = ExtClassLoader.getExtClassLoader();
        }
        catch(IOException ioexception)
        {
            throw new InternalError("Could not create extension class loader");
        }
        try
        {
            loader = AppClassLoader.getAppClassLoader(extclassloader);
        }
        catch(IOException ioexception1)
        {
            throw new InternalError("Could not create application class loader");
        }
        Thread.currentThread().setContextClassLoader(loader);
        String s = System.getProperty("java.security.manager");
        if(s != null)
        {
            SecurityManager securitymanager = null;
            if("".equals(s) || "default".equals(s))
                securitymanager = new SecurityManager();
            else
                try
                {
                    securitymanager = (SecurityManager)loader.loadClass(s).newInstance();
                }
                catch(IllegalAccessException illegalaccessexception) { }
                catch(InstantiationException instantiationexception) { }
                catch(ClassNotFoundException classnotfoundexception) { }
                catch(ClassCastException classcastexception) { }
            if(securitymanager != null)
                System.setSecurityManager(securitymanager);
            else
                throw new InternalError((new StringBuilder()).append("Could not create SecurityManager: ").append(s).toString());
        }