Classloader委托加载模型的反向问题
Classloader委托加载模型的反向问题 英文就是 back door around the classloading delegation scheme
当A类引用到B类，那么加载B类的classloader就是当时A的classloader，下面一个例子
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public class A {public static void main(String[] args) { System.out.println("class A loaded by " + A.class.getClassLoader()); //在调用下面的语句是，B的class会被加载,而加载器则是加载A的classloader B b = new B(); b.methodB();}}?public class B {public void methodB() { System.out.println("class B loaded by " + B.class.getClassLoader());}}??? 上面打印输出如下：
? ? class A loaded by sun.misc.Launcher$AppClassLoader@192d342
??? class B loaded by sun.misc.Launcher$AppClassLoader@192d342
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??? 可是假使我们有3个类，A,B,C,? A和C在classpath下面，B不在classpath下面，而在C:\jdk1.6.0_29\jre\lib\ext?
??? \classes下面，上代码
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public class A {public static void main(String[] args) {System.out.println("class A loaded by " + A.class.getClassLoader());B b = new B();b.methodB();}}?
public class B {public void methodB() {System.out.println("class B loaded by " + B.class.getClassLoader());C c = new C();c.methodc();}}?public class C {public void methodc() {System.out.println("class C loaded by " + C.class.getClassLoader());}}??? 将A和C放入C盘根目录，B放入C:\jdk1.6.0_29\jre\lib\ext\classes
进入cmd，执行结果如下
???? C:\>java A
class A loaded by sun.misc.Launcher$AppClassLoader@192d342
class B loaded by sun.misc.Launcher$ExtClassLoader@6b97fd
Exception in thread "main" java.lang.NoClassDefFoundError: C
??????? at B.methodB(B.java:8)
??????? at A.main(A.java:6)
Caused by: java.lang.ClassNotFoundException: C
??????? at java.net.URLClassLoader$1.run(URLClassLoader.java:202)
??????? at java.security.AccessController.doPrivileged(Native Method)
??????? at java.net.URLClassLoader.findClass(URLClassLoader.java:190)
??????? at sun.misc.Launcher$ExtClassLoader.findClass(Launcher.java:229)
??????? at java.lang.ClassLoader.loadClass(ClassLoader.java:306)
??????? at java.lang.ClassLoader.loadClass(ClassLoader.java:247)
??????? ... 2 more
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之所以出现A和B都加载了，而C加载不了，是因为委托模型不能反向，因为加载B的classloader是ExtClassLoader，所以接下来的C也由此classloader加载,按照委托模型，ExtClassLoader会先交由BootStrapClassLoader搜其管辖目录搜索然后由本身在在C:\jdk1.6.0_29\jre\lib\ext\下搜索，即便如果都搜索不到C，也不会交由AppClassloader加载,因为委托模型只能由当前classloader向上不能向下(而这里的C只能由Appclassloader加载)
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如何能让B加载到C呢？只能变换classloader
修改后的B代码如下
public class B {public void methodB() {System.out.println("class B loaded by " + B.class.getClassLoader());ClassLoader cl = Thread.currentThread().getContextClassLoader();System.out.println("the thread context classloader was " + cl);try {Class clazzc = Class.forName("C", true, cl);//强制cl去加载C类Method m = clazzc.getMethod("methodC", null);Object obj = clazzc.newInstance();m.invoke(obj, null);} catch (Exception e) {e.printStackTrace();}}}?? Every thread has a classloader associated with it. If the thread is a main thread then the class??
?? loader associated with it a System Class Loader. 具体Thread.currentThread(). getContextClassLoader
?? 方法参考文档，在这里他会返回AppClassloader
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同上方法运行A类在cmd，得到
C:\>java A
class A loaded by sun.misc.Launcher$AppClassLoader@192d342
class B loaded by sun.misc.Launcher$ExtClassLoader@6b97fd
the thread context classloader was sun.misc.Launcher$AppClassLoader@192d342
class C loaded by sun.misc.Launcher$AppClassLoader@192d342
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这种需求哪里会用到？
比如java.sql.Drivermanager是rt.jar中的类，而用户classpath下面的具体driver则是classpath下面的类，则需要用到此方法，具体参见DriverManager.getConnection(x,x,x)方法
也就是说核心类库中调用用户代码之前都需要变更classloader
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下面文章可以参考：
http://stackoverflow.com/questions/13307885/could-you-please-explain-a-code-snippet-in-drivermanager-class-related-to-classl/13308729#13308729
http://stackoverflow.com/questions/1771679/difference-between-threads-context-class-loader-and-normal-classloader