This document discusses Java classloaders and classloading in Java. It covers the core classloader hierarchy in Java including the bootstrap, extension, and system classloaders. It also discusses how tools like jps and jinfo can be used to view classloading information. Additional topics covered include classloading in OSGi, Maven, Spring, dynamic classloading using reflection, and causes of ClassNotFoundExceptions. The presentation provides examples and demos of different classloading scenarios.

1. Java ClassLoader
java.lang.ClassNotFoundException
Xuefeng.Wu

2. 明明就在这里啊！

3. ClassNotFoundException

4. When
 Class.forName()
 findSystemClass()
 loadClass()
 NoClassDefFoundError: compile time
Run time

5. Agenda
 JVM class and execute
 Core ClassLoader and MyClassLoader
 Tools: jps, jinfo
 Eclipse-OSGi
 Maven
 Spring
 Dynamic Classes and Method (Reflect, Java 8)
 Other: MethodNotDef

6. Agenda
 JVM class and execute
 Core ClassLoader and MyClassLoader
 Tools: jps, jinfo
 Eclipse-OSGi
 Maven
 Spring
 Dynamic Classes and Method (Reflect, Java 8)
 Other: MethodNotDef

7. Key JVM Components

8. Agenda
 JVM class and execute
 Core ClassLoader and MyClassLoader
 Tools: jps, jinfo
 Eclipse-OSGi
 Maven
 Spring
 Dynamic Classes and Method (Reflect, Java 8)
 Other: MethodNotDef

9. Default class loader
sun.misc.Launcher$ExtClassLoader
sun.misc.Launcher$AppClassLoader
CLASSPATH environment variable,
-classpath or -cp command line option,
Class-Path attribute of Manifest file inside JAR

10. Tools: jps, jinfo

11. ClassLoader works

12. MyClassLoader
DEMO:MyClassLoader

13. Three principles
 Delegation principles
 Visibility Principle
 Uniqueness Principle
Child ClassLoader can see class loaded by Parent ClassLoad
but vice-versa is not true.
According to this principle a class loaded by Parent
should not be loaded by Child ClassLoader again.
DEMO:demo. ExplicitlyLoadClassByExtension

14. Is it easy?

15. Agenda
 JVM class and execute
 Core ClassLoader and MyClassLoader
 Tools: jps, jinfo
 Eclipse-OSGi
 Maven
 Spring
 Dynamic Classes and Method (Reflect, Java 8)
 Other: MethodNotDef

16. Eclipse-OSGi

17. OSGi Class Loader

18. Demo
 Run Plugin_ListAll_Normal
 Jinfo

19. Demo
 Debug Plugin_ListAll_Normal

20. Demo
 Debug HelloWorld in remote project

21. Issue & fix
 Try to find csdm/client/StartUp.class is exist.

22. Agenda
 JVM class and execute
 Core ClassLoader and MyClassLoader
 Tools: jps, jinfo
 Eclipse-OSGi
 Maven
 Spring
 Dynamic Classes and Method (Reflect, Java 8)
 Other: MethodNotDef

23. Maven
 System Classloader: Classworlds classloading
framework

 Core Classloader:
 Plugin Classloaders
 Custom Classloaders
boot the classloader graph: ${maven.home}/boot
down the graph contains the core requirements of
Maven:
${maven.home}/lib
each plugin has its own classloader
that is a child of Maven's core classloader
plugins/plugin

24. Demo: jetty plugin
 mvn jetty:run @ remote

25. a limit on how long you can make your
command line
 Isolaed Classloader: your classpath
isn't really correct. try to escape the confines of an
isolated classloader.
 Manifest-Only JAR:your app may be confused if it
finds only our booter.jar there!

26. Demo: surefire plugin
 mvn -Dtest=* test

27. Agenda
 JVM class and execute
 Core ClassLoader and MyClassLoader
 Tools: jps, jinfo
 Eclipse-OSGi
 Maven
 Spring
 Dynamic Classes and Method (Reflect, Java 8)
 Other: MethodNotDef

28. Spring
String bean = readerBeanClass()
Class clz = classLoader.loadClass(bean)
clz .newInstance()

29. Spring ResourceLoader
 ClassPathXmlApplicationContext
 DefaultResourceLoader.
 setClassLoader(ClassLoader classLoader)
XmlBeanDefinitionReader reader = new XmlBeanDefinitionReader();
reader.setBeanClassLoader([Class LoaderB here!]);
DefaultListableBeanFactory factory = new DefaultListableBeanFactory(reader);
reader.loadBeanDefinitions([xml resource here!]);
Default: thread context class loader: Thread.currentThread().getContextClassLoader()

30. Issue & fix

31. Agenda
 JVM class and execute
 Core ClassLoader and MyClassLoader
 Tools: jps, jinfo
 Eclipse-OSGi
 Maven
 Spring
 Dynamic Classes and Method (Reflect, Java 8)
 Other: MethodNotDef

32. Agenda
 JVM class and execute
 Core ClassLoader and MyClassLoader
 Tools: jps, jinfo
 Eclipse-OSGi
 Maven
 Spring
 Dynamic Classes and Method (Reflect, Java 8)
 Other: MethodNotDef

33. Other: MethodNotDef
 Jar version

34. Class.forName(“...HelloWorld")
public class com.carestreamhealth.pas.RemoteWS.utils.HelloWorld {
public com.carestreamhealth.pas.RemoteWS.utils.HelloWorld();
Code:
0: aload_0
1: invokespecial #8 // Method java/lang/Object."<init>":()V
4: return
public static void main(java.lang.String[]) throws java.lang.ClassNotFoundException;
Code:
0: ldc #19 // String
com.carestreamhealth.pas.RemoteWS.utils.HelloWorld
2: invokestatic #21 // Method
java/lang/Class.forName:(Ljava/lang/String;)Ljava/lang/Class;
5: pop
6: return
}

35. getClassLoader().loadClass(“..HelloWorld")
public class com.carestreamhealth.pas.RemoteWS.utils.HelloWorld {
public com.carestreamhealth.pas.RemoteWS.utils.HelloWorld();
Code:
0: aload_0
1: invokespecial #8 // Method java/lang/Object."<init>":()V
4: return
public static void main(java.lang.String[]) throws java.lang.ClassNotFoundExce
ption, java.lang.InstantiationException, java.lang.IllegalAccessException;
Code:
0: ldc #1 // class
com/carestreamhealth/pas/RemoteWS/utils/HelloWorld
2: invokevirtual #23 // Method
java/lang/Class.getClassLoader:()Ljava/lang/ClassLoader;
5: ldc #29 // String
com.carestreamhealth.pas.RemoteWS.utils.HelloWorld
7: invokevirtual #31 // Method
java/lang/ClassLoader.loadClass:(Ljava/lang/String;)Ljava/lang/Class;
10: pop
11: return
}

36. new HelloWorld
public class com.carestreamhealth.pas.RemoteWS.utils.HelloWorld {
public com.carestreamhealth.pas.RemoteWS.utils.HelloWorld();
Code:
0: aload_0
1: invokespecial #8 // Method java/lang/Object."<init>":
()V
4: return
public static void main(java.lang.String[]);
Code:
0: new #1 // class com/carestreamhealth/pas/Re
moteWS/utils/HelloWorld
3: invokespecial #16 // Method "<init>":()V
6: return
}

37. Call Site-VM Operation

38. JVM Spec: 5.3 Creation and Loading
http://docs.oracle.com/javase/specs/jvms/se7/html/jvms-5.html

39. Dynamic Classes and Method (Reflect, Java
8)
 invokedynamic
http://www.slideshare.net/CharlesNutter/jax-2012-invoke-dynamic-keynote

40. Java bytecode:
Understanding bytecode makes you a
better programmer
Example:
http://www.ibm.com/developerworks/ibm/library/it-haggar_bytecode/
Java Class:
http://en.wikipedia.org/wiki/Java_class_file
Java bytecodes:
http://en.wikipedia.org/wiki/Java_bytecode_instruction_listings