Java Class Loader http://bd.linkedin.com/in/imranhasanhira https://www.linkedin.com/company/kona-software-lab-ltd- Regular java programs doesn't need to mess with class loader. But if someone want to make a framework then it may be helpful to get these classloader things clear in mind. Most java frameworks use their custom classloader to make coding more dynamic. So understanding the the concepts clearly will definitely boost up the confidence, as well as knowledge about Class Loader.

1. @KonaTechAdda - 6
What is Class Loader
ন িঃস্বার্থ ক্লাশ-ল াডার
Md Imran Hasan Hira
Software Engineer
Kona Software Lab Ltd.
http://bd.linkedin.com/in/imranhasanhira
https://www.linkedin.com/company/kona-software-lab-ltd-

2. Part 1 – Brunch
Part 2 – Buffet Dinner
Java Class Bytecodes JVM
ClassLoading Example Loading Phases
Delegation Model Class Loading exceptions

3. References
Title Reference
Demystifying class loading problems, Part 1: An
introduction to class loading and debugging tools
http://www.ibm.com/developerworks/library/j-dclp1/
The basics of Java class loaders http://www.javaworld.com/article/2077260/learn-java/learn-
java-the-basics-of-java-class-loaders.html?null
Java Classloaders Tutorial http://zeroturnaround.com/rebellabs/rebel-labs-tutorial-do-
you-really-get-classloaders/
Core Java Security: Class Loaders, Security
Managers, and Encryption
http://www.informit.com/articles/article.aspx?p=1187967
Java Class Loader - Java Tutorial http://javapapers.com/core-java/java-class-loader/
Inside Class Loaders - O'Reilly Media http://www.onjava.com/pub/a/onjava/2003/11/12/classloader.
html
Discovering Class Members (The Java™ Tutorials >
The Reflection API > Classes)
https://docs.oracle.com/javase/tutorial/reflect/class/classMem
bers.html

4. Index
• What is Java ? How it runs on PC ?
• Different Java Virtual Machines
• A close look at the bytecodes
• Introduction to class loader
• Building the information from bytecode
• Maintaining the class loading chain

5. Part-1
What is Java
- ন িঃস্বার্থ জাভা

6. What is Java

7. Java iLand
http://vimeo.com/46871479

8. What we know java is
Java
Java
Technology
Java
Programming
Language
Object
Oriented
Write once, Run
Everywhere

9. GreenTalk > Oak > Java
A detail history can be found in http://oracle.com.edgesuite.net/timeline/java/
GreenTalk
• James Gosling initiated a project as Green Team
• Firstly it was called GreenTalk and file extension
was .gt
Oak
• After that it was called Oak and developed as part
of the Green Project
• Oak is a symbol for strength
Java
• Oak renamed to Java for trademark issue with Oak
Technologies
• Java was chosen amongst Silk, Jolt, DNA etc.

10. How java works
Main.java
Main.cpp Operating
System
.class
Operating
System
Java Virtual
Machine
(JVM)
C/C++ source file
Native executable
.exe
OS executes the
machine code
Java source file Java intermediate
bytecodes
JVM interprets* the
bytecodes

11. Let’s write some java
Welcome.java

12. Compile it
javac Welcome.java
Welcome.java
Compile command
Compile output Welcome.class

13. The Bytecodes (simplified view)

14. Run it
Welcome.java
java WelcomeRun command

15. What does JVM do?
bytecodes
Amazing
Things
Code Interpretation
Native Instruction mapping Type Conversion
Memory Management
Garbage Collection
Disk/Network access
Security Management

16. There is a specification about What JVM
should do
http://docs.oracle.com/javase/specs/

17. Virtual Machines
• Hotspot JVM (OpenJDK)
• Hotspot JVM (Oracle JDK)
• J9 by IBM
• Apache Harmony
• Kaffe OpenVM
• NanoVM
…

18. An Important part is to load the classes into
memory

19. Part-2
Class Loader

20. Takes classname
(i.e. com.konasl.test.Welcome)
Find the bytecode data
associated with this classname
Use defineClass() to decode
raw bytecode into Class
Return the Class
Simple Class Loading Procedure

21. Takes classname
(i.e. com.konasl.test.Welcome)
Find the bytecode data
associated with this classname
Use defineClass() to decode
raw bytecode into Class
Return the Class
Find in self cache for this class
Put the Class in self cache
Simple Class Loading Procedure (cont.)

22. Takes classname
(i.e. com.konasl.test.Welcome)
Find the bytecode data
associated with this classname
Use defineClass() to decode
raw bytecode into Class
Use resolveClass() to resolve
the other referenced classes
Return the Class
Find in super classloaders for
this class
Find in self cache for this class
Put the Class in self cache
Simple Class Loading Procedure (cont.)

23. Class Loader chain
• Bootstrap Class Loader
• <JAVA_HOME>/jre/lib
• Part of the core JVM, written in native code
• Extension Class Loader
• <JAVA_HOME>/jre/lib/ext
• java.ext.dirs.
• Implemented by sun.misc.Launcher$ExtClassLoader
• System Class Loader
• <CLASSPATH>
• java.class.path
• Implemented by the sun.misc.Launcher$AppClassLoader

24. Class Loader Delegation Model
Bootstrap
Class Loader
Extension
Class Loader
System Class
Loader
User Defined
Class Loader
User Defined
Class Loader
User Defined
Class Loader

25. Class Loader Delegation Model
Bootstrap
Class Loader
Extension
Class Loader
System Class
Loader
User Defined
Class Loader
User Defined
Class Loader
User Defined
Class Loader
$JAVAHOME/jre/lib/rt.jar
Known as
Primordial
Class Loader

26. Class Loader Delegation Model
Bootstrap
Class Loader
Extension
Class Loader
System Class
Loader
User Defined
Class Loader
User Defined
Class Loader
User Defined
Class Loader
$JAVAHOME/jre/lib/rt.jar
$JAVAHOME/jre/lib/ext/*.jar
$CLASSPATH

27. defineClass()
resolveClass()
findInParentClass()
findLoadedClass()
Put the Class in self cache
Simple class loading procedure
loadClass()
findClass()

29. Insight of defineClass() magic
grepcode.com, openJDK 6, ClassLoader.java

30. Phases of Class Loading
Loading
Verifying
Preparing
Resolving
Initializing
Linking

31. Phases of Class Loading
Loading
Verifying
Preparing
Resolving
Initializing
Linking
1. Bringing Binary
data from a class into
JVM

32. Phases of Class Loading
Loading
Verifying
Preparing
Resolving
Initializing
Linking
1. Bringing Binary
data from a class into
JVM
2. Incorporating the binary data
into the runtime state of the JVM

33. Phases of Class Loading
Loading
Verifying
Preparing
Resolving
Initializing
Linking
1. Bringing Binary
data from a class into
JVM
2. Incorporating the binary data
into the runtime state of the JVM
2.1 Ensure class is properly
formed and fit for use by the
JVM

34. Phases of Class Loading
Loading
Verifying
Preparing
Resolving
Initializing
Linking
1. Bringing Binary
data from a class into
JVM
2. Incorporating the binary data
into the runtime state of the JVM
2.1 Ensure class is properly
formed and fit for use by the
JVM
2.2 Allocating memory
needed by the Class

35. Phases of Class Loading
Loading
Verifying
Preparing
Resolving
Initializing
Linking
1. Bringing Binary
data from a class into
JVM
2. Incorporating the binary data
into the runtime state of the JVM
2.1 Ensure class is properly
formed and fit for use by the
JVM
2.2 Allocating memory
needed by the Class 3. Class variables are given
their proper initial values

36. Phases of Class Loading
Loading
Verifying
Preparing
Resolving
Initializing
Linking
1. Bringing Binary
data from a class into
JVM
2. Incorporating the binary data
into the runtime state of the JVM
2.1 Ensure class is properly
formed and fit for use by the
JVM
2.2 Allocating memory
needed by the Class
2.3 Transforming symbolic
references in the constant
pool, into direct references
3. Class variables are given
their proper initial values

37. Explicit Loading vs Implicit Loading
ClassLoader.loadClass()
Class.forName()
Class A
Class B Class C
Reference
Inheritance
Instantiation
Explicit Loading
Implicit Loading
…
…

38. ClassNotFoundException
NoClassDefFoundException
ClassFormatError
ClassCastException
ClassCircularityError
The Exceptions

39. ClassNotFoundException
• These calls below and the class is not found
• Class.forName()
• ClassLoader.findSystemClass()
• ClassLoader.loadClass()
• N.B. Unsuccessful Explicit attempt to load class

40. NoClassDefFoundException
• JVM or CL instance tries to load class and the class is not found while
• in regular method call
• creating a new instance using the ‘new’
• N.B. Unsuccessful Implicit class load

41. ClassCastException
• Casting an object to a subclass of which it is not an instance.
• i.e. Integer val = (Integer) new String(“1234”);
• or

42. UnsatisfiedLinkError
•Java Virtual Machine cannot find an appropriate
native language definition of a method declared
native. i.e.

43. ClassCircularityError
•Java Virtual Machine cannot find an appropriate
native language definition of a method declared
native. i.e.
Class A
Class B extends A
Class B
Class A extends B
A.class B.class A.class B.class
B.class A.classTry to load this two classes

44. IncompatibleClassChangeError
ClassFormatError
LinkageError
IllegalAccessError
More…

45. Power of Class Loader
•Dynamically loading desired classes
•Loading different versions of the classes
•Generating new class definitions on the fly
•Verify custom code signature before executing
code
•Use encrypted* bytecodes in program
Most of the programs don’t need to mess with Class Loader.

46. Quick Question Answer
&
Open Discussion

47. Thank You 