Diving into Java Class Loader
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The document provides an overview of Java class loaders, detailing their functionality, phases of class loading, and various types of class loading exceptions. It explains the process of loading classes into the Java Virtual Machine (JVM), including the delegation model and the roles of different class loaders. Additionally, it highlights the importance and power of class loaders in dynamically managing class definitions and ensuring security in Java applications.
Diving into Java 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
- 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
- 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 … …
- 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
- 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