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Diving into Java Class Loader
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.
In this document
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Introduction to class loading in Java, its phases, and examples of class loading and its references.
Overview of Java, its history, and how it operates through JVM, including compiling and running Java programs.
Explanation of class loading procedures, delegation model, and types of class loaders in Java.
Detailed examination of the phases of class loading: loading, verification, preparation, resolution, and initialization.
Common exceptions encountered during class loading in Java such as ClassNotFoundException and ClassCastException.
Advantages and capabilities of class loaders including dynamic loading and managing class versions.
Open discussion on class loaders and a thank you note to the audience.
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Diving into Java Class Loader
- 1. @KonaTechAdda - 6 Whatis 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 classloading 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 isJava ? 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
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- 8. What we knowjava 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.cppOperating 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 somejava Welcome.java
- 12. Compile it javac Welcome.java Welcome.java Compilecommand Compile output Welcome.class
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- 15. What does JVMdo? bytecodes Amazing Things Code Interpretation Native Instruction mapping Type Conversion Memory Management Garbage Collection Disk/Network access Security Management
- 16. There is aspecification about What JVM should do http://docs.oracle.com/javase/specs/
- 17. Virtual Machines • HotspotJVM (OpenJDK) • Hotspot JVM (Oracle JDK) • J9 by IBM • Apache Harmony • Kaffe OpenVM • NanoVM …
- 18. An Important partis to load the classes into memory
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- 20. Takes classname (i.e. com.konasl.test.Welcome) Findthe 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) Findthe 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) Findthe 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 DelegationModel Bootstrap Class Loader Extension Class Loader System Class Loader User Defined Class Loader User Defined Class Loader User Defined Class Loader
- 25. Class Loader DelegationModel 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 DelegationModel 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 Classin self cache Simple class loading procedure loadClass() findClass()
- 29. Insight of defineClass()magic grepcode.com, openJDK 6, ClassLoader.java
- 30. Phases of ClassLoading Loading Verifying Preparing Resolving Initializing Linking
- 31. Phases of ClassLoading Loading Verifying Preparing Resolving Initializing Linking 1. Bringing Binary data from a class into JVM
- 32. Phases of ClassLoading 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 ClassLoading 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 ClassLoading 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 ClassLoading 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 ClassLoading 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 vsImplicit Loading ClassLoader.loadClass() Class.forName() Class A Class B Class C Reference Inheritance Instantiation Explicit Loading Implicit Loading … …
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- 39. ClassNotFoundException • These callsbelow and the class is not found • Class.forName() • ClassLoader.findSystemClass() • ClassLoader.loadClass() • N.B. Unsuccessful Explicit attempt to load class
- 40. NoClassDefFoundException • JVM orCL 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 anobject to a subclass of which it is not an instance. • i.e. Integer val = (Integer) new String(“1234”); • or
- 42. UnsatisfiedLinkError •Java Virtual Machinecannot find an appropriate native language definition of a method declared native. i.e.
- 43. ClassCircularityError •Java Virtual Machinecannot 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
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- 45. Power of ClassLoader •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 & OpenDiscussion
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