Java virtual machine
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The document discusses Java virtual machines and how Java code is executed. It defines a virtual machine as an abstraction layer that allows programs to interact with different operating systems. It then describes how the Java virtual machine works, executing Java bytecode and allowing code to run on any platform. It outlines the components of the JVM, including the class loader, execution engine, and garbage collector.
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Java virtual machine
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- 3. Disclaimer: This presentation is prepared by trainees of baabtra as a part of mentoring program. This is not official document of baabtra –Mentoring Partner Baabtra-Mentoring Partner is the mentoring division of baabte System Technologies Pvt . Ltd
- 4. Virtual Machine “A virtual machine is a layer of abstraction that gives a program, one simplified interface for interacting with a variety of physical computers and their operating systems.”
- 5. Java Virtual Machine • JVM is not a real hardware machine but a software layer which resembles an hardware platform. • JVM converts Java byte code into machine language and executes it. • The byte code can be executed on any platform where there exist JVM. • JVM‘s are available for many hardware and software platforms. • The use of the same byte code for all JVM’s on all platforms allows Java to be described as a "write once, run anywhere" programming language. • Thus, the JVM is a crucial component of the Java platform.
- 6. Diagram Of JVM Java Source Code(.java) Java Object Code(.class) Java Compiler (javac) Computer Operating System Java Interpreter Java Virtual Machine Object Code is loaded into Java Virtual Machine
- 7. Components Of JVM Class Loader Execution Engine Run-time data areas Java Virtual Machine .class file
- 8. Components Of JVM 1. Byte code verifier 2. Class loader 3. Execution engine 4. Garbage collector 5. Security Manager
- 9. 1. Byte code Verifier: • Byte code verifier is used to verify the byte code. • Also checks for unusual code. • So it is a crucial component for security. 2. Class loader: • Class loader loads java classes into java virtual machine and creates the instance of java.lang.class. • The main feature is that JVM doesn’t need to have any knowledge about the classes that will be loaded at runtime. • It performs three main functions of JVM, namely: loading, linking and initialization
- 10. 3. Execution Engine: • The execution engine helps JVM to convert bytecode into machine code. • It has two parts: a) Interpreter b) Just-in-time-interpreter • Execution engine is responsible for executing the instructions contained in the methods of loaded classes. 4. Garbage Collector: • Garbage collection is the process of automatically freeing objects that are no longer referenced by the program. • It periodically check for the object on heap , whose link is broken so it can collect garbage from heap. • Garbage collection relieves java programmer from memory management.
- 11. 5. Security Manager: • Security manager constantly monitors the code. • It is a special java object that is responsible for guarding security policies for java applications. • It is always consulted before any potentially dangerous operation is requested by a java application.
- 12. JDK ‘n’ JRE Diagram to show the relations between JVM, JRE & JDK
- 13. Java Runtime Environment(JRE): • Provides the libraries, the Java Virtual Machine, and other components to run applets and applications written in the Java programming language. • The JRE does not contain tools and utilities such as compilers or debuggers for developing applets and applications. Java Development Kit(JDK): • JDK is a bundle of software that is used to develop Java based applications. • The JDK is a superset of the JRE, and contains everything that is in the JRE, plus set of API classes, Java compiler and debugger, and additional files needed to write Java applets and applications. • Needs more disk space than JRE
- 14. Just In Time(JIT): • JIT are advanced part of Java Virtual machine • It optimize byte code to machine instruction conversion by compiling similar byte codes at same time • Thus reducing overall execution time. • JIT performs several optimizations such as in-lining function. • JIT compiler improves performance
- 15. In Short.... • Java Virtual Machine (JVM) is platform dependent. • Java is platform independent. • JVM does not really exists but is created within another environment. • The JVM stands on top of the operating system (OS).
- 16. Is Java a Compiled or Interpreted Language
- 17. Java is both compiled and interpreted language. Java compiles using javac and JIT, and interprets using Java virtual machine.
- 19. 1 2 “Programmer write source code and store that into a .java file.” Note: Name of your Java source file must be same with the public class declared inside that file, for example if there is public class called Order inside Java file, then it name must be Order.java “javac compiles .java fie and creates class files, which contains byte code”
- 20. 3 4 “JVM executes these class files and gathers statistics of execution run.” “When JVM has enough data to make decision, JIT compiles frequently used byte codes into native code, which is then directly executed by platform. This is how JIT provides performance boost to Java application.”
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Editor's Notes
- #5: Before understanding what is JVM let us first know what virtual machine is
- #6: When you run a Java program, the OS is actually running the Java Runtime Engine, or JRE, as an executable program; it processes your compiled code through the Java Virtual Machine (usually referred to as the JVM).
- #10: All Java virtual machines include one class loader that is embedded in the virtual machine. • The linking process consists of three sub-tasks, namely, verification, preparation, and resolution