IBM User Guide for Java V7 on Windows > IBM SDK for Java

SDK components

The IBM Virtual Machine for Java (JVM) is a core component of the JRE from IBM. The JVM is a virtualized computing machine that follows a well-defined specification for the runtime requirements of the Java programming language.

The JVM is called "virtual" because it provides a machine interface that does not depend on the underlying operating system and machine hardware architecture. This independence from hardware and operating system is a cornerstone of the write-once run-anywhere value of Java programs. Java programs are compiled into "bytecodes" that target the abstract virtual machine; the JVM is responsible for executing the bytecodes on the specific operating system and hardware combinations.

The JVM specification also defines several other runtime characteristics.

All JVMs:

• Execute code that is defined by a standard known as the class file format
• Provide fundamental runtime security such as bytecode verification
• Provide intrinsic operations such as performing arithmetic and allocating new objects

JVMs that implement the specification completely and correctly are called "compliant". The IBM Virtual Machine for Java is certified as compliant. Not all compliant JVMs are identical. JVM implementers have a wide degree of freedom to define characteristics that are beyond the scope of the specification. For example, implementers might choose to favour performance or memory footprint; they might design the JVM for rapid deployment on new platforms or for various degrees of serviceability.

All the JVMs that are currently used commercially come with a supplementary compiler that takes bytecodes and produces platform-dependent machine code. This compiler works with the JVM to select parts of the Java program that could benefit from the compilation of bytecode, and replaces the JVM's virtualized interpretation of these areas of bytecode with concrete code. This is called just-in-time (JIT) compilation.

The diagnostic information in this guide discusses the characteristics of the IBM JRE that might affect the non-functional behavior of your Java program. This guide also provides information to assist you with tracking down problems and offers advice, from the point of view of the JVM implementer, on how you can tune your applications. There are many other sources for good advice about Java performance, descriptions of the semantics of the Java runtime libraries, and tools to profile and analyze in detail the execution of applications.

See

1. Java application stack
   
2. Components of the IBM Virtual Machine for Java
   

Parent: IBM SDK for Java

Error 404 - Not Found

The document you are looking for may have been removed or re-named. Please contact the web site owner for further assistance.