Java Virtual Machine (JVM)

JVM Implementation:

JVM is platform dependent that means there are different implementation of JVM on different OS.

Type of JVM implementations:

The primary reference Java VM implementation is HotSpot, produced by Oracle Corporation.

Other list of JVM for various O/S and hardware:

• Hewlett-Packard, Java for HP-UX, OpenVMS, Tru64 and Reliant (Tandem) UNIX platforms
• J9 (IBM), for AIX, Linux, MVS, OS/400, Pocket PC, z/OS
• JBlend, (Aplix) is a Java ME implementation
• JRockit (originally from Appeal Virtual Machines) acquired by Oracle for Linux, Windows and Solaris
• Mac OS Runtime for Java (MRJ)
• Microsoft Java Virtual Machine (discontinued in 2001)

SAPJVM (SAP) is a licensed and modified SUN JVM ported to all supported platforms of SAP NetWeaver, started as Java 5, in the meantime Java 6 compatible (Windows i386, x64, IA-64; Linux x86, IA-64, PowerPC; AIX PowerPC; HP-UX SPARC IA-64; Solaris SPARC x86-64; i5/OS PowerPC)

Java’s Magic – Just In Time (JIT):

Just in time (JIT) is a part of Java Virtual Machine (JVM) architecture. The job of JIT inside JVM is to compile bytecode into machine executable code in real time, on a piece-by-piece, demand basis.

When Java programs are executed, JVM does not read the entire Bytecode and converts it into machine instructions. If JVM tries to do this approach then the program execution time will be delayed for hours. Java has overcome the latency of program execution time by interpreting the required bytecode and keep the rest of the code aside.

Just in time (JIT) helps to compile code that is only needed and at the same time boost the program performance. Wheather the Java programs are interpreted traditional way or on the fly the functionality and features like portability and security remains the same.

Summary:Java compiler converts the Java source code that you write into a binary program consisting of bytecodes. Bytecodes are machine instructions for the Java Virtual Machine. When you execute a Java program, a program called the Java interpreter (JIT) inspects and decipher the bytecodes into machine executable language.