Java Virtual Machine
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The Java Virtual Machine (JVM) is a software emulation of a hypothetical computing machine that executes Java bytecodes, licensed by Sun Microsystems, with strict testing and core class modification rules. It supports various environments, managing threads, memory, and automatic garbage collection, utilizing a stack model for execution and a method area for storing bytecodes. Built to work across multiple operating systems, the JVM addresses limitations such as a 32 KB method size and 256 local variables per stack.
Java Virtual Machine
- 1. The Java Virtual Machine
- 2. Taha Malampatti - 140050107052By:-
- 3. A software emulation of a hypothetical computing machine that runs Java bytecodes. (Java compiler output) ISV’s can license the JVM from Sun Microsystems
- 4. To have a JVM licensed Sun must approve the implementation Rigid test procedure with published suite of test code. Core classes must not be modified ◦ this was the root of the Sun/Microsoft Lawsuit
- 5. Separate versions for command line and browser environments Many restrictions on browser version to ensure the Java security model and robustness of the JVM.
- 7. Target Hardware - all CISC and RISC Machine type - stack (similar to Forth VM) A number of companies have promised a Java chip but haven’t delivered…yet
- 9. As threads are created each thread get a Java Stack and a pc. Creates a stack frame for each method of a class ◦ Local Variables ◦ Execution Environment ◦ Operand Stack
- 10. Array of 32 bit variables ◦ Types longer than 32 bit (double) use consecutive cells ◦ Loaded onto and stored from operand stack
- 11. 32 bit FIFO holds the arguments for the opcodes a subsection of the Java Stack ◦ primary area for current status of bytecode execution
- 12. Memory from which class instances are allocated Interpreter monitors memory usage and reclaims memory when no longer in use. Garbage collection is automatic (unlike C/C++). System.gc()
- 13. Method Area - bytecodes for all Java Methods Constant Pool - class names, method and field names, string constants
- 14. 4 Gb internal addressing due to 32 bit wide stack implementation Methods are limited to 32 Kb due to 16 bit offset addressing used for branching 256 local variables /stack (8 bit field) 32k constant pool entries per method
- 15. Unix (many) Windows/NT/95/95/2000/XP OS/2 Linux VM/CMS (1Q98) MVS / S390 (1Q98)