Tuesday, October 31, 2006

The Java Virtual Machine History

The Java virtual machine is the cornerstone of the Java and Java 2 platforms. It is the component of the technology responsible for its hardware- and operating system- independence, the small size of its compiled code, and its ability to protect users from malicious programs.

The Java virtual machine is an abstract computing machine. Like a real computing machine, it has an instruction set and manipulates various memory areas at run time. It is reasonably common to implement a programming language using a virtual machine; the best-known virtual machine may be the P-Code machine of UCSD Pascal.

The first prototype implementation of the Java virtual machine, done at Sun Microsystems, Inc., emulated the Java virtual machine instruction set in software hosted by a handheld device that resembled a contemporary Personal Digital Assistant (PDA). Sun's current Java virtual machine implementations, components of its JavaTM 2 SDK and JavaTM 2 Runtime Environment products, emulate the Java virtual machine on Win32 and Solaris hosts in much more sophisticated ways. However, the Java virtual machine does not assume any particular implementation technology, host hardware, or host operating system. It is not inherently interpreted, but can just as well be implemented by compiling its instruction set to that of a silicon CPU. It may also be implemented in microcode or directly in silicon.

The Java virtual machine knows nothing of the Java programming language, only of a particular binary format, the class file format. A class file contains Java virtual machine instructions (or bytecodes) and a symbol table, as well as other ancillary information.

For the sake of security, the Java virtual machine imposes strong format and structural constraints on the code in a class file. However, any language with functionality that can be expressed in terms of a valid class file can be hosted by the Java virtual machine. Attracted by a generally available, machine-independent platform, implementors of other languages are turning to the Java virtual machine as a delivery vehicle for their languages.

Taken From: http://java.sun.com/docs/books/vmspec

How to get Java Memory Heap Size

// Get current size of heap in bytes

long heapSize = Runtime.getRuntime().totalMemory();

// Get maximum size of heap in bytes. The heap cannot grow beyond this size.
// Any attempt will result in an OutOfMemoryException.

long heapMaxSize = Runtime.getRuntime().maxMemory();

// Get amount of free memory within the heap in bytes. This size will increase
// after garbage collection and decrease as new objects are created.

long heapFreeSize = Runtime.getRuntime().freeMemory();