A Java virtual machine is a program which executes certain other programs, namely those containing Java bytecode instructions. JVMs are most often implemented to run on an existing operating system, but can also be implemented to run directly on hardware. A JVM provides a run-time environment in which Java bytecode can be executed, enabling features such as automated exception handling, which provides root-cause debugging information for every software error (exception). A JVM is distributed along with Java Class Library, a set of standard class libraries (in Java bytecode) that implement the Java application programming interface (API). These libraries, bundled together with the JVM, form the Java Runtime Environment (JRE).
JVMs are available for many hardware and software platforms. The use of the same bytecode for all JVMs on all platforms allows Java to be described as a write once, run anywhere programming language, versus write once, compile anywhere, which describes cross-platform compiled languages. Thus, the JVM is a crucial component of the Java platform.
Java bytecode is an intermediate language which is typically compiled from Java, but it can also be compiled from other programming languages. For example, Ada source code can be compiled to Java bytecode and executed on a JVM.
Oracle Corporation, the owner of the Java trademark, produces the most widely used JVM, named HotSpot, that is written in the C++ programming language. JVMs using the Java trademark may also be developed by other companies as long as they adhere to the JVM specification published by Oracle Corporation and to related contractual obligations.
Following diagram summarizes the key components in a JVM. In the JVM architecture, two main components that are related to garbage collection are heap memory and garbage collector. Heap memory is the runtime data area where the instances will be store and the garbage collector will operate on. Now we know how these things fit in the larger scheme.
Java Heap Memory
It is essential to understand the role of heap memory in JVM memory model. At runtime the Java instances are stored in the heap memory area. When an object is not referenced anymore it becomes eligible for eviction from heap memory. During garbage collection process, those objects are evicted from heap memory and the space is reclaimed. Heap memory has three major areas,
- Young Generation
- Eden Space (any instance enters the runtime memory area through eden)
- S0 Survivor Space (older instances moved from eden to S0)
- S1 Survivor Space (older instances moved from S0 to S1)
- Old Generation (instances promoted from S1 to tenured)
- Permanent Generation (contains meta information like class, method detail)