The Java Virtual Machine (JVM) defines various run-time data areas that are used during execution of a program. Some of these data areas are created on Java Virtual Machine start-up and are destroyed only when the Java Virtual Machine exits. Other data areas are per thread. Per-thread data areas are created when a thread is created and destroyed when the thread exits.

There are various areas or segments available in run-time data areas as described below:

Program Counter Register

JVM supports many threads of execution at once. Each JVM thread has its own pc register. At point, each JVM thread executes the code of single method and this method is current method of the thread. If the method is not native, the value of the Java Virtual Machine’s pc register is undefined, otherwise the pc register contains the address of the Java Virtual Machine instruction currently being executed. The Java Virtual Machine’s pc register is wide enough to hold a returnAddress or a native pointer on the specific platform.

Java Virtual Machine Stacks

Each Java Virtual Machine thread has a private Java Virtual Machine stack, created at the same time when a thread is created. A Java Virtual Machine stack stores frames. The memory for a Java Virtual Machine stack does not need to be contiguous. Java Virtual Machine stack is never manipulated directly except to push and pop frames. Frames may be allocated into heap.

A frame is used to store data and partial results, as well as to perform dynamic linking, return values for methods, and dispatch exceptions. A new frame is created each time a method is invoked. A frame is destroyed when its method invocation completes, whether that completion is normal or abrupt (it throws an uncaught exception). Frames are allocated from the Java Virtual Machine stack of the thread creating the frame. Each frame has its own array of local variables, its own operand stack, and a reference to the run-time constant pool of the class of the current method.

Java Virtual Machine stacks either to be of a fixed size or to dynamically expand and contract as required by the computation. If the Java Virtual Machine stacks are of a fixed size, the size of each Java Virtual Machine stack may be chosen independently when that stack is created.

The following exceptional conditions are associated with Java Virtual Machine stacks:

If the computation in a thread requires a larger Java Virtual Machine stack than is permitted, the Java Virtual Machine throws a StackOverflowError.

If Java Virtual Machine stacks can be dynamically expanded, and expansion is attempted but insufficient memory can be made available to effect the expansion, or if insufficient memory can be made available to create the initial Java Virtual Machine stack for a new thread, the Java Virtual Machine throws an OutOfMemoryError.

Heap

The Java Virtual Machine has a heap that is shared among all Java Virtual Machine threads. The heap is the run-time data area from which memory for all class instances and arrays is allocated.

The heap is created on virtual machine start-up. Heap storage for objects is reclaimed by an automatic storage management system (known as a garbage collector). So objects are never explicitly deallocated. The Java Virtual Machine assumes no particular type of automatic storage management system, and the storage management technique may be chosen according to the implementor’s system requirements. The heap may be of a fixed size or may be expanded as required by the computation and may be contracted if a larger heap becomes unnecessary. The memory for the heap does not need to be contiguous.

A Java Virtual Machine implementation may provide the programmer or the user control over the initial size of the heap, as well as, if the heap can be dynamically expanded or contracted, control over the maximum and minimum heap size. Though PermGen space has been removed from Java version 8, and replaced by MetaSpace.

The following exceptional condition is associated with the heap:

If a computation requires more heap than can be made available by the automatic storage management system, the Java Virtual Machine throws an OutOfMemoryError.

Method Area

The Java Virtual Machine has a method area that is shared among all Java Virtual Machine threads. It stores per-class structures such as the run-time constant pool, field and method data, and the code for methods and constructors, including the special methods used in class and instance initialization and interface initialization.

The method area is created on virtual machine start-up. Although the method area is logically part of the heap, simple implementations may choose not to either garbage collect or compact it. The method area may be of a fixed size or may be expanded as required by the computation and may be contracted if a larger method area becomes unnecessary. The memory for the method area does not need to be contiguous.

A Java Virtual Machine implementation may provide the programmer or the user control over the initial size of the method area, as well as, in the case of a varying-size method area, control over the maximum and minimum method area size.

The following exceptional condition is associated with the method area:

If memory in the method area cannot be made available to satisfy an allocation request, the Java Virtual Machine throws an OutOfMemoryError.

Run-Time Constant Pool

A run-time constant pool is a per-class or per-interface run-time representation of the constant_pool table in a class file. It contains several kinds of constants, ranging from numeric literals known at compile-time to method and field references that must be resolved at run-time. The run-time constant pool serves a function similar to that of a symbol table for a conventional programming language, although it contains a wider range of data than a typical symbol table.

Each run-time constant pool is allocated from the Java Virtual Machine’s method area. The run-time constant pool for a class or interface is constructed when the class or interface is created by the Java Virtual Machine.

The following exceptional condition is associated with the construction of the run-time constant pool for a class or interface:

When creating a class or interface, if the construction of the run-time constant pool requires more memory than can be made available in the method area of the Java Virtual Machine, the Java Virtual Machine throws an OutOfMemoryError.

Native Method Stacks

An implementation of the Java Virtual Machine may use conventional stacks, colloquially called “C stacks,” to support native methods (methods written in a language other than the Java programming language). Native method stacks may also be used by the implementation of an interpreter for the Java Virtual Machine’s instruction set in a language such as C. Java Virtual Machine implementations that cannot load native methods and that do not themselves rely on conventional stacks need not supply native method stacks. If supplied, native method stacks are typically allocated per thread when each thread is created.

This specification permits native method stacks either to be of a fixed size or to dynamically expand and contract as required by the computation. If the native method stacks are of a fixed size, the size of each native method stack may be chosen independently when that stack is created.

A Java Virtual Machine implementation may provide the programmer or the user control over the initial size of the native method stacks, as well as, in the case of varying-size native method stacks, control over the maximum and minimum method stack sizes.

The following exceptional conditions are associated with native method stacks:

If the computation in a thread requires a larger native method stack than is permitted, the Java Virtual Machine throws a StackOverflowError.

If native method stacks can be dynamically expanded and native method stack expansion is attempted but insufficient memory can be made available, or if insufficient memory can be made available to create the initial native method stack for a new thread, the Java Virtual Machine throws an OutOfMemoryError.

You can find the original documentation in Oracle Docs.

Thanks for reading.

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