JVM: Stack VS. Heap VS. Method Area

data area

Similar to Operating System, Java Virtual Machine offers a runtime data area. This time, I make a summery about stack, hep and method.

Stack Vs. Heap

Stack

  1. LIFO, last in first out
  2. Local variables, bookkeeping data
  3. Size is set when a thread is created.
  4. Stores local data, return addresses, used for parameter passing.

Heap

  1. Dynamic allocation at runtime
  2. Size is initialized on application startup but grow dynamically

Like OS, Each thread gets a stack, only one heap for the application

Each Java Virtual Machine thread has a private Java Virtual Machine stack, created at the same time as the thread. A Java Virtual Machine stack stores frames. A Java Virtual Machine stack is analogous to the stack of a conventional language such as C: it holds local variables and partial results, and plays a part in method invocation and return. Because the Java Virtual Machine stack is never manipulated directly except to push and pop frames, frames may be heap allocated. The memory for a Java Virtual Machine stack does not need to be contiguous.

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

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

  2. 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.

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); 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.

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 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. This version of the Java Virtual Machine specification does not mandate the location of the method area or the policies used to manage compiled code. 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.

For each type it loads, a Java virtual machine must store the following kinds of information in the method area:

  • The fully qualified name of the type
  • The fully qualified name of the type’s direct superclass (unless the type is an interface or class java.lang.Object, neither of which have a superclass)
  • Whether or not the type is a class or an interface
  • The type’s modifiers ( some subset of` public, abstract, final)
  • An ordered list of the fully qualified names of any direct superinterfaces
  • The constant pool for the type
  • Field information
  • Method information
  • All class (static) variables declared in the type, except constants
  • A reference to class ClassLoader
  • A reference to class Class

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