JVM Stack and Heap.


Stack

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. 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 Java virtual machine specification permits 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. A Java virtual machine implementation may provide the programmer or the user control over the initial size of Java virtual machine stacks, as well as, in the case of dynamically expanding or contracting Java virtual machine stacks, control over the maximum and minimum sizes.

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 runtime 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); 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 heap mainly store objects create using or class level variables.

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

 The JVM size can be increased or decreased as needed:

The old generation’s default heap size can be overridden by using the -Xms and -Xmx switches to specify the initial and maximum sizes respectively:
java -Xms
<initial size> -Xmx <maximum size> program
For example:
java -Xms64m -Xmx128m program

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What happens with Class.forName(str) ?


The terse answer will be the class name mentioned as parameter will be loaded. Especially if we see in the JDBC, the driver is loaded and initialized.

If its asked to me, I will say : The method attempts to locate,load and link the class.

The JLS specification states that

“Loading refers to the process of finding the binary form of a class or interface type with a particular name, perhaps by computing it on the fly, but more typically by retrieving a binary representation previously computed from source code by a compiler, and constructing, from that binary form, a Class object to represent the class or interface.”

According to the JLS, it must be transformed from it’s binary representation to something the Java virtual machine can use, this process is called linking. Finally, the class is initialized, which is the process that executes the static initializer and the initializers for static fields declared in the class.

Well its lots of stuff done before the class is loaded.

Now in terms of JDBC, Class.forName( new Driver(“sqldriver”));

The call to Driver class is made, which contains the static method:

static{

java.sql.DriverManager.registerDriver(new Driver());

} // with try catch block.

The above line will register the driver and add it in the list of Vector. The elements are referred when DriverManager.getConnection() is done