What is garbage collection? Explain the difference between garbage collections in .NET 4.0 and earlier versions

Garbage collection prevents memory leaks during execution of programs.
Garbage collector is a low-priority process that manages the allocation and deallocation of memory for your application.
It checks for the unreferenced variables and objects.
If GC finds any object that is no longer used by the application, it frees up the memory from that object.
GC has changed a bit with the introduction of .NET 4.0. In .NET 4.0, the GC.Collect() method contains the following overloaded methods:
GC.Collect(int)
GC.Collect(int, GCCollectionMode)
Another new feature introduced in .NET is to notify you when the GC.Collect() method is invoked and completed successfully by using different methods.
The .NET 4.0 supports a new background garbage collection that replaces the concurrent garbage collection used in earlier versions.
This concurrent GC allocates memory while running and uses current segment (which is 16 MB on a workstation) for that.
After that, all threads are suspended. In case of background GC, a separate ephemeral GC - gen0 and gen1 can be started, while the full GC - gen0, 1, and 2 - is already running.

2. Discuss Java Virtual Machine

JVM (Java Virtual Machine)

JVMs are available for many hardware and software platforms (i.e. JVM is platform dependent).JVM (Java Virtual Machine) is an abstract machine. It is a specification that provides runtime environment in which java bytecode can be executed.

What is JVM

It is:

A specification where working of Java Virtual Machine is specified. But implementation provider is independent to choose the algorithm. Its implementation has been provided by Sun and other companies.
An implementation Its implementation is known as JRE (Java Runtime Environment).
Runtime Instance Whenever you write java command on the command prompt to run the java class, an instance of JVM is created.

What it does

The JVM performs following operation:

Loads code
Verifies code
Executes code
Provides runtime environment

JVM provides definitions for the:

Memory area
Class file format
Register set
Garbage-collected heap
Fatal error reporting etc.

Internal Architecture of JVM

Let's understand the internal architecture of JVM. It contains classloader, memory area, execution engine etc.

1) Classloader

Classloader is a subsystem of JVM that is used to load class files.

2) Class(Method) Area

Class(Method) Area stores per-class structures such as the runtime constant pool, field and method data, the code for methods.

3) Heap

It is the runtime data area in which objects are allocated.

4) Stack

Java Stack stores frames.It holds local variables and partial results, and plays a part in method invocation and return.

Each thread has a private JVM stack, created at the same time as thread.

A new frame is created each time a method is invoked. A frame is destroyed when its method invocation completes.

5) Program Counter Register

PC (program counter) register. It contains the address of the Java virtual machine instruction currently being executed.

6) Native Method Stack

It contains all the native methods used in the application.

7) Execution Engine

It contains:

1) A virtual processor

2) Interpreter: Read bytecode stream then execute the instructions.

3) Just-In-Time(JIT) compiler: It is used to improve the performance.JIT compiles parts of the byte code that have similar functionality at the same time, and hence reduces the amount of time needed for compilation.Here the term ?compiler? refers to a translator from the instruction set of a Java virtual machine (JVM) to the instruction set of a specific CPU.

Sajib Biswas

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