Execution Engine


Execution Engine Phase 

Once .class files are loaded, linked, and initialized, the JVM moves into execution phase.

Execution Engine is the place Where Code actually Runs

It contains:-

  • Interpreter
  • JIT (Just in Time)
  • Garbage Collector

1. Interpreter  

Interpreter reads bytecode one instruction at a time and executes it immediately.

Interpreter works like:

  • Read instruction 1 → execute
  • Read instruction 2 → execute
  • Read instruction 3 → execute


Very fast startup, No waiting for compilation and Slower for long-running programs


2. JIT Compiler (Just-in-time)

  • Detects hot methods(frequently executed)
  • Converts bytecode - native machine code
  • Stores optimized code for reuse
  • It results, Much faster execution and JVM becomes faster over time


What are “Hot Methods”?

  • Methods inside loops
  • Frequently called methods
  • Business logic methods in servers


Why JIT makes JVM faster

  • Once compiled:
  • JVM does not interpret again
  • CPU runs native machine code directly
  • Execution becomes much faster


When you run a Java program, JVM does three big things:

  1. Loads the .class files
  2. Prepares them (linking + initialization)
  3. Executes the code


Real objects are created only during the Execution Phase, when the new keyword is actually executed by the Execution Engine.


Step by Step

1. Compile Time

User u = new User();

  • It convert .java file into .class

No object is created here


2. Class Loading / Linking / Initialization

  • JVM loads User.class
  • Prepares class metadata
  • Initializes static variables
  • Static variables initialized

Object NOT created yet


3. Execution 

  • When JVM executes this line:
    • User u = new User();

Now the real object is created


What Exactly Happens When new Executes?

  • Internally:
  • JVM checks: Is User class loaded?
  • If not → load it
  • JVM allocates memory in Heap
  • Object fields are initialized
  • Constructor is called
  • Reference u is stored in Stack


Stack                   Heap

u  ─────────▶  User object

Now the real object exists

3.  Garbage Collector

  • Automatically removes unused objects from Heap
  • Frees memory
  • Prevents memory leaks

GC types:

  • Minor GC → Young Gen
  • Major/Full GC → Old Gen

Native Libraries

When Java needs OS-level functionality:

  • File IO
  • Threading
  • Memory allocation

➡️ JVM calls Native Libraries using JNI


Garbage Collector :

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