JVM Architecture in Java

 The Java Virtual Machine (JVM) architecture is a core component of the Java Runtime Environment (JRE) and enables Java's "Write Once, Run Anywhere" capability. It provides a runtime environment for executing Java bytecode. 

The JVM architecture consists of several key components:

1. Class Loader Subsystem:

• Responsible for loading, linking, and initializing Java classes.
• Loading: Finds and loads the class file (.class) into memory.
• Linking: Verifies the loaded bytecode, prepares static fields with default values, and resolves symbolic references.
• Initialization: Executes class static initializers and assigns actual values to static variables.
• It has a hierarchical structure with Bootstrap, Extension, and Application Class Loaders.

2. Runtime Data Areas (Memory Areas):

• These are the memory areas used by the JVM during program execution.
• Method Area: Stores class-level data like class structure, method data, constructor information, and static variables. It is shared among all threads.
• Heap Area: Stores all objects and instance variables. It is also shared among all threads and is subject to garbage collection.
• Stack Area: Each thread has its own private JVM stack, which stores method calls (stack frames) and local variables.
• PC Registers: Each thread has its own Program Counter (PC) register, which stores the address of the next instruction to be executed.
• Native Method Stack: Stores information about native methods (methods written in languages other than Java).

3. Execution Engine:

• Responsible for executing the bytecode loaded by the Class Loader.
• Interpreter: Reads and executes bytecode instruction by instruction.
• Just-In-Time (JIT) Compiler: Identifies "hot" code (frequently executed sections) and compiles it into native machine code for faster execution.
• Garbage Collector: Automatically reclaims memory occupied by unused objects in the Heap.

4. Java Native Interface (JNI):

• Provides an interface for Java code to interact with native applications and libraries written in other languages (like C/C++).

5. Native Method Libraries:

• These are the native libraries (e.g., .dll on Windows, .so on Linux) that contain the implementations of native methods called through JNI.

Reference  from Google  AI Review