Java Multithreading | Part 1

Understanding Processes and Threads in Java

Process

A process is an instance of a program that is being executed.

It has its own resources like memory and threads, which the operating system allocates when the process is created.

Compilation: Running javac Test.java generates bytecode that can be executed by the JVM.
Execution: Using java Test.class starts a new process where Test is the class with the public static void main(String[] args) method.

Thread

A thread is the smallest sequence of instructions executed independently by the CPU.

A process can have multiple threads. When a process is created, it starts with one main thread, from which multiple threads can be created to perform tasks concurrently.

Example:

public class Main {
    public static void main(String[] args) {
        System.out.println("Inside main method, thread - " + Thread.currentThread().getName());
    }
}
//output - Inside main method, thread - main

Memory Segments in JVM

Code Segment

• Contains the compiled bytecode of the Java program.
• Read-only.
• Shared by all threads within the same process.

Data Segment

• Contains global and static variables.
• Shared by all threads within the same process.
• Requires synchronization for thread safety.

Heap

• Allocated for objects created at runtime using the new keyword.
• Shared among all threads of the same process (but not within Processes).
• Requires synchronization for thread safety.

Stack

• Each thread has its own stack.
• Manages method calls and local variables.

Register

• Used by the JIT compiler to optimize generated machine code.
• Each thread has its own register, aiding in context switching.

Program Counter

• Also known as the program counter, it points to the currently executing instruction.
• Increments after the successful execution of an instruction.

java -Xms256m -Xmx2g MainClassName

Multithreading in Java

Definition: Allows a program to perform multiple tasks simultaneously. Multiple threads share the same resources but can perform tasks independently.

Benefits:

• Improved performance through task parallelism.
• Increased responsiveness.
• Efficient resource sharing.

Challenges:

• Concurrency issues like deadlock and data inconsistency.
• Overhead of synchronization.
• Difficulty in testing and debugging.

Multitasking vs. Multithreading

Multitasking involves running multiple processes, while multithreading involves running multiple threads within a single process, allowing more efficient use of resources.