🧠 What Is a Monitor in Java?
A monitor is a synchronization construct that allows threads to have mutually exclusive access to critical sections of code or data.
In Java, every object has an implicit monitor — yes, even a new Object()
has one!
A monitor is a synchronization construct that allows threads to have mutually exclusive access to critical sections of code or data.
In Java, every object has an implicit monitor — yes, even a new Object()
has one!
Scenario | Can you do it? | How? |
---|---|---|
Start a thread without calling start() | ❌ No | start() is required |
Start a thread from another thread externally | ❌ No | Caller must control it |
Force a paused/sleeping thread to resume | ❌ Not directly | You can interrupt it |
Force execution of run() logic manually | ✅ Yes | Just call run() (but runs in current thread) |
Use thread pools to schedule execution | ✅ Yes | Use ExecutorService |
✅ The Thread
class in Java just implements the Runnable
interface. so…
Method | What it Does |
---|---|
start() | Starts a new thread of execution (calls run() on a new thread) |
run() | Just a normal method call, executes in the current thread |
Let’s break it down step by step:
Concept | Thread | Runnable |
---|---|---|
What is it? | A class that represents a thread of execution | A functional interface (task to run in a thread) |
Role | Executes the code | Defines the code to run |
Relationship | Thread is both a task and the runner | Runnable is just the task; needs a thread to run it |
Flexibility | ❌ Less flexible (extends Thread) | ✅ More flexible (can be reused/shared) |
Usage | Thread t = new MyThread(); t.start(); | Thread t = new Thread(new MyRunnable()); t.start(); |
🔨 1. Extending Thread
Class
class MyThread extends Thread {
public void run() {
System.out.println("Hello from thread!");
}
}
public class Main {
public static void main(String[] args) {
MyThread t = new MyThread();
t.start(); // Don't call run() directly!
}
}
✅ Easy to use
❌ Can’t extend another class (Java has single inheritance)
Green threads are threads that are scheduled and managed entirely by the JVM (or user-level runtime), not the OS.
That means:
🧠 Think of green threads as “user-mode” threads.
Continue readingFeature | Process | Thread |
---|---|---|
Definition | A program in execution (independent) | A lightweight unit of execution within a process |
Memory | Has its own memory space | Shares memory with other threads in same process |
Isolation | Fully isolated from others | Not isolated — shares heap & static data |
Communication | Expensive (IPC: sockets, pipes) | Cheap (shared memory, synchronized access) |
Overhead | Heavy | Light |
Crash impact | Crash of one process doesn’t affect others | Crash of one thread can affect whole process |
Scheduling | Handled by OS | Also scheduled by OS (as lightweight processes) |
Ordering is about the sequence of execution of operations (like reads and writes). It matters because compilers, JVM, and CPUs may reorder instructions to optimize performance.
x = 1;
y = 2;
You’d assume x
is set before y
, but the JVM might reorder this if it thinks it’s safe to do so — unless there’s a rule (like happens-before) that prevents it.
🧩 Cooperative multitasking means:
Tasks (threads/processes) voluntarily yield control of the CPU so others can run.
yield()
or similar.🛠 Used in:
ThreadPoolExecutor
to run on different cores?⚠️ Short Answer: Not directly in Java — you cannot manually bind threads to specific cores using
ThreadPoolExecutor
or Java alone.
But…
Continue reading✅ Long Answer: You can influence it indirectly and even hack it a bit if you’re determined 😎