Thread Scheduler

Lesson 43

The Thread Scheduler decides which runnable thread executes next, allocating CPU time based on priority and system availability.

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Lesson content

The Thread Scheduler is a JVM component that controls which runnable thread executes and when.

Definition List

Thread Scheduler: JVM component that determines which thread executes next.
Thread Scheduling: Selecting a thread from runnable threads for execution.
CPU Time: Processor time allocated to a thread.
Context Switching: Switching CPU execution from one thread to another.
Runnable Thread: A thread ready to execute, waiting for CPU.

Types of Thread Scheduling

Preemptive Scheduling — Scheduler can pause a running thread.
Priority-Based Scheduling — Higher-priority threads get preference.

Priority Constants

Thread.MIN_PRIORITY   // 1
Thread.NORM_PRIORITY  // 5
Thread.MAX_PRIORITY   // 10

Example 01: Java Thread Scheduler

package com.java.Multi_threading;

class MyThread extends Thread {

    public MyThread(String name) {
        super(name);
    }

    public void run() {
        for (int i = 1; i <= 5; i++) {
            System.out.println(getName() + " - Priority: " + getPriority() + " - Count: " + i);
            try {
                Thread.sleep(100);
            } catch (InterruptedException e) {
                e.printStackTrace();
            }
        }
    }
}

public class ThreadSchedulerExample {

    public static void main(String[] args) {

        MyThread t1 = new MyThread("Thread-1 (Low Priority)");
        MyThread t2 = new MyThread("Thread-2 (High Priority)");
        MyThread t3 = new MyThread("Thread-3 (Medium Priority)");

        t1.setPriority(Thread.MIN_PRIORITY);
        t2.setPriority(Thread.MAX_PRIORITY);
        t3.setPriority(Thread.NORM_PRIORITY);

        t1.start();
        t2.start();
        t3.start();
    }
}

Output

Thread-2 (High Priority) - Priority: 10 - Count: 1
Thread-3 (Medium Priority) - Priority: 5 - Count: 1
Thread-1 (Low Priority) - Priority: 1 - Count: 1
Thread-2 (High Priority) - Priority: 10 - Count: 2
Thread-3 (Medium Priority) - Priority: 5 - Count: 2
...

Thread Scheduler and Life Cycle

New
  ↓
Runnable ← Thread Scheduler Selects
  ↓
Running
  ↓
Waiting/Blocked
  ↓
Runnable
  ↓
Terminated

Context Switching

Thread.sleep(100);

Pauses the thread temporarily, releasing CPU to others.

Best Practices

Never depend on a fixed execution order.
Use synchronization to protect shared resources.
Keep threads lightweight for better scheduling.
Treat priorities as hints, not strict controls.

FAQ

1. What is a thread scheduler in Java?

A JVM component that assigns CPU time to runnable threads.

2. Does Java always run higher-priority threads first?

No. Priority increases likelihood but doesn't guarantee order.

3. What is context switching?

Pausing one thread and switching CPU to another thread.

Thread Scheduler allocates CPU to runnable threads.

Java uses preemptive, priority-based scheduling.

Priority ranges from 1 (min) to 10 (max).

Context switching lets threads share the CPU.

Thread.sleep() releases CPU to other threads.

Execution order is never fully guaranteed.

Scheduling behavior varies by JVM and OS.

Where Java is used

Java in real-world development

Thread Scheduler

JVM component that picks which runnable thread executes next.

Preemptive Scheduling

Running thread can be paused for another thread.

Priority-Based Scheduling

Higher-priority threads are generally preferred by the scheduler.

Context Switching

CPU stops one thread and starts another. Thread.sleep() triggers it.

Execution Order

Not guaranteed. JVM and OS both influence final scheduling.

Best Practice

Use synchronization, not priority, to control resource access.

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