Thread Scheduler in Java

The Thread Scheduler in Java is part of the Java Virtual Machine (JVM) responsible for managing threads. It determines the execution order of threads in a multithreaded application. Since Java uses a preemptive and time-slicing-based scheduling model, the scheduler ensures that each thread gets CPU time according to its priority and availability.

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Key Responsibilities of the Thread Scheduler

1. Thread Selection:
   1. Chooses which thread will run when multiple threads are in the ready-to-run state.
2. Time Sharing:
   1. Allocates CPU time to threads in a fair and efficient manner.
3. Priority Consideration:
   1. Considers thread priority to decide the execution order.

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Thread States Relevant to Scheduling

A thread can be in one of the following states:

1. New:
   1. The thread is created but hasn’t started yet.
2. Runnable:
   1. The thread is ready to run and waiting for CPU time.
3. Running:
   1. The thread is currently being executed by the CPU.
4. Blocked or Waiting:
   1. The thread is waiting for a resource or another thread to complete.
5. Terminated:
   1. The thread has completed its execution.

The Thread Scheduler works with threads in the Runnable state to decide which thread will move to the Running state.

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Thread Priority

In Java, each thread has a priority represented by an integer value. The scheduler uses this priority to determine thread execution order.

• Priority Levels:
  • MIN_PRIORITY (1): Lowest priority.
  • NORM_PRIORITY (5): Default priority for threads.
  • MAX_PRIORITY (10): Highest priority.

The priority of a thread can be set using the setPriority() method:

thread.setPriority(Thread.MAX_PRIORITY);

Example of Thread Priority

class MyThread extends Thread {

    public void run() {

        System.out.println(Thread.currentThread().getName() + ” is running with priority ” + Thread.currentThread().getPriority());

    }

}

public class ThreadPriorityExample {

    public static void main(String[] args) {

        MyThread t1 = new MyThread();

        MyThread t2 = new MyThread();

        MyThread t3 = new MyThread();

        t1.setPriority(Thread.MIN_PRIORITY);

        t2.setPriority(Thread.NORM_PRIORITY);

        t3.setPriority(Thread.MAX_PRIORITY);

        t1.start();

        t2.start();

        t3.start();

    }

}

Output (example):

Thread-0 is running with priority 1

Thread-2 is running with priority 10

Thread-1 is running with priority 5

Note: The actual order of execution depends on the JVM implementation and operating system.

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Types of Thread Scheduling

1. Preemptive Scheduling:
   1. Higher-priority threads preempt lower-priority threads.
2. Time-Slicing:
   1. Threads share CPU time equally in a round-robin fashion.

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Limitations of Thread Scheduler

1. Platform Dependency:
   1. The behavior of the thread scheduler depends on the JVM implementation and the underlying operating system.
2. Priority Isn’t Guaranteed:
   1. While the scheduler considers thread priority, it does not guarantee execution order based on priority.
3. Unpredictable Behavior:
   1. The exact order of thread execution is unpredictable, as it depends on the scheduler’s algorithm.

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Example: Thread Scheduling in Action

class PrintNumbers extends Thread {

    public void run() {

        for (int i = 1; i <= 5; i++) {

            System.out.println(Thread.currentThread().getName() + “: ” + i);

            try {

                Thread.sleep(500); // Simulate some work

            } catch (InterruptedException e) {

                System.out.println(“Thread interrupted.”);

            }

        }

    }

}

public class ThreadSchedulerDemo {

    public static void main(String[] args) {

        PrintNumbers t1 = new PrintNumbers();

        PrintNumbers t2 = new PrintNumbers();

        PrintNumbers t3 = new PrintNumbers();

        t1.setPriority(Thread.MIN_PRIORITY);

        t2.setPriority(Thread.NORM_PRIORITY);

        t3.setPriority(Thread.MAX_PRIORITY);

        t1.start();

        t2.start();

        t3.start();

    }

}

Output (example):

Thread-2: 1

Thread-1: 1

Thread-0: 1

Thread-2: 2

Thread-1: 2

Thread-0: 2

…

Explanation:

• The scheduler alternates between threads, but due to platform dependencies, the output order may vary.

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Best Practices for Thread Scheduling

1. Avoid Priority-Based Assumptions:
   1. Do not rely on thread priorities to enforce a specific execution order.
2. Use Synchronization:
   1. For precise thread coordination, use synchronization mechanisms like wait(), notify(), or join().
3. Minimize Blocking Operations:
   1. Avoid long blocking operations in threads to improve overall system responsiveness.

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Conclusion

The Thread Scheduler in Java is an essential component of multithreading that decides the execution order of threads. While it uses priorities and scheduling policies, its behavior is platform-dependent, making it important to avoid assuming specific execution sequences. Instead, combine scheduling with robust synchronization for effective multithreaded programming.