Thread scheduling and priority

Thread scheduling and priority in Java determine the order in which threads are executed by the CPU. Java provides a way to specify the priority of threads to influence their scheduling, although the exact behavior depends on the underlying operating system. Here's an overview of thread scheduling and priority in Java:

Thread Scheduling:

1. Preemptive Scheduling:

   - Operating systems use preemptive scheduling to switch between threads based on priority and time-slicing.

   - Higher priority threads are given preference over lower priority threads.

   - Threads with the same priority are scheduled in a round-robin fashion.

2. Time-Slicing:

   - The CPU allocates a small time slice to each thread, and then switches to the next thread in the queue.

   - Time-slicing ensures that each thread gets a fair share of CPU time.

Thread Priority:

1. Thread Priority Levels:

   - Java assigns each thread a priority level ranging from 1 to 10.

   - The default priority level is 5.

   - Use the constants `Thread.MIN_PRIORITY`, `Thread.NORM_PRIORITY`, and `Thread.MAX_PRIORITY` for minimum, normal, and maximum priorities, respectively.

2. Setting Thread Priority:

   - You can set the priority of a thread using the `setPriority(int priority)` method.

   - Priority values are integers ranging from 1 (lowest priority) to 10 (highest priority).

3. Priority Inheritance:

   - Java provides priority inheritance, where if a higher priority thread is waiting for a resource held by a lower priority thread, the priority of the lower priority thread is temporarily increased to that of the higher priority thread.

Example:

class MyThread extends Thread {

    public void run() {

        for (int i = 0; i < 5; i++) {

            System.out.println(Thread.currentThread().getName() + " - Priority: " + Thread.currentThread().getPriority() + " - Counter: " + i);

        }

    }

}

public class Main {

    public static void main(String[] args) {

        MyThread thread1 = new MyThread();

        MyThread thread2 = new MyThread();

        thread1.setPriority(Thread.MIN_PRIORITY);

        thread2.setPriority(Thread.MAX_PRIORITY);

        thread1.start();

        thread2.start();

    }

}

Output (Example):

Thread-0 - Priority: 1 - Counter: 0

Thread-0 - Priority: 1 - Counter: 1

Thread-0 - Priority: 1 - Counter: 2

Thread-0 - Priority: 1 - Counter: 3

Thread-0 - Priority: 1 - Counter: 4

Thread-1 - Priority: 10 - Counter: 0

Thread-1 - Priority: 10 - Counter: 1

Thread-1 - Priority: 10 - Counter: 2

Thread-1 - Priority: 10 - Counter: 3

Thread-1 - Priority: 10 - Counter: 4

Conclusion:

Understanding thread scheduling and priority is important for writing concurrent programs in Java. While thread priority can influence scheduling, it's not always guaranteed due to differences in operating systems and JVM implementations. It's generally recommended to use thread priorities judiciously and rely on other synchronization mechanisms for controlling thread execution.