Thread Priority

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Thread priority is a hint to the thread scheduler about which thread should get CPU preference.

In Java:

Thread.MIN_PRIORITY = 1
Thread.NORM_PRIORITY = 5 (default)
Thread.MAX_PRIORITY = 10

public class Main {
    public static void main(String[] args) {

        Thread t1 = new Thread(() -> {
            System.out.println("Low priority thread");
        });

        Thread t2 = new Thread(() -> {
            System.out.println("High priority thread");
        });

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

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

Expectation:

• t2 runs first (higher priority)

Reality:

• Not guaranteed

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Why Thread Priority Exists

Historically, it was introduced to:

1. Influence Scheduling

• Suggest which threads are more important

2. Optimize CPU usage

• High priority → critical tasks
• Low priority → background tasks

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How It Works Internally

Thread → JVM → OS Scheduler → CPU

Important:

• Java does NOT control scheduling directly
• OS scheduler makes final decision

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Note:

Thread priority is just a hint, NOT a guarantee.

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Why You Should NOT Rely on Priority

❌ 1. OS dependent

Windows vs Linux → different behavior

❌ 2. JVM implementation dependent

❌ 3. Can cause starvation

Low priority threads may never run

Thread priority is a hint to the scheduler, but since scheduling is OS-dependent, it is not reliable and should not be used for controlling execution order in real applications.