How Thread Priorities Work

Every thread in Java has a priority. By default, a thread takes the priority of the thread that created it. You can change a thread’s priority using the setPriority method. The priority must be a number between MIN_PRIORITY (1) and MAX_PRIORITY (10). The normal priority, NORM_PRIORITY, is 5.

The thread scheduler in Java usually prefers threads with higher priority when deciding which thread to run next. However, thread priorities do not work exactly the same on every computer. This is because Java threads rely on the operating system’s thread system. Some systems may have more or fewer priority levels than Java, so Java priorities are “mapped” to the closest operating system priority.

For example:

On Windows, there are seven priority levels. Multiple Java priorities may end up using the same Windows priority.
On Linux with OpenJDK, all threads are treated equally, and priorities are ignored.

In early versions of Java, thread priorities could help control which thread ran first, especially when Java did not use operating system threads. Today, they are not very useful, and most programs do not need to set thread priorities.

For virtual threads, the priority is always NORM_PRIORITY, and trying to change it will have no effect.

Important Methods and Constants

void setPriority(int newPriority) – Changes the priority of the thread. The value must be between Thread.MIN_PRIORITY and Thread.MAX_PRIORITY. Use Thread.NORM_PRIORITY for a normal priority.
Thread.MIN_PRIORITY – The lowest possible priority (1).
Thread.NORM_PRIORITY – The default priority (5).
Thread.MAX_PRIORITY – The highest possible priority (10).

Example:

Thread highThread = new Thread(() -> {
    System.out.println("This thread has high priority.");
});
highThread.setPriority(Thread.MAX_PRIORITY);

Thread lowThread = new Thread(() -> {
    System.out.println("This thread has low priority.");
});
lowThread.setPriority(Thread.MIN_PRIORITY);

highThread.start();
lowThread.start();

In this example, the scheduler may run highThread before lowThread, but the behavior can change depending on the operating system. On Linux, both threads may run in any order because Linux ignores Java thread priorities.