Priority of a Thread (Thread Priority)

In a Multi threading environment, thread scheduler assigns processor to a thread based on priority of thread. Whenever we create a thread in Java, it always has some priority assigned to it. Priority can either be given by JVM while creating the thread or it can be given by programmer explicitly.
Accepted value of priority for a thread is in range of 1 to 10.

There are are 3 basic priority constants defined in the Thread Class –

1. public static int MIN_PRIORITY (1)
2. public static int NORM_PRIORITY (5)
3. public static int MAX_PRIORITY (10)

Default priority of a thread is 5 (NORM_PRIORITY). The value of MIN_PRIORITY is 1 and the value of MAX_PRIORITY is 10.

Get and Set Thread Priority –

1. public final int getPriority(): java.lang.Thread.getPriority() method returns priority of given thread.
2. public final void setPriority(int newPriority): java.lang.Thread.setPriority() method changes the priority of thread to the value newPriority. This method throws IllegalArgumentException if value of parameter newPriority goes beyond minimum(1) and maximum(10) limit.

Lets take a program example to understand the working of Thread Priority –

class TestMultiPriority1 extends Thread{  
 public void run(){  
   System.out.println("running thread name is:"+Thread.currentThread().getName());  
   System.out.println("running thread priority is:"+Thread.currentThread().getPriority());  
  
  }  
 public static void main(String args[]){  
  TestMultiPriority1 m1=new TestMultiPriority1();  
  TestMultiPriority1 m2=new TestMultiPriority1();  
  m1.setPriority(Thread.MIN_PRIORITY);  
  m2.setPriority(Thread.MAX_PRIORITY);  
  m1.start();  
  m2.start();  
   
 }  
}

Output:running thread name is:Thread-0
       running thread priority is:10
       running thread name is:Thread-1
       running thread priority is:1

Some Important Points to remember – 

• Thread with highest priority will get execution chance prior to other threads. Suppose there are 3 threads t1, t2 and t3 with priorities 4, 6 and 1. So, thread t2 will execute first based on maximum priority 6 after that t1 will execute and then t3.
• Default priority for main thread is always 5, it can be changed later. Default priority for all other threads depends on the priority of parent thread.
• If two threads have same priority then we can’t expect which thread will execute first. It depends on thread scheduler’s algorithm(Round-Robin, First Come First Serve, etc)
• If we are using thread priority for thread scheduling then we should always keep in mind that underlying platform should provide support for scheduling based on thread priority.