Thread Prioritization

By giving priorities to threads, we tell the JVM, give more importance to Highest priority threads, and low importance to low priority threads.

1. How to get the priority of a Thread

    java provides getPriority(), to get the priority of a running thread.

    Syntax

    public final int getPriority()

Returns this thread's priority. This method is final, so we can't override this method

2. By Default main thread runs at the priority of 5.

Example 

class ThreadPriorityEx{
 public static void main(String args[]){
  System.out.println("Main Thread Priority is " + Thread.currentThread().getPriority());
 }
}

   
 Output

Main Thread Priority is 5

   

3. How to set the priority to a Thread

java provides setPriority(), to set the priority of a running thread.

Syntax

public final void setPriority(int newPriority)

Changes the priority of this thread.

4. when a new Thread is constructed, it runs at the same priority as the thread that constructed it.

Example

class ThreadPriorityEx implements Runnable{
 public void run(){
  System.out.println(Thread.currentThread().getName() + "Priority is" + Thread.currentThread().getPriority());
 }
 
 public static void main(String args[]){
  ThreadPriorityEx task = new ThreadPriorityEx();
  Thread.currentThread().setPriority(8);
  
  System.out.println("Main Thread Priority is " + Thread.currentThread().getPriority());
  
  Thread t1 = new Thread(task);
  t1.setName("MyThread");
  t1.start();
 }
}

   
 Output

Main Thread Priority is 8
MyThreadPriority is 8

   

5. Thread Priority Constants

Thread API provides three constants that specify thread priorities

static int MAX_PRIORITY

The maximum priority that a thread can have. Generally this value is 10.

static int MIN_PRIORITY

The minimum priority that a thread can have. Generally this value is 1.

static int NORM_PRIORITY

The default priority that is assigned to a thread. Generally this value is 5.

Example

class ThreadPriorityEx{
 public static void main(String args[]){
  System.out.println("Maximum priority of a thread is " + Thread.MAX_PRIORITY);
  System.out.println("Minimum priority of a thread is " + Thread.MIN_PRIORITY);
  System.out.println("Normal priority of a thread is " + Thread.NORM_PRIORITY);
 }
}

 

 Output

Maximum priority of a thread is 10
Minimum priority of a thread is 1
Normal priority of a thread is 5

6. If you try to set a priority which is less than MIN_PRIORITY and greater than MAX_PRIORITY, then program compiles fine, but throws “IllegalArgumentException” at run time.

Example  

class ThreadPriorityEx implements Runnable{
 public void run(){
 
 }
 
 public static void main(String args[]){
  ThreadPriorityEx task1 = new ThreadPriorityEx();
  
  int priority1 = Thread.MAX_PRIORITY + 1;
  int priority2 = Thread.MIN_PRIORITY - 1;
  
  Thread t1 = new Thread(task1);
  Thread t2 = new Thread(task1);
  
  t1.setPriority(priority1);
  t2.setPriority(priority2);
 }
}

   
 Output

Exception in thread "main" java.lang.IllegalArgumentException
        at java.lang.Thread.setPriority(Unknown Source)
        at ThreadPriorityEx.main(ThreadPriorityEx.java:16)

   

Thread group priorities reflect the thread priorities. Will discuss about more on these in the Thread Group post.

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