Java multithreading - thread priority

Thread priority is just a hint to OS task scheduler and is dependent on the underlying OS. OS will try to allocate more resources to a high priority thread but it does not guarantee it. So if your program is dependent on thread priorities than you are in-turn binding your program to underlying OS, which is bad.

From Java Concurrency in Practice:

Avoid the temptation to use thread priorities, since they increase platform dependence and can cause liveness problems. Most concurrent applications can use the default priority for all threads.

Thread priority is only a hint to OS task scheduler. Task scheduler will only try to allocate more resources to a thread with higher priority, however there are no explicit guarantees.

In fact, it is not only relevant to Java or JVM. Most non-real time OS use thread priorities (managed or unmanaged) only in a suggestive manner.

Also very important, Priorties are different to every underlying plattform. Therefore you kind of loose your plattform freedom of java. See this summary as well:

Indeed, some priority levels can map to the same "native" priority level. Here's the list (based on the Hotspot code in OpenJDK 6):

Solaris
1 ⇒ 0
2 ⇒ 32
3 ⇒ 64
4 ⇒ 96
5 – 10 ⇒ 127
Of note is that on Solaris, you can't raise the thread priority above normal, only lower it: the priority value for 5 is the same as any of the higher values.

Linux
1 – 10 ⇒ 4 – -5 (nice values)Of note is that on Linux, different thread priorities in Java do map to distinct priority values at native level.

Windows
1 – 2 ⇒ THREAD_PRIORITY_LOWEST
3 – 4 ⇒ THREAD_PRIORITY_BELOW_NORMAL
5 – 6 ⇒ THREAD_PRIORITY_NORMAL
7 – 8 ⇒ THREAD_PRIORITY_ABOVE_NORMAL
9 – 10 ⇒ THREAD_PRIORITY_HIGHEST

I've developed a lot of multi threaded java applications, and in my opinion if you have to set thead priorities you have another problem. Like a bad algorythm that consumes to much cpu time etc.... I always suggest to not change the java thread prio, since you can't rely on it anyways. (of course there are some scenarios where it makes sense)

There are several situations where setting a priority for a Thread is useful, you just must not start believing in any guarantees that come form such settings, as always: the order of execution with threads is undefined. If you think that your thread will be done earlier if you give it a higher priority, please read the statement above again until that's out of your head. ;)

You should define the priority based on how important it is for this thread to gain execution time compared to the others. Even though the underlaying system guarantees nothing, it still can provide a benefit for those JVMs that support it.

Example: You have a program that does some heavy calculation based on user input in your JFrame. It is a good practice to set those background threads to a low priority, because it is more important that the JFrame is responsive than working on the calculation (which will take a long time to finish anyways, a few more millis won't hurt). The OS will still assign most CPU time to the low priority threads, unless something more important needs it. But then it is good that this more important stuff gets the priority.

Now it could happen that on some systems the priority setting is ignored and the JFrame is again unresponsive, but then this does nothing worse to your code then not setting the priority to low in the first place.