How to change the priority of a running java process?

Question

In a related question we explored using ProcessBuilder to start external processes in low priority using OS-dependant commands. I also discovered that if a parent process is low priority, then all of its spawned processes start in low priority. So my new question is about starting a java file (run via double-clicking an executable jar in windows) in low priority or changing its priority programmatically during the run. I have tried altering the thread priority, but this has no effect on the windows process priority.

I have tried the following, but it does not change the process priority in the task manager

public class hello{
    public hello(){
        try{
            Thread.currentThread().setPriority(1);
            Thread.sleep(10000);    
        }catch(Exception e){e.printStackTrace();}
    }
}

The only other thing I can think of is to run the program using a batch file, but I would rather keep this in the family so to speak. So does anyone know of a java-based way to change the current process priority? Ideally, it would be nice to be able to change the priority of the process in response to user input while the program is running.

Answer 1

https://stackoverflow.com/questions/257859 discusses how to change the priority of a thread in Windows. I don't know of any Java API to do this, so you're going to have to fall back on JNI to call into the Windows API. In your shoes I think I'd start with JNA which will let you map the functions easily, or find a ready-written Java wrapper to the API if there is one.

Answer 2

Perhaps you are trying to do something the OS does for you.

In Unix, under load, each process is given a short time slice to do its work. If it uses all its time slice it is assume the process is CPU bound it priority is lowers. If it blocks on IO, it is assumed to be IO bound and its priority is raised (because it didn't use all its time slice)

All this only matters if there isn't enough CPU. If you keep you CPU load below 100% most of the time, every process will get as much CPU as it needs and the priority doesn't make much difference.

Answer 3

(The title does not address windows specifically, but the tags do. However I think it might be relevant to know the differences.)

In general scheduling of threads an processes is a kernel dependent feature, there is hardly a portable way to do this. In fact what priority means varies greatkly. For example on NT a high value of 24 means realtime and a value of 1 means idle. On unix this is the opposite: 1 is fastest and larger values are slower.

Of course Java abstracts this information away using .setPriority with a range of 1 (lowest) to 10 (highest).

Something not pointed out yet, but a pretty big problem on many unixes is: By default a user can not increase the priority of a process (that is reduce the nice value), even if the user itself decreased the priority right before.

In contrast on NT I think you can reraise your priority back to default priority.

Simply put: .setPriority may work on windows, but will most likely not work on unix.