freeMemory() going down every one hour in tomcat without any requests

Question

I encountered a problem when I deploy a war to a cloud server(PasS,128Mb), and then I have two timers in my application.

1. Timer1 will be executing a task to log my memory(as well as a Map described in #2) every one hour.
2. Every time I create a Room object, I put this room into a Map, so I have a Timer2 to create a task when I initiate a Roomto remove that object from a map.

I happen to know that that I should not use a timer.

But the problem is that I check the log logged by Timer1, my freeMemory is going down every hour and the total memory is the same.

I have read What are Runtime.getRuntime().totalMemory() and freeMemory()?, so I know what the total memory and freeMemory means. And according to the log, my used memory is going up!

PS: I do NOT use any new Runnable or Thread to handle any long-running logic.

Logs are like:

2014-07-22 04:39:27  [ Timer-10:44100005 ] - [ INFO ]  
maxMemory()=128
totalMemory() = 75
freeMemory()= 44
2014-07-22 04:39:27  [ Timer-10:44100005 ] - [ INFO ]  Now there are Existing Room of : 0

2014-07-22 05:39:27  [ Timer-10:47700005 ] - [ INFO ]  
maxMemory()=128
totalMemory() = 75
freeMemory()= 41
2014-07-22 05:39:27  [ Timer-10:47700005 ] - [ INFO ]  Now there are Existing Room of : 0

2014-07-22 06:39:27  [ Timer-10:51300006 ] - [ INFO ]  
maxMemory()=128
totalMemory() = 75
freeMemory()= 39

I am logging every time when this sevlet receive a request, so I am sure that no requests is handed,but why free memory going down.

1.This is how Timer1 works:

@Override
public void init() throws ServletException {
    super.init();
    LogUtil.info("CoreServlet inits");
    memoryTimer=new Timer(true);
    //check the memory every 1 hour,first time will be delayed to 15 mins later
    memoryTimer.schedule(new checkMemoryTask(),TimeConstant.FIFTHTEEN_MINUTES_MILISECONDS,TimeConstant.ONE_HOUR_MILISECONDS);
    commandService = CommandService.getInstance();
}

2.This is how Timer2 works.

public Room newRoom() {
    Integer randomNumObject;

    LogUtil.debug("Trying to new a room with the random number");
    //generate a random number util the number is not a existing room number    
    do {
        randomNumObject=generateRandom(4);//4 digits room number
    } while (roomMap.containsKey(randomNumObject));//roomNum is existing, continue to find  

    Room newRoom=new Room(randomNumObject);//use this number to create a new room

    roomMap.put(randomNumObject, newRoom);
    //abandon the room after 15 MINTES
    roomClearTimer.schedule(new abandonRoomTask(randomNumObject.intValue()), TimeConstant.FIFTHTEEN_MINUTES_MILISECONDS);
    LogUtil.info(newRoom+" is scheduled to be abandoned 15 MINUTES later");
    return newRoom;
}

Answer 1

There isn't necessarily a real problem here at all.

As the Javadoc for freeMemory() states:

Returns the amount of free memory in the Java Virtual Machine. Calling the gc method may result in increasing the value returned by freeMemory.

In fact, what typically happens in a Java program is that as you allocate new objects, the amount of free memory decreases by the same amount. When the amount of free memory hits a threshold (determined by the JVM), the GC runs behinds the scenes, and reclaims a bunch of garbage. That causes the free memory to go back up again.

You only have a problem (e.g. a memory leak) if the amount of free memory immediately after garbage collection is continually trending downwards.

---

In your specific case, the reason that freeMemory() is trending downwards is that objects are being allocated. (Timer2 seems to be one possible source of new objects, but it could also be something the web container code.)

This is only a problem if those objects don't get garbage collected ... eventually.