Finding Memory Usage in Java

Question

Following is the scenario i need to solve. I have struck with two solutions.

I need to maintain a cache of data fetched from database to be shown on a Swing GUI. Whenever my JVM memory exceeds 70% of its allocated memory, i need to warn user regarding excessive usage. And once JVM memory usage exceeds 80%, then i have to halt all the database querying and clean up the existing cache fetched as part of the user operations and notifying the user. During cleanup process, i will manually handle deleting some data based up on some rules and instructs JVM for a GC. Whenever GC occurs, if memory cleans up and reaches 60% of the allocated memory, I need to restart all the Database handling and giving back control to the user.

For checking JVM memory statistics i found following two solutions. Could not able to decide which is best way and why.

1. Runtime.freeMemory() - Thread created to run every 10 seconds and check for the free memory and if memory exceeds the limits mentioned, necessary popups will intimate user and will call the methods to halt the operations and freeing up the memory.

2. MemoryPoolMXBean.getUsage() - Java 5 has introduced JMX to get the snapshot of the memory at runtime. In, JMX i cannot use Threshold notification since it will only notify when memory reaches/exceeds the given threshhold. Only way to use is Polling in MemoryMXBean and check the memory statistics over a period.

In case of using polling, it seems for me both the implementations are going to be same.

Please suggest the advantages of the methods and if there are any other alternatives/any corrections to the methods using.

Answer 1

Just a side note: Runtime.freeMemory() doesn't state the amount of memory that's left of allocating, it's just the amount of memory that's free within the currently allocated memory (which is initially smaller than the maximum memory the VM is configured to use), but grows over time.

When starting a VM, the max memory (Runtime.maxMemory()) just defines the upper limit of memory that the VM may allocate (configurable using the -Xmx VM option). The total memory (Runtime.totalMemory()) is the initial size of the memory allocated for the VM process (configurable using the -Xms VM option), and will dynamically grow every time you allocate more than the currently free portion of it (Runtime.freeMemory()), until it reaches the max memory.

The metric you're interested in is the memory available for further allocation:

long usableFreeMemory= Runtime.getRuntime().maxMemory()
    -Runtime.getRuntime().totalMemory()
    +Runtime.getRuntime().freeMemory()

or:

double usedPercent=(double)(Runtime.getRuntime().totalMemory()
    -Runtime.getRuntime().freeMemory())/Runtime.getRuntime().maxMemory()