Getting JVM heap size, used memory, total memory using Java Runtime

java-textReading runtime information about Java is useful sometime when your application is struggling in getting resources. System Memory is one of the main resource that an application developer has to consider while managing the application. Hence sometimes it is beneficial to see what amount of memory is your application using and what is the free memory in system. Java’s Runtime class provide lot of information about the resource details of Java Virtual Machine or JVM. The memory consumed by the JVM can be read by different methods in Runtime class. Following is the small example of getting/reading JVM Heap Size, Total Memory and used memory using Java Runtime api.
/** * Class: TestMemory * @author: Viral Patel * @description: Prints JVM memory utilization statistics */ public class TestMemory { public static void main(String [] args) { int mb = 1024*1024; //Getting the runtime reference from system Runtime runtime = Runtime.getRuntime(); System.out.println("##### Heap utilization statistics [MB] #####"); //Print used memory System.out.println("Used Memory:" + (runtime.totalMemory() - runtime.freeMemory()) / mb); //Print free memory System.out.println("Free Memory:" + runtime.freeMemory() / mb); //Print total available memory System.out.println("Total Memory:" + runtime.totalMemory() / mb); //Print Maximum available memory System.out.println("Max Memory:" + runtime.maxMemory() / mb); } }
Code language: Java (java)
You may also want to print the memory utilization in Kilobytes KBs. For that just assign 1024 value to the int variable mb.
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  1. scottyab says:

    Thanks for this, saved me typing ;)

  2. Vish says:

    Hi Viral,
    I want to get JVM heap size of WebS portal server pre/post its restarts.

    Is there any way I can run the above script on RHEL linu box and get it working.

    Please advise.


    • Rob says:

      I believe you can do this with Sun/Oracle’s jmap utility
      find the pid(process Id or Task Id of the JVM process/task and e.g
      e.g jmap 1477
      It should work try it?

  3. Rob says:

    Can you explain to me why the settings do not match the reported amounts?
    java -Xms65m -Xmx256m TestMemory
    comes out:

    ##### Heap utilization statistics [MB] #####
    Used Memory:0
    Free Memory:61
    Total Memory:62
    Max Memory:227

    Since no memory is used shouldn’t I have 65MB yet I see only 61MB what happened to 4MB?
    Also the Maximum was set to 256 and yet the max shows 227MB. What is happening here?
    Overhead? From what ???

  4. rkr says:

    It’s just a bug, and one that won’t be resolved, as per:

  5. redstar says:

    in jvm initially i calculated the free memory.. then i created many variables and used it.. after garbage collection.. i again calculated free memory.. it is not equal to the initial free memory.. why this mismatch???????? plz ans me…

  6. rams says:

    public class Add extends java.lang.object{
    public Add();
    0: aload-0
    1: invoke special #1;//Method java/lang/object.””()u
    4: return
    public static void main(java.lang.String[]);
    0: bipush 10
    2: istore-1
    3: bipush 20
    5: istore-2
    6: iload-1
    7: iload-2
    8: iadd
    9: istore-1
    10: return

    i want to run command to see the jvm flags pls help me out

  7. vakeel says:

    its very useful information to get the memory information by using application.

  8. JECII says:

    Thank you – used this for debugging today. Very helpful.

  9. srikanth says:

    Used Memory is always less than the values shows up on Linux top command. Why is this difference?
    top command values are not even close to TotalMemory when we give the spike. How do I get the top command values through JAVA program.

  10. Mark says:

    Very helpful — thank you for posting.

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