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I am studying about Java JMX and JConsole. I am curious about the memory management abilities of JConsole. For example, there is a "Perform GC" button in the Memory tab :

enter image description here

Suppose I have simple Java app that eats up memory, something like this :

public class MemoryEater
{
  public static void main(String[] args)
  {
    Vector v = new Vector();
    while (true)
    {
      byte b[] = new byte[1048576];
      v.add(b);
      Runtime rt = Runtime.getRuntime();
      System.out.println( "free memory: " + rt.freeMemory() );
    }
  }
}

Would there be a way to configure JConsole to prevent this app from consuming X amount of memory? Or do I need to make a new MBean via JMX ? thanks

Caffeinated
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    You can configure how much memory the Java process can consume as a startup commandline option. I don't think this can be changed at runtime. http://stackoverflow.com/questions/763295/setting-jvm-heap-size-at-runtime – Thilo Mar 01 '16 at 01:42
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    If you are worried about a JVM instance eating up all your OS memory, you can limit the heap size of the JVM with the -Xmx64m where you can replace 64m to specify your own maximum allowed heap size in megabytes. – Nándor Előd Fekete Mar 01 '16 at 01:44

3 Answers3

3

Would there be a way to configure JConsole to prevent this app from consuming X amount of memory?

From JConsole we cannot tune/control the memory limits. Only option I see is using -Xmx during startup of java process. Looking at the below screenshot, JConsole exposes memory parameters as only as readable but not writable.

JConsole memory parameters readonly

Or do I need to make a new MBean via JMX ?

Even if we write our own MBean I don't think it is possible to change the memory limits of java process at runtime. The only way is to configure the memory limits during the start up using -Xms and -Xmx.

Unless the question is specific to your example code, where in you want to limit the number of elements that can be added to the Vector through JMX Bean and there by limiting the memory consumption, which is do-able, but I doubt if that is what you are looking for.

Madhusudana Reddy Sunnapu
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No, you can't. Java memory management is a responsibility of garbage collector and actual software developers. You can tweak memory allocation with different reference types (like Soft references), tune GC parameters and algorithm or try to force GC invocation with JMX or jcmd GC.run utility command. But you can't alter memory boundaries for running JVM or set high-water-mark for a heap occupancy.

If you need another strategy for memory management in Java take a look at products like Terracotta BigMemory.

nukie
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you can only add MBean functions and call on them from JConsole - under operations.

public void add(){
   byte b[] = new byte[1048576];
   v.add(b);
   Runtime rt = Runtime.getRuntime();
   System.out.println( "free memory: " + rt.freeMemory() );
}

see here https://docs.oracle.com/javase/tutorial/jmx/mbeans/standard.html

LiozM
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