Printing class histogram programmaticly...

Question

is there any way to print top used N classes on the current java application programmaticly?

sample output: N=10

num   #instances    #bytes  class name
--------------------------------------
  1:        23     4723136  [I
  2:        19     4718928  [J
  3:        18     4718880  [D
  4:     73925     1774200  java.lang.String
  5:       208     1226400  [C
  6:        28     1205064  [B
  7:        18     1179936  [F
  8:        68      297040  [Ljava.lang.String;
  9:       332       14136  [Ljava.lang.Object;
 10:        32       10240  <objArrayKlassKlass>

Answer 1

Probably not without going through JVM Tool Interface (JVM TI) or tampering with the implementation of Object (which is a tricky business).

This article is perhaps useful: Creating a Debugging and Profiling Agent with JVMTI.

Answer 2

If you mean by top used, classes that are initialized most, you can define a pointcut around the constructors and keep track of the initializations for each type. You can use AspectJ for that matter.

Answer 3

I don't think you can do it within the same JVM because you need to traverse the object heap, and you might end up going in an infinite loop. Just out of curiosity, I tried spawning jmap using Runtime.exec against the same JVM and even against a different JVM and it just hangs?

String pid = ManagementFactory.getRuntimeMXBean().getName().split("@")[0];
//pid=2520
System.out.println("PID: " + pid);
Process p = Runtime.getRuntime().exec("jmap -histo " + pid);
p.waitFor();
BufferedReader br = new BufferedReader(new InputStreamReader(p.getInputStream()));
String line ;
while((line = br.readLine())!=null){
    System.out.println(line);
}

br = new BufferedReader(new InputStreamReader(p.getErrorStream()));
while((line = br.readLine())!=null){
    System.out.println(line);
}

Answer 4

You could kick off jmap as part of your java wrapper script and run it continuously in a loop:

For example, if you are on Unix, you could do something like:

java MyMainClass ... &

pid=$!
while [ ! -z $pid ]
do
    jmap -histo $pid | head -13
    sleep 60

    #check pid
    kill -0 $pid > /dev/null 2>&1   
    if [ $? -ne 0 ]
    then
       pid=""
    fi  
done