java heap analysis with oql: Count unique strings

Question

Im doing a memory analysis of an existing java software. Is there a sql \'group by\' equivalent in oql to see the count of objects with same values but different instances.<

Answer 1

Sadly, there isn't an equivalent to "group by" in OQL. I'm assuming you're talking about the OQL that is used in jhat and VisualVM.

There is an alternative, though. If you use pure JavaScript syntax instead of the "select x from y" syntax then you have the full power of JavaScript to work with.

Even so, the alternative way of getting the information you're looking for isn't simple. For example, here's an OQL "query" that will perform the same task as your query:

var set={};
sum(map(heap.objects("java.lang.String"),function(heapString){
  if(set[heapString.toString()]){
    return 0;
  }
  else{
    set[heapString.toString()]=true;
    return 1;
  }
}));

In this example a regular JavaScript object mimics a set (collection with no duplicates). As the the map function goes through each string, the set is used to determine if the string has already been seen. Duplicates don't count toward the total (return 0) but new strings do (return 1).