Dadidadida: 05/01/2007

Monday, May 21, 2007

Heap Dump in Java 6

Just experimenting with the use of jmap and jhat in Java 6. The Object Query Language in jhat that allows the use of javascript closure seems pretty powerful, though also pretty slow.

Practices
# Find out the jvm pid
jps
# Trigger heap dump
jmap -dump:format=b,file=/tmp/java_app-heap.bin <pid>

# Analyzing the heap dump
jhat -J-Xmx326m /tmp/java_app-heap.bin

http://localhost:7000/showInstanceCounts/
http://localhost:7000/oql

#Find out the total size of MyClass instances
select sum(map(heap.objects('foo.bar.MyClass'), 'sizeof(it)'))

# List out all class names matching "foo.bar"
select filter(heap.classes(), "/foo.bar/(it.name)")

# Ditto
select map(filter(heap.classes(), "/foo.bar/(it.name)"),
    function(c) {
      return "<br>"+ c.name;
    })

# List classes that match "foo.bar" with instances > 0
select map(filter(heap.classes(), "/foo.bar/(it.name)"),
    function(c) {
      var len = length(heap.objects(c));
      if (len == 0) return "";
      return "<br>" + c.name + ", instances:" + len;
    })

# List classes that match "foo.bar" with instance sizes > 0
select map(filter(heap.classes(), "/foo.bar/(it.name)"),
    function(c) {
      var totalsize = sum(map(heap.objects(c), 
          function(o) {
              return sizeof(o);
          }
      ));
      if (totalsize == 0) return "";
      return "<br>" + c.name + ", size:" + totalsize;
    })