NativeJavaAgent

The NativeJavaAgent is a JVMTI library supported by a Java library that allows you to:

Get a count of the number of objects in the heap of a specific type or a type and any type that inherit/implement from that type.
Acquire an object array of all the objects in the heap of a specific type or a type and any type that inherit/implement from that type.

Instance Counts

Here's an simple example of getting instance counts:

	import com.heliosapm.jvmti.agent.Agent;
    
    public static void main(String[] args) {
		
		final Agent agent = Agent.getInstance();
        System.gc();  // a lot of garbage created at boot time
		final long startTime = System.currentTimeMillis();
		
		final int objectCount = agent.getInstanceCountOf(Object.class);
		log("Object instance count:%s", objectCount);
		
		final int charSequenceCount = agent.getInstanceCountOfAny(CharSequence.class);
		log("CharSequence instance count:%s", charSequenceCount);
		
		log("Elapsed:%s ms.", System.currentTimeMillis()-startTime);
	}

The output is:

Initializing Agent OnAttach...
Agent Initialized
Object instance count:467
CharSequence instance count:3196
Elapsed:3 ms.

What's going on here:

Acquire the (singleton) agent using com.heliosapm.jvmti.agent.Agent.getInstance(). In this case, the underlying native JVMTI library was loaded at runtime. The library can be loaded at boot time using -agentpath:oif_agent.so or if not loaded on boot, it will be loaded dynamically when the agent is first called.
Calling agent.getInstanceCountOf(Object.class) returns the number of Object instances found in the heap. The getInstanceCountOf method only counts objects of the exact type so it will not count instances of any type inherrited from java.lang.Object (basically everything else).
On the other hamd, in the next call, agent.getInstanceCountOfAny(CharSequence.class), counts all instances in the heap that extend java.lang.CharSequence.
Since there's not much else going on in the JVM, the number of object instances is fairly small, but agent is fairly quick. Even so, warmup makes a difference. If I add this code to the example above:

		int total = 0;
		for(int i = 0; i < 100; i++) {
			final int oc = agent.getInstanceCountOf(Object.class);
			final int cc = agent.getInstanceCountOfAny(CharSequence.class);
			total += (oc + cc);
			System.gc();
		}

		final long startTime2 = System.currentTimeMillis();
		
		final int objectCount2 = agent.getInstanceCountOf(Object.class);
		log("Object instance count:%s", objectCount2);
		
		final int charSequenceCount2 = agent.getInstanceCountOfAny(CharSequence.class);
		log("CharSequence instance count:%s", charSequenceCount2);
		
		log("Elapsed:%s ms.", System.currentTimeMillis()-startTime2);

.... then the output of the second timing is:

Object instance count:450
CharSequence instance count:3174
Elapsed:1 ms.

Note that background activity and the agent itself generate some number of objects, so for "accurate" counts, I am calling System.gc() at specific points so we're not counting unreachable but uncleared objects.

Instance References

In this example, the agent acquires the actual references to the first 20 java.lang.String instances found on the heap. The maximum number of instances supplied as 20 is optional. If not supplied, it will default to Integer.MAX_VALUE. It then prints a selection of those strings so we can see examples of the sort of strings hanging out in the heap.

	public static void main(String[] args) {
		final Agent agent = Agent.getInstance();
		System.gc();
		String[] strings = agent.getInstancesOf(String.class, 20);
		for(int i = 10; i < 15; i++) {
			log("String #%s: [%s]", i, strings[i]);
		}
		strings = null;  // Don't prevent gc of these objects !
	}

Your output will vary, but in my last test I saw:

Initializing Agent OnAttach...
Agent Initialized
Aborting Instance Tagging after 20 Instances
String #10: [Unexpected vector type encounterd: entry_offset = ]
String #11: [ (0x]
String #12: [), units = ]
String #13: [Unexpected variability attribute: entry_offset = 0x]
String #14: [ name = ]

As with the counting calls, the instance calls come in 2 flavours where getInstancesOf only retrieves objects of the exact supplied type, whereas the getInstancesOfAny retrieves instances of the specified type or any type that inherrits/extends that type. Repeating the same example for CharSequences:

		CharSequence[] charSeqs = agent.getInstancesOfAny(CharSequence.class, 20);
		for(int i = 10; i < 15; i++) {
			log("CharSequence#%s: Type:%s, Value:[%s]", 
            i, charSeqs[i].getClass().getName(), charSeqs[i].toString());
		}
		charSeqs = null;

The output:

CharSequence#10: Type:java.lang.String, Value:[%(\d+\$)?([-#+ 0,(\<]*)?(\d+)?(\.\d+)?([tT])?([a-zA-Z%])]
CharSequence#11: Type:java.lang.String, Value:[DISPLAY]
CharSequence#12: Type:java.lang.String, Value:[user.language.display]
CharSequence#13: Type:java.lang.String, Value:[user.script.display]
CharSequence#14: Type:java.lang.String, Value:[user.country.display]

Top N Type Instances

Top N supports finding the types for which there are the most instances, so you can determine what your top n types in the heap are. For example:

	public static void main(String[] args) {
		final Agent agent = Agent.getInstance();
		System.gc();
		final Map<Class<Object>, Long> topMap = agent.getTopNInstanceCounts(Object.class, 10, true);
		for(Map.Entry<Class<Object>, Long> entry: topMap.entrySet()) {
			log("%s  :  %s", Agent.renderClassName(entry.getKey()), entry.getValue());
		}
		topMap.clear();  // don't prevent gc !
	}

My output was:

Initializing Agent OnAttach...
Agent Initialized
java.lang.String  :  3171
java.lang.Class  :  1105
java.lang.Object[]  :  1052
java.lang.reflect.Method  :  550
java.lang.Object  :  467
java.util.concurrent.ConcurrentHashMap$Node  :  424
java.util.HashMap$Node  :  418
java.lang.Class[]  :  395
java.lang.String[]  :  313
java.lang.reflect.Field  :  295

Note that the Agent.renderClassName method switches the internal array type names to the more visually clear <type>[]... syntax.

The getTopNInstanceCounts parameters are:

The type. Matching is Any since specific type matching is not useful.
The n in TopN.
Indicates if primitives and primitive array types should be ignored in computing the TopN.

Ignoring primitives and primitve arrays can make a big difference when values of N are small. In the example above, if we switch to not ignoring....

Initializing Agent OnAttach...
Agent Initialized
char[]  :  3185
java.lang.String  :  3171
java.lang.Class  :  1105
java.lang.Object[]  :  1052
java.lang.reflect.Method  :  550
java.lang.Object  :  467
java.util.concurrent.ConcurrentHashMap$Node  :  424
java.util.HashMap$Node  :  418
java.lang.Class[]  :  395
java.lang.String[]  :  313

which is not too bad, but for real applications, you may find byte and byte arrays dominate instance counts. Plus, all (non-empty?) strings will always contain char[] instances so will typically outnumber Strings.

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