Java Object Layout Dumpers
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These are the very tiny tools to analyze object layout schemes in JVMs.
These tools are using Unsafe heavily to deduce the *actual* object
layout and footprint. This makes the tools much more accurate than
others relying on heap dumps, specification assumptions, etc.
Prepare:
$ mvn clean install
Dump the fields of some class:
$ java -jar target/java-object-layout.jar java.util.HashMap
Running 64-bit HotSpot VM.
Using compressed references with 3-bit shift.
Objects are 8 bytes aligned.
java.util.HashMap
offset size type description
0 12 (object header + first field alignment)
12 4 Set AbstractMap.keySet
16 4 Collection AbstractMap.values
20 4 int HashMap.size
24 4 int HashMap.threshold
28 4 float HashMap.loadFactor
32 4 int HashMap.modCount
36 4 int HashMap.hashSeed
40 1 boolean HashMap.useAltHashing
41 3 (alignment/padding gap)
44 4 Entry[] HashMap.table
48 4 Set HashMap.entrySet
52 4 (loss due to the next object alignment)
56 (object boundary, size estimate)
VM reports 56 bytes per instance
Traverse the reachability graph and count instances and their sizes
(this should be more convenient to use once we publish the Maven artifact):
$ java -jar target/java-object-graph.jar java.util.HashMap
Running 64-bit HotSpot VM.
Using compressed references with 3-bit shift.
Objects are 8 bytes aligned.
java.util.HashMap instance:
count size avg description
1 80 80 [Ljava.util.HashMap$Entry;
1 56 56 java.util.HashMap
2 136 (total)