Java Object Layout (JOL)
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These are the 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.
Source compile:
$ mvn clean install
The project workspace is arranged as follows:
a) jol-core: the core functionality, which is suitable to
be used as the dependency for the headless tools
b) jol-cli: the command line utilities using the core, generally
suitable for end users, see the usages below
c) jol-samples: sample programs showing off the kinds of information
available with the help of JOL
Command line tools:
*) Get the object internals, that is, the field layout within the object,
header information, field values, the data on alignment losses.
$ java -jar jol-cli/target/jol-internals.jar java.util.HashMap
Running 64-bit HotSpot VM.
Using compressed references with 3-bit shift.
Objects are 8 bytes aligned.
java.util.HashMap object internals:
OFFSET SIZE TYPE DESCRIPTION VALUE
0 4 (object header) 01 00 00 00 (00000001 00000000 00000000 00000000)
4 4 (object header) 00 00 00 00 (00000000 00000000 00000000 00000000)
8 4 (object header) 0f 0f 3e e0 (00001111 00001111 00111110 11100000)
12 4 Set AbstractMap.keySet null
16 4 Collection AbstractMap.values null
20 4 int HashMap.size 0
24 4 int HashMap.threshold 16
28 4 float HashMap.loadFactor 0.75
32 4 int HashMap.modCount 0
36 4 int HashMap.hashSeed 0
40 4 Entry[] HashMap.table []
44 4 Set HashMap.entrySet null
Instance size: 48 bytes (estimated, add this JAR via -javaagent: to get accurate result)
Space losses: 0 bytes internal + 0 bytes external = 0 bytes total
*) Get the object externals, that is, the objects reachable from the instance,
their addresses, paths through the reachability grapsh, etc (is is more
convenient with API though):
$ java -jar jol-cli/target/jol-externals.jar java.util.concurrent.ConcurrentHashMap
Running 64-bit HotSpot VM.
Using compressed references with 3-bit shift.
Objects are 8 bytes aligned.
java.util.concurrent.ConcurrentHashMap@1436183764 object externals:
ADDRESS SIZE TYPE PATH
7ad137ff8 48 java.util.concurrent.ConcurrentHashMap
7ad138028 40 java.util.concurrent.ConcurrentHashMap$Segment .segments[0]
7ad138050 24 [Ljava.util.concurrent.ConcurrentHashMap$HashEntry; .segments[0].table
7ad138068 32 java.util.concurrent.locks.ReentrantLock$NonfairSync .segments[0].sync
7ad138088 80 [Ljava.util.concurrent.ConcurrentHashMap$Segment; .segments
*) Get the object footprint estimate, same as the object externals, but tabulated:
$ java -jar jol-cli/target/jol-footprint.jar java.lang.Thread
Running 64-bit HotSpot VM.
Using compressed references with 3-bit shift.
Objects are 8 bytes aligned.
java.lang.Thread instance footprint:
COUNT AVG SUM DESCRIPTION
4 22 88 [B
155 69 10720 [C
2 24 48 [Ljava.lang.Class;
(.................)
1 24 24 sun.nio.cs.UTF_8
1 40 40 sun.nio.cs.UTF_8$Decoder
2 48 96 sun.nio.cs.UTF_8$Encoder
952 37464 (total)
*) Get the estimated object layout in different VM modes (EXPERIMENTAL):
$ java -jar jol-cli/target/jol-estimates.jar java.math.BigInteger
***** 32-bit VM: **********************************************************
java.math.BigInteger object internals:
OFFSET SIZE TYPE DESCRIPTION VALUE
0 8 (object header) N/A
8 4 int BigInteger.signum N/A
12 4 int[] BigInteger.mag N/A
16 4 int BigInteger.bitCount N/A
20 4 int BigInteger.bitLength N/A
24 4 int BigInteger.lowestSetBit N/A
28 4 int BigInteger.firstNonzeroIntNum N/A
Instance size: 32 bytes (estimated, the sample instance is not available)
Space losses: 0 bytes internal + 0 bytes external = 0 bytes total
***** 64-bit VM: **********************************************************
java.math.BigInteger object internals:
OFFSET SIZE TYPE DESCRIPTION VALUE
0 16 (object header) N/A
16 8 int[] BigInteger.mag N/A
24 4 int BigInteger.signum N/A
28 4 int BigInteger.bitCount N/A
32 4 int BigInteger.bitLength N/A
36 4 int BigInteger.lowestSetBit N/A
40 4 int BigInteger.firstNonzeroIntNum N/A
44 4 (loss due to the next object alignment)
Instance size: 48 bytes (estimated, the sample instance is not available)
Space losses: 0 bytes internal + 4 bytes external = 4 bytes total
***** 64-bit VM, compressed references enabled: ***************************
java.math.BigInteger object internals:
OFFSET SIZE TYPE DESCRIPTION VALUE
0 12 (object header) N/A
12 4 int BigInteger.signum N/A
16 4 int[] BigInteger.mag N/A
20 4 int BigInteger.bitCount N/A
24 4 int BigInteger.bitLength N/A
28 4 int BigInteger.lowestSetBit N/A
32 4 int BigInteger.firstNonzeroIntNum N/A
36 4 (loss due to the next object alignment)
Instance size: 40 bytes (estimated, the sample instance is not available)
Space losses: 0 bytes internal + 4 bytes external = 4 bytes total
*) Consume the heap dump and figure the perceived footprint in different VM
modes (EXPERIMENTAL):
$ java -jar jol-cli/target/jol-heapdump.jar heapdump.hprof
Heap Dump: heapdump.hprof
Estimated heap consumed, bytes:
173,564,985: Raw data (X32 model)
179,480,339: VM Layout Simulation (X32 model)
-0.000% 179,480,067: VM Layout Simulation (X32 model, hierarchy gaps)
-0.000% 179,480,043: VM Layout Simulation (X32 model, super gaps)
-2.692% 174,649,167: VM Layout Simulation (X32 model, autoalign)
-2.693% 174,647,815: VM Layout Simulation (X32 model, hierarchy gaps, autoalign)
-2.693% 174,647,803: VM Layout Simulation (X32 model, super gaps, autoalign)
232,120,233: Raw data (X64 model)
237,629,451: VM Layout Simulation (X64 model)
-0.000% 237,629,155: VM Layout Simulation (X64 model, hierarchy gaps)
-0.001% 237,626,779: VM Layout Simulation (X64 model, super gaps)
-0.059% 237,488,735: VM Layout Simulation (X64 model, autoalign)
-0.059% 237,488,439: VM Layout Simulation (X64 model, hierarchy gaps, autoalign)
-0.060% 237,486,063: VM Layout Simulation (X64 model, super gaps, autoalign)
190,397,793: Raw data (X64 model (compressed oops))
199,205,031: VM Layout Simulation (X64 model (compressed oops))
-0.001% 199,202,303: VM Layout Simulation (X64 model (compressed oops), hierarchy gaps)
-0.014% 199,177,791: VM Layout Simulation (X64 model (compressed oops), super gaps)
-3.845% 191,544,859: VM Layout Simulation (X64 model (compressed oops), autoalign)
-3.846% 191,543,211: VM Layout Simulation (X64 model (compressed oops), hierarchy gaps, autoalign)
-3.859% 191,518,687: VM Layout Simulation (X64 model (compressed oops), super gaps, autoalign)