代码工具：jol
JOL（Java对象布局）是用于分析JVM中的对象布局方案的微型工具箱。这些工具大量使用Unsafe，JVMTI和Serviceability Agent（SA）来解码实际的 对象布局，占用空间和引用。这使得JOL比依赖堆转储的其他工具更加准确。
作为maven包引入
OpenJDK社区将这些发行版本定期地推送到Maven Central，你可以通过在pom中添加以下的依赖项来使用它：
<dependency> <groupId> org.openjdk.jol </ groupId> <artifactId> jol-core </ artifactId> <version>version</ version> </ dependency>
在使用之前建议你先看下 JOL Samples 和 CLI tools source 。
作为命令行工具
如果你是通过源代码构建的JOL，那么会生成一个可执行JAR包 jol-cli/target/jol-cli.jar。已发布在Maven的组件也包含这个JAR，您可以立即下载并开始使用它。请在此处查找最新版本。
以下是我们的简要操作清单：
- 获取对象的内部信息，即对象内的字段布局，标题信息，字段值，对齐损失数据。
$ java -jar jol-cli/target/jol-cli.jar internals java.util.HashMap
Running 64-bit HotSpot VM.
Using compressed oop with 3-bit shift.
Using compressed klass with 3-bit shift.
Objects are 8 bytes aligned.
Field sizes by type: 4, 1, 1, 2, 2, 4, 4, 8, 8 [bytes]
Array element sizes: 4, 1, 1, 2, 2, 4, 4, 8, 8 [bytes]
java.util.HashMap object internals:
OFFSET SIZE TYPE DESCRIPTION VALUE
0 4 (object header) 05 00 00 00 (0000 0101 0000 0000 0000 0000 0000 0000)
4 4 (object header) 00 00 00 00 (0000 0000 0000 0000 0000 0000 0000 0000)
8 4 (object header) 8c 3b 00 f8 (1000 1100 0011 1011 0000 0000 1111 1000)
12 4 Set AbstractMap.keySet null
16 4 Collection AbstractMap.values null
20 4 int HashMap.size 0
24 4 int HashMap.modCount 0
28 4 int HashMap.threshold 0
32 4 float HashMap.loadFactor 0.75
36 4 Node[] HashMap.table null
40 4 Set HashMap.entrySet null
44 4 (loss due to the next object alignment)
Instance size: 48 bytes (reported by Instrumentation API)
Space losses: 0 bytes internal + 4 bytes external = 4 bytes total
- 获取对象的externals，即从实例可访问的对象，它们的地址，通过可达性图的路径等：
$ java -jar jol-cli/target/jol-cli.jar externals java.util.PriorityQueue
Running 64-bit HotSpot VM.
Using compressed oop with 3-bit shift.
Using compressed klass with 3-bit shift.
Objects are 8 bytes aligned.
Field sizes by type: 4, 1, 1, 2, 2, 4, 4, 8, 8 [bytes]
Array element sizes: 4, 1, 1, 2, 2, 4, 4, 8, 8 [bytes]
java.util.PriorityQueue object externals:
ADDRESS SIZE TYPE PATH VALUE
719231ad0 32 java.util.PriorityQueue (object)
719231af0 64 [Ljava.lang.Object; .queue [null, null, null, null, null, null, null, null, null, null, null]
获取对象 footprint 估算值，与获取对象的externals元素相同，但列表如下 ：
$ java -jar jol-cli/target/jol-cli.jar footprint java.util.Hashtable
Running 64-bit HotSpot VM.
Using compressed oop with 3-bit shift.
Using compressed klass with 3-bit shift.
Objects are 8 bytes aligned.
Field sizes by type: 4, 1, 1, 2, 2, 4, 4, 8, 8 [bytes]
Array element sizes: 4, 1, 1, 2, 2, 4, 4, 8, 8 [bytes]
java.util.Hashtable instance footprint:
COUNT AVG SUM DESCRIPTION
1 64 64 [Ljava.util.Hashtable$Entry;
1 48 48 java.util.Hashtable
2 112 (total)
- 获取不同VM模式下的对象布局（EXPERIMENTAL） ：
$ java -jar jol-cli/target/jol-cli.jar estimates java.math.BigInteger
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
***** 64-bit VM, compressed references enabled, 16-byte align: ************
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 12 (loss due to the next object alignment)
Instance size: 48 bytes (estimated, the sample instance is not available)
Space losses: 0 bytes internal + 12 bytes external = 12 bytes total
- 使用堆转储计算在不同VM模式下可感知的占用空间（EXPERIMENTAL）：
$ java -jar jol-cli/target/jol-cli.jar heapdump 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)
