使用JOL工具查看java中类的字段分布情况
本文是郑雨迪老师的“深入拆解 Java 虚拟机”第10节 | Java对象的内存布局 的课后作业
下载jol-cli-0.9-full.jar
JOL简介：
可以查阅java虚拟机中对象的内存分布，是OpenJDK的Code Tools项目中的工具，具体可以通过如下两套指令来实现：
$ java -jar /path/to/jol-cli-0.9-full.jar internals java.util.HashMap
$ java -jar /path/to/jol-cli-0.9-full.jar estimates java.util.HashMap
D:\CQQ>java -cp jol-cli-0.9-full.jar org.openjdk.jol.Main internals java.lang.String
# WARNING: Unable to get Instrumentation. Dynamic Attach failed. You may add this JAR as -javaagent manually, or supply -Djdk.attach.allowAttachSelf
# WARNING: Unable to attach Serviceability Agent. Unable to attach even with module exceptions: [org.openjdk.jol.vm.sa.SASupportException: Sense failed., org.openjdk.jol.vm.sa.SASupportException: Sense failed., org.openjdk.jol.vm.sa.SASupportException: Sense failed.]
# Running 64-bit HotSpot VM.
# Using compressed oop with 3-bit shift.
# Using compressed klass with 3-bit shift.
# WARNING | Compressed references base/shifts are guessed by the experiment!
# WARNING | Therefore, computed addresses are just guesses, and ARE NOT RELIABLE.
# WARNING | Make sure to attach Serviceability Agent to get the reliable addresses.
# 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]
Instantiated the sample instance via default constructor.
java.lang.String object internals:
OFFSET SIZE TYPE DESCRIPTION VALUE
0 4 (object header) 01 00 00 00 (00000001 00000000 00000000 00000000) (1)
4 4 (object header) 00 00 00 00 (00000000 00000000 00000000 00000000) (0)
8 4 (object header) d0 16 c8 16 (11010000 00010110 11001000 00010110) (382211792)
12 4 char[] String.value []
16 4 int String.hash 0
20 4 (loss due to the next object alignment)
Instance size: 24 bytes
Space losses: 0 bytes internal + 4 bytes external = 4 bytes total
D:\CQQ>java -cp jol-cli-0.9-full.jar org.openjdk.jol.Main estimates java.lang.String
# WARNING: Unable to get Instrumentation. Dynamic Attach failed. You may add this JAR as -javaagent manually, or supply -Djdk.attach.allowAttachSelf
# WARNING: Unable to attach Serviceability Agent. Unable to attach even with module exceptions: [org.openjdk.jol.vm.sa.SASupportException: Sense failed., org.openjdk.jol.vm.sa.SASupportException: Sense failed., org.openjdk.jol.vm.sa.SASupportException: Sense failed.]
# Running 64-bit HotSpot VM.
# Using compressed oop with 3-bit shift.
# Using compressed klass with 3-bit shift.
# WARNING | Compressed references base/shifts are guessed by the experiment!
# WARNING | Therefore, computed addresses are just guesses, and ARE NOT RELIABLE.
# WARNING | Make sure to attach Serviceability Agent to get the reliable addresses.
# 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]
***** 32-bit VM: **********************************************************
java.lang.String object internals:
OFFSET SIZE TYPE DESCRIPTION VALUE
0 8 (object header) N/A
8 4 char[] String.value N/A
12 4 int String.hash N/A
Instance size: 16 bytes
Space losses: 0 bytes internal + 0 bytes external = 0 bytes total
***** 64-bit VM: **********************************************************
java.lang.String object internals:
OFFSET SIZE TYPE DESCRIPTION VALUE
0 16 (object header) N/A
16 8 char[] String.value N/A
24 4 int String.hash N/A
28 4 (loss due to the next object alignment)
Instance size: 32 bytes
Space losses: 0 bytes internal + 4 bytes external = 4 bytes total
***** 64-bit VM, compressed references enabled: ***************************
java.lang.String object internals:
OFFSET SIZE TYPE DESCRIPTION VALUE
0 12 (object header) N/A
12 4 char[] String.value N/A
16 4 int String.hash N/A
20 4 (loss due to the next object alignment)
Instance size: 24 bytes
Space losses: 0 bytes internal + 4 bytes external = 4 bytes total
***** 64-bit VM, compressed references enabled, 16-byte align: ************
java.lang.String object internals:
OFFSET SIZE TYPE DESCRIPTION VALUE
0 12 (object header) N/A
12 4 char[] String.value N/A
16 4 int String.hash N/A
20 12 (loss due to the next object alignment)
Instance size: 32 bytes
Space losses: 0 bytes internal + 12 bytes external = 12 bytes total下面是看hashmap
D:\CQQ>java -cp jol-cli-0.9-full.jar org.openjdk.jol.Main internals java.util.HashMap
# WARNING: Unable to get Instrumentation. Dynamic Attach failed. You may add this JAR as -javaagent manually, or supply -Djdk.attach.allowAttachSelf
# WARNING: Unable to attach Serviceability Agent. Unable to attach even with module exceptions: [org.openjdk.jol.vm.sa.SASupportException: Sense failed., org.openjdk.jol.vm.sa.SASupportException: Sense failed., org.openjdk.jol.vm.sa.SASupportException: Sense failed.]
# Running 64-bit HotSpot VM.
# Using compressed oop with 3-bit shift.
# Using compressed klass with 3-bit shift.
# WARNING | Compressed references base/shifts are guessed by the experiment!
# WARNING | Therefore, computed addresses are just guesses, and ARE NOT RELIABLE.
# WARNING | Make sure to attach Serviceability Agent to get the reliable addresses.
# 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]
Instantiated the sample instance via default constructor.
java.util.HashMap object internals:
OFFSET SIZE TYPE DESCRIPTION VALUE
0 4 (object header) 01 00 00 00 (00000001 00000000 00000000 00000000) (1)
4 4 (object header) 00 00 00 00 (00000000 00000000 00000000 00000000) (0)
8 4 (object header) 18 bd c8 16 (00011000 10111101 11001000 00010110) (382254360)
12 4 java.util.Set AbstractMap.keySet null
16 4 java.util.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 java.util.HashMap.Node[] HashMap.table null
40 4 java.util.Set HashMap.entrySet null
44 4 (loss due to the next object alignment)
Instance size: 48 bytes
Space losses: 0 bytes internal + 4 bytes external = 4 bytes total
D:\CQQ>java -cp jol-cli-0.9-full.jar org.openjdk.jol.Main estimates java.util.HashMap
# WARNING: Unable to get Instrumentation. Dynamic Attach failed. You may add this JAR as -javaagent manually, or supply -Djdk.attach.allowAttachSelf
# WARNING: Unable to attach Serviceability Agent. Unable to attach even with module exceptions: [org.openjdk.jol.vm.sa.SASupportException: Sense failed., org.openjdk.jol.vm.sa.SASupportException: Sense failed., org.openjdk.jol.vm.sa.SASupportException: Sense failed.]
# Running 64-bit HotSpot VM.
# Using compressed oop with 3-bit shift.
# Using compressed klass with 3-bit shift.
# WARNING | Compressed references base/shifts are guessed by the experiment!
# WARNING | Therefore, computed addresses are just guesses, and ARE NOT RELIABLE.
# WARNING | Make sure to attach Serviceability Agent to get the reliable addresses.
# 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]
***** 32-bit VM: **********************************************************
java.util.HashMap object internals:
OFFSET SIZE TYPE DESCRIPTION VALUE
0 8 (object header) N/A
8 4 java.util.Set AbstractMap.keySet N/A
12 4 java.util.Collection AbstractMap.values N/A
16 4 int HashMap.size N/A
20 4 int HashMap.modCount N/A
24 4 int HashMap.threshold N/A
28 4 float HashMap.loadFactor N/A
32 4 java.util.HashMap.Node[] HashMap.table N/A
36 4 java.util.Set HashMap.entrySet N/A
Instance size: 40 bytes
Space losses: 0 bytes internal + 0 bytes external = 0 bytes total
***** 64-bit VM: **********************************************************
java.util.HashMap object internals:
OFFSET SIZE TYPE DESCRIPTION VALUE
0 16 (object header) N/A
16 8 java.util.Set AbstractMap.keySet N/A
24 8 java.util.Collection AbstractMap.values N/A
32 4 int HashMap.size N/A
36 4 int HashMap.modCount N/A
40 4 int HashMap.threshold N/A
44 4 float HashMap.loadFactor N/A
48 8 java.util.HashMap.Node[] HashMap.table N/A
56 8 java.util.Set HashMap.entrySet N/A
Instance size: 64 bytes
Space losses: 0 bytes internal + 0 bytes external = 0 bytes total
***** 64-bit VM, compressed references enabled: ***************************
java.util.HashMap object internals:
OFFSET SIZE TYPE DESCRIPTION VALUE
0 12 (object header) N/A
12 4 java.util.Set AbstractMap.keySet N/A
16 4 java.util.Collection AbstractMap.values N/A
20 4 int HashMap.size N/A
24 4 int HashMap.modCount N/A
28 4 int HashMap.threshold N/A
32 4 float HashMap.loadFactor N/A
36 4 java.util.HashMap.Node[] HashMap.table N/A
40 4 java.util.Set HashMap.entrySet N/A
44 4 (loss due to the next object alignment)
Instance size: 48 bytes
Space losses: 0 bytes internal + 4 bytes external = 4 bytes total
***** 64-bit VM, compressed references enabled, 16-byte align: ************
java.util.HashMap object internals:
OFFSET SIZE TYPE DESCRIPTION VALUE
0 12 (object header) N/A
12 4 java.util.Set AbstractMap.keySet N/A
16 4 java.util.Collection AbstractMap.values N/A
20 4 int HashMap.size N/A
24 4 int HashMap.modCount N/A
28 4 int HashMap.threshold N/A
32 4 float HashMap.loadFactor N/A
36 4 java.util.HashMap.Node[] HashMap.table N/A
40 4 java.util.Set HashMap.entrySet N/A
44 4 (loss due to the next object alignment)
Instance size: 48 bytes
Space losses: 0 bytes internal + 4 bytes external = 4 bytes total