jcmd 的 GC.class_stats 子命令
jcmd 的 GC.class_stats 子命令 输出 JVM 里metaspace (元数据)区的 class 相关的信息. 这里必须记住, 所有的 java.lang.Class 及其子类都在heap, 元数据区只是这些class的元数据, 元数据区存的也不是这些类编译后的字节码(bytescode).
GC.class_stats 能输出每个class 在 metaspace 空间占用的大小.
这个子命令在JDK 14 deprecated, 15去掉.
默认只输出下面这些列, 还有更多列可以显示, 需要指定列名称:
InstBytes,KlassBytes,CpAll,annotations,MethodCount,Bytecodes,MethodAll,ROAll,RWAll,Total$ bin/jcmd <pid> GC.class_stats由于类太多, 并且长度会折叠, 这里截图开头和结尾的截图:
最好结合 jcmd <pid> VM.metaspace 一起看
关于任意每列的具体含义, 使用 jcmd <pid> GC.class_stats -help 查看:
InstSize:
Size of each object instance of the Java class. 每个实例的shallow size
InstCount:
Number of object instances of the Java class. heap 实例个数
InstBytes:
This is usually (InstSize * InstNum). The only exception is java.lang.Class,
whose InstBytes also includes the slots used to store static
fields. InstBytes is not counted in ROAll, RWAll or Total. 所有实例占用heap的大小
Mirror:
Size of the Klass::java_mirror() object.
KlassBytes:
Size of the InstanceKlass or ArrayKlass for this class. Note
that this includes VTab, ITab, OopMap.
K_secondary_supers:
Number of bytes used by the Klass::secondary_supers() array.
VTab:
Size of the embedded vtable in InstanceKlass.
ITab:
Size of the embedded itable in InstanceKlass.
OopMap:
Size of the embedded nonstatic_oop_map in InstanceKlass.
IK_methods:
Number of bytes used by the InstanceKlass::methods() array.
IK_method_ordering:
Number of bytes used by the InstanceKlass::method_ordering()
array.
IK_default_methods:
Number of bytes used by the InstanceKlass::default_methods()
array.
IK_default_vtable_indices:
Number of bytes used by the InstanceKlass::default_vtable_indices()
array.
IK_local_interfaces:
Number of bytes used by the InstanceKlass::local_interfaces()
array.
IK_transitive_interfaces:
Number of bytes used by the InstanceKlass::transitive_interfaces()
array.
IK_fields:
Number of bytes used by the InstanceKlass::fields() array.
IK_inner_classes:
Number of bytes used by the InstanceKlass::inner_classes() array.
IK_nest_members:
Number of bytes used by the InstanceKlass::nest_members() array.
IK_signers:
Number of bytes used by the InstanceKlass::singers() array.
class_annotations:
Size of class annotations.
class_type_annotations:
Size of class type annotations.
fields_annotations:
Size of field annotations.
fields_type_annotations:
Size of field type annotations.
methods_annotations:
Size of method annotations.
methods_parameter_annotations:
Size of method parameter annotations.
methods_type_annotations:
Size of methods type annotations.
methods_default_annotations:
Size of methods default annotations.
annotations:
Size of all annotations.
Cp:
Size of InstanceKlass::constants().
CpTags:
Size of InstanceKlass::constants()->tags().
CpCache:
Size of InstanceKlass::constants()->cache().
CpOperands:
Size of InstanceKlass::constants()->operands().
CpRefMap:
Size of InstanceKlass::constants()->reference_map().
CpAll:
Sum of Cp + CpTags + CpCache + CpOperands + CpRefMap.
MethodCount:
Number of methods in this class.
MethodBytes:
Size of the Method object.
ConstMethod:
Size of the ConstMethod object.
MethodData:
Size of the MethodData object.
StackMap:
Size of the stackmap_data.
Bytecodes:
Of the MethodBytes column, how much are the space taken up by
bytecodes.
MethodAll:
Sum of MethodBytes + Constmethod + Stackmap + Methoddata.
ROAll:
Size of all class meta data that could (potentially) be placed
in read-only memory. (This could change with CDS design).
RWAll:
Size of all class meta data that must be placed in read/write
memory. (This could change with CDS design) .
Total:
ROAll + RWAll. Note that this does NOT include InstBytes..