JVM（9）：虚拟机性能分析和故障解决工具之jmap工具

1 jmap(Memory Map for Java)作用

一个多功能的命令，它可以生成 java 程序的 dump 文件，也可以查看堆内对象信息、查看 ClassLoader 的信息以及 finalizer 队列

2 命令格式

jmap [options]

参数解释：

第一个参数：options

no option：查看进程的内存映像信息,类似 Solaris pmap 命令。
heap：显示Java堆详细信息
histo[:live]：显示堆中对象的统计信息
clstats：打印类加载器信息
finalizerinfo：显示在F-Queue队列等待Finalizer线程执行finalizer方法的对象
dump：生成堆转储快照

3 命令演示

代码如下：

package com.example.demo;import java.io.IOException;/*jmap:命令像信息使用不带选项参数的jmap打印共享对象映射，将会打印目标虚拟机中加载的每个共享对象的起始地址、映射大小以及共享对象文件的路径全称命令：jmap -heap pid描述：显示Java堆详细信息打印一个堆的摘要信息，包括使用的GC算法、堆配置信息和各内存区域内存使用信息命令：jmap -histo:live pid命令：jmap -histo pid描述：显示堆中对象的统计信息其中包括每个Java类、对象数量、内存大小(单位：字节)、完全限定的类名。打印的虚拟机内部的类名称将会带有一个’*’前缀。如果指定了live子选项，则只计算活动的对象命令：jmap -clstats pid描述：打印类加载器信息-clstats是-permstat的替代方案，在JDK8之前，-permstat用来打印类加载器的数据打印Java堆内存的永久保存区域的类加载器的智能统计信息。对于每个类加载器而言，它的名称、活跃度、地址、父类加载器、它所加载的类的数量和大小都会被打印。此外，包含的字符串数量和大小也会被打印。命令：jmap -finalizerinfo pid描述：打印等待终结的对象信息Number of objects pending for finalization:0 说明当前F-Queue队列中并没有等待Finalizer线程执行finalizer方法的对象。命令：jmap -dump:live,format=b,file=jmap.bin pid描述：生成堆转储快照dump文件以二进制格式转储Java堆到指定filename的文件中。live子选项是可选的。如果指定了live子选项，堆中只有活动的对象会被转储。想要浏览heap dump，你可以使用jhat(Java堆分析工具)读取生成的文件。这个命令执行，JVM会将整个heap的信息dump写入到一个文件，heap如果比较大的话，就会导致这个过程比较耗时，并且执行的过程中为了保证dump的信息是可靠的，所以会暂停应用， 线上系统慎用*/
public class jmapdemo {public static void main(String[] args) throws IOException {jmapdemo jmapdemo1 = new jmapdemo();System.out.println("jmap");System.in.read();}
}

运行后进入命令行工具输入命令。

（1）命令一

jmap pid

描述：查看进程的内存映像信息

使用不带选项参数的jmap打印共享对象映射，将会打印目标虚拟机中加载的每个共享对象的起始地址、映射大小以及共享对象文件的路径全称

如下：

H:\project\jvmtestdemo>jmap 28016
Attaching to process ID 28016, please wait...
Debugger attached successfully.
Server compiler detected.
JVM version is 25.261-b12
0x000000005b830000      8624K   C:\Program Files\Java\jdk1.8.0_261\jre\bin\server\jvm.dll
0x000000005f400000      52K     D:\Program Files (x86)\360\360Safe\safemon\SafeWrapper.dll
0x00007ff7fddc0000      284K    C:\Program Files\Java\jdk1.8.0_261\bin\java.exe
0x00007ff862220000      620K    C:\Program Files\Java\jdk1.8.0_261\jre\bin\msvcp140.dll
0x00007ff87d6e0000      172K    C:\Program Files\Java\jdk1.8.0_261\jre\bin\java.dll
0x00007ff87fd80000      112K    C:\Program Files\Java\jdk1.8.0_261\jre\bin\net.dll
0x00007ff880190000      96K     C:\Program Files\Java\jdk1.8.0_261\jre\bin\zip.dll
0x00007ff880500000      84K     C:\Program Files\Java\jdk1.8.0_261\jre\bin\vcruntime140.dll
0x00007ff892be0000      104K    D:\Program Files\JetBrains\IntelliJ IDEA 2019.3.2\bin\breakgen64.dll
0x00007ff896ed0000      64K     C:\Program Files\Java\jdk1.8.0_261\jre\bin\verify.dll
0x00007ff89d3d0000      36K     C:\WINDOWS\SYSTEM32\WSOCK32.dll
0x00007ff89f940000      200K    C:\Program Files\Java\jdk1.8.0_261\jre\bin\instrument.dll
0x00007ff8a3fd0000      2664K   C:\WINDOWS\WinSxS\amd64_microsoft.windows.common-controls_6595b64144ccf1df_6.0.19041.4355_none_60b8b9eb71f62e16\COMCTL32.dll
0x00007ff8a4720000      1036K   D:\Program Files (x86)\360\360Safe\safemon\libzdtp64.dll
0x00007ff8a4f90000      156K    C:\WINDOWS\SYSTEM32\WINMM.dll
0x00007ff8b2650000      72K     C:\WINDOWS\SYSTEM32\kernel.appcore.dll
0x00007ff8b2850000      7796K   C:\WINDOWS\SYSTEM32\windows.storage.dll
0x00007ff8b3f50000      424K    C:\WINDOWS\system32\mswsock.dll
0x00007ff8b41f0000      184K    C:\WINDOWS\SYSTEM32\Wldp.dll
0x00007ff8b4390000      40K     C:\WINDOWS\SYSTEM32\VERSION.dll
0x00007ff8b4770000      144K    C:\WINDOWS\SYSTEM32\profapi.dll
0x00007ff8b48f0000      1116K   C:\WINDOWS\System32\gdi32full.dll
0x00007ff8b4a60000      3032K   C:\WINDOWS\System32\KERNELBASE.dll
0x00007ff8b4dd0000      156K    C:\WINDOWS\System32\bcrypt.dll
0x00007ff8b4e00000      136K    C:\WINDOWS\System32\win32u.dll
0x00007ff8b4e30000      1024K   C:\WINDOWS\System32\ucrtbase.dll
0x00007ff8b4f30000      628K    C:\WINDOWS\System32\msvcp_win.dll
0x00007ff8b51c0000      756K    C:\WINDOWS\System32\KERNEL32.DLL
0x00007ff8b5430000      1164K   C:\WINDOWS\System32\RPCRT4.dll
0x00007ff8b55e0000      820K    C:\WINDOWS\System32\OLEAUT32.dll
0x00007ff8b5830000      172K    C:\WINDOWS\System32\GDI32.dll
0x00007ff8b5860000      32K     C:\WINDOWS\System32\PSAPI.DLL
0x00007ff8b5870000      704K    C:\WINDOWS\System32\ADVAPI32.dll
0x00007ff8b5a60000      7596K   C:\WINDOWS\System32\SHELL32.dll
0x00007ff8b62c0000      1660K   C:\WINDOWS\System32\USER32.dll
0x00007ff8b6460000      632K    C:\WINDOWS\System32\msvcrt.dll
0x00007ff8b6b20000      692K    C:\WINDOWS\System32\SHCORE.dll
0x00007ff8b6bd0000      428K    C:\WINDOWS\System32\WS2_32.dll
0x00007ff8b6c40000      200K    C:\WINDOWS\System32\IMM32.DLL
0x00007ff8b6c80000      3404K   C:\WINDOWS\System32\combase.dll
0x00007ff8b7050000      640K    C:\WINDOWS\System32\sechost.dll
0x00007ff8b70f0000      340K    C:\WINDOWS\System32\SHLWAPI.dll
0x00007ff8b7190000      2016K   C:\WINDOWS\SYSTEM32\ntdll.dll

（2）命令二

jmap -heap pid

描述：显示Java堆详细信息

打印一个堆的摘要信息，包括使用的GC算法、堆配置信息和各内存区域内存使用信息

如下：

H:\project\jvmtestdemo>jmap -heap 28016
Attaching to process ID 28016, please wait...
Debugger attached successfully.
Server compiler detected.
JVM version is 25.261-b12using thread-local object allocation.
Parallel GC with 10 thread(s)Heap Configuration:MinHeapFreeRatio         = 0MaxHeapFreeRatio         = 100MaxHeapSize              = 10697572352 (10202.0MB)NewSize                  = 222822400 (212.5MB)MaxNewSize               = 3565682688 (3400.5MB)OldSize                  = 446169088 (425.5MB)NewRatio                 = 2SurvivorRatio            = 8MetaspaceSize            = 21807104 (20.796875MB)CompressedClassSpaceSize = 1073741824 (1024.0MB)MaxMetaspaceSize         = 17592186044415 MBG1HeapRegionSize         = 0 (0.0MB)Heap Usage:
PS Young Generation
Eden Space:capacity = 167247872 (159.5MB)used     = 13379896 (12.760063171386719MB)free     = 153867976 (146.73993682861328MB)8.00003960588509% used
From Space:capacity = 27787264 (26.5MB)used     = 0 (0.0MB)free     = 27787264 (26.5MB)0.0% used
To Space:capacity = 27787264 (26.5MB)used     = 0 (0.0MB)free     = 27787264 (26.5MB)0.0% used
PS Old Generationcapacity = 446169088 (425.5MB)used     = 0 (0.0MB)free     = 446169088 (425.5MB)0.0% used3185 interned Strings occupying 261152 bytes.

（3）命令三

jmap -histo:live pid

描述：显示堆中对象的统计信息

其中包括每个Java类、对象数量、内存大小(单位：字节)、完全限定的类名。打印的虚拟机内部的类名称将会带有一个’*’前缀。如果指定了live子选项，则只计算活动的对象

如下：

H:\project\jvmtestdemo>jmap -histo:live 28016num     #instances         #bytes  class name
----------------------------------------------1:          4586         439440  [C2:           420         135856  [B3:          4437         106488  java.lang.String4:           706          80896  java.lang.Class5:           634          41568  [Ljava.lang.Object;6:           791          31640  java.util.TreeMap$Entry7:           629          25160  java.util.LinkedHashMap$Entry8:           425          19040  [Ljava.lang.String;9:           373          11936  java.util.HashMap$Node10:            24           8640  [Ljava.util.HashMap$Node;11:           113           8136  java.lang.reflect.Field12:           132           7664  [I13:            87           5568  java.net.URL14:           110           4400  java.lang.ref.SoftReference15:           256           4096  java.lang.Integer16:           121           3872  java.util.Hashtable$Entry17:           107           3424  java.util.concurrent.ConcurrentHashMap$Node18:             8           3008  java.lang.Thread19:            42           2352  sun.misc.URLClassPath$JarLoader20:            26           2080  java.lang.reflect.Constructor21:            15           1968  [Ljava.util.concurrent.ConcurrentHashMap$Node;22:            39           1872  sun.util.locale.LocaleObjectCache$CacheEntry23:            43           1720  java.lang.ref.Finalizer24:            34           1632  java.util.HashMap25:             1           1568  [[B26:            93           1488  java.lang.Object27:            37           1480  java.io.ObjectStreamField28:            26           1456  java.lang.Class$ReflectionData29:            20           1280  java.util.concurrent.ConcurrentHashMap30:            15           1200  [Ljava.util.WeakHashMap$Entry;31:             9           1184  [Ljava.util.Hashtable$Entry;32:             2           1064  [Ljava.lang.invoke.MethodHandle;33:             1           1040  [Ljava.lang.Integer;34:             1           1040  [[C35:            41            984  java.io.ExpiringCache$Entry36:            15            840  sun.nio.cs.UTF_8$Encoder37:            12            768  java.util.jar.JarFile38:            19            760  sun.util.locale.BaseLocale$Key39:            15            720  java.util.WeakHashMap40:            22            704  java.lang.ref.ReferenceQueue41:             7            672  java.util.jar.JarFile$JarFileEntry42:            12            672  java.util.zip.ZipFile$ZipFileInputStream43:            11            648  [Ljava.lang.reflect.Field;44:             8            640  [S45:            19            608  java.io.File46:            19            608  java.util.Locale47:            19            608  sun.util.locale.BaseLocale48:            13            520  java.security.AccessControlContext49:            21            504  java.util.jar.Attributes$Name50:            19            456  java.util.Locale$LocaleKey51:            18            432  sun.misc.MetaIndex52:            10            400  java.io.FileDescriptor53:            10            400  sun.nio.cs.UTF_8$Decoder54:            13            392  [Ljava.io.ObjectStreamField;55:             1            384  com.intellij.rt.execution.application.AppMainV2$156:             1            384  java.lang.ref.Finalizer$FinalizerThread57:            24            384  java.lang.ref.ReferenceQueue$Lock58:             6            384  java.nio.DirectByteBuffer59:            12            384  java.util.zip.ZipCoder60:             1            376  java.lang.ref.Reference$ReferenceHandler61:            18            352  [Ljava.lang.Class;62:             6            336  java.nio.DirectLongBufferU63:            10            320  java.lang.OutOfMemoryError64:             3            312  [D65:            13            312  [Ljava.lang.reflect.Constructor;66:            13            312  sun.reflect.NativeConstructorAccessorImpl67:            12            288  java.util.ArrayDeque68:             3            264  java.lang.reflect.Method69:            11            264  java.net.StandardSocketOptions$StdSocketOption70:            11            264  java.util.ArrayList71:             8            256  java.util.Vector72:             3            240  [Ljava.lang.ThreadLocal$ThreadLocalMap$Entry;73:             5            240  java.util.Hashtable74:             6            240  java.util.WeakHashMap$Entry75:             7            224  java.lang.ThreadLocal$ThreadLocalMap$Entry76:             4            224  java.util.LinkedHashMap77:             4            224  sun.util.calendar.ZoneInfo78:             9            216  java.util.LinkedList$Node79:            13            208  sun.reflect.DelegatingConstructorAccessorImpl80:             5            200  java.security.ProtectionDomain81:             6            192  java.io.FileInputStream82:             4            192  java.util.Properties83:             4            192  java.util.TreeMap84:             2            160  [[Ljava.lang.String;85:             4            160  java.lang.ClassLoader$NativeLibrary86:             5            160  java.security.CodeSource87:             5            160  java.util.LinkedList88:             5            160  sun.util.locale.provider.LocaleProviderAdapter$Type89:             3            144  java.nio.HeapByteBuffer90:             6            144  sun.misc.PerfCounter91:             3            144  sun.misc.URLClassPath92:             2            128  java.io.ExpiringCache$193:             4            128  java.util.Stack94:             2            128  sun.nio.cs.ext.DoubleByte$Encoder95:             1            120  java.net.SocksSocketImpl96:             5            120  java.util.Collections$UnmodifiableRandomAccessList97:             5            120  sun.misc.FloatingDecimal$PreparedASCIIToBinaryBuffer98:             2            112  java.lang.Package99:             7            112  java.util.HashSet100:             2            112  java.util.ResourceBundle$CacheKey101:             2            112  java.util.zip.ZipFile$ZipFileInflaterInputStream102:             2            112  sun.nio.cs.ext.DoubleByte$Decoder103:             3             96  java.io.FileOutputStream104:             4             96  java.lang.RuntimePermission105:             3             96  java.lang.StringCoding$StringEncoder106:             2             96  java.lang.ThreadGroup107:             6             96  java.lang.ThreadLocal108:             1             96  java.net.DualStackPlainSocketImpl109:             2             96  java.util.ResourceBundle$BundleReference110:             2             96  java.util.zip.Inflater111:             4             96  jdk.net.ExtendedSocketOptions$ExtSocketOption112:             1             96  sun.misc.Launcher$AppClassLoader113:             3             96  sun.net.spi.DefaultProxySelector$NonProxyInfo114:             2             96  sun.nio.cs.StreamEncoder115:             1             88  sun.misc.Launcher$ExtClassLoader116:             5             80  [Ljava.security.Principal;117:             2             80  java.io.BufferedWriter118:             2             80  java.io.ExpiringCache119:             5             80  java.security.ProtectionDomain$Key120:             2             80  sun.misc.FloatingDecimal$BinaryToASCIIBuffer121:             3             72  java.lang.ThreadLocal$ThreadLocalMap122:             3             72  java.net.Proxy$Type123:             3             72  java.util.Arrays$ArrayList124:             1             72  java.util.ResourceBundle$RBClassLoader125:             3             72  java.util.concurrent.atomic.AtomicLong126:             3             72  sun.misc.FloatingDecimal$ExceptionalBinaryToASCIIBuffer127:             1             72  sun.util.locale.provider.JRELocaleProviderAdapter128:             1             64  [F129:             2             64  [Ljava.lang.Thread;130:             2             64  java.io.FilePermission131:             2             64  java.io.PrintStream132:             2             64  java.lang.ClassValue$Entry133:             2             64  java.lang.StringCoding$StringDecoder134:             2             64  java.lang.VirtualMachineError135:             2             64  java.lang.ref.ReferenceQueue$Null136:             2             64  java.lang.ref.WeakReference137:             2             64  java.security.BasicPermissionCollection138:             2             64  java.security.Permissions139:             2             64  java.util.ResourceBundle$LoaderReference140:             1             48  [J141:             2             48  [Ljava.lang.reflect.Method;142:             2             48  java.io.BufferedOutputStream143:             1             48  java.io.BufferedReader144:             2             48  java.io.File$PathStatus145:             2             48  java.io.FilePermissionCollection146:             2             48  java.io.OutputStreamWriter147:             2             48  java.net.InetAddress$Cache148:             2             48  java.net.InetAddress$Cache$Type149:             1             48  java.net.SocketInputStream150:             1             48  java.nio.HeapCharBuffer151:             2             48  java.nio.charset.CoderResult152:             3             48  java.nio.charset.CodingErrorAction153:             3             48  java.util.Collections$UnmodifiableSet154:             3             48  java.util.HashMap$KeySet155:             2             48  java.util.zip.ZStreamRef156:             2             48  sun.misc.NativeSignalHandler157:             2             48  sun.misc.Signal158:             3             48  sun.net.www.protocol.jar.Handler159:             1             48  sun.nio.cs.StreamDecoder160:             1             48  sun.nio.cs.US_ASCII$Decoder161:             1             48  sun.util.resources.TimeZoneNames162:             1             48  sun.util.resources.en.TimeZoneNames_en163:             1             40  [Lsun.util.locale.provider.LocaleProviderAdapter$Type;164:             1             40  java.io.BufferedInputStream165:             1             40  java.util.ResourceBundle$1166:             1             40  sun.nio.cs.StandardCharsets$Aliases167:             1             40  sun.nio.cs.StandardCharsets$Cache168:             1             40  sun.nio.cs.StandardCharsets$Classes169:             1             40  sun.nio.cs.ext.ExtendedCharsets170:             1             32  [Ljava.lang.OutOfMemoryError;171:             2             32  [Ljava.lang.StackTraceElement;172:             1             32  [Ljava.lang.ThreadGroup;173:             1             32  [Ljava.net.Proxy$Type;174:             1             32  java.io.WinNTFileSystem175:             1             32  java.lang.ArithmeticException176:             2             32  java.lang.Boolean177:             1             32  java.lang.NullPointerException178:             1             32  java.net.InetAddress$InetAddressHolder179:             1             32  java.net.Socket180:             2             32  java.nio.ByteOrder181:             2             32  java.util.LinkedHashMap$LinkedKeySet182:             2             32  java.util.concurrent.atomic.AtomicInteger183:             1             32  java.util.concurrent.atomic.AtomicReferenceFieldUpdater$AtomicReferenceFieldUpdaterImpl184:             1             32  jdk.net.ExtendedSocketOptions$1185:             1             32  sun.instrument.InstrumentationImpl186:             1             32  sun.nio.cs.StandardCharsets187:             1             32  sun.util.locale.provider.LocaleResources188:             1             32  sun.util.locale.provider.LocaleServiceProviderPool189:             1             24  [Ljava.io.File$PathStatus;190:             1             24  [Ljava.lang.ClassValue$Entry;191:             1             24  [Ljava.net.InetAddress$Cache$Type;192:             1             24  [Ljava.security.ProtectionDomain;193:             1             24  [Lsun.launcher.LauncherHelper;194:             1             24  java.io.InputStreamReader195:             1             24  java.lang.ClassValue$Version196:             1             24  java.lang.StringBuilder197:             1             24  java.lang.invoke.MethodHandleImpl$4198:             1             24  java.lang.reflect.ReflectPermission199:             1             24  java.net.Inet4Address200:             1             24  java.net.Inet6AddressImpl201:             1             24  java.net.Proxy202:             1             24  java.util.BitSet203:             1             24  java.util.Collections$EmptyMap204:             1             24  java.util.Collections$SetFromMap205:             1             24  java.util.Collections$SynchronizedSet206:             1             24  java.util.Locale$Cache207:             1             24  java.util.ResourceBundle$Control$CandidateListCache208:             1             24  java.util.jar.Manifest209:             1             24  sun.instrument.TransformerManager210:             1             24  sun.launcher.LauncherHelper211:             1             24  sun.misc.JarIndex212:             1             24  sun.misc.URLClassPath$FileLoader213:             1             24  sun.nio.cs.ISO_8859_1214:             1             24  sun.nio.cs.ThreadLocalCoders$1215:             1             24  sun.nio.cs.ThreadLocalCoders$2216:             1             24  sun.nio.cs.US_ASCII217:             1             24  sun.nio.cs.UTF_16218:             1             24  sun.nio.cs.UTF_16BE219:             1             24  sun.nio.cs.UTF_16LE220:             1             24  sun.nio.cs.UTF_8221:             1             24  sun.nio.cs.ext.GBK222:             1             24  sun.reflect.NativeMethodAccessorImpl223:             1             24  sun.util.locale.BaseLocale$Cache224:             1             24  sun.util.locale.provider.TimeZoneNameProviderImpl225:             1             16  [Ljava.lang.Throwable;226:             1             16  [Ljava.security.cert.Certificate;227:             1             16  [Lsun.instrument.TransformerManager$TransformerInfo;228:             1             16  com.example.demo.jmapdemo229:             1             16  java.io.FileDescriptor$1230:             1             16  java.lang.CharacterDataLatin1231:             1             16  java.lang.ClassValue$Identity232:             1             16  java.lang.Runtime233:             1             16  java.lang.String$CaseInsensitiveComparator234:             1             16  java.lang.System$2235:             1             16  java.lang.Terminator$1236:             1             16  java.lang.invoke.MemberName$Factory237:             1             16  java.lang.invoke.MethodHandleImpl$2238:             1             16  java.lang.invoke.MethodHandleImpl$3239:             1             16  java.lang.ref.Reference$1240:             1             16  java.lang.ref.Reference$Lock241:             1             16  java.lang.reflect.ReflectAccess242:             1             16  java.net.InetAddress$2243:             1             16  java.net.URLClassLoader$7244:             1             16  java.nio.Bits$1245:             1             16  java.nio.charset.CoderResult$1246:             1             16  java.nio.charset.CoderResult$2247:             1             16  java.security.ProtectionDomain$2248:             1             16  java.security.ProtectionDomain$JavaSecurityAccessImpl249:             1             16  java.util.Collections$EmptyIterator250:             1             16  java.util.Collections$EmptyList251:             1             16  java.util.Collections$EmptySet252:             1             16  java.util.HashMap$EntrySet253:             1             16  java.util.Hashtable$EntrySet254:             1             16  java.util.ResourceBundle$Control255:             1             16  java.util.WeakHashMap$KeySet256:             1             16  java.util.concurrent.atomic.AtomicBoolean257:             1             16  java.util.jar.Attributes258:             1             16  java.util.jar.JavaUtilJarAccessImpl259:             1             16  java.util.zip.ZipFile$1260:             1             16  jdk.net.ExtendedSocketOptions$PlatformSocketOptions261:             1             16  sun.misc.ASCIICaseInsensitiveComparator262:             1             16  sun.misc.FloatingDecimal$1263:             1             16  sun.misc.Launcher264:             1             16  sun.misc.Launcher$Factory265:             1             16  sun.misc.Perf266:             1             16  sun.misc.Unsafe267:             1             16  sun.net.spi.DefaultProxySelector268:             1             16  sun.net.www.protocol.file.Handler269:             1             16  sun.reflect.DelegatingMethodAccessorImpl270:             1             16  sun.reflect.ReflectionFactory271:             1             16  sun.util.calendar.Gregorian272:             1             16  sun.util.locale.provider.AuxLocaleProviderAdapter$NullProvider273:             1             16  sun.util.locale.provider.SPILocaleProviderAdapter274:             1             16  sun.util.locale.provider.TimeZoneNameUtility$TimeZoneNameGetter275:             1             16  sun.util.resources.LocaleData276:             1             16  sun.util.resources.LocaleData$LocaleDataResourceBundleControl
Total         15227         994896

（4）命令四

jmap -histo pid

描述：显示堆中对象的统计信息

（5）命令五

jmap -clstats pid

描述：打印类加载器信息

-clstats是-permstat的替代方案，在JDK8之前，-permstat用来打印类加载器的数据

打印Java堆内存的永久保存区域的类加载器的智能统计信息。对于每个类加载器而言，它的名称、活跃度、地址、父类加载器、它所加载的类的数量和大小都会被打印。此外，包含的字符串数量和大小也会被打印。

（6）命令六

jmap -finalizerinfo pid

描述：打印等待终结的对象信息

Number of objects pending for finalization:0 说明当前F-Queue队列中并没有等待Finalizer线程执行finalizer方法的对象。

（7）命令七

jmap -dump:live,format=b,file=d:\jmap.bin pid

描述：生成堆转储快照dump文件

以hprof二进制格式转储Java堆到指定filename的文件中。live子选项是可选的。如果指定了live子选项，堆中只有活动的对象会被转储。想要浏览heap dump，你可以使用jhat(Java堆分析工具)读取生成的文件。这个命令执行，JVM会将整个heap的信息dump写入到一个文件，heap如果比较大的话，就会导致这个过程比较耗时，并且执行的过程中为了保证dump的信息是可靠的，所以会暂停应用，线上系统慎用