jmap
jmap命令用于生成heap dump文件，如果不使用这个命令，还可以使用-XX:+HeapDumpOnOutOfMemoryError参数来让虚拟机出现OOM的时候自动生成dump文件。jmap不仅能生成dump文件，还可以查询finalize执行队列、Java堆和永久代的详细信息，如当前使用率、当前使用的是哪种收集器等。
命令格式： jmap [ option ] pid jmap [ option ] executable core jmap [ option ] [[email protected]]remote-hostname-or-IP
参数
option：选项参数，不可同时使用多个选项参数
pid：java进程id，命令ps -ef | grep java获取
executable：产生核心dump的java可执行文件
core：需要打印配置信息的核心文件
remote-hostname-or-ip：远程调试的主机名或ip
server-id：可选的唯一id，如果相同的远程主机上运行了多台调试服务器，用此选项参数标识服务器
options参数
heap : 显示Java堆详细信息
histo : 显示堆中对象的统计信息
permstat :Java堆内存的永久保存区域的类加载器的统计信息
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
F : 当-dump没有响应时，强制生成dump快照
示例
-dump
dump堆到文件,format指定输出格式，live指明是活着的对象,file指定文件名
[[email protected] jdk1.7.0_79]# jmap -dump:live,format=b,file=dump.hprof 24971 Dumping heap to /usr/local/java/jdk1.7.0_79/dump.hprof ... Heap dump file created
-heap
打印heap的概要信息，GC使用的算法，heap的配置及使用情况，可以用此来判断内存目前的使用情况以及垃圾回收情况
[[email protected] jdk1.7.0_79]# jmap -heap 24971 Attaching to process ID 24971, please wait... Debugger attached successfully. Server compiler detected. JVM version is 24.79-b02 using thread-local object allocation. Parallel GC with 4 thread(s) Heap Configuration: MinHeapFreeRatio = 0 MaxHeapFreeRatio = 100 MaxHeapSize = 4146069504 (3954.0MB) NewSize = 1310720 (1.25MB) MaxNewSize = 17592186044415 MB OldSize = 5439488 (5.1875MB) NewRatio = 2 SurvivorRatio = 8 PermSize = 21757952 (20.75MB) MaxPermSize = 85983232 (82.0MB) G1HeapRegionSize = 0 (0.0MB) Heap Usage: PS Young Generation Eden Space: capacity = 517996544 (494.0MB) used = 151567520 (144.54605102539062MB) free = 366429024 (349.4539489746094MB) 29.26033421566612% used From Space: capacity = 41943040 (40.0MB) used = 0 (0.0MB) free = 41943040 (40.0MB) 0.0% used To Space: capacity = 40370176 (38.5MB) used = 0 (0.0MB) free = 40370176 (38.5MB) 0.0% used PS Old Generation capacity = 115343360 (110.0MB) used = 32927184 (31.401809692382812MB) free = 82416176 (78.59819030761719MB) 28.54709972034801% used PS Perm Generation capacity = 85983232 (82.0MB) used = 54701200 (52.16712951660156MB) free = 31282032 (29.832870483398438MB) 63.6184506300019% used 20822 interned Strings occupying 2441752 bytes.
-finalizerinfo
打印等待回收的对象信息，
[[email protected] jdk1.7.0_79]# jmap -finalizerinfo 24971 Attaching to process ID 24971, please wait... Debugger attached successfully. Server compiler detected. JVM version is 24.79-b02 Number of objects pending for finalization: 0
Number of objects pending for finalization: 0 说明当前F-QUEUE队列中并没有等待Fializer线程执行final
izer方法的对象。
-histo
打印堆的对象统计，包括对象数、内存大小等等。jmap -histo:live 这个命令执行，JVM会先触发gc，然后再统计信息
[[email protected] jdk1.7.0_79]# jmap -histo:live 24971 | more num #instances #bytes class name ---------------------------------------------- 1: 100134 14622728 <constMethodKlass> 2: 100134 12830128 <methodKlass> 3: 88438 12708392 [C 4: 8271 10163584 <constantPoolKlass> 5: 27806 9115784 [B 6: 8271 6225312 <instanceKlassKlass> 7: 6830 5632192 <constantPoolCacheKlass> 8: 86717 2081208 java.lang.String 9: 2264 1311720 <methodDataKlass> 10: 10880 870400 java.lang.reflect.Method 11: 8987 869888 java.lang.Class 12: 13330 747264 [[I 13: 11808 733872 [S 14: 20110 643520 java.util.concurrent.ConcurrentHashMap$HashEntry 15: 18574 594368 java.util.HashMap$Entry 16: 3668 504592 [Ljava.util.HashMap$Entry; 17: 30698 491168 java.lang.Integer 18: 2247 486864 [I 19: 7486 479104 java.net.URL 20: 8032 453616 [Ljava.lang.Object; 21: 10259 410360 java.util.LinkedHashMap$Entry 22: 699 380256 <objArrayKlassKlass> 23: 5782 277536 org.apache.catalina.loader.ResourceEntry 24: 8327 266464 java.lang.ref.WeakReference 25: 2374 207928 [Ljava.util.concurrent.ConcurrentHashMap$HashEntry; 26: 3440 192640 java.util.LinkedHashMap 27: 4779 191160 java.lang.ref.SoftReference 28: 3576 171648 java.util.HashMap 29: 10080 161280 java.lang.Object
jmap -histo:live 24971 | grep com.yuhuo 查询类名包含com.yuhuo的信息
jmap -histo:live 24971 | grep com.yuhuo > histo.txt 保存信息到histo.txt文件
附 - jmap输出中class name非自定义类的说明：
| BaseType Character | Type | Interpretation |
|---|---|---|
| B | byte | signed byte |
| C | char | Unicode character |
| D | double | double-precision floating-point value |
| F | float | single-precision floating-point value |
| I | int | integer |
| J | long | long integer |
| L; | reference | an instance of class |
| S | short | signed short |
| Z | boolean | true or false |
| [ | reference | one array dimension，[I表示int[] |
-permstat
打印Java堆内存的永久区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。
[[email protected] jdk1.7.0_79]# jmap -permstat 24971 Attaching to process ID 24971, please wait... Debugger attached successfully. Server compiler detected. JVM version is 24.79-b02 finding class loader instances ..done. computing per loader stat ..done. please wait.. computing liveness....................................................liveness analysis may be inaccurate ... class_loader classes bytes parent_loader alive? type <bootstrap> 3034 18149440 null live <internal> 0x000000070a88fbb8 1 3048 null dead sun/reflect/[email protected] 0x000000070a914860 1 3064 0x0000000709035198 dead sun/reflect/[email protected] 0x000000070a9fc320 1 3056 0x0000000709035198 dead sun/reflect/[email protected] 0x000000070adcb4c8 1 3064 0x0000000709035198 dead sun/reflect/[email protected] 0x000000070a913760 1 1888 0x0000000709035198 dead sun/reflect/[email protected] 0x0000000709f3fd40 1 3032 null dead sun/reflect/[email protected] 0x000000070923ba78 1 3088 0x0000000709035260 dead sun/reflect/[email protected] 0x000000070a88fff8 1 3048 null dead sun/reflect/[email protected] 0x000000070adcbc58 1 1888 0x0000000709035198 dead sun/reflect/[email protected]
-F
强制模式。如果指定的pid没有响应，请使用jmap -dump或jmap -histo选项。此模式下，不支持live子选项。
原文地址：https://www.cnblogs.com/what-/p/9057883.html