jmap命令能够获取运行中的jvm的快照,从而离线剖析,查看内存透露,查看一些重大影响性能的大对象的创立,查看零碎中最多的对象,各种对象所占用的内存大小.能够应用jmap生成Heap Dump.
什么是堆Dump
堆Dump是反馈Java堆应用状况的内存镜像,其中次要蕴含零碎信息,虚拟机属性,残缺的线程Dump,所有类和对象的状态等。个别，在内存不足,GC异样等状况下,咱们就会狐疑内存透露,这个时候就能够制作堆(Dump)来查问具体情况。
常见的内存谬误
> outOfMemoryError 年轻代内存不足。> outOfMemoryError:PermGen Space 永恒代内存不足。> outOfMemoryError:GC overhead limit exceed 垃圾回收工夫占用零碎运行工夫的98%或以上。jmap -heap pid
查看java堆信息
Attaching to process ID 18378, please wait...Debugger attached successfully.Server compiler detected.JVM version is 25.261-b12using thread-local object allocation.Parallel GC with 4 thread(s)Heap Configuration: MinHeapFreeRatio = 0 # JVM最小闲暇比率 MaxHeapFreeRatio = 100 MaxHeapSize = 4164943872 (3972.0MB) NewSize = 87031808 (83.0MB) MaxNewSize = 1388314624 (1324.0MB) OldSize = 175112192 (167.0MB) NewRatio = 2 SurvivorRatio = 8 MetaspaceSize = 21807104 (20.796875MB) CompressedClassSpaceSize = 1073741824 (1024.0MB) MaxMetaspaceSize = 17592186044415 MB G1HeapRegionSize = 0 (0.0MB)Heap Usage:PS Young GenerationEden Space: capacity = 1314914304 (1254.0MB) used = 794405392 (757.6040191650391MB) free = 520508912 (496.39598083496094MB) 60.41499355383087% usedFrom Space: capacity = 36175872 (34.5MB) used = 23758320 (22.657699584960938MB) free = 12417552 (11.842300415039062MB) 65.6744915506114% usedTo Space: capacity = 37224448 (35.5MB) used = 0 (0.0MB) free = 37224448 (35.5MB) 0.0% usedPS Old Generation capacity = 352321536 (336.0MB) used = 206117376 (196.56884765625MB) free = 146204160 (139.43115234375MB) 58.502633231026785% used应用了Parallel GC垃圾收集器- MinHeapFreeRatio 参数用来设置堆空间最小闲暇比例，默认值是 0。当堆空间的闲暇内存小于这个数值时，JVM 便会扩大堆空间。
- MaxHeapFreeRatio 参数用来设置堆空间最大闲暇比例，默认值是 100。当堆空间的闲暇内存大于这个数值时，便会压缩堆空间，失去一个较小的堆。
当-Xmx 和-Xms 相等时MinHeapFreeRatio和MaxHeapFreeRatio 两个参数有效。
- MaxHeapSize 最大堆内存3972MB
- NewSize 新生代默认大小 83MB
- MaxNewSize 新生代最大大小为 1324MB
- OldSize 老年代大小 167MB
- NewRatio 新生代和老年代的大小比率 2
- SurvivorRatio 年老代中Eden和Survivor的比率 8
- MetaspaceSize 元空间大小 20.796875MB
- CompressedClassSpaceSize 如果开启了-XX:+UseCompressedOops及-XX:+UseCompressedClassesPointers(默认是开启)，则UseCompressedOops会应用32-bit的offset来代表java object的援用，而UseCompressedClassPointers则应用32-bit的offset来代表64-bit过程中的class pointer；能够应用CompressedClassSpaceSize来设置这块的空间大小
- MaxMetaspaceSize 最大元空间大小 1073741824
- G1HeapRegionSize G1收集器启用,一个Region的大小能够通过参数-XX:G1HeapRegionSize设定，取值范畴从1M到32M，且是2的指数。
jmap pid
查看过程的内存映像信息,相似 Solaris pmap 命令应用不带选项参数的jmap打印共享对象映射，将会打印指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门路全称。这与Solaris的pmap工具比拟类似。
[root@rumenz ~]# jmap 18378Attaching to process ID 18378, please wait...Debugger attached successfully.Server compiler detected.JVM version is 25.261-b120x000056248b911000 8K /usr/local/jdk1.8/bin/java0x00007f35f8b2e000 487K /usr/local/jdk1.8/jre/lib/amd64/libfontmanager.so0x00007f35f8d91000 38K /usr/local/jdk1.8/jre/lib/amd64/libawt_headless.so0x00007f35f8f99000 741K /usr/local/jdk1.8/jre/lib/amd64/libawt.so0x00007f35f9669000 86K /usr/lib64/libgcc_s-4.8.5-20150702.so.10x00007f35f987f000 276K /usr/local/jdk1.8/jre/lib/amd64/libsunec.so0x00007f36189ed000 110K /usr/local/jdk1.8/jre/lib/amd64/libnet.so0x00007f3618c04000 91K /usr/local/jdk1.8/jre/lib/amd64/libnio.so0x00007f361991f000 50K /usr/local/jdk1.8/jre/lib/amd64/libmanagement.so0x00007f363caee000 124K /usr/local/jdk1.8/jre/lib/amd64/libzip.so0x00007f363cd0a000 60K /usr/lib64/libnss_files-2.17.so0x00007f363cf1d000 226K /usr/local/jdk1.8/jre/lib/amd64/libjava.so0x00007f363d14c000 64K /usr/local/jdk1.8/jre/lib/amd64/libverify.so0x00007f363d35b000 42K /usr/lib64/librt-2.17.so0x00007f363d563000 1110K /usr/lib64/libm-2.17.so0x00007f363d865000 16698K /usr/local/jdk1.8/jre/lib/amd64/server/libjvm.so0x00007f363e85a000 2105K /usr/lib64/libc-2.17.so0x00007f363ec28000 18K /usr/lib64/libdl-2.17.so0x00007f363ee2c000 106K /usr/local/jdk1.8/lib/amd64/jli/libjli.so0x00007f363f044000 138K /usr/lib64/libpthread-2.17.so0x00007f363f260000 159K /usr/lib64/ld-2.17.sojmap -histo:live pid
堆中对象统计
其中包含每个Java类、对象数量、内存大小(单位：字节)、齐全限定的类名。打印的虚拟机外部的类名称将会带有一个*前缀。如果指定了live子选项，则只计算流动的对象。
[root@Server-i-b2l6uawirw ~]# jmap -histo:live 18378 | more num #instances #bytes class name---------------------------------------------- 1: 221097 19915168 [C 2: 220601 5294424 java.lang.String 3: 44932 3954016 java.lang.reflect.Method 4: 67973 3769400 [Ljava.lang.Object; 5: 98145 3140640 java.util.HashMap$Node 6: 115945 2782680 java.util.concurrent.atomic.AtomicLong 7: 19301 2126520 java.lang.Class 8: 61752 1976064 java.util.concurrent.ConcurrentHashMap$Node 9: 14120 1665080 [Ljava.util.HashMap$Node; 10: 8997 1548592 [B 11: 15452 1236160 com.google.common.cache.LocalCache$Segment 12: 34289 1097248 java.util.Hashtable$Entry 13: 37325 895800 java.util.ArrayList 14: 2899 790728 [Z 15: 49080 785280 java.lang.Object 16: 19315 772600 com.google.common.cache.AbstractCache$SimpleStatsCounter 17: 21096 675072 java.lang.ref.WeakReference 18: 16237 649480 java.lang.ref.SoftReference 19: 13368 641664 java.util.HashMap 20: 15945 637800 java.util.LinkedHashMap$Entry 21: 114 588392 [F 22: 18238 583616 java.util.concurrent.locks.ReentrantLock$NonfairSync 23: 486 533312 [Ljava.util.concurrent.ConcurrentHashMap$Node; 24: 16521 528672 java.lang.ref.ReferenceQueue 25: 6928 498816 java.lang.reflect.Field 26: 3863 494464 com.google.common.cache.LocalCache 27: 7680 491520 java.net.URL 28: 5948 475840 java.lang.reflect.Constructor 29: 8359 463928 [I[B 代表 byte[C 代表 char[D 代表 double[F 代表 float[I 代表 int[J 代表 long[Z 代表 booleanjmap -clstats pid
打印类加载信息
-clstats是-permstat的代替计划，在JDK8之前，-permstat用来打印类加载器的数据
打印Java堆内存的永恒保留区域的类加载器的智能统计信息。对于每个类加载器而言，它的名称、活跃度、地址、父类加载器、它所加载的类的数量和大小都会被打印。此外，蕴含的字符串数量和大小也会被打印。
[root@rumenz ~]# jmap -clstats 18378Attaching to process ID 18378, please wait...Debugger attached successfully.Server compiler detected.JVM version is 25.261-b12finding class loader instances ..done.computing per loader stat ..done.please wait.. computing liveness.liveness analysis may be inaccurate ...class_loader classes bytes parent_loader alive? type<bootstrap> 3365 5881890 null live <internal>0x00000006cb8abb88 1 880 0x00000006c7c2a4c8 dead sun/reflect/DelegatingClassLoader@0x00000007c000a0a00x00000006cafd0af0 1 878 0x00000006c8520248 dead sun/reflect/DelegatingClassLoader@0x00000007c000a0a00x00000006c8600870 0 0 0x00000006c82362b8 dead hudson/ClassicPluginStrategy$DependencyClassLoader@0x00000007c02753080x00000006c86a9878 81 74744 0x00000006c86a98e8 dead hudson/ClassicPluginStrategy$AntClassLoader2@0x00000007c0274dc80x00000006ca739a60 1 880 0x00000006c7c2a4c8 dead sun/reflect/DelegatingClassLoader@0x00000007c000a0a00x00000006cafceae8 1 880 0x00000006c7c2a4c8 dead sun/reflect/DelegatingClassLoader@0x00000007c000a0a00x00000006cb541b40 1 880 0x00000006c7c2a4c8 dead sun/reflect/DelegatingClassLoader@0x00000007c000a0a00x00000006cb75fb68 1 880 0x00000006c86b09b8 dead sun/reflect/DelegatingClassLoader@0x00000007c000a0a00x00000006ca5f9a40 1 1474 null dead sun/reflect/DelegatingClassLoader@0x00000007c000a0a00x00000006cb07fb18 1 880 0x00000006c82bcba8 dead sun/reflect/DelegatingClassLoader@0x00000007c000a0a00x00000006cb083b28 1 1474 0x00000006c7c2a4c8 dead sun/reflect/DelegatingClassLoader@0x00000007c000a0a00x00000006c86ad848 8 8716 0x00000006c86ad8b8 dead hudson/ClassicPluginStrategy$AntClassLoader2@0x00000007c0274dc80x00000006c8bde898 49 155511 0x00000006c8237a80 dead com/google/inject/internal/BytecodeGen$BridgeClassLoader@0x00000007c05f9028jmap -dump:format=b,file=dump.phrof pid
生成堆转储快照文件这个命令执行，JVM会将整个heap的信息dump写入到一个文件，heap如果比拟大的话，就会导致这个过程比拟耗时，并且执行的过程中为了保障dump的信息是牢靠的，所以会暂停利用， 线上零碎慎用。
[root@rumenz ~]# jmap -dump:format=b,file=dump.hprof 18378Dumping heap to /root/dump.hprof ...Heap dump file created