jmap -heap 命令详解

1. jmap -heap pid 查看java 堆（heap）使用状况

using thread-local object allocation. Parallel GC with 4 thread(s) //GC 方式 Heap Configuration: //堆内存初始化配置 MinHeapFreeRatio=40 //对应jvm启动参数-XX:MinHeapFreeRatio设置JVM堆最小空闲比率(default 40) MaxHeapFreeRatio=70 //对应jvm启动参数 -XX:MaxHeapFreeRatio设置JVM堆最大空闲比率(default 70) MaxHeapSize=512.0MB //对应jvm启动参数-XX:MaxHeapSize=设置JVM堆的最大大小 NewSize = 1.0MB //对应jvm启动参数-XX:NewSize=设置JVM堆的‘新生代’的默认大小 MaxNewSize =4095MB //对应jvm启动参数-XX:MaxNewSize=设置JVM堆的‘新生代’的最大大小 OldSize = 4.0MB //对应jvm启动参数-XX:OldSize=<value>:设置JVM堆的‘老生代’的大小 NewRatio = 8 //对应jvm启动参数-XX:NewRatio=:‘新生代’和‘老生代’的大小比率 SurvivorRatio = 8 //对应jvm启动参数-XX:SurvivorRatio=设置年轻代中Eden区与Survivor区的大小比值 PermSize= 16.0MB //对应jvm启动参数-XX:PermSize=<value>:设置JVM堆的‘永生代’的初始大小 MaxPermSize=64.0MB //对应jvm启动参数-XX:MaxPermSize=<value>:设置JVM堆的‘永生代’的最大大小

Heap Usage: //堆内存分步 PS Young Generation Eden Space: //Eden区内存分布 capacity = 20381696 (19.4375MB) //Eden区总容量 used = 20370032 (19.426376342773438MB) //Eden区已使用 free = 11664 (0.0111236572265625MB) //Eden区剩余容量 99.94277218147106% used //Eden区使用比率 From Space: //其中一个Survivor区的内存分布 capacity = 8519680 (8.125MB) used = 32768 (0.03125MB) free = 8486912 (8.09375MB) 0.38461538461538464% used To Space: //另外一个Survivor区的内存分布 capacity = 9306112 (8.875MB) used = 0 (0.0MB) free = 9306112 (8.875MB) 0.0% used PS Old Generation //当前的Old区内存分布 capacity = 366280704 (349.3125MB) used = 322179848 (307.25464630126953MB) free = 44100856 (42.05785369873047MB) 87.95982001825573% used PS Perm Generation //当前的 “永生代” 内存分布 capacity = 32243712 (30.75MB) used = 28918584 (27.57891082763672MB) free = 3325128 (3.1710891723632812MB) 89.68751488662348% used

2. jmap -histo pid

查看堆内存(histogram)中的对象数量，大小

num #instances #bytes class name