因为在Linux环境，用jdk自带的jmap工具(Linux/Unix环境特有的)，可以对进程中的内存对象监视，然后就运行命令jmap -histo [pid]，找内存中的对象数目变化,如下图所示：
num #instances #bytes class name
----------------------------------------------
1: 1169837 131659368 [C
2: 25945 38337824 [I
3: 31548 29407968 [B
4: 1164546 27949104 java.lang.String
6: 91313 12829072 <constMethodKlass>
7: 12395 12404880 [S
8: 91313 11700288 <methodKlass>
9: 7525 9303112 <constantPoolKlass>
10: 7525 5606808 <instanceKlassKlass>
11: 6043 5028288 <constantPoolCacheKlass>
12: 10048 2007888 [Ljava.lang.Object;
14: 3507 1707048 <methodDataKlass>
15: 8132 980616 java.lang.Class
16: 26854 859328 java.util.HashMap$Entry
17: 12368 699296 [[I
18: 14135 452320 java.util.concurrent.ConcurrentHashMap$HashEntry
19: 20883 334128 java.lang.Object
20: 590 316240 <objArrayKlassKlass>
21: 1757 305904 [Ljava.util.HashMap$Entry;
22: 2809 224720 net.sf.ehcache.Element
23: 1992 223104 java.net.SocksSocketImpl
24: 2668 213440 java.lang.reflect.Method
26: 5932 183928 [Ljava.lang.String;
27: 7588 182112 java.util.concurrent.ConcurrentSkipListMap$Node
28: 7317 175608 java.lang.Long
29: 5303 169696 java.util.Hashtable$Entry
30: 6778 162672 java.util.ArrayList
31: 3931 157240 java.lang.ref.SoftReference
32: 2972 118880 java.util.LinkedHashMap$Entry
33: 1565 112680 org.apache.commons.pool2.impl.DefaultPooledObject
34: 2817 112680 net.sf.ehcache.store.chm.SelectableConcurrentHashMap$HashEntry
35: 2243 107664 java.util.HashMap
36: 2592 103680 java.util.TreeMap$Entry
37: 3214 102848 java.lang.ref.WeakReference
38: 1565 100160 redis.clients.jedis.Client
39: 4155 99720 java.util.LinkedList$Node
40: 1986 95328 java.net.SocketInputStream
41: 414 92952 [Ljava.util.concurrent.ConcurrentHashMap$HashEntry;
42: 2275 91000 java.lang.ref.Finalizer
43: 1161 83592 java.lang.reflect.Constructor
44: 757 78728 java.io.ObjectStreamClass
45: 1587 76176 java.net.SocketOutputStream
46: 1189 66584 java.beans.MethodDescriptor
47: 2770 66480 org.apache.commons.pool2.impl.LinkedBlockingDeque$Node
48: 388 66368 [Ljava.util.Hashtable$Entry;
49: 1989 63648 java.net.Socket
50: 749 53928 java.lang.reflect.Field
...
...
2947: 1 16 sun.misc.Launcher
2948: 1 16 org.codehaus.jackson.map.ser.std.DateSerializer
2949: 1 16 org.apache.phoenix.schema.types.PDataType$2
2950: 1 16 org.springframework.data.redis.connection.convert.StringToRedisClientInfoConverter
Total 3090439 316004152
[C is a char[]
[S is a short[]
[I is a int[]
[B is a byte[]
[[I is a int[][]
上面的输出中[C对象占用Heap这么多，往往跟String有关，String其内部使用final char[]数组来保存数据的
constMethodKlass/ methodKlass/ constantPoolKlass/ constantPoolCacheKlass/instanceKlassKlass/ methodDataKlass与Classloader相关，常驻与Perm区。其中最后一行(total行），分别记录了实例总数、程序占用总内存数，本例显示的程序总占用内存约300M