Shallow Heap Size
指对象自身所占用的内存大小，不包含其引用的对象所占的内存大小。
1、数组类型
数组元素对象所占内存的大小总和。
2、非数组类型
对象与它所有的成员变量大小的总和。当然这里面还会包括一些Java语言特性的数据存储单元。
Retained Heap Size
前对象大小+当前对象可直接或间接引用到的对象的大小总和。
(间接引用的含义：A->B->C, C就是间接引用)
换句话说，Retained Size就是当前对象被GC后，从Heap上总共能释放掉的内存。
不过，释放的时候还要排除被GC Roots直接或间接引用的对象。他们暂时不会被被当做Garbage。GC Roots直接引用了A和B两个对象。
A对象的Retained Size=A对象的Shallow Size
B对象的Retained Size=B对象的Shallow Size + C对象的Shallow Size这里不包括D对象，因为D对象被GC Roots直接引用。
如果GC Roots不引用D对象呢？
GC Roots
The so-called GC (Garbage Collector) roots are objects special for
garbage collector. Garbage collector collects those objects that are not
GC roots and are not accessible by references from GC roots.There are several kinds of GC roots. One object can belong to more than one kind of root. The root kinds are:
- Class - class loaded by system class loader. Such
classes can never be unloaded. They can hold objects via static fields.
Please note that classes loaded by custom class loaders are not roots,
unless corresponding instances ofjava.lang.Class
happen to be roots of other kind(s).- Thread - live thread
- Stack Local - local variable or parameter of Java method
- JNI Local - local variable or parameter of JNI method
- JNI Global - global JNI reference
- Monitor Used - objects used as a monitor for synchronization
- Held by JVM
- objects held from garbage collection by JVM for its purposes.
Actually the list of such objects depends on JVM implementation.
Possible known cases are: the system class loader, a few important
exception classes which the
JVM knows about, a few pre-allocated objects for exception handling,
and custom class loaders when they are in the process of loading
classes.Unfortunately, JVM provides absolutely no additional detail for such objects. Thus it is up to the analyst
to decide to which case a certain "Held by JVM" belongs.If an object is a root, it is specially marked in all views showing
individual objects. For example, the following picture shows a fragment
ofpaths view: