轉載自:http://www.yourkit.com/docs/90/help/sizes.jsp?

YourKit Java Profiler is capable of measuring?shallow?and?retained?sizes of objects.

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Shallow size?of an object is the amount of memory allocated to store the object itself, not taking into account the referenced objects. Shallow size of a regular (non-array) object depends on the number and types of its fields. Shallow size of an array depends on the array length and the type of its elements (objects, primitive types). Shallow size of a set of objects represents the sum of shallow sizes of all objects in the set.

Retained size?of an object is its shallow size plus the shallow sizes of the objects that are accessible, directly or indirectly,?only?from this object. In other words, the retained size represents the amount of memory that will be freed by the garbage collector when this object is collected.

To better understand the notion of the?retained size, let us look at the following examples:

In order to measure the retained sizes, all objects in memory are treated as nodes of a graph where its edges represent references from objects to objects. There are also special nodes -?GC root?objects, which will not be collected by?Garbage Collector?at the time of measuring (read more about?GC roots).

The pictures below show the same set of objects, but with varying internal references.

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Figure 1:

Figure 2:

Let us consider?obj1.
As you can see, in both pictures we have highlighted all of the objects that are directly or indirectly accessed?onlyby?obj1. If you look at Figure 1, you will see that?obj3?is not highlighted, because it is also referenced by a?GC rootobject. On Figure 2, however, it is already included into the retained set, unlike?obj5, which is still referenced by?GC root.

Thus, the retained size of?obj1?will represent the following respective values:

• For Figure 1: the sum of shallow sizes of?obj1,?obj2?and?obj4
• For Figure 2: the sum of shallow sizes of?obj1,?obj2,?obj3?and?obj4

Looking at?obj2, however, we see that its retained size in the above cases will be:

• For Figure 1: the sum of shallow sizes of?obj2?and?obj4
• For Figure 2: the sum of shallow sizes of?obj2,?obj3?and?obj4

In general, retained size is an integral measure, which helps to understand the structure (clustering) of memory and the dependencies between object subgraphs, as well as find potential roots of those subgraphs.

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轉載于:https://blog.51cto.com/yaoyinjie/697924

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