17 October 2013

Recognising Object Types in JVM Memory Histograms

Here's an example of valuable information being available in the documentation - but not easily so (as in not easy to find)... For instance, a Google search for "[C" didn't throw up anything of relevance.

Backdrop to the search for this information was a JVM Heap memory allocation issue that prompted me to do a Histogram of the Heap using JMAP. It showed that 2.2GB of type "[C" were allocated. My guess was array of chars and it turns out that I was right...


"If this class object represents a class of arrays, then the internal form of the name consists of the name of the element type preceded by one or more '[' characters representing the depth of the array nesting. The encoding of element type names is as follows:

Element TypeEncoding
boolean   Z
byte   B
char   C
class or interface   Lclassname;
double   D
float   F
int   I
long   J
short   S

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