What do those strange class names in a java heap dump mean?

Question

I'm trying to track down a memory leak in a java process, using jmap and jhat. Every time I do this I see those weird notation for specific object types, like [S for string arrays and [C for Character arrays. I never remember what means what, and it's very hard to google this stuff.

(EDIT: to prove my point, it turns out that [S is array of short and [C is array of char.)

Anyone care to make a table listing all the different class names and what they mean? Or point me to such table?

Specifically I'd like to know what [Ljava.lang.Object; means.

Platus

Array of string should be [Ljava/lang/String;. – Hyperbaton

In my defense, googling led me to an article on java.sun.com which contains this line: "The String and Character array objects, [S and [C, are always going to be high on this list". Somewhat misleading. – Platus

This site also has some good information: communities.ca.com/web/ca-wily-global-user-community/wiki/-/… – Reasonless

This link is broken. I hate to post links, but I'll post the updated link for the previous comment - communities.ca.com/docs/DOC-99575135. – Granary

Answer 1

You'll find the complete list documented under Class.getName():

If this class object represents a reference type that is not an array type then the binary name of the class is returned, as specified by the Java™ Language Specification, Second Edition.

If this class object represents a primitive type or void, then the name returned is a String equal to the Java language keyword corresponding to the primitive type or void.

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

Thistly

Answer 2

it is an array of objects as specified by JVM Specifications for internal representation of class names:

• a single [ means an array of
• L followed by a fully qualified class name (e.g. java/lang/Object) is the class name terminated by semicolon ;

so [Ljava.lang.object; means Object[]

Afro

Section 4.3.2 is probably a better reference. – Hyperbaton

that link doesn't actually describe the conventions used by Class.getName() -- in fact, the internal naming representation is quite different – Thistly

furthermore the internal naming is same of Class.getName(), just replace / with . – Afro

I saw this: org.slf4j.spi.LocationAwareLogger.log(Lorg/slf4j/Marker;Ljava/lang/String;ILjava/lang/String;[Lj ava/lang/Object;Ljava/lang/Throwable;)V -- What does "IL" mean? The methods that closely match that signature are log(Marker, String, int, String, Object[], Throwable), do primitives not get separated by semi-colon? – Oruro

Answer 3

The rules are listed in the API doc of Class.getName().

[Ljava.lang.Object; would be an instance of Object[]. Note that multidimensional arrays are displayed with multiple opening braces.

Plausive

Answer 4

Means Object[]

Eggnog