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.

Solution 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 

Solution 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[]

Solution 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.

Comments

[Brett]

Section 4.3.2 is probably a better reference.

[dfa]

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

[Neil]

I saw this: org.slf4j.spi.LocationAwareLogger.log(Lorg/slf4j/Marker;Ljav‌​a/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?