What are GC roots for classes?

Question

In Java, there are special objects called Garbage Collection Roots (GC roots). They serve as a root objects for Garbage Collection marking mechanism (see picture).

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This article describes four types of GC roots:

local variables
active threads
static variables
JNI references

It is also mentioned, that:

Classes themselves can be garbage-collected.

GC roots aren't collected thus classes themselves are not GC roots.

So what are GC roots for the classes?

The GC roots are those that you listed for everything. Your question really is about what kind of objects typically hold references to class objects. — biziclop, Nov 28 '14 at 10:40

score 33 · Accepted Answer · edited Jul 12 '20 at 05:18

33

So what are GC roots for the classes?

Classloaders, effectively - via other GC roots.

If there is nothing which can reach a classloader - which means nothing can reach any classes created by that classloader or any instances of those classes - then both the classloader and the classes it created are eligible for garbage collection. Keeping them alive until then is necessary so that Class::forName and ClassLoader::findClass can be idempotent even when the class's static initializers are not.

Hidden classes (see https://openjdk.java.net/jeps/371) are exceptions to this rule. As an implementation detail of OpenJDK, so are the classes of method references, lambdas, and proxies created with the static methods of java.lang.reflect.Proxy. The classloader does not hold a strong reference to these classes.

edited Jul 12 '20 at 05:18

Pr0methean

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answered Nov 28 '14 at 10:38

Jon Skeet

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classloader instance resides at heap? – gstackoverflow Dec 28 '16 at 16:34
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@gstackoverflow: Well they're certainly not on the stack. Which precise part of the heap is a different matter. – Jon Skeet Dec 28 '16 at 17:50
@gstackoverflow: Well that's part of the heap. That's what I mean by "which precise part of the heap is a different matter".... – Jon Skeet Dec 28 '16 at 21:21
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just official oracle documentation call permgen as part of heap, but usually in community **heap = [young gen] + [old gen]** – gstackoverflow Dec 29 '16 at 07:41

score 20 · Answer 2 · edited Jun 24 '19 at 09:01

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A garbage collection root is an object that is accessible from outside the heap.

Memory Analyzer categorizes garbage collection roots according to the following list:

Class loaded by system ClassLoader
- static field in JDK classes(java.* etc)
Live thread
- stack -local vars, method params
- java.lang.Thread instance
Object held as synchronization monitor
JNI references
JVM specials...

Source 1 Source 2

edited Jun 24 '19 at 09:01

Vladimir Vagaytsev

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answered Jul 09 '17 at 08:45

Premraj

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There's also good reference in YourKit docs: https://www.yourkit.com/docs/java/help/gc_roots.jsp – Juraj Martinka Oct 08 '19 at 13:23

What are GC roots for classes?

2 Answers2

Linked