The so-called GC (Garbage Collector) roots are objects special for garbage
collector. Garbage collector collects those objects that are not GC roots and
are not accessible by references from GC roots.

There are several kinds of GC roots. One object can belong to more than one
kind of root. The root kinds are:

• Class - class loaded by system class loader. Such classes can never
  be unloaded. They can hold objects via static fields. Please note that classes
  loaded by custom class loaders are not roots, unless corresponding instances of
  java.lang.Class happen to be roots of other kind(s).
• Thread - live thread
• Stack Local - local variable or parameter of Java method
• JNI Local - local variable or parameter of JNI method
• JNI Global - global JNI reference
• Monitor Used - objects used as a monitor for synchronization
• Held by JVM - objects held from garbage collection by JVM for its
  purposes. Actually the list of such objects depends on JVM implementation.
  Possible known cases are: the system class loader, a few important exception
  classes which the JVM knows about, a few pre-allocated objects for exception
  handling, and custom class loaders when they are in the process of loading
  classes. Unfortunately, JVM provides absolutely no additional detail for such
  objects. Thus it is up to the analyst to decide to which case a certain "Held by
  JVM" belongs.

If an object is a root, it is specially marked in all views showing
individual objects. For example, the following picture shows a fragment of paths view: