Question

From this IBM article on handling memory leaks in Java :

The garbage collector starts at the root nodes, classes that persist
throughout the life of a Java application, and sweeps through all of
the nodes that are referenced. As it traverses the nodes, it keeps
track of which objects are actively being referenced. Any classes that
are no longer being referenced are then eligible to be garbage
collected. The memory resources used by these objects can be returned
to the Java virtual machine (JVM) when the objects are deleted.

What are the classes that persist through out the life of a Java application ?

Answer 1

What you are asking is also called Garbage Collection Roots. From Yourkit’s documentation of 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.

You can also read it from The Truth About Garbage Collection (dated).