12.6.1

Implementing Finalization

EXECUTION

The

finalize

 method declared in class

Object

 takes no action. However, the

fact that class

Object

 declares a

finalize

 method means that the

finalize

method for any class can always invoke the

finalize

 method for its superclass,

which is usually good practice. (Unlike constructors, finalizers do not automati 

cally invoke the finalizer for the superclass; such an invocation must be coded

explicitly.)

For efficiency, an implementation may keep track of classes that do not over 

ride the

finalize

 method of class

Object

, or override it in a trivial way, such as:

protected void finalize() throws Throwable {

super.finalize();

}

We encourage implementations to treat such objects as having a finalizer that is

not overridden, and to finalize them more efficiently, as described in  12.6.1.

A finalizer may be invoked explicitly, just like any other method.

12.6.1   Implementing Finalization

Every object can be characterized by two attributes: it may be

reachable

,

finalizer 

reachable

, or

unreachable

, and it may also be

unfinalized

,

finalizable

, or

finalized

.

A

reachable

 object is any object that can be accessed in any potential continu 

ing computation from any live thread. Optimizing transformations of a program

can be designed that reduce the number of objects that are reachable to be less

than those which would naively be considered reachable. For example, a compiler

or code generator may choose, explicitly or implicitly, to set a variable or parame 

ter that will no longer be used to

null

 to cause the storage for such an object to be

potentially reclaimable sooner. A

finalizer reachable

 object can be reached from

some finalizable object through some chain of references, but not from any live

thread. An

unreachable

 object cannot be reached by either means.

An

unfinalized

 object has never had its finalizer automatically invoked; a

finalized

 object has had its finalizer automatically invoked. A

finalizable

 object

has never had its finalizer automatically invoked, but the Java Virtual Machine

may eventually automatically invoke its finalizer.

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