The class Object has a protected method called finalize (§20.1.11); this
method can be overridden by other classes. The particular definition of finalize
that can be invoked for an object is called the finalizer of that object. Before the
storage for an object is reclaimed by the garbage collector, the Java Virtual
Machine will invoke the finalizer of that object.
Finalizers provide a chance to free up resources (such as file descriptors or operating system graphics contexts) that cannot be freed automatically by an automatic storage manager. In such situations, simply reclaiming the memory used by an object would not guarantee that the resources it held would be reclaimed.
The Java language does not specify how soon a finalizer will be invoked, except to say that it will happen before the storage for the object is reused. Also, the Java language does not specify which thread will invoke the finalizer for any given object. If an uncaught exception is thrown during the finalization, the exception is ignored and finalization of that object terminates.
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 automatically 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 override 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.