Java How to Program Fourth Edition - DCC

426 Object-Based Programming Chapter 8
8.14 Finalizers
We have seen that constructor methods are capable of initializing data in an object of a class
when the class is created. Often, constructors acquire various system resources such as
memory (when the new operator is used). We need a disciplined way to give resources
back to the system when they are no longer needed to avoid resource leaks. The most common
resource acquired by constructors is memory. Java performs automatic garbage collection
of memory to help return memory back to the system. When an object is no longer
used in the program (i.e., there are no references to the object), the object is marked for garbage
collection. The memory for such an object can be reclaimed when the garbage collector
executes. Therefore, memory leaks that are common in other languages like C and
C++ (because memory is not automatically reclaimed in those languages) are less likely to
happen in Java. However, other resource leaks can occur.
Performance Tip 8.5
Extensive use of local variables that refer to objects can degrade performance. When a local
variable is the only reference to an object, the object is marked for garbage collection as the
local variable goes out of scope. If this happens frequently in short time periods, large numbers
of objects could be marked for garbage collection, thus placing a performance burden
on the garbage collector. 8.5
Every class in Java can have a finalizer method that returns resources to the system.
The finalizer method for an object is guaranteed to be called to perform termination housekeeping
on the object just before the garbage collector reclaims the memory for the object.
A class’s finalizer method always has the name finalize, receives no parameters and
returns no value (i.e., its return type is void). A class should have only one finalize
method that takes no arguments. Method finalize is defined originally in class Object
as a placeholder that does nothing. This guarantees that every class has a finalize
method for the garbage collector to call.
Good Programming Practice 8.10
The last statement in a finalize method should always be super.finalize(); to ensure
that the superclass’s finalize method is called. 8.10
Software Engineering Observation 8.20
The garbage collector is not guaranteed to execute; therefore, an object’s finalize method
is not guaranteed to get called. You should not architect classes that rely on the garbage
collector calling an object’s finalize method to deallocate resources. 8.20
Finalizers have not been provided for the classes presented so far. Actually, finalizers
are rarely used in industrial Java applications. We will see a sample finalize method
and discuss the garbage collector further in Figure 8.20.
Testing and Debugging Tip 8.4
Several professional Java developers who reviewed this book indicated that method finalize
is not useful in industrial Java applications, because it is not guaranteed to get called.
For this reason, you should search your Java programs for any use of method finalize
to ensure that the program does not rely on calls to method finalize for proper deallocation
of resources. In fact, you might consider removing method finalize entirely and
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