Java Reference
In-Depth Information
Listing 4.2. Using Confinement to Ensure Thread Safety.
This example makes no assumptions about the thread-safety of
Person
, but if it is mutable,
additional synchronization will be needed when accessing a
Person
retrieved from a
Per-
sonSet
. The most reliable way to do this would be to make
Person
thread-safe; less reli-
able would be to guard the
Person
objects with a lock and ensure that all clients follow the
protocol of acquiring the appropriate lock before accessing the
Person
.
Instance confinement is one of the easiest ways to build thread-safe classes. It also allows
flexibility in the choice of locking strategy;
PersonSet
happened to use its own intrinsic
lock to guard its state, but any lock, consistently used, would do just as well. Instance con-
finement also allows different state variables to be guarded by different locks. (For an ex-
ample of a class that uses multiple lock objects to guard its state, see
ServerStatus
on
There are many examples of confinement in the platform class libraries, including some
classes that exist solely to turn non-thread-safe classes into threadsafe ones. The basic col-
lection classes such as
ArrayList
and
HashMap
are not thread-safe, but the class library
provides wrapper factory methods (
Collections.synchronizedList
and friends) so
they can be used safely in multithreaded environments. These factories use the Decorator
pattern (
Gamma et al., 1995
) to wrap the collection with a synchronized wrapper object; the
wrapper implements each method of the appropriate interface as a synchronized method that
