Example 11-2 Protecting a HashMap
public synchronized Object tweek(Object arg) {
HashMap.put(arg);
}
Example 11-3 Subclassing an Unsafe Object
public class MyFoo extends Foo {
...
public synchronized Object frob(Object arg) {
return foo.super(arg);
}
}
When Should a Class Be Synchronized?
There is a tendency when writing a threaded program to declare all methods synchronized. This
would seem a good thing to do, as then they could all be safely called from any thread. But it's not.
The vast majority of objects that a program uses are called from code that already contains proper
protection. In our producer/ consumer examples, there was no need to make workpile.add()
and workpile.remove() synchronized because they were only called from producer and
consumer.
Synchronized Collections in Java 2
In Java 2 several classes that previously had synchronized methods have gotten replacement
classes that don't [e.g., HashTable has synchronized methods, HashMap (new in Java 2) doesn't].
This is a good thing because HashMap runs faster. It is a good thing because it is fairly unlikely
that you need it to be synchronized anyway. But sometimes you do.
Java 2 has a clever method of dealing with this situation. Instead of making subclasses of the
various collection classes and synchronizing them, Java 2 provides a static factory method for
each collection class which will return a synchronized version of the collection. This is known to
design pattern folks as a decorator pattern.
So the idea is that you create a class, then call the factory method on it to get back a synchronized
version of that same class. Obviously, you must now use only the synchronized version of the
class. Each of the core collections has such a factory method:
public
static
Collection synchronizedCollection(Collection c)
public
static
Set synchronizedSet(Set c)
public
static
Map synchronizedMap(Map c)
public
static
List synchronizedList(List c)
public
static
SortedSet synchronizedSortedSet(SortedSet c)
public
static
SortedMap synchronizedSortedMap(SortedMap c)
So you use these methods to obtain synchronized collections (Code Example 11-4):
Example 11-4 Making Synchronized Collections
List syncdList = Collections.synchronizedList(list);
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