cuncurrency problems with class Properties

[Andy Jung]

Hi,

having a class Properties (to work with user configuration properties) causes concurrency problems in my mutlithreaded Swing application.
This is due to the fact that methods setProperty and getProperty are not synchronized.

I tried to solve this problem by creating a synchronized version of my Properties class

    private static final Properties props
            = (Properties) Collections.synchronizedMap(new Properties());

however, at runtime this results in a ClassCastException:

java.lang.ClassCastException: java.util.Collections$SynchronizedMap cannot be cast to java.util.Properties
        at suncertify.URLyBirdProperties.<clinit>(URLyBirdProperties.java:94)

What's wrong and how can I produce a synchronized version of class Properties then?

Thanks,
Andy

[Andy Jung]

got the solution:

Properties implements interface Map<String, String>.
Collections.synchronizedMap(), however, returns a Map<Object, Object>, so casting to Properties won't be possible (-> covariance of generics).

Since Properties also extends class Hashtable<String, String> and Hashtable is synchronized use its put() and get()-methods instead of
the unsynchronized setProperty()- and getProperty()-methods of class Properties.

[Ajay Saxena]

Watch out for check-then-put pattern in your code which requires external synchronization.
e.g let's say we have the following piece of code

// declare and initialize map
Map map = Collections.synchronizedMap(new HashMap<A,List<B>>())
....

 public void foo(A key,B element ){
   List<B> l;
   if(! map.containsKey(key)){
       l = new ArrayList<B>();
       l.add(element);
       map.put(key,l);
    }else{
       l = map.get(key);
       l.add(element);
   }
}

If the application is so designed that the method foo() could be invoked on the same object in multiple threads,it's not sufficient to just declare the map as a synchronized collection,as done in line # 02
Try reasoning out why.