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Reference Map and Set

Probably you are already familiar with the existence of WeakHashMap: a hashmap implementation with weak keys. Sadly, that is the only reference based collection available in the JDK. Fortunately, Jodd gives you more power with two reference-aware collections: ReferenceMap and ReferenceSet, where user can choose the 'strength' of the used keys and/or values.

ReferenceMap

ReferenceMap usage is quite simple: constructor takes two arguments that defines the reference strength of the keys and values:

ReferenceMap<String, String> rm = new ReferenceMap<String, String>(WEAK, STRONG);
String key = new String("key");			// enforce instance creation
rm.put(key, "value");
System.out.println(rm.isEmpty());		// false

Lets assume that key becomes null and that garbage collector has destroyed the instance. We may simulate this with the following code (although there are no guarantees that it will work):

key = null;
System.gc();
System.gc();
ThreadUtil.sleep(5000);
		

Now rm.isEmpty() returns true.

Reference Types

Following reference types can be used, defined in the ReferenceTypes enumeration:

• STRONG - Prevents referent from being reclaimed by the garbage collector,
• SOFT - Referent reclaimed in an LRU fashion when the VM runs low on memory and no strong references exist,
• WEAK - Referent reclaimed when no strong or soft references exist,
• PHANTOM - Similar to weak references except the garbage collector doesn't actually reclaim the referent. More flexible alternative to finalization.

ReferenceSet

ReferenceSet is reference-aware set, where user can define the weakness of the used elements. Example:

ReferenceSet rs = new ReferenceSet<String>(WEAK);
String value = new String("value");
rs.add(value);
System.out.println(rs.isEmpty());		// false

After value is nulled and garbage collected, rs.isEmpty() returns true.