Synchronization of Nested Data Structures between Threads in Java
- by Dominik
I have a cache implementation like this:
class X
{
private final Map<String, ConcurrentMap<String, String>> structure = new HashMap...();
public String getValue(String context, String id)
{
// just assume for this example that there will be always an innner map
final ConcurrentMap<String, String> innerStructure = structure.get(context);
String value = innerStructure.get(id);
if(value == null)
{
synchronized(structure)
{
// can I be sure, that this inner map will represent the last updated
// state from any thread?
value = innerStructure.get(id);
if(value == null)
{
value = getValueFromSomeSlowSource(id);
innerStructure.put(id, value);
}
}
}
return value;
}
}
Is this implementation thread-safe? Can I be sure to get the last updated state from any thread inside the synchronized block? Would this behaviour change if I use a java.util.concurrent.ReentrantLock instead of a synchronized block, like this:
...
if(lock.tryLock(3, SECONDS))
{
try
{
value = innerStructure.get(id);
if(value == null)
{
value = getValueFromSomeSlowSource(id);
innerStructure.put(id, value);
}
}
finally
{
lock.unlock();
}
}
...
I know that final instance members are synchronized between threads, but is this also true for the objects held by these members?
Maybe this is a dumb question, but I don't know how to test it to be sure, that it works on every OS and every architecture.
