如何正确生成唯一的ID并将其存储在ConcurrentHashMap中

Question

I'm currently using a ConcurrentHashMap in which I store some files based on a unique id that I generate using org.apache.commons.lang.RandomStringUtils#randomAlphanumeric .

My current approach is this:

private ConcurrentHashMap <String, CustomFile> fileIdMap = 
                     new ConcurrentHashMap <String, SwitchConfigurationFile>();

public void importFile () {       
    CustomFile file = new CustomFile (generateFileID(), param1, param2, param3, param4);
    fileIdMap.put (file.getID(), file);       
}

private String generateFileID () {
   String generatedValue = RandomStringUtils.randomAlphanumeric(5);
   while (fileIdMap.containsKey(generatedValue)) {
       generatedValue = RandomStringUtils.randomAlphanumeric(5);
   }
   //I was thinking here to put the generated value into the Map
   //but at this moment I don't have the CustomFile instance
   //and null values are not allowed
   //maybe:
   //fileIdMap.put (generatedValue, new CustomFile());
   return generatedValue;
}

I was thinking: What if fileIdMap.containsKey(generatedValue) returns false and before I add it to the map another thread comes and adds the same key, I will have one CustomFile in the map instead of two. I know the chances are very small, but I want to take this into consideration.

So, what is the best approach in this situation and how can I ensure that I also have a unique id for each file?

Answer 1

You can use putIfAbsent(K key, V value) :

private String generateFileID () {
   CustomFile file = new CustomFile();
   String generatedValue = RandomStringUtils.randomAlphanumeric(5);
   while (fileIdMap.putIfAbsent(generatedValue, file) != null) {
       generatedValue = RandomStringUtils.randomAlphanumeric(5);
   }
   return generatedValue;
}

The implementation of this method in ConcurrentHashMap is correctly synchronized to avoid concurrence problems.

Answer 2

You can use #putIfAbsent() , which will atomically place the item in the map or return null otherwise, so you could put in your while loop while not null.
You will need to refactor your code a bit, since you don't yet have the CustomFile instance.
One thing you could do is place a dummy item in first, then replace it with your real object once it is created, but this might cause another synchronization issues if other threads read that object in the mean time.