Does java have a “LinkedConcurrentHashMap” data structure?

Question

I need a data structure that is a LinkedHashMap and is thread safe.

How can I do that ?

Answer 1

You can use a ConcurrentSkipListMap, only available in Java SE/EE 6 or later. It is order presevering in that keys are sorted according to their natural ordering. You need to have a Comparator or make the keys Comparable objects. In order to mimik a linked hash map behavior (iteration order is the order in time in which entries were added) I implemented my key objects to always compare to be greater than a given other object unless it is equal (whatever that is for your object). A wrapped synchronized linked hash map did not suffice because as stated in http://www.ibm.com/developerworks/java/library/j-jtp07233.html: "The synchronized collections wrappers, synchronizedMap and synchronizedList, are sometimes called conditionally thread-safe -- all individual operations are thread-safe, but sequences of operations where the control flow depends on the results of previous operations may be subject to data races. The first snippet in Listing 1 shows the common put-if-absent idiom -- if an entry does not already exist in the Map, add it. Unfortunately, as written, it is possible for another thread to insert a value with the same key between the time the containsKey() method returns and the time the put() method is called. If you want to ensure exactly-once insertion, you need to wrap the pair of statements with a synchronized block that synchronizes on the Map m."

So what only helps is a ConcurrentSkipListMap which is 3-5 times slower than a normal ConcurrentHashMap.