创建在Java中按创建/更新时间排序的有序HashMap

Question

I am working on data tuples in the following format: [IP, number-of-bytes-served, time]. I created a HashMap on IP to count the number of bytes served for each IP. Then, I realised that I need to remove some least recently used key-value pairs to create some more space. I want to create a time constraint, let's say 1 hour, and remove the key-value pairs with no action in that period. So I need to save the update time for each pair. In fact, for a good performance having the pairs sorted by timestamps seems reasonable.

Thus, what I would like to do is to maintain a sorted list based on the creation or update time of key-value pairs. I need to know these creation and update times explicitly. I came up with two different ideas, but now exactly sure which one to use and how. Here are my two ideas:

• I need a LinkedList with head pointing to the timestamp for the recently updated key-value pair and have this key-value pairs point list nodes.
• I need to maintain HashMap in sorted order based on their creation/update time. Maybe I need to change value from integer to Object with the integer value and a long indicating timestamp.

And the question is how to implement these in Java for efficient add/delete/get performance? Or which libraries I can use to get a HashMap sorted by creation/update time?

Answer 1

Offering my two cents... I recently did something similar (I didn't know about LinkedHashMap at the time) where I needed to keep track of some session info.

I ended up using a ConcurrentHashMap because multiple user sessions can be active at a time, and run a cleanup every 30 minutes to clear stale session data. My thought process was, since the app is going to need snappier performance for dealing with session data, I kept the session id as the key. When I need to clear the oldest data, just grab a list of the values and sort (provided that the class implements Comparable or you can write a Comparator for it) because this is not done "that often".

Hope that helps.

PS. I'm curious how this compares to the LinkedHashMap implementation?

Answer 2

This is a case where LinkedHashMap is appropriate, overriding the removeEldestEntry() method. The key of this map will be the IP address, the value will be a tuple of (bytes, last_update) .

First, you need to create your map using with "access order": this means that any access to a map entry will move that entry to the end of the list (MRU). Then, do the following when you get a new record:

• If map does not contain an entry for an IP, create a new one with number of bytes.
• If map does contain an entry, get the number of bytes from it, and create a new entry by adding the new bytes to old. Then put() the new entry into the map, replacing the old.

Your tuple should automatically set the time field to current time. But, the thing to understand is that you don't really care about the time, it's simply an attribute used to remove items from the list.

Override removeEldestEntry() , and return true if the time is outside your bounds.

Although, to be honest I think you'll be better off using a size-based eviction strategy (limit your map to a fixed number of entries). A time-based strategy opens you up to a DDOS attack, in which case you have a large number of entries that come in at once, exhausting your memory.

Answer 3

To sort by Creation/Update time, you'll need to have that time to do the comparation. That means your object will have to know when was it created/updated. This can be accomplished relatively easy by having a version field set by default when the object is created and set with a new Date() when the object is updated.

There are structures you can base yourself in ( TreeSet and TreeMap , in particular) who implement an order defined by the objects themselves ( Comparable interface) or a Comparator . If you store items that save the date when they were created ir updated, you can implement a comparator that can aid in the sorting process.

If you're restricted to LinkedList and HashMap , you'll have to sort the list by the use of, for example, Collections#Sort . In the case of the HashMap , you'll have to sort its Entry Set but since you can't modify it, you'll have to generate a new sorted map this way.

Still, a HashMap is a structure that has nothing to do with ordering, so you'll still have some issues when iterating through it. A LinkedHashMap could solve this, but again, it all depends on your data type restrictions.