Hard coding a hashtable inside a class, how to do this in a thread safe manner?

Question

I want to initialize a hashtable inside my class only once for my web application.

How can you do this in a thread-safe manner in case?

public class MySettings {

   private HashTable<int, SomeObject> settings;

}

Should it be marked static or final or volatile? Do I wrap it in a synchronize?

Also, how can I set the properties of SomeObject inline? Is it possible in java?

You're got an illegal declaration there: You can't specify `int` as a generic type. Try `Integer` — Bohemian, Dec 05 '11 at 00:09
*"Also, how can I set the properties of SomeObject inline? Is it possible in java?"* - totally orthogonal to your real question. Ask it in a different Question ... and take the time to expand on what you are really asking ... — Stephen C, Dec 05 '11 at 01:14

DarthVader · Answer 1 · 2011-12-05T00:29:39.037

3

Make it static, final, initialize it and use ConcurrentHashMap.

You can even have a singleton class for this, and make it lazy.

Something like this. But again, use ConcurrentHashmap. Dont use Hashtable.

import java.util.*;

class Foo { 
        private static final Foo foo = new Foo(); 
        public static final Hashtable table = new Hashtable(); 
        private Foo(){} 

        public static Foo Instance(){
            return foo;
        }

}

edited Dec 05 '11 at 00:29

answered Dec 05 '11 at 00:05

DarthVader

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FYI, here's a fun example: class Foo { private static final Foo foo = new Foo(); private static final HashTable table = new HashTable(); private Foo(){ System.out.println(table); } } Guess what that prints? – LazyCubicleMonkey Dec 05 '11 at 00:17
doenst print. there is no HashTable, should be Hashtable. – DarthVader Dec 05 '11 at 00:19
I edited and put the code. that must work right? you made me typed that. i m lazier than u . – DarthVader Dec 05 '11 at 00:30
That works. I was just pointing out that if you reference table in the constructor of Foo, it would be null. – LazyCubicleMonkey Dec 05 '11 at 00:35
well that s a private contsructor, to ensure that the class wont be initalized by another class and even subclassed. – DarthVader Dec 05 '11 at 00:36

score 0 · Answer 2 · edited May 23 '17 at 12:11

I want to initialize a hashtable inside my class only once for my web application.

If you only want there to be one instance for your webapp then your choices are:

a bare static (nasty!!),
a static wrapped by a class with static methods (nasty, and awkward to use),
an static wrapped using the Singleton class pattern (not so nasty / awkward, but not without problems), or
a shared instance that is created and injected using a dependency injection (DI) framework.

The last is best from a number of perspectives, but it entails changing your webapp to use DI, which will be disruptive.

So I suggest that you read up on the Singleton class pattern and how to implement it in Java so that it is created and initialized in a thread-safe fashion:

Then there is the separate question/issue of how you use the hash table in a thread-safe fashion. And the answers/solutions depend on how your application is going to use the hash table once it has been initialized.

If the webapp only ever reads the hash table, then a plain HashMap is sufficient.
If the webapp needs to read and update the hash table, then the choice is between a ConcurrentHashMap or a synchronized wrapper for a regular HashMap.
- If the table is likely to have lots of threads wanting to perform operations (i.e. lock contention is a likely issue), OR if you need to be able to iterate the table entries while updates might be happening, then use a ConcurrentHashMap.
- Otherwise, a HashMap with a synchronization wrapper will be sufficient.

Hard coding a hashtable inside a class, how to do this in a thread safe manner?

2 Answers2