proper usage of synchronized singleton?

java multithreading singleton synchronized
Dan.StackOverflow picture Dan.StackOverflow · Apr 22, 2009 · Viewed 24.6k times · Source

So I am thinking about building a hobby project, one off kind of thing, just to brush up on my programming/design.

It's basically a multi threaded web spider, updating the same data structure object->int.

So it is definitely overkill to use a database for this, and the only thing I could think of is a thread-safe singleton used to contain my data structure. http://web.archive.org/web/20121106190537/http://www.ibm.com/developerworks/java/library/j-dcl/index.html

Is there a different approach I should look in to?

Answer

Steve Kuo picture Steve Kuo · Apr 22, 2009

Double-checked locking has been proven to be incorrect and flawed (as least in Java). Do a search or look at Wikipedia's entry for the exact reason.

First and foremost is program correctness. If your code is not thread-safe (in a multi-threaded environment) then it's broken. Correctness comes first before performance optimization.

To be correct you'll have to synchronize the whole getInstance method

public static synchronized Singleton getInstance() {
   if (instance==null) ...
}

or statically initialize it

private static final Singleton INSTANCE = new Singleton();

Related questions

Singleton Pattern in Multi threaded environment

During my interview, interviewer started his question with singleton pattern. I wrote below. Then, he asked Shouldn't we check for Nullity inside getInstance method? I replied with, It is NOT necessary, since member is static type and is being initialized …

Read More
Java Singleton and Synchronization

Please clarify my queries regarding Singleton and Multithreading: What is the best way to implement Singleton in Java, in a multithreaded environment? What happens when multiple threads try to access getInstance() method at the same time? Can we make singleton's …

Read More
Thread Safe singleton class

I wrote a below Singleton class. I am not sure whether this is thread safe singleton class or not? public class CassandraAstyanaxConnection { private static CassandraAstyanaxConnection _instance; private AstyanaxContext<Keyspace> context; private Keyspace keyspace; private ColumnFamily<String, String&…

Read More