Hi, Please help me to find out what is the best way to implement thread-safe singleton (for DB connection)? Here are the variants:

object DBProvider {
    @Volatile private var dbSession: Session? = null

    fun getDBSession(): Session = dbSession ?: synchronized(this) {
        dbSession ?: when (dbType) {
            DBTypes.MYSQL.type -> MySqlSession(host = dbHost, port = dbPort).getSession()
            DBTypes.ORACLE.type -> OracleSession(host = dbHost, port = dbPort).getSession()
            else -> throw Exception("Unsupported DB type - $dbType.")
        }.also { dbSession = it }
    }
...

Simplest way would be

val dbSession by lazy { /* create session here */ }

, which is thread-safe by default.

lateinit is the same?

No, lateinit is simply syntax-sugar for a nullable field.

Thanks, I will try

Kotlin does not have the concept of static fields

I know, but I want concept of singleton to work

Since you are doing this in an

object

, there will only be one instance of that

object

and thus only one value for each of it's properties

but what about properties

That's why

lazy

has synchronization built in. It guarantees only one value will ever be created

So, I do not need custom getter for

dbSession

Yes, if you use

by lazy { }

I can just put this into

by lazy {  }

init

fun getDBSession(): Session {
dbSession = when (dbType) {
                DBTypes.MYSQL.type -> MySqlSession(host = dbHost, port = dbPort).getSession()
                DBTypes.ORACLE.type -> OracleSession(host = dbHost, port = dbPort).getSession()
                else -> throw Exception("Unsupported DB type - $dbType.")
            }
        }
        return dbSession
}

Yes

Thanks