Hi, Please help me to find out what is the best wa...

# getting-started

[mike_shysh]

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 }
    }
...

[diesieben07]

Simplest way would be val dbSession by lazy { /* create session here */ } , which is thread-safe by default.

[mike_shysh]

lateinit is the same?

[mike_shysh]

private lateinit var dbSession: Session

[diesieben07]

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

[diesieben07]

lazy has synchronization built-in

[mike_shysh]

Thanks, I will try

[mike_shysh]

Should this dbSession be a static variable, or it does not matter?

[diesieben07]

Kotlin does not have the concept of static fields

[mike_shysh]

I know, but I want concept of singleton to work

[mike_shysh]

object DBProvider its a singleton class

[diesieben07]

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

[mike_shysh]

but what about properties

[mike_shysh]

yeah

[mike_shysh]

Will it be ok when one thread use dbSession from object DBProvider and at this very time another thread tries to obtain dbSession

[diesieben07]

That's why lazy has synchronization built in. It guarantees only one value will ever be created

[diesieben07]

Even in concurrent scenarios

[mike_shysh]

So, I do not need custom getter for dbSession

[mike_shysh]

Am I right?

[diesieben07]

Yes, if you use by lazy { }

[mike_shysh]

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
}

[diesieben07]

Yes

[mike_shysh]

Thanks

[mike_shysh]

There are lots of info on the internet, but its hard to catch the idea)