How can I get the current SparkSession in any place of the codes?

Once a session was created (anywhere), you can safely use:

SparkSession.builder.getOrCreate()

To get the (same) session anywhere in the code, as long as the session is still alive. Spark maintains a single active session so unless it was stopped or crashed, you'll get the same one.

Edit: builder is not callable, as mentioned in the comments.

Since 2.2.0 you can access the active SparkSession through:

/**
 * Returns the active SparkSession for the current thread, returned by the builder.
 *
 * @since 2.2.0
 */
def getActiveSession: Option[SparkSession] = Option(activeThreadSession.get)

or default SparkSession:

/**
 * Returns the default SparkSession that is returned by the builder.
 *
 * @since 2.2.0
 */
def getDefaultSparkSession: Option[SparkSession] = Option(defaultSession.get)