feat: allow setting sessionId in initial configuration

android/src/main/java/com/amplitude/android/Amplitude.kt

@@ -33,7 +33,7 @@ open class Amplitude(
     }
 
     override fun createTimeline(): Timeline {
-        return Timeline().also { it.amplitude = this }
+        return Timeline(configuration.sessionId).also { it.amplitude = this }
     }
 
     override fun createIdentityConfiguration(): IdentityConfiguration {

android/src/main/java/com/amplitude/android/Configuration.kt

@@ -48,6 +48,7 @@ open class Configuration @JvmOverloads constructor(
     var migrateLegacyData: Boolean = true,
     override var offline: Boolean? = false,
     override var deviceId: String? = null,
+    override var sessionId: Long? = null,
 ) : Configuration(
     apiKey,
     flushQueueSize,
@@ -70,6 +71,7 @@ open class Configuration @JvmOverloads constructor(
     identityStorageProvider,
     offline,
     deviceId,
+    sessionId,
 ) {
     companion object {
         const val MIN_TIME_BETWEEN_SESSIONS_MILLIS: Long = 300000

android/src/main/java/com/amplitude/android/Timeline.kt

@@ -7,24 +7,31 @@ import kotlinx.coroutines.channels.Channel
 import kotlinx.coroutines.launch
 import java.util.concurrent.atomic.AtomicLong
 
-class Timeline : Timeline() {
+class Timeline(
+    private val initialSessionId: Long? = null,
+) : Timeline() {
     private val eventMessageChannel: Channel<EventQueueMessage> = Channel(Channel.UNLIMITED)
 
-    private val _sessionId = AtomicLong(-1)
+    private val _sessionId = AtomicLong(initialSessionId ?: -1L)
     val sessionId: Long
         get() {
             return _sessionId.get()
         }
 
     internal var lastEventId: Long = 0
-    var lastEventTime: Long = -1
+    var lastEventTime: Long = -1L
 
     internal fun start() {
         amplitude.amplitudeScope.launch(amplitude.storageIODispatcher) {
             // Wait until build (including possible legacy data migration) is finished.
             amplitude.isBuilt.await()
 
-            _sessionId.set(amplitude.storage.read(Storage.Constants.PREVIOUS_SESSION_ID)?.toLongOrNull() ?: -1)
+            if (initialSessionId == null) {
+                _sessionId.set(
+                    amplitude.storage.read(Storage.Constants.PREVIOUS_SESSION_ID)?.toLongOrNull()
+                        ?: -1
+                )
+            }
             lastEventId = amplitude.storage.read(Storage.Constants.LAST_EVENT_ID)?.toLongOrNull() ?: 0
             lastEventTime = amplitude.storage.read(Storage.Constants.LAST_EVENT_TIME)?.toLongOrNull() ?: -1
 

android/src/test/java/com/amplitude/android/AmplitudeTest.kt

@@ -78,6 +78,7 @@ class AmplitudeTest {
         minTimeBetweenSessionsMillis: Long? = null,
         storageProvider: StorageProvider = InMemoryStorageProvider(),
         deviceId: String? = null,
+        sessionId: Long? = null,
     ): Configuration {
         val configuration = Configuration(
             apiKey = "api-key",
@@ -88,6 +89,7 @@ class AmplitudeTest {
             loggerProvider = ConsoleLoggerProvider(),
             identifyInterceptStorageProvider = InMemoryStorageProvider(),
             identityStorageProvider = IMIdentityStorageProvider(),
+            sessionId = sessionId,
         )
 
         if (deviceId != null) {
@@ -217,6 +219,18 @@ class AmplitudeTest {
         }
     }
 
+    @Test
+    fun amplitude_should_set_sessionId_from_configuration() = runTest {
+        val testSessionId = 1337L
+        // set device Id in the config
+        amplitude = Amplitude(createConfiguration(sessionId = testSessionId))
+        setDispatcher(testScheduler)
+
+        if (amplitude?.isBuilt!!.await()) {
+            Assertions.assertEquals(testSessionId, amplitude?.sessionId)
+        }
+    }
+
     companion object {
         const val instanceName = "testInstance"
     }

core/src/main/java/com/amplitude/core/Configuration.kt

@@ -32,6 +32,7 @@ open class Configuration @JvmOverloads constructor(
     open var identityStorageProvider: IdentityStorageProvider = IMIdentityStorageProvider(),
     open var offline: Boolean? = false,
     open var deviceId: String? = null,
+    open var sessionId: Long? = null,
 ) {
 
     companion object {