Media3 App Crash: Internal Player Calls & Non-Main Looper

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Media3 App Crash: Internal Player Calls & Non-Main Looper

Encountering crashes in your Media3-based Android app when using a non-main application Looper? You're not alone! This article dives deep into a common issue where internal calls to the player lead to unexpected app crashes, especially when the app is removed from recents. Let's explore the problem, understand the root cause, and discuss potential workarounds.

The Problem: Internal Calls on the Wrong Thread

The core of the issue lies in how Media3 handles thread management when a non-main Looper is used for the player. Specifically, if you configure your player to run on a background thread (using setLooper(playerThread.looper)), you might expect all player-related operations to occur on that thread. However, internal calls within the Media3 library can sometimes inadvertently be executed on the main thread, leading to a java.lang.IllegalStateException. This exception clearly states: "Player is accessed on the wrong thread." The stack trace pinpoints the ExoPlayerImpl.verifyApplicationThread method as the culprit, indicating a thread mismatch during player access. This problem is triggered when your app is removed from the recents, and the onTaskRemoved() method is called.

When the system removes your app from the recent tasks list, the onTaskRemoved() lifecycle method is invoked in your Service. The default implementation of onTaskRemoved() in MediaSessionService attempts to pause all players and stop itself. This pausing action involves calling setPlayWhenReady, which, if executed on the wrong thread, will trigger the aforementioned exception. The crash occurs because Media3's internal logic, particularly within MediaSessionService, isn't consistently using the player's designated Looper when handling onTaskRemoved(). This inconsistency leads to thread conflicts and application instability. Therefore, understanding the implications of using a non-main looper with Media3 and carefully managing thread execution is crucial for preventing unexpected crashes and ensuring a smooth user experience.

Decoding the Stack Trace

Let's break down the stack trace to understand the flow of execution leading to the crash:

  1. The crash originates from android.app.ActivityThread.handleServiceArgs, indicating an issue within the Android system's service management.
  2. The MediaSessionService.onTaskRemoved method is identified as the source of the problem.
  3. The pauseAllPlayersAndStopSelf method within MediaSessionService is called, which subsequently calls setPlayWhenReady on the player.
  4. Finally, ExoPlayerImpl.verifyApplicationThread detects that setPlayWhenReady is being called on the main thread instead of the expected player thread, resulting in the IllegalStateException.

This stack trace paints a clear picture: the onTaskRemoved event triggers a sequence of actions that ultimately lead to a thread conflict when interacting with the player. The key takeaway is that the call to setPlayWhenReady, initiated during the task removal process, is not being executed on the correct thread, highlighting a potential flaw in Media3's internal thread management.

The Hacky Workaround (and Why It's Not Ideal)

One might attempt a workaround by overriding onTaskRemoved() and manually posting the super call to the player's Looper. While this might seem to solve the immediate crash, it's far from a robust solution. The problem persists, manifesting as a crash in the paused state.

Even with this workaround, a different IllegalStateException arises: "MediaController method is called from a wrong thread." This indicates that while the initial crash might be avoided, other parts of the Media3 library are still making calls on the wrong thread. Specifically, the MediaNotificationManager attempts to access MediaController.getPlayWhenReady on the main thread, leading to the crash. This secondary crash highlights the pervasive nature of the threading issue within Media3. It emphasizes that simply patching the onTaskRemoved() method is insufficient to address the underlying problem of inconsistent thread usage. A more comprehensive solution is needed to ensure that all Media3 components correctly interact with the player's designated thread, preventing unexpected crashes and ensuring a stable and predictable application behavior.

Digging Deeper: Why Does This Happen?

The root cause of this issue appears to be an inconsistency in how Media3 manages thread context during specific lifecycle events, particularly onTaskRemoved(). When onTaskRemoved() is triggered, the MediaSessionService attempts to pause the player. However, the code responsible for this action doesn't consistently ensure that it's executed on the player's designated thread. This can happen when the MediaSessionService posts a task using the main thread's Handler instead of the player thread's Handler. This inconsistency likely stems from the fact that the MediaSessionService may be operating within the context of the main thread when onTaskRemoved() is called, and it doesn't properly switch to the player's thread before interacting with the player. This issue highlights the complexity of managing threads in Android applications, especially when dealing with lifecycle events and background services. It also emphasizes the importance of carefully reviewing and testing thread-sensitive code to ensure that operations are always executed on the correct thread, preventing unexpected crashes and ensuring the stability of the application.

Potential Solutions and Recommendations

  1. Ensure Consistent Threading: The most crucial step is to ensure that all player-related operations are consistently executed on the player's designated thread. This includes calls made within onTaskRemoved() and any other lifecycle methods.
  2. Careful Use of Handlers: When posting tasks to the player, always use the Handler associated with the player's Looper. Avoid using the main thread's Handler unless absolutely necessary.
  3. Review Media3's Threading Model: Thoroughly understand Media3's threading model and how it interacts with Android's lifecycle events. Pay close attention to the documentation and examples related to background playback and thread management.
  4. Report the Issue: Make sure the Media3 team are aware by reporting the bug. Include the details from this guide and your own additional findings.

Example Implementation

Let's illustrate with an example of the correct way to override onTaskRemoved():

class PlaybackService : MediaSessionService() {

    private val playerThread = HandlerThread("PlayerThread").apply { start() }
    private val playerHandler = Handler(playerThread.looper)
    private lateinit var player: ExoPlayer

    override fun onCreate() {
        super.onCreate()
        player = ExoPlayer.Builder(this)
            .setLooper(playerThread.looper)
            .build()
        // ... other initialization
    }

    override fun onTaskRemoved(rootIntent: Intent) {
        playerHandler.post {
            super.onTaskRemoved(rootIntent)
        }
    }

    override fun onDestroy() {
        playerThread.quitSafely()
        // ... other cleanup
        super.onDestroy()
    }

    override fun onGetSession(controllerInfo: MediaSession.ControllerInfo): MediaSession? {
        return mediaSession
    }
}

In this example, we ensure that super.onTaskRemoved(rootIntent) is called on the playerThread using playerHandler.post. This guarantees that the internal calls within onTaskRemoved are executed on the correct thread.

Conclusion

Dealing with threading issues in Android media playback can be tricky, but understanding the underlying causes and applying the correct solutions can save you a lot of headaches. By ensuring consistent thread usage and carefully managing Handlers, you can avoid the dreaded "Player accessed on the wrong thread" crash and create a more robust and reliable media playback experience. Remember to thoroughly test your application on various devices and scenarios to catch any potential threading issues early on.