bumptech/glide · error · IllegalArgumentException

You must call this method on a background thread

Error message

You must call this method on a background thread

What it means

Thrown by Util.assertBackgroundThread() when the calling code is on the main (UI) thread. Some Glide internals and APIs (notably certain cache/disk operations and RequestBuilder.asDrawable().submit() style synchronous fetches run on background executors) require a background thread to avoid blocking the UI; calling them on the main thread is rejected.

Source

Thrown at library/src/main/java/com/bumptech/glide/util/Util.java:192

      }
    }
    return mainThreadHandler;
  }

  /**
   * Throws an {@link java.lang.IllegalArgumentException} if called on a thread other than the main
   * thread.
   */
  public static void assertMainThread() {
    if (!isOnMainThread()) {
      throw new IllegalArgumentException("You must call this method on the main thread");
    }
  }

  /** Throws an {@link java.lang.IllegalArgumentException} if called on the main thread. */
  public static void assertBackgroundThread() {
    if (!isOnBackgroundThread()) {
      throw new IllegalArgumentException("You must call this method on a background thread");
    }
  }

  /** Returns {@code true} if called on the main thread, {@code false} otherwise. */
  public static boolean isOnMainThread() {
    return Looper.myLooper() == Looper.getMainLooper();
  }

  /** Returns {@code true} if called on a background thread, {@code false} otherwise. */
  public static boolean isOnBackgroundThread() {
    return !isOnMainThread();
  }

  /** Creates a {@link java.util.Queue} of the given size using Glide's preferred implementation. */
  @NonNull
  public static <T> Queue<T> createQueue(int size) {
    return new ArrayDeque<>(size);
  }

View on GitHub (pinned to eb14a895d8)

Solutions

1. Move the call off the main thread: withContext(Dispatchers.IO) { ... } or an AsyncTask/Executor
2. For clearDiskCache specifically, run it on a background executor and post UI updates back
3. For fetching an image synchronously, use submit().get() only on a background thread, or switch to an async into() with a Target
4. Audit the stack trace to find the UI-thread caller of the background-only method

Example fix

// before
fun onClearCacheClicked() {
  Glide.get(this).clearDiskCache() // throws: main thread
}

// after
fun onClearCacheClicked() {
  lifecycleScope.launch(Dispatchers.IO) {
    Glide.get(this@Activity).clearDiskCache()
  }
}

fun isBackgroundThread(): Boolean =
  Looper.myLooper() != Looper.getMainLooper()

if (isBackgroundThread()) {
  Glide.get(ctx).clearDiskCache()
} else {
  // move to a background executor
}

When it happens

Trigger: Calling a Glide API marked as background-only from the UI thread directly; e.g. certain disk-cache methods, or synchronous submit().get() patterns; helper methods inside custom decoders/modules that assert background execution.

Common situations: Calling Glide's disk cache inspection APIs (e.g. Glide.get(ctx).clearDiskCache()) from an onClick handler; synchronous fetch via RequestBuilder.submit().get() on the main thread; custom logic invoking assertBackgroundThread-bearing utilities during bind.

Related errors

• You must call this method on the main thread
• You can't start or clear loads in RequestListener or Target
• You cannot start a load on a fragment before it is attached
• Unable to find GlideModule implementation
• Expected instanceof GlideModule, but found: {module}

AI-assisted analysis of bumptech/glide@eb14a895d8 (2026-08-14). Data as JSON: /api/errors/e70170e56589d4d1. Report an issue: GitHub.