ANR Debugging & Prevention

📖 Concept

ANR (Application Not Responding) occurs when the main thread is blocked for too long:

Input event: 5 seconds without response
BroadcastReceiver.onReceive(): 10 seconds
Service.onCreate()/onStartCommand(): 20 seconds (foreground), 200 seconds (background)

Common causes:

Disk I/O on main thread — SharedPreferences.commit(), file reads, database queries
Network on main thread — synchronous HTTP calls (rare now but still happens)
Heavy computation — JSON parsing, image processing, complex algorithms
Deadlocks — Two threads waiting for each other's locks
Slow ContentProvider queries — Unindexed database queries
Binder calls — Synchronous IPC to a slow system service

Debugging ANRs:

Logcat: Look for "ANR in" messages with the process and reason
traces.txt: adb pull /data/anr/traces.txt — Shows stack trace of ALL threads at ANR time
StrictMode: Enable disk/network detection in debug builds
Android Vitals (Play Console): Shows ANR rate and stack traces from production
Perfetto: System-wide tracing for complex threading issues

Google's guideline: ANR rate should be below 0.47% (less than 1 ANR per 200 user sessions).

💻 Code Example

codeTap to expand ⛶

1// Common ANR causes and fixes
2
3// ANR: SharedPreferences.commit() on main thread ❌
4fun saveUserPreference(key: String, value: String) {
5    prefs.edit().putString(key, value).commit() // Blocks main thread!
6}
7// FIX: Use apply() or DataStore
8fun saveUserPreference(key: String, value: String) {
9    prefs.edit().putString(key, value).apply() // ✅ Async write
10}
11
12// Even better: Use DataStore
13class PreferencesRepository @Inject constructor(
14    private val dataStore: DataStore<Preferences>
15) {
16    suspend fun savePreference(key: String, value: String) {
17        dataStore.edit { prefs -> prefs[stringPreferencesKey(key)] = value }
18    }
19    
20    fun getPreference(key: String): Flow<String?> {
21        return dataStore.data.map { it[stringPreferencesKey(key)] }
22    }
23}
24
25// ANR: Database query on main thread ❌
26fun getUser(id: String): User {
27    return database.userDao().getById(id) // Blocks if DB is slow!
28}
29// FIX: Use suspend + Dispatchers.IO
30suspend fun getUser(id: String): User = withContext(Dispatchers.IO) {
31    database.userDao().getById(id)
32}
33
34// ANR: Deadlock detection
35// Thread A holds lock1, waits for lock2
36// Thread B holds lock2, waits for lock1
37// Both threads blocked forever — ANR if main thread is involved
38
39// Prevention: Always acquire locks in the same order
40class SafeLocking {
41    private val lock1 = ReentrantLock()
42    private val lock2 = ReentrantLock()
43    
44    // Always acquire lock1 before lock2 — prevents deadlock
45    fun operation() {
46        lock1.withLock {
47            lock2.withLock {
48                // Safe — consistent lock ordering
49            }
50        }
51    }
52}
53
54// StrictMode for development-time detection
55if (BuildConfig.DEBUG) {
56    StrictMode.setThreadPolicy(
57        StrictMode.ThreadPolicy.Builder()
58            .detectDiskReads()
59            .detectDiskWrites()
60            .detectNetwork()
61            .detectCustomSlowCalls()
62            .penaltyLog()
63            .penaltyDeath() // Crash on violation in debug
64            .build()
65    )
66}

🏋️ Practice Exercise

Practice:

Enable StrictMode and find all main-thread disk/network violations in your app
Analyze an ANR traces.txt file — identify the blocked thread and root cause
Replace all SharedPreferences.commit() calls with apply() or DataStore
Implement a custom ANR watchdog that detects main thread blocking
Check your app's ANR rate on Google Play Console

⚠️ Common Mistakes

Using SharedPreferences.commit() instead of apply() — commit() writes synchronously to disk on the calling thread
Querying ContentProviders without a background thread — some providers (Contacts, MediaStore) can be slow
Not monitoring ANR rates in production — by the time users complain, your ANR rate may be critical
Running Room database operations without specifying a background dispatcher

💼 Interview Questions

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