What is ANR in Android and how to diagnose and solve ANR issues - 面试题

Android ANR Explained

ANR (Application Not Responding) is an error state in Android when an application fails to respond to user input or broadcasts within a specific time period.

ANR Trigger Conditions

Type	Trigger Condition	Time Limit
Input ANR	Key or touch event not responding	5 seconds
BroadcastReceiver ANR	onReceive() timeout	Foreground 10s, Background 60s
Service ANR	Service lifecycle method timeout	20 seconds
ContentProvider ANR	ContentProvider operation timeout	10 seconds

Causes of ANR

1. Main Thread Time-consuming Operations

Network requests on main thread
Large database operations
Complex calculation tasks
Loading large files or images

2. Deadlock

Main thread waiting for child thread to release lock
Child thread waiting for main thread to release lock

3. Insufficient Memory

System resource shortage
Frequent GC causing lag

4. Binder Communication Timeout

Cross-process call timeout
Server process not responding

ANR Diagnosis Methods

1. Check Logcat

shell
E/ActivityManager: ANR in com.example.app
E/ActivityManager: Reason: Input dispatching timed out

2. Analyze traces.txt

shell
/data/anr/traces.txt

Contains:

Stack traces of all threads
ANR occurrence timestamp
Blocked code location

3. Use Android Studio Profiler

CPU Profiler to check main thread status
Identify time-consuming methods

4. StrictMode Detection

java
StrictMode.setThreadPolicy(new StrictMode.ThreadPolicy.Builder()
    .detectDiskReads()
    .detectDiskWrites()
    .detectNetwork()
    .penaltyLog()
    .build());

ANR Solutions

1. Asynchronous Processing

java
// Use thread pool
ExecutorService executor = Executors.newFixedThreadPool(4);
executor.execute(() -> {
    // Time-consuming operations
});

// Use Kotlin coroutines
lifecycleScope.launch(Dispatchers.IO) {
    // Time-consuming operations
}

2. Use HandlerThread

java
HandlerThread handlerThread = new HandlerThread("background");
handlerThread.start();
Handler backgroundHandler = new Handler(handlerThread.getLooper());

3. Optimize Database Operations

Use transactions for batch operations
Create appropriate indexes
Avoid querying large data on main thread

4. Avoid Deadlocks

Unified lock acquisition order
Use tryLock() instead of lock()
Reduce lock holding time

Key Points

ANR is a system protection mechanism, not a crash
traces.txt is key to diagnosing ANR
Main thread cannot perform time-consuming operations
Use Systrace for performance analysis
Monitor lag and collect online ANR data