Matter AI | Code Reviewer Documentation

Memory Leaks Overview

Unclosed Resources

// Anti-pattern: Not closing resources properly
public void readFile(String filePath) {
    try {
        FileInputStream fileStream = new FileInputStream(filePath);
        BufferedReader reader = new BufferedReader(new InputStreamReader(fileStream));
        
        String line;
        while ((line = reader.readLine()) != null) {
            processLine(line);
        }
        
        // Missing reader.close() and fileStream.close()
        // This will leak file handles
    } catch (IOException e) {
        e.printStackTrace();
    }
}

// Better approach: Using try-with-resources
public void readFileCorrectly(String filePath) {
    try (FileInputStream fileStream = new FileInputStream(filePath);
         BufferedReader reader = new BufferedReader(new InputStreamReader(fileStream))) {
        
        String line;
        while ((line = reader.readLine()) != null) {
            processLine(line);
        }
        
        // Resources automatically closed when the try block exits
    } catch (IOException e) {
        e.printStackTrace();
    }
}

// Anti-pattern: Not closing database connections
function queryDatabase(query) {
  const connection = openDatabaseConnection();
  
  // Run query
  const results = connection.execute(query);
  
  // Return results without closing the connection
  return results;
  
  // Missing connection.close()
}

// Better approach: Ensuring connections are closed
async function queryDatabaseCorrectly(query) {
  const connection = await openDatabaseConnection();
  
  try {
    // Run query
    const results = await connection.execute(query);
    return results;
  } finally {
    // Always close the connection, even if an error occurs
    await connection.close();
  }
}

Failing to close resources like file handles, database connections, network sockets, or streams leads to resource leaks that can exhaust system resources and cause memory leaks.

To prevent resource leaks:

Always close resources in a finally block or use language features like try-with-resources in Java
Use the appropriate pattern for your language (e.g., using statement in C#, with statement in Python)
Consider using resource management libraries or utilities
Implement proper error handling to ensure resources are closed even when exceptions occur
Use connection pooling for database connections
Consider implementing AutoCloseable interface for custom resources

Event Listener Leaks

// Anti-pattern: Not removing event listeners
class ComponentWithLeak {
  constructor(element) {
    this.element = element;
    this.handleClick = this.handleClick.bind(this);
    
    // Add event listener
    this.element.addEventListener('click', this.handleClick);
  }
  
  handleClick() {
    console.log('Element clicked');
  }
  
  // Missing cleanup method to remove the event listener
}

// Better approach: Properly removing event listeners
class ComponentWithoutLeak {
  constructor(element) {
    this.element = element;
    this.handleClick = this.handleClick.bind(this);
    
    // Add event listener
    this.element.addEventListener('click', this.handleClick);
  }
  
  handleClick() {
    console.log('Element clicked');
  }
  
  // Cleanup method to remove the event listener
  destroy() {
    this.element.removeEventListener('click', this.handleClick);
    this.element = null; // Remove reference to DOM element
  }
}

// Anti-pattern: Not removing listeners in Android
public class LeakyActivity extends Activity {
    private SensorManager sensorManager;
    private SensorEventListener sensorListener;
    
    @Override
    protected void onCreate(Bundle savedInstanceState) {
        super.onCreate(savedInstanceState);
        setContentView(R.layout.activity_main);
        
        sensorManager = (SensorManager) getSystemService(SENSOR_SERVICE);
        Sensor accelerometer = sensorManager.getDefaultSensor(Sensor.TYPE_ACCELEROMETER);
        
        sensorListener = new SensorEventListener() {
            @Override
            public void onSensorChanged(SensorEvent event) {
                // Handle sensor data
            }
            
            @Override
            public void onAccuracyChanged(Sensor sensor, int accuracy) {
                // Handle accuracy changes
            }
        };
        
        sensorManager.registerListener(sensorListener, accelerometer, SensorManager.SENSOR_DELAY_NORMAL);
        
        // Missing unregisterListener in onPause or onDestroy
    }
}

// Better approach: Properly unregistering listeners
public class NonLeakyActivity extends Activity {
    private SensorManager sensorManager;
    private SensorEventListener sensorListener;
    
    @Override
    protected void onCreate(Bundle savedInstanceState) {
        super.onCreate(savedInstanceState);
        setContentView(R.layout.activity_main);
        
        sensorManager = (SensorManager) getSystemService(SENSOR_SERVICE);
        Sensor accelerometer = sensorManager.getDefaultSensor(Sensor.TYPE_ACCELEROMETER);
        
        sensorListener = new SensorEventListener() {
            @Override
            public void onSensorChanged(SensorEvent event) {
                // Handle sensor data
            }
            
            @Override
            public void onAccuracyChanged(Sensor sensor, int accuracy) {
                // Handle accuracy changes
            }
        };
        
        sensorManager.registerListener(sensorListener, accelerometer, SensorManager.SENSOR_DELAY_NORMAL);
    }
    
    @Override
    protected void onPause() {
        super.onPause();
        // Unregister listener when activity is paused
        sensorManager.unregisterListener(sensorListener);
    }
}

Event listener leaks occur when listeners are registered but not properly unregistered, particularly in component-based architectures or UI frameworks.

To prevent event listener leaks:

Always unregister listeners when they’re no longer needed
Implement proper cleanup methods in components
Use weak references for listeners when appropriate
Follow the lifecycle methods of your framework (e.g., componentWillUnmount in React)
Consider using event delegation patterns to reduce the number of listeners
Use tools like Chrome DevTools Memory panel to detect listener leaks

Circular References

// Anti-pattern: Creating circular references
function createCircularReference() {
  let parent = {};
  let child = {};
  
  // Create circular reference
  parent.child = child;
  child.parent = parent;
  
  return parent;
}

// Better approach: Using weak references
function createNonCircularReference() {
  let parent = {};
  let child = {};
  
  // Create reference from parent to child
  parent.child = child;
  
  // Use WeakRef for the reverse reference (in modern JavaScript)
  child.parent = new WeakRef(parent);
  
  return parent;
}

# Anti-pattern: Creating circular references in Python
def create_circular_reference():
    parent = {}
    child = {}
    
    # Create circular reference
    parent['child'] = child
    child['parent'] = parent
    
    return parent

# Better approach: Using weak references
import weakref

def create_non_circular_reference():
    parent = {}
    child = {}
    
    # Create reference from parent to child
    parent['child'] = child
    
    # Use weak reference for the reverse reference
    child['parent'] = weakref.ref(parent)
    
    return parent

Circular references occur when two or more objects reference each other, creating a cycle that can prevent garbage collection in environments without cycle-detecting garbage collectors.

To prevent circular reference leaks:

Use weak references for back-references
Implement explicit cleanup methods to break cycles
Design object relationships to avoid unnecessary bidirectional references
Consider using observer patterns instead of direct references
Use data structures that don’t create cycles
Manually set references to null when they’re no longer needed

Static Collections Growing Unbounded

// Anti-pattern: Unbounded static collection
public class CacheWithLeak {
    // Static cache that grows without bounds
    private static final Map<String, Object> CACHE = new HashMap<>();
    
    public static void addToCache(String key, Object value) {
        CACHE.put(key, value);
    }
    
    public static Object getFromCache(String key) {
        return CACHE.get(key);
    }
    
    // No method to remove items or clear the cache
}

// Better approach: Bounded cache with size limits
public class BoundedCache {
    // Using LinkedHashMap with access order and maximum size
    private static final int MAX_ENTRIES = 1000;
    private static final Map<String, Object> CACHE = new LinkedHashMap<String, Object>(MAX_ENTRIES + 1, 0.75f, true) {
        @Override
        protected boolean removeEldestEntry(Map.Entry<String, Object> eldest) {
            return size() > MAX_ENTRIES;
        }
    };
    
    public static void addToCache(String key, Object value) {
        CACHE.put(key, value);
    }
    
    public static Object getFromCache(String key) {
        return CACHE.get(key);
    }
    
    public static void removeFromCache(String key) {
        CACHE.remove(key);
    }
    
    public static void clearCache() {
        CACHE.clear();
    }
}

// Anti-pattern: Unbounded cache in a singleton service
class LoggerService {
  constructor() {
    if (LoggerService.instance) {
      return LoggerService.instance;
    }
    
    this.logCache = {};
    LoggerService.instance = this;
  }
  
  logMessage(id, message) {
    // Storing every log message without limits
    if (!this.logCache[id]) {
      this.logCache[id] = [];
    }
    
    this.logCache[id].push({
      message,
      timestamp: Date.now()
    });
  }
  
  // No method to clear old logs
}

// Better approach: Bounded cache with cleanup
class ImprovedLoggerService {
  constructor() {
    if (ImprovedLoggerService.instance) {
      return ImprovedLoggerService.instance;
    }
    
    this.logCache = {};
    this.maxLogsPerKey = 100;
    this.maxAgeMs = 24 * 60 * 60 * 1000; // 24 hours
    
    // Set up periodic cleanup
    setInterval(() => this.cleanupOldLogs(), 60 * 60 * 1000); // Every hour
    
    ImprovedLoggerService.instance = this;
  }
  
  logMessage(id, message) {
    if (!this.logCache[id]) {
      this.logCache[id] = [];
    }
    
    this.logCache[id].push({
      message,
      timestamp: Date.now()
    });
    
    // Trim if too many logs for this ID
    if (this.logCache[id].length > this.maxLogsPerKey) {
      this.logCache[id] = this.logCache[id].slice(-this.maxLogsPerKey);
    }
  }
  
  cleanupOldLogs() {
    const now = Date.now();
    
    Object.keys(this.logCache).forEach(id => {
      // Remove logs older than maxAgeMs
      this.logCache[id] = this.logCache[id].filter(log => 
        (now - log.timestamp) < this.maxAgeMs
      );
      
      // Remove empty arrays
      if (this.logCache[id].length === 0) {
        delete this.logCache[id];
      }
    });
  }
}

Static collections that grow unbounded are a common source of memory leaks, especially in long-running applications or singleton services.

To prevent unbounded collection growth:

Use bounded collections with a maximum size
Implement eviction policies (LRU, LFU, FIFO)
Set up periodic cleanup of old or unused entries
Use weak references for values to allow garbage collection
Consider using specialized caching libraries
Implement time-based expiration for cached items
Monitor collection sizes in production

Forgotten Timers and Intervals

// Anti-pattern: Not clearing timers
class DataPoller {
  constructor(url) {
    this.url = url;
    this.startPolling();
  }
  
  startPolling() {
    // Set up interval that's never cleared
    this.intervalId = setInterval(() => {
      this.fetchData();
    }, 5000);
  }
  
  fetchData() {
    fetch(this.url)
      .then(response => response.json())
      .then(data => this.processData(data));
  }
  
  processData(data) {
    console.log('Processing data:', data);
  }
  
  // Missing stopPolling method to clear the interval
}

// Better approach: Providing cleanup method
class ImprovedDataPoller {
  constructor(url) {
    this.url = url;
    this.intervalId = null;
  }
  
  startPolling() {
    // Clear any existing interval first
    this.stopPolling();
    
    this.intervalId = setInterval(() => {
      this.fetchData();
    }, 5000);
  }
  
  stopPolling() {
    if (this.intervalId) {
      clearInterval(this.intervalId);
      this.intervalId = null;
    }
  }
  
  fetchData() {
    fetch(this.url)
      .then(response => response.json())
      .then(data => this.processData(data));
  }
  
  processData(data) {
    console.log('Processing data:', data);
  }
}

// Anti-pattern: Not invalidating timers in iOS
class LeakyViewController: UIViewController {
    var timer: Timer?
    
    override func viewDidLoad() {
        super.viewDidLoad()
        
        // Create timer that's never invalidated
        timer = Timer.scheduledTimer(timeInterval: 1.0, target: self, selector: #selector(updateUI), userInfo: nil, repeats: true)
    }
    
    @objc func updateUI() {
        // Update UI elements
    }
    
    // Missing timer invalidation in deinit or viewWillDisappear
}

// Better approach: Properly invalidating timers
class NonLeakyViewController: UIViewController {
    var timer: Timer?
    
    override func viewDidLoad() {
        super.viewDidLoad()
        
        // Create timer
        timer = Timer.scheduledTimer(timeInterval: 1.0, target: self, selector: #selector(updateUI), userInfo: nil, repeats: true)
    }
    
    @objc func updateUI() {
        // Update UI elements
    }
    
    override func viewWillDisappear(_ animated: Bool) {
        super.viewWillDisappear(animated)
        
        // Invalidate timer when view disappears
        timer?.invalidate()
        timer = nil
    }
    
    deinit {
        // Safety check to ensure timer is invalidated
        timer?.invalidate()
    }
}

Forgotten timers and intervals are a common source of memory leaks, especially in single-page applications or mobile apps where components are created and destroyed frequently.

To prevent timer leaks:

Always clear intervals and timeouts when they’re no longer needed
Implement proper cleanup methods in components
Follow component lifecycle methods to clear timers
Use weak references in timer callbacks when possible
Consider using timer management utilities
In iOS, invalidate timers in viewWillDisappear and deinit
In Android, cancel handlers in onPause or onDestroy

DOM Element References

// Anti-pattern: Storing DOM references that outlive elements
class ElementManager {
  constructor() {
    this.elements = {};
  }
  
  registerElement(id, element) {
    // Storing reference to DOM element
    this.elements[id] = element;
  }
  
  // Missing method to unregister elements
}

// Usage that causes a leak
const manager = new ElementManager();

function createTemporaryUI() {
  const tempDiv = document.createElement('div');
  document.body.appendChild(tempDiv);
  
  // Register with manager
  manager.registerElement('temp', tempDiv);
  
  // Later, we remove the element from DOM
  document.body.removeChild(tempDiv);
  
  // But the reference still exists in the manager!
}

// Better approach: Providing cleanup method
class ImprovedElementManager {
  constructor() {
    this.elements = {};
  }
  
  registerElement(id, element) {
    this.elements[id] = element;
  }
  
  unregisterElement(id) {
    delete this.elements[id];
  }
  
  // Optional: Use weak references
  registerElementWeak(id, element) {
    this.elements[id] = new WeakRef(element);
  }
}

// Improved usage
const betterManager = new ImprovedElementManager();

function createTemporaryUIImproved() {
  const tempDiv = document.createElement('div');
  document.body.appendChild(tempDiv);
  
  // Register with manager
  betterManager.registerElement('temp', tempDiv);
  
  // Later, we remove the element from DOM and unregister
  document.body.removeChild(tempDiv);
  betterManager.unregisterElement('temp');
}

// Anti-pattern: Storing View references in Android
public class ViewCache {
    private static Map<String, View> viewCache = new HashMap<>();
    
    public static void cacheView(String id, View view) {
        viewCache.put(id, view);
    }
    
    public static View getView(String id) {
        return viewCache.get(id);
    }
    
    // Missing method to remove views
}

// Better approach: Using WeakReferences
public class ImprovedViewCache {
    private static Map<String, WeakReference<View>> viewCache = new HashMap<>();
    
    public static void cacheView(String id, View view) {
        viewCache.put(id, new WeakReference<>(view));
    }
    
    public static View getView(String id) {
        WeakReference<View> weakRef = viewCache.get(id);
        return weakRef != null ? weakRef.get() : null;
    }
    
    public static void removeView(String id) {
        viewCache.remove(id);
    }
    
    public static void clearCache() {
        viewCache.clear();
    }
}

Storing references to DOM elements or Views that outlive their visual representation is a common cause of memory leaks in web and mobile applications.

To prevent DOM/View reference leaks:

Use weak references when storing DOM/View references
Implement proper cleanup methods to remove references
Follow component lifecycle methods
Avoid storing references in long-lived objects or singletons
Consider using data attributes instead of direct references
Use tools like Chrome DevTools Memory panel to detect leaks
In Android, avoid storing Activity or View references in static fields

Closure Memory Leaks

// Anti-pattern: Closure capturing large objects
function createLeakyProcessor(data) {
  // data might be a very large object
  
  return function process() {
    // This closure captures the entire data object
    return data.items.map(item => item.value * 2);
  };
}

// Usage that causes a leak
const largeData = { items: Array(1000000).fill().map((_, i) => ({ value: i })) };
const processor = createLeakyProcessor(largeData);

// Even if we don't need largeData anymore, it's still referenced by the closure
largeData = null; // This doesn't free the original object

// Better approach: Extracting only what's needed
function createEfficientProcessor(data) {
  // Extract only the necessary data
  const items = data.items.map(item => item.value);
  
  return function process() {
    // This closure only captures the values, not the entire objects
    return items.map(value => value * 2);
  };
}

# Anti-pattern: Closure capturing large objects in Python
def create_leaky_processor(data):
    # data might be a very large dictionary
    
    def process():
        # This closure captures the entire data object
        return [item['value'] * 2 for item in data['items']]
    
    return process

# Better approach: Extracting only what's needed
def create_efficient_processor(data):
    # Extract only the necessary data
    items = [item['value'] for item in data['items']]
    
    def process():
        # This closure only captures the values, not the entire objects
        return [value * 2 for value in items]
    
    return process

Closures can cause memory leaks when they capture and retain references to large objects or the entire parent scope, even when only a small portion of that data is needed.

To prevent closure memory leaks:

Extract only the specific data needed by the closure
Avoid capturing large objects or collections
Be careful with event handlers that capture the parent scope
Consider using partial application or currying instead of closures
Explicitly set captured variables to null when they’re no longer needed
Be aware of the scope chain and what variables are being captured
Use memory profiling tools to detect closure-related leaks

Memory-Intensive Caching

// Anti-pattern: Inefficient image caching
public class ImageCache {
    // Unbounded cache that stores full-size images
    private static final Map<String, Bitmap> CACHE = new HashMap<>();
    
    public static void cacheImage(String url, Bitmap image) {
        CACHE.put(url, image);
    }
    
    public static Bitmap getImage(String url) {
        return CACHE.get(url);
    }
}

// Better approach: Size-limited cache with memory-sensitive handling
public class ImprovedImageCache {
    private static final int MAX_MEMORY = (int) (Runtime.getRuntime().maxMemory() / 1024);
    private static final int CACHE_SIZE = MAX_MEMORY / 8;
    
    private static LruCache<String, Bitmap> mMemoryCache;
    
    static {
        mMemoryCache = new LruCache<String, Bitmap>(CACHE_SIZE) {
            @Override
            protected int sizeOf(String key, Bitmap bitmap) {
                // The cache size will be measured in kilobytes
                return bitmap.getByteCount() / 1024;
            }
            
            @Override
            protected void entryRemoved(boolean evicted, String key, Bitmap oldValue, Bitmap newValue) {
                // Optional: handle evicted bitmaps (e.g., add to disk cache)
            }
        };
    }
    
    public static void cacheImage(String url, Bitmap bitmap) {
        if (bitmap != null && url != null) {
            mMemoryCache.put(url, bitmap);
        }
    }
    
    public static Bitmap getImage(String url) {
        return mMemoryCache.get(url);
    }
    
    public static void clearCache() {
        mMemoryCache.evictAll();
    }
}

// Anti-pattern: Inefficient caching of large objects
class DataCache {
  constructor() {
    this.cache = {};
  }
  
  storeData(key, data) {
    // Storing potentially large objects without limits
    this.cache[key] = data;
  }
  
  getData(key) {
    return this.cache[key];
  }
}

// Better approach: Memory-sensitive caching
class ImprovedDataCache {
  constructor(maxItems = 100, maxSizeMB = 50) {
    this.cache = new Map();
    this.maxItems = maxItems;
    this.maxSizeBytes = maxSizeMB * 1024 * 1024;
    this.currentSizeBytes = 0;
  }
  
  estimateSize(data) {
    // Rough estimation of object size in bytes
    const jsonString = JSON.stringify(data);
    return new Blob([jsonString]).size;
  }
  
  storeData(key, data) {
    // Check if key already exists
    if (this.cache.has(key)) {
      const oldSize = this.cache.get(key).size;
      this.currentSizeBytes -= oldSize;
    }
    
    const size = this.estimateSize(data);
    
    // Check if this item would exceed our max size
    if (size > this.maxSizeBytes) {
      console.warn(`Item ${key} is too large to cache (${size} bytes)`);  
      return false;
    }
    
    // Make room if necessary
    while (this.cache.size >= this.maxItems || 
           (this.currentSizeBytes + size) > this.maxSizeBytes) {
      // Remove oldest entry (first item in Map)
      const oldestKey = this.cache.keys().next().value;
      const oldSize = this.cache.get(oldestKey).size;
      this.cache.delete(oldestKey);
      this.currentSizeBytes -= oldSize;
    }
    
    // Store the data with its size
    this.cache.set(key, {
      data,
      size,
      timestamp: Date.now()
    });
    
    this.currentSizeBytes += size;
    return true;
  }
  
  getData(key) {
    const item = this.cache.get(key);
    return item ? item.data : null;
  }
  
  clear() {
    this.cache.clear();
    this.currentSizeBytes = 0;
  }
}

Memory-intensive caching can lead to excessive memory consumption, especially when caching large objects like images, datasets, or complex structures.

To implement memory-efficient caching:

Set appropriate size limits based on available memory
Use LRU (Least Recently Used) or similar eviction policies
Consider the memory footprint of cached objects
Implement multi-level caching (memory, disk, network)
Use weak references for non-critical cache entries
Monitor cache size and memory usage
Consider using specialized caching libraries
Implement proper cache invalidation strategies
Use memory-efficient data structures for cached items

Memory Leak Detection and Prevention

// Example: Using Chrome DevTools for memory leak detection

// 1. Take heap snapshot before suspected leak
// In Chrome DevTools: Memory tab > Heap Snapshot > Take Snapshot

// 2. Perform actions that might cause a leak
function potentiallyLeakyOperation() {
  // Create and destroy components, perform operations, etc.
}

// 3. Take another heap snapshot
// In Chrome DevTools: Memory tab > Heap Snapshot > Take Snapshot

// 4. Compare snapshots to identify retained objects
// In Chrome DevTools: Select second snapshot > Filter by "Objects allocated between Snapshot 1 and Snapshot 2"

// Example: Using LeakCanary for Android memory leak detection

// In build.gradle
dependencies {
  // Debug dependency
  debugImplementation 'com.squareup.leakcanary:leakcanary-android:2.7'
}

// LeakCanary automatically watches for Activity and Fragment leaks
// For custom objects:

class MyApplication extends Application {
  @Override
  public void onCreate() {
    super.onCreate();
    
    // Watch for leaks in custom objects
    AppWatcher.Config config = AppWatcher.getConfig()
        .newBuilder()
        .watchActivities(true)
        .watchFragments(true)
        .watchViewModels(true)
        .build();
    AppWatcher.setConfig(config);
  }
}

Detecting and preventing memory leaks requires a combination of tools, practices, and awareness.

Memory leak detection strategies:

Use memory profiling tools (Chrome DevTools, Android Profiler, etc.)
Implement automated memory leak detection in CI/CD pipelines
Use specialized libraries like LeakCanary for Android
Monitor memory usage over time to identify gradual increases
Implement proper logging and metrics for memory usage
Conduct regular code reviews focused on memory management
Use static analysis tools to identify potential leaks
Test applications under memory pressure

Prevention best practices:

Follow proper resource management patterns
Implement appropriate cleanup methods
Use weak references for back-references
Be cautious with static collections and singletons
Understand object lifecycle in your framework
Implement proper caching strategies
Educate team members about common memory leak patterns

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