Memory leak trong Java là gì? GC có thu hồi được không?

Memory leak Java: object vẫn còn reference nhưng không còn cần — GC không thể collect (vì nguyên tắc GC: chỉ thu hồi object unreachable).

// Static collection — leak điển hình
static List<Object> list = new ArrayList<>();
list.add(obj);     // không bao giờ remove → object sống mãi

Nguyên nhân phổ biến:
- Unclosed resource: file handle, DB connection, network socket.
- Listener không unregister: subject giữ reference đến listener.
- Static collection phình mãi (cache không evict).
- ThreadLocal không remove() — thread pool reuse thread → ThreadLocal giữ object cũ.
- Inner class giữ ref ngầm đến enclosing object.
- String pool — intern() quá nhiều String.

Phòng tránh:
- try-with-resources cho AutoCloseable (file, DB connection):

try (var stmt = conn.prepareStatement(sql)) { /* ... */ }   // auto close

• Unregister listener trong @PreDestroy hoặc destroy().
• Cache có eviction policy (Caffeine, Guava).
• WeakReference/SoftReference cho cache.
• ThreadLocal: luôn remove() trong finally.

Debug: heap dump (jmap -dump) → Eclipse MAT phân tích "Leak Suspects".

Java memory leak: an object is still referenced but no longer needed — GC cannot collect it (GC only reclaims unreachable objects).

// Static collection — classic leak
static List<Object> list = new ArrayList<>();
list.add(obj);     // never removed → object lives forever

Common causes:
- Unclosed resources: file handles, DB connections, network sockets.
- Unregistered listeners: the subject still holds a reference.
- Unbounded static collections (caches with no eviction).
- ThreadLocal not remove()-d — thread pools reuse threads → ThreadLocal keeps old objects.
- Inner classes implicitly retain the enclosing object.
- String pool — too many intern() calls.

Prevention:
- try-with-resources for AutoCloseable (files, DB connections):

try (var stmt = conn.prepareStatement(sql)) { /* ... */ }   // auto close

• Unregister listeners in @PreDestroy or destroy().
• Caches need an eviction policy (Caffeine, Guava).
• WeakReference/SoftReference for cache values.
• ThreadLocal: always remove() in a finally.

Debug: heap dump (jmap -dump) → Eclipse MAT, look at "Leak Suspects".