Memory leaks in Java refer to objects that are no longer used but cannot be recycled by GC because the reference is not released. Common scenarios include the collection class not being cleaned, the listener not being logged out, the cache is not invalidated, and the internal class holding external class references, etc. 1. Uncleaned collection classes will cause continuous memory occupancy. The solution is to clean up or use weak references regularly; 2. If the listener and callback are not logged out, the weak reference mechanism should be removed or used; 3. If the internal class holds external class references, it should use static internal classes and manually manage the references; 4. If the cache is not set to expire policies, it is recommended to use mature cache libraries such as Caffeine or Ehcache; in addition, you should also pay attention to log objects, ThreadLocal usage and ClassLoader uninstallation issues. To identify memory leaks, you need to combine heap memory trends and tool analysis, such as VisualVM, MAT, JProfiler, etc. Developing good habits during the development stage will help prevent memory leaks.

Although memory leaks in Java applications are not as obvious as C/C, once they occur, they often cause serious performance degradation or even crashes. The most common manifestation is that the longer the application runs, the more memory it consumes, and the OutOfMemoryError is eventually triggered.

What is a memory leak in Java?

Many people think that Java has a garbage collection mechanism (GC), so there will be no memory leaks. Actually, it is not the case. Memory leaks in Java refer to objects no longer being used, but because some references are not released, GC cannot recycle them. These "useless but accessible" objects will continue to occupy memory, causing problems over time.

Common scenarios include: collection class not being cleaned, listener not being logged out, cache not being invalidated, internal class holding external class references, etc.

How to identify memory leaks?

To discover memory leaks, you cannot rely solely on code searches, but you must judge them in combination with tools and phenomena.

• Observe the heap memory trend : If the memory in the old age does not drop significantly after Full GC, and the heap memory is on the rise, there is a memory leak likely.
• Use the memory analysis tool :
  • VisualVM: Lightweight, suitable for local debugging
  • Eclipse MAT (Memory Analyzer): Powerful, suitable for in-depth analysis of heap dump
  • JProfiler: Commercial software, powerful visualization, suitable for complex problem positioning

These tools allow you to see which class instances grow abnormally, whether there are multiple ClassLoaders loading repeatedly, or a Map/Set that continues to grow but is not cleared.

Common memory leak types and response methods

1. Collection class not cleaned

   • For example, a global Map or List is constantly added without deleting old data
   • Workaround: Clean up or use weak references regularly (such as WeakHashMap )

2. Listeners and callbacks are not logged out

   • Especially in event-driven systems, listeners are registered but not removed when components are destroyed
   • Recommendation: Actively log out the listener at the end of the object life cycle, or use a weak reference listening mechanism

3. The inner class holds the outer class reference

   • Non-static inner classes (such as anonymous classes) hold references to external classes by default
   • If the inner class is held for a long time (such as submitting it to the thread pool as a task), the outer class will not be recycled
   • Solution: Use static inner class instead and manually manage external class references

4. The cache has not set an expiration policy

   • The cache is not set to an upper limit or is not automatically cleaned, which can easily become a memory black hole.
   • It is recommended to use mature cache libraries such as Caffeine or Ehcache, which come with expiration and elimination mechanisms.

Details to pay attention to during development

• Don't treat log objects as something that "does not leak". If MDC data like Log4j is not cleared, it may also lead to the accumulation of local variables in threads.
• Be careful when using ThreadLocal, especially when using thread pools. If ThreadLocal variables are not cleaned when multiplexing threads, it is easy to cause data residues and memory accumulation across requests.
• For web applications, pay attention to whether ClassLoader is properly uninstalled during frequent deployment/rebooting. Otherwise, PermGen/Metaspace memory leak may occur.

Basically that's it. Memory leaks don't happen every time, but once they occur, the cost of troubleshooting is very high. It is easier to develop good habits in the development stage than to remedy them after they are launched.

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