Why is it bad practice to call System.gc()?

by

The reason everyone always says to avoid System.gc() is that it is a pretty good indicator of fundamentally broken code. Any code that depends on it for correctness is certainly broken; any that rely on it for performance are most likely broken.

You don’t know what sort of garbage collector you are running under. There are certainly some that do not “stop the world” as you assert, but some JVMs aren’t that smart or for various reasons (perhaps they are on a phone?) don’t do it. You don’t know what it’s going to do.

Also, it’s not guaranteed to do anything. The JVM may just entirely ignore your request.

The combination of “you don’t know what it will do,” “you don’t know if it will even help,” and “you shouldn’t need to call it anyway” are why people are so forceful in saying that generally you shouldn’t call it. I think it’s a case of “if you need to ask whether you should be using this, you shouldn’t”

EDIT to address a few concerns from the other thread:

After reading the thread you linked, there’s a few more things I’d like to point out.
First, someone suggested that calling gc() may return memory to the system. That’s certainly not necessarily true – the Java heap itself grows independently of Java allocations.

As in, the JVM will hold memory (many tens of megabytes) and grow the heap as necessary. It doesn’t necessarily return that memory to the system even when you free Java objects; it is perfectly free to hold on to the allocated memory to use for future Java allocations.

To show that it’s possible that System.gc() does nothing, view
JDK bug 6668279
and in particular that there’s a -XX:DisableExplicitGC VM option:

By default calls to System.gc() are enabled (-XX:-DisableExplicitGC). Use -XX:+DisableExplicitGC to disable calls to System.gc(). Note that the JVM still performs garbage collection when necessary.

More Related Contents:

  1. What is the difference between Java Garbage collection and C++ object destruction? [closed]
  2. How to force garbage collection in Java?
  3. When does System.gc() do something?
  4. Java Thread Garbage collected or not
  5. Java heap terminology: young, old and permanent generations?
  6. How to free memory in Java?
  7. Java 7 (JDK 7) garbage collection and documentation on G1
  8. Does setting Java objects to null do anything anymore?
  9. Java Garbage Collection Log messages
  10. How to monitor Java memory usage?
  11. Garbage collector in Android
  12. GC overhead of Optional in Java
  13. Analyzing gc logs
  14. kubernetes pod memory – java gc logs
  15. Garbage collection behaviour for String.intern()
  16. Empty an ArrayList or just create a new one and let the old one be garbage collected? [duplicate]
  17. Java G1 garbage collection in production
  18. Is WeakHashMap ever-growing, or does it clear out the garbage keys?
  19. Know of any Java garbage collection log analysis tools? [closed]
  20. Does using final for variables in Java improve garbage collection?
  21. Can you get basic GC stats in Java?
  22. Will stream classes or connections considered as a resource leak in Java
  23. Why is finalize not being called?
  24. How to calculate HashMap memory usage in Java?
  25. Java Collections and Garbage Collector
  26. “Island of isolation” of Garbage Collection
  27. What is the Metadata GC Threshold and how do I tune it?
  28. Java GC (Allocation Failure)
  29. Static references are cleared–does Android unload classes at runtime if unused?
  30. How to read a verbose:GC output?

Leave a Comment