JVM.GC.How do you trigger garbage collection manually, and why is it discouraged?

Although Garbage Collection (GC) in Java is automatic, you can suggest it manually using the following methods:

1️⃣ Using System.gc()

System.gc();
  • This requests the JVM to run garbage collection.
  • The JVM may or may not execute GC immediately.

2️⃣ Using Runtime.getRuntime().gc()

Runtime.getRuntime().gc();
  • Similar to System.gc(), but explicitly calls the gc() method of the JVM’s Runtime instance.
  • Again, GC execution is not guaranteed.

3️⃣ Calling finalize() (Discouraged)

  • Before Java 9, objects could override finalize(), which was called before an object was garbage collected.
  • However, it does not guarantee immediate GC execution and is deprecated since Java 9.

Example:

@Override
protected void finalize() throws Throwable {
    System.out.println("Finalizing object...");
}

🔴 Deprecated in Java 9 and removed in Java 18.

Why Is Manually Triggering GC Discouraged?

1️⃣ GC Is Optimized by the JVM

  • The JVM automatically manages memory efficiently.
  • Manual GC calls disrupt optimization and can cause unnecessary overhead.

2️⃣ System.gc() Causes Stop-the-World Pauses

  • Calling System.gc() forces a full garbage collection (if the JVM allows it).
  • All application threads stop (Stop-the-World event) while GC runs.
  • This reduces performance and can cause noticeable lag in real-time applications.

3️⃣ It Does Not Guarantee Immediate Collection

  • System.gc() is only a suggestion.
  • The JVM may ignore it if it determines GC is not necessary.

4️⃣ Full GC Can Lead to Performance Issues

  • A Full GC (triggered by System.gc()) can be very expensive in large applications.
  • It can cause latency spikes, making applications unresponsive.

5️⃣ Encourages Bad Coding Practices

  • If your application relies on manual GC, it may indicate poor memory management.
  • Proper memory handling (object reuse, weak references, GC tuning) is a better alternative.

When Should You Trigger GC Manually?

Although discouraged, manual GC triggering can be useful in some cases:

1. Before a Large Memory-Intensive Task

  • If an application is about to load a large dataset, forcing GC beforehand may free up space.
  • Example:
System.gc(); // Free memory before loading a large file
loadLargeFile();

2. In Testing & Benchmarking

  • In performance testing, you might use System.gc() to measure memory usage before and after an operation.
  • Example:
System.gc();
long beforeMemory = Runtime.getRuntime().freeMemory();

performTask(); // Run task

long afterMemory = Runtime.getRuntime().freeMemory();
System.out.println("Memory used: " + (beforeMemory - afterMemory));

3. In Embedded or Low-Memory Systems

  • In memory-constrained environments, a manual GC call might help prevent OutOfMemoryErrors.

How to Optimize GC Without Manual Calls

🚀 Instead of calling System.gc(), use these optimizations:Use Proper Data Structures – Avoid unnecessary object creation.
Use Weak References – Prevent memory leaks with WeakReference or SoftReference.
Tweak JVM GC Settings – Use -XX:+UseG1GC or -XX:+UseZGC for optimized GC.
Monitor Memory Usage – Use VisualVM, JConsole, or JProfiler to analyze GC behavior.


Conclusion

  • Manual GC triggering (System.gc()) is discouraged because it disrupts JVM optimizations and can slow down applications.
  • The JVM automatically manages memory, making manual GC unnecessary in most cases.
  • Use GC tuning and better coding practices instead of relying on explicit garbage collection.

📌 In short: Let the JVM handle GC—manual calls do more harm than good! 🚀

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