What does System.gc() and Runtime.gc() methods do?

The System.gc() and Runtime.gc() methods in Java are used to suggest to the Java Virtual Machine (JVM) that it is an opportune time to perform garbage collection. Garbage collection is the process of identifying and disposing of objects that are no longer in use by a program, thereby reclaiming memory resources.

When you invoke System.gc(), you are effectively making a request to the JVM to initiate garbage collection. However, it's important to note that this method does not guarantee immediate garbage collection; it merely suggests that this might be a good time for the JVM to consider running its garbage collection routines[1][2][4][7][11]. The JVM has its own garbage collection algorithms and may decide to ignore this suggestion based on its current state or internal thresholds.

The Runtime.gc() method serves the same purpose as System.gc() but is called on the Runtime instance associated with the current Java application. It is an instance method of the Runtime class, whereas System.gc() is a static method and can be called without needing an instance of any object[2][3][4].

Both methods are generally used in the same way and have the same effect. Internally, System.gc() calls Runtime.getRuntime().gc(), which in turn suggests to the JVM that garbage collection should be performed[2][4][11]. The JVM may then perform garbage collection at its discretion, which includes the recycling of unused objects and, if necessary, finalization.

It is also worth mentioning that the use of these methods is generally discouraged unless there is a very good reason to do so, such as in the context of benchmarking or before a known idle period. This is because invoking garbage collection can cause a pa...