Rule Definition
The garbage collector should not used explicitly in the code. It is an automated process scheduled by the Java Runtime Environment. System.gc() triggers a full collection, which includes tracing all live objects in the heap and sweeping and compacting the old generation. Lot of time can be spent during this operation. In general, it is better to let the system decide when it needs to collect the heap, and whether or not to do a full collection.

Remediation
Ensure to let the system decide when it needs to collect the heap, and whether or not to do a full collection

Violation Code Sample

public class DemoApplication {       private static final Map<String, String> cache = new HashMap<String, String>();       public static void main(String[] args) {         Scanner scanner = new Scanner(System.in);           while (scanner.hasNext()) {             final String next = scanner.next();             if ("fill".equals(next)) {                 for (int i = 0; i < 1000000; i++) {                      cache.put(randomUUID().toString(), randomUUID().toString());                  }              } else if ("invalidate".equals(next)) {                 Runtime.gc();             } else if ("gc".equals(next)) {                 System.gc();             } else if ("exit".equals(next)) {                 System.exit(0);             } else {                 System.out.println("unknown");             }         }     } }   The Java Language Specification does not guarantee that the JVM will start a GC when you call System.gc(). Most of JVM has a garbage collector that runs continuously, so a call to System.gc() won't do anything

Fixed Code Sample

public class DemoApplication {       private static final Map<String, String> cache = new HashMap<String, String>();       public static void main(String[] args) {         Scanner scanner = new Scanner(System.in);           while (scanner.hasNext()) {             final String next = scanner.next();             if ("fill".equals(next)) {                 for (int i = 0; i < 1000000; i++) {                      cache.put(randomUUID().toString(), randomUUID().toString());                  }              } else if ("invalidate".equals(next)) {                 cache.clear();             } else if ("gc".equals(next)) {                  System.out.println("known")             } else if ("exit".equals(next)) {                 System.exit(0);             } else {                 System.out.println("unknown");             }         }     } }

https://help.semmle.com/wiki/display/JAVA/Explicit+garbage+collection#:~:text=You%20should%20avoid%20making%20calls,that%20lead%20to%20decreased%20performance.