Java Performance Tuning

Monitoring Gross Memory Usage

The JDK provides two methods for monitoring the amount of memory used by the runtime system. The methods are freeMemory( ) and totalMemory( ) in the java.lang.Runtime class.

totalMemory( ) returns a long, which is the number of bytes currently allocated to the runtime system for this particular Java VM process. Within this memory allocation, the VM manages its objects and data. Some of this allocated memory is held in reserve for creating new objects. When the currently allocated memory gets filled and the garbage collector cannot allocate sufficiently more memory, the VM requests more memory to be allocated to it from the underlying system. If the underlying system cannot allocate any further memory, an OutOfMemoryError error is thrown. Total memory can go up and down; some Java runtimes can return sections of unused memory to the underlying system while still running.

freeMemory( ) returns a long, which is the number of bytes available to the VM to create objects from the section of memory it controls (i.e., memory already allocated to the runtime by the underlying system). The free memory increases when a garbage collection successfully reclaims space used by dead objects, and also increases when the Java runtime requests more memory from the underlying operating system. The free memory reduces each time an object is created, and also when the runtime returns memory to the underlying system.

It can be useful to monitor memory usage while an application ...