In Java, allocation and de-allocation of memory space for objects are done by the garbage collection process in an automated way by the JVM. Unlike C language the developers need not write code for garbage collection in Java. This is one among the many features that made Java popular and helps programmers write better Java applications.
This is a four part tutorial series to know about the basics of garbage collection in Java,
This tutorial is the first part in the series. It will explain the basic terminologies like JDK, JVM, JRE, HotSpot VM then understand the JVM architecture and Java heap memory structure. It is important to understand these basic things before going into the Garbage collection tutorial.
Each JVM implementation may be different in the way the garbage collection principles are implemented. Prior to SUN takeover Oracle had JRockit JVM and after takeover it got the HotSpot JVM. Presently Oracle maintains both the JVM implementations and it has declared that over a period these two JVM implementations will be converged to one.
HotSpot JVM is available as a core component as part of the standard Oracle SE platform. In this garbage collection tutorial, we will see the garbage collection principles based on the HotSpot Virtual Machine.
Following diagram summarizes the key components in a JVM. In the JVM architecture, two main components that are related to garbage collection are heap memory and garbage collector. Heap memory is the runtime data area where the instances will be store and the garbage collector will operate on. Now we know how these things fit in the larger scheme.
It is essential to understand the role of heap memory in JVM memory model. At runtime the Java instances are stored in the heap memory area. When an object is not referenced anymore it becomes eligible for eviction from heap memory. During garbage collection process, those objects are evicted from heap memory and the space is reclaimed. Heap memory has three major areas,
Update: Permanent Generation (Permgen) space is removed from Java SE 8 features.
In this next part of this tutorial series we will see about how garbage collection works in Java.