JVM Memory Manager - Garbage Collector

This section describes what is a garbage collector and its role and responsibilities for managing the JVM memroy.

In previous sections, we have observed how JVM manages the Heap memory automatically for us. In following sections, we are going to learn some basic concepts on how the JVM actually manages the memory.

What Is a Garbage Collector? A garbage collector is JVM component that is responsible for managing memory space primarily in the Head data area. It also manages memory space in Method Area and Direct Memory data areas. More specifically, a garbage collector has 3 major responsibilities.

1. Allocating Memory for New Objects - When the Java application is requesting to create a new data object (an instance of a class or array), the garbage collector must find a block of free space of a certain size in the memory. It is an easy task at the beginning, when the free space is a single continuous chuck. But when the free space becomes fragmented because of old objects are deallocated not in the same order as them were allocated. The garbage collector may have to move data objects around to put free spaces together into large chunks.

2. Ensuring Live Objects Remaining in Memory - Once a data object is allocated, the garbage collector must keep tracking it to know where this object is actually located in the memory, and how many live references that are pointing this object.

3. Deallocating Memory for Dead Objects - When a data object loses its last live reference, it becomes a dead object, which is also called garbage. The garbage collector can leave dead objects in the memory for some time. But it must deallocate them when there is not enough free space to allocate new objects. The process of finding dead objects and freeing the space used by these objects is known as garbage collection.

Common design goals of a garbage collector:

Last update: 2014.

Table of Contents

 About This Book

 Downloading and Installing JDK 1.8.0 on Windows

 Downloading and Installing JDK 1.7.0 on Windows

 java.lang.Runtime Class - The JVM Instance

 java.lang.System Class - The Operating System

 ClassLoader Class - Class Loaders

 Class Class - Class Reflections

 Sun's JVM - Java HotSpot VM

 JRockit JVM 28.2.7 by Oracle Corporation

 JVM Runtime Data Areas

Memory Management and Garbage Collectors

 Memory Management General Rules

 Java Exception: "java.lang.OutOfMemoryError: Java heap space"

 OutOfMemoryError Comparison of HotSpot and JRockit

 Garbage Collection Demonstration

JVM Memory Manager - Garbage Collector

 Generational Garbage Collection in HotSpot

 Young Generation Collection - Minor Collection

 Tenured Generation Collection - Full Collection

 HotSpot Default Garbage Collector - Serial Collector

 "-XX:+PrintGCDetails" - Garbage Collection Logging

 GC Log Messages on GarbageCollection.java

 Serial, Parallel, Concurrent, and Regionalized Collectors

 Parallel Collector GC Log Message Format

 Parallel Compacting Collector GC Log Message Format

 Concurrent Mark-Sweep Collector GC Log Message Format

 Garbage First GC Log Message Format

 Garbage Collection Tests

 JVM Stack, Frame and Stack Overflow

 Thread Testing Program and Result

 CPU Impact of Multi-Thread Applications

 I/O Impact of Multi-Thread Applications

 CDS (Class Data Sharing)

 Micro Benchmark Runner and JVM Options

 Micro Benchmark Tests on "int" Operations

 Micro Benchmark Tests on "long" Operations

 Micro Benchmark Tests in JIT Compilation Mode

 Micro Benchmark Tests on "float" and "double" Operations

 Outdated Tutorials


 PDF Printing Version