Featured White Papers

• Measuring Risk to Improve Java Software Quality
• Do You Really Get Classloaders?
• Pragmatic Continuous Delivery with Jenkins, Nexus, and LiveRebel
• Your Next Java Web App: Less XML, No Long Restarts, Fewer Hassles
• Coding with JRebel, Java Forever Changed
• 2012 Developer Productivity Report: Java Tools, Tech, Devs, and Data

Sponsored Links

• Graph databases are 1000x faster than relational.

  Learn More!

• Career opportunities at Harman- Careers That'll Rock Your Socks Off.

  Learn More!

• Developer-friendly Java PDF libraries by Qoppa Software

  Live Demos

• Stop blaming your app! Diagnose Java performance issues.

  Free tool >

• UrbanCode: Go From Continuous Integration to
  Continuous Delivery

  Learn More!

Sponsored Links

Optimize with a SATA RAID Storage Solution
Range of capacities as low as $1250 per TB. Ideal if you currently rely on servers/disks/JBODs

Java's garbage-collected heap

An introduction to the garbage-collected <br>heap of the Java virtual machine

By Bill Venners, JavaWorld.com, 08/01/96

Page 6 of 7

Swimming fish
Heap Of Fish has five modes, which are selectable via radio buttons at the bottom left of the applet. When the applet starts it is in swim mode. Swim mode is just a gratuitous animation. The animation is vaguely reminiscent of the familiar image of a big fish about to eat a medium-sized fish, which is about to eat a small fish.

The other four modes -- allocate fish, assign references, garbage collect, and compact heap -- allow you to interact with the heap. You can instantiate new fish objects in the allocate fish mode. The new fish objects go on the heap as all Java objects do. In the assign references mode you can build a network of local variables and fish that refer to other fish. In garbage collect mode, a mark and sweep operation will free any unreferenced fish. The compact heap mode allows you to slide heap objects so that they are side by side at one end of the heap, leaving all free memory as one large contiguous block at the other end of the heap.

Allocate fish
The allocate fish mode shows the two parts that make up the heap, the object pool and handle pool. The object pool is a contiguous block of memory from which space is taken for new objects. The object pool is structured as a series of memory blocks. Each memory block has a four-byte header which indicates the length of the memory block and whether it is free. The headers are shown in the applet as black horizontal lines in the object pool.

The object pool in Heap Of Fish is implemented as an array of ints. The first header is always at objectPool[0]. The object pool's series of memory blocks can be traversed by hopping from header to header. Each header gives the length of its memory block, which also reveals where the next header is going to be. The header of the next memory block will be the first int immediately following the current memory block. When a new object is allocated the object pool is traversed until a memory block is encountered with enough space to accommodate the new object. Allocated objects in the object pool are shown as colored bars. YellowFish objects are shown in yellow, BlueFish objects are shown in blue, and RedFish objects are shown in red. Free memory blocks, those that currently contain no fish, are shown in white.

The handle pool in Heap Of Fish is implemented as an array of objects of a class named ObjectHandle. An ObjectHandle contains information about an object, including the vital index into the object pool array. The object pool index functions as a reference to the actual allocated object's instance data in the object pool. The ObjectHandle also reveals information about the class of the fish object. In a real JVM, each allocated object would need to be associated with the information read in from the class file such as the method bytecodes, names of the class, its superclass, any interfaces it implements, its fields, and the type signatures of its methods and fields. In Heap Of Fish, the ObjectHandle associates each allocated object with information such as its class -- whether it is a RedFish, BlueFish, or YellowFish -- and some data used in displaying the fish in the applet user interface.