10 Things to Know about Memory Mapped File in Java

What is Memory Mapped File and IO in Java

Memory mapped files are special, which allows Java program to access contents directly from memory, this is achieved by mapping whole file or portion of file into memory and operating system takes care of loading page requested and writing into file while application only deals with memory which results in very fast IO operations. Memory used to load Memory mapped file is outside of Java heap Space. Java programming language supports memory mapped file with the java.nio package and has MappedByteBuffer to read and write from memory.

Advantage and Disadvantage of Memory Mapped File

Possibly the main advantage of Memory Mapped IO is the performance which is important to build high-frequency electronic trading system. Memory Mapped Files are way faster than standard file access via normal IO. Another big advantage of memory mapped IO is that it allows you to load a potentially larger file which is not otherwise accessible. Experiments show that memory mapped IO performs better with large files. Though it has a disadvantage in terms of increasing number of page faults. Since operating system only loads a portion of the file into memory if a page requested is not present in memory, then it would result in the page fault. It can also be used to share data between two processes.

Memory Mapped IO Support in Operating System

Most of the major operating systems like Windows platform, UNIX, Solaris and other UNIX like operating system supports memory mapped IO and with 64 bit architecture you can map almost any file into memory and access it directly using Java programming language.

Important Points of Memory Mapped IO in Java

Here are some of the important facts to know about Memory Mapped File in Java:

1. Java supports Memory mapped IO with java.nio package.
2. Memory mapped files is used in performance sensitive application, e.g. high-frequency electronic trading platforms.
3. By using memory mapped IO, you can load portion of large files in memory.
4. Memory mapped file can result in page fault if requested page is not in memory.
5. Ability to map a region of the file in memory depends on the addressable size of memory. In a 32 bit machine, you cannot access beyond 4GB or 2^32.
6. Memory mapped IO is much faster than Stream IO in Java.
7. Memory used to load File is outside of Java heap and resides on shared memory which allows two different processes to access File.
8. Reading and writing on memory mapped file is done by the operating system, so even if your Java program crashes after putting content into memory, it will make to File until OS is fine.
9. Prefer Direct Byte buffer over NonDirect Buffer for faster performance.
10. Don't call MappedByteBuffer.force() method too often, this method is meant to force operating system to write content of memory into disk, So if you call force() method each time you write into memory mapped file, you will not see true benefit of using mapped byte buffer, instead it will be similar to disk IO.
11. In case of power failure or host failure, there is a slim chance that content of memory mapped file is not written into the disk, which means you could lose critical data.

That's all folks. Memory mapped IO is an important concept for advanced Java developers, especially for writing high-performance applications in Java. If you like this tip, then you may like to visit my blog http://javarevisited.blogspot.com for more such tips.

Thank you!