Thinking in Java (memory mapped file)

When dealing with large files, frequent read and write operations using ordinary FileInputStream or FileOutputStream or randomaccessfile will cause the process to slow down due to frequent read and write external memory. The following is a comparison experiment.

package test;import java.io.bufferedinputstream;import java.io.fileinputstream;import  Java.io.filenotfoundexception;import java.io.ioexception;import java.io.randomaccessfile;import  java.nio.mappedbytebuffer;import java.nio.channels.filechannel;public class test { Public static void main (String[] args)  {try {FileInputStream fis=new  FileInputStream ("/home/tobacco/test/res.txt"); Int sum=0;int n;long t1=system.currenttimemillis ( ); Try {while ((N=fis.read ()) >=0) {sum+=n;}}  catch  (ioexception e)  {// TODO Auto-generated catch  Blocke.printstacktrace ();} Long t=system.currenttimemillis ()-t1; System.out.println ("Sum:" +sum+ "  time:" +t);}  catch  (filenotfoundexception e)  {// TODO Auto-generated catch  Blocke.printstacktrace ();} Try {fileinputstream fis=new fileinputstream ("/home/toBacco/test/res.txt "); Bufferedinputstream bis=new bufferedinputstream (FIS); int sum=0;int n;long t1= System.currenttimemillis (); Try {while ((N=bis.read ()) >=0) {sum+=n;}}  catch  (ioexception e)  {// TODO Auto-generated catch  Blocke.printstacktrace ();} Long t=system.currenttimemillis ()-t1; System.out.println ("Sum:" +sum+ "  time:" +t);}  catch  (filenotfoundexception e)  {// TODO Auto-generated catch  Blocke.printstacktrace ();} Mappedbytebuffer buffer=null;try {buffer=new randomaccessfile ("/home/tobacco/test/res.txt", "RW" ). Getchannel (). Map (filechannel.mapmode.read_write, 0, 1253244);int sum=0;int n;long  T1=system.currenttimemillis (); for (int i=0;i<1253244;i++) {n=0x000000ff&buffer.get (i); sum+=n;} Long t=system.currenttimemillis ()-t1; System.out.println ("Sum:" +sum+ "  time:" +t);}  catch  (filenotfoundexception e)  {// todo auto-generated catch blocke.printstacktrace ();}  catch  (ioexception e)  {// TODO Auto-generated catch  Blocke.printstacktrace ();}}}

The test file is a file with a size of 1253244 bytes. Test results:

sum:220152087 time:1464
sum:220152087 time:72
sum:220152087 time:25

Indicates that the read data is correct. Delete the data processing part thereof.

package test;import java.io.bufferedinputstream;import java.io.fileinputstream;import  Java.io.filenotfoundexception;import java.io.ioexception;import java.io.randomaccessfile;import  java.nio.mappedbytebuffer;import java.nio.channels.filechannel;public class test { Public static void main (String[] args)  {try {FileInputStream fis=new  FileInputStream ("/home/tobacco/test/res.txt"); Int sum=0;int n;long t1=system.currenttimemillis ( ); Try {while ((N=fis.read ()) >=0) {//sum+=n;}}  catch  (ioexception e)  {// TODO Auto-generated catch  Blocke.printstacktrace ();} Long t=system.currenttimemillis ()-t1; System.out.println ("Sum:" +sum+ "  time:" +t);}  catch  (filenotfoundexception e)  {// TODO Auto-generated catch  Blocke.printstacktrace ();} Try {fileinputstream fis=new fileinputstream ("/home/Tobacco/test/res.txt "); Bufferedinputstream bis=new bufferedinputstream (FIS); int sum=0;int n;long t1= System.currenttimemillis (); Try {while ((N=bis.read ()) >=0) {//sum+=n;}}  catch  (ioexception e)  {// TODO Auto-generated catch  Blocke.printstacktrace ();} Long t=system.currenttimemillis ()-t1; System.out.println ("Sum:" +sum+ "  time:" +t);}  catch  (filenotfoundexception e)  {// TODO Auto-generated catch  Blocke.printstacktrace ();} Mappedbytebuffer buffer=null;try {buffer=new randomaccessfile ("/home/tobacco/test/res.txt", "RW" ). Getchannel (). Map (filechannel.mapmode.read_write, 0, 1253244);int sum=0;int n;long  T1=system.currenttimemillis (); for (int i=0;i<1253244;i++) {//n=0x000000ff&buffer.get (i);//sum+=n;} Long t=system.currenttimemillis ()-t1; System.out.println ("Sum:" +sum+ "  time:" +t);}  catch  (Filenotfoundexception e)  {// todo auto-generated catch blocke.printstacktrace ();}  catch  (ioexception e)  {// TODO Auto-generated catch  Blocke.printstacktrace ();}}}

Test results:

sum:0 time:1458
sum:0 time:67
sum:0 Time:8

This shows that some or all of the files are mapped to memory after reading and writing, the speed will be much higher.

This is because the memory-mapped file first maps the file on the external memory to a contiguous area in memory, is treated as a byte array, the read-write operation directly to the memory, and then re-map the memory area to the external memory file, which saves the intermediate time to read and write, greatly reducing the read and write time.

Ps: This article is only in my study to retrieve the relatively professional article, but has not been verified by the practice.

Thinking in Java (memory mapped file)