Java中用内存映射处理大文件.docx

/art/201206/343690.htm在处理大文件时，如果利用普通的FileInputStream 或者FileOutputStream 抑或RandomAccessFile 来进行频繁的读写操作，都将导致进程因频繁读写外存而降低速度.如下为一个对比实验。1. packagetest; 2. 3. importjava.io.BufferedInputStream; 4. importjava.io.FileInputStream; 5. importjava.io.FileNotFoundException; 6. importjava.io.IOException; 7. importjava.io.RandomAccessFile; 8. importjava.nio.MappedByteBuffer; 9. importjava.nio.channels.FileChannel; 10. 11. publicclassTest 12. 13. publicstaticvoidmain(Stringargs) 14. try 15. FileInputStreamfis=newFileInputStream(/home/tobacco/test/res.txt); 16. intsum=0; 17. intn; 18. longt1=System.currentTimeMillis(); 19. try 20. while(n=fis.read()=0) 21. sum+=n; 22. 23. catch(IOExceptione) 24. /TODOAuto-generatedcatchblock 25. e.printStackTrace(); 26. 27. longt=System.currentTimeMillis()-t1; 28. System.out.println(sum:+sum+time:+t); 29. catch(FileNotFoundExceptione) 30. /TODOAuto-generatedcatchblock 31. e.printStackTrace(); 32. 33. try 34. FileInputStreamfis=newFileInputStream(/home/tobacco/test/res.txt); 35. BufferedInputStreambis=newBufferedInputStream(fis); 36. intsum=0; 37. intn; 38. longt1=System.currentTimeMillis(); 39. try 40. while(n=bis.read()=0) 41. sum+=n; 42. 43. catch(IOExceptione) 44. /TODOAuto-generatedcatchblock 45. e.printStackTrace(); 46. 47. longt=System.currentTimeMillis()-t1; 48. System.out.println(sum:+sum+time:+t); 49. catch(FileNotFoundExceptione) 50. /TODOAuto-generatedcatchblock 51. e.printStackTrace(); 52. 53. MappedByteBufferbuffer=null; 54. try 55. buffer=newRandomAccessFile(/home/tobacco/test/res.txt,rw).getChannel().map(FileChannel.MapMode.READ_WRITE,0,1253244); 56. intsum=0; 57. intn; 58. longt1=System.currentTimeMillis(); 59. for(inti=0;i=0) 20. /sum+=n; 21. 22. catch(IOExceptione) 23. /TODOAuto-generatedcatchblock 24. e.printStackTrace(); 25. 26. longt=System.currentTimeMillis()-t1; 27. System.out.println(sum:+sum+time:+t); 28. catch(FileNotFoundExceptione) 29. /TODOAuto-generatedcatchblock 30. e.printStackTrace(); 31. 32. 33. try 34. FileInputStreamfis=newFileInputStream(/home/tobacco/test/res.txt); 35. BufferedInputStreambis=newBufferedInputStream(fis); 36. intsum=0; 37. intn; 38. longt1=System.currentTimeMillis(); 39. try 40. while(n=bis.read()=0) 41. /sum+=n; 42. 43. catch(IOExceptione) 44. /TODOAuto-generatedcatchblock 45. e.printStackTrace(); 46. 47. longt=System.currentTimeMillis()-t1; 48. System.out.println(sum:+sum+time:+t); 49. catch(FileNotFoundExceptione) 50. /TODOAuto-generatedcatchblock 51. e.printStackTrace(); 52. 53. 54. MappedByteBufferbuffer=null; 55. try 56. buffer=newRandomAccessFile(/home/tobacco/test/res.txt,rw).getChannel().map(FileChannel.MapMode.READ_WRITE,0,1253244); 57. intsum=0; 58. intn; 59. longt1=System.currentTimeMillis(); 60. for(inti=0;i1253244;i+) 61. /n=0x000000ff&buffer.get(i); 62. /sum+=n; 63. 64. longt=System.currentTimeMillis()-t1; 65. System.out.println(sum:+sum+time:+t); 66. catch(FileNotFoundExceptione) 67. /TODOAuto-generatedcatchblock 68. e.printStackTrace(); 69. catch(IOExceptione) 70. /TODOAuto-generatedcatchblock 71. e.printStackTrace(); 72. 73. 74. 75. 测试结果：1. sum:0time:1458 2. sum:0time:67 3. sum:0time:8由此可见，将文件部分或者全部映射到内存后进行读写，速度将提高很多。这是因