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本篇內(nèi)容介紹了“Java中如何用內(nèi)存映射處理大文件”的有關(guān)知識,在實際案例的操作過程中,不少人都會遇到這樣的困境,接下來就讓小編帶領(lǐng)大家學習一下如何處理這些情況吧!希望大家仔細閱讀,能夠?qū)W有所成!
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 block e.printStackTrace(); } long t=System.currentTimeMillis()-t1; System.out.println("sum:"+sum+" time:"+t); } catch (FileNotFoundException e) { // TODO Auto-generated catch block e.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 block e.printStackTrace(); } long t=System.currentTimeMillis()-t1; System.out.println("sum:"+sum+" time:"+t); } catch (FileNotFoundException e) { // TODO Auto-generated catch block e.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 block e.printStackTrace(); } catch (IOException e) { // TODO Auto-generated catch block e.printStackTrace(); } } }
測試文件為一個大小為1253244字節(jié)的文件。測試結(jié)果:
sum:220152087 time:1464 sum:220152087 time:72 sum:220152087 time:25
說明讀數(shù)據(jù)無誤。刪去其中的數(shù)據(jù)處理部分。
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 block e.printStackTrace(); } long t=System.currentTimeMillis()-t1; System.out.println("sum:"+sum+" time:"+t); } catch (FileNotFoundException e) { // TODO Auto-generated catch block e.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 block e.printStackTrace(); } long t=System.currentTimeMillis()-t1; System.out.println("sum:"+sum+" time:"+t); } catch (FileNotFoundException e) { // TODO Auto-generated catch block e.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 block e.printStackTrace(); } catch (IOException e) { // TODO Auto-generated catch block e.printStackTrace(); } } }
測試結(jié)果:
sum:0 time:1458 sum:0 time:67 sum:0 time:8
由此可見,將文件部分或者全部映射到內(nèi)存后進行讀寫,速度將提高很多。
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