Question

While googling, I see that using java.io.File#length() can be slow. FileChannel has a size() method that is available as well.

Is there an efficient way in java to get the file size?

Answer 1

Well, I tried to measure it up with the code below:

For runs = 1 and iterations = 1 the URL method is fastest most times followed by channel. I run this with some pause fresh about 10 times. So for one time access, using the URL is the fastest way I can think of:

LENGTH sum: 10626, per Iteration: 10626.0  CHANNEL sum: 5535, per Iteration: 5535.0  URL sum: 660, per Iteration: 660.0 

For runs = 5 and iterations = 50 the picture draws different.

LENGTH sum: 39496, per Iteration: 157.984  CHANNEL sum: 74261, per Iteration: 297.044  URL sum: 95534, per Iteration: 382.136 

File must be caching the calls to the filesystem, while channels and URL have some overhead.

Code:

import java.io.*; import java.net.*; import java.util.*;  public enum FileSizeBench {      LENGTH {         @Override         public long getResult() throws Exception {             File me = new File(FileSizeBench.class.getResource(                     'FileSizeBench.class').getFile());             return me.length();         }     },     CHANNEL {         @Override         public long getResult() throws Exception {             FileInputStream fis = null;             try {                 File me = new File(FileSizeBench.class.getResource(                         'FileSizeBench.class').getFile());                 fis = new FileInputStream(me);                 return fis.getChannel().size();             } finally {                 fis.close();             }         }     },     URL {         @Override         public long getResult() throws Exception {             InputStream stream = null;             try {                 URL url = FileSizeBench.class                         .getResource('FileSizeBench.class');                 stream = url.openStream();                 return stream.available();             } finally {                 stream.close();             }         }     };      public abstract long getResult() throws Exception;      public static void main(String[] args) throws Exception {         int runs = 5;         int iterations = 50;          EnumMap<FileSizeBench, Long> durations = new EnumMap<FileSizeBench, Long>(FileSizeBench.class);          for (int i = 0; i < runs; i++) {             for (FileSizeBench test : values()) {                 if (!durations.containsKey(test)) {                     durations.put(test, 0l);                 }                 long duration = testNow(test, iterations);                 durations.put(test, durations.get(test) + duration);                 // System.out.println(test + ' took: ' + duration + ', per iteration: ' + ((double)duration / (double)iterations));             }         }          for (Map.Entry<FileSizeBench, Long> entry : durations.entrySet()) {             System.out.println();             System.out.println(entry.getKey() + ' sum: ' + entry.getValue() + ', per Iteration: ' + ((double)entry.getValue() / (double)(runs * iterations)));         }      }      private static long testNow(FileSizeBench test, int iterations)             throws Exception {         long result = -1;         long before = System.nanoTime();         for (int i = 0; i < iterations; i++) {             if (result == -1) {                 result = test.getResult();                 //System.out.println(result);             } else if ((result = test.getResult()) != result) {                  throw new Exception('variance detected!');              }         }         return (System.nanoTime() - before) / 1000;     }  }