查询Java I / O字节流类

Question

Below is the program which created separate byte streams for reading from and writing into same file.

import java.io.File;
import java.io.FileInputStream;
import java.io.FileOutputStream;
import java.io.IOException;

public class FileCopyNoBuffer{
    public static void main(String[] args){
        FileInputStream in = null;
        FileOutputStream out = null;
        Long startTime, elapsedTime;

        //String inFileStr = "C:\\project\\books\\java-Preparation\\main_docs\\practice.jpg";
        //String outFileStr = "C:\\project\\books\\java-Preparation\\main_docs\\practice_out.jpg";
        String fileStr = "C:\\project\\books\\java-Preparation\\main_docs\\practice.jpg";

        File file = new File(fileStr);
        System.out.println("File size before - r/w is: " + file.length() + " bytes");
        try{
            in = new FileInputStream(fileStr);
            out = new FileOutputStream(fileStr);

            startTime = System.nanoTime();
            int byteRead;

            while((byteRead = in.read()) != -1){
                out.write(byteRead);
            }

            elapsedTime = System.nanoTime() - startTime;
            System.out.println("Elapsed Time is: " + (elapsedTime/1000000.0) + " msec");
            System.out.println("File size after - r/w is: " + file.length() + " bytes");

        }catch(IOException ex){
            ex.printStackTrace();
        }finally{
            try{
                if(in != null){
                    in.close();
                }
                if(out != null){
                    out.close();
                }
            }catch(IOException ex){
                ex.printStackTrace();
            }
        }


    }
}

Below is the observation:

File size before - r/w is: 1115512 bytes
Elapsed Time is: 0.040711 msec
File size after - r/w is: 0 bytes

I know that FileInputStream and FileOutputStream are non-buffer byte stream I/O classes.

I expect that the file size to remain same after writing to same file.

What could be under-hood reason that file size goes to zero?

Note : am learning java 1.6 I/O

Answer 1

[...]for reading from and writing into same file.

NEVER DO THAT. EVER .

The results are unpredictable.

If you want to modify the contents of a file, write the new contents into a new file and then atomically rename to the old -- after you have ensured that the new content was successfully written.

Also, this is 2014, so unless you REALLY have to use Java 6, use java.nio.file instead , especially if you have to rename, since File will leave you stranded more often than not .

Sample code with java.nio.file:

final Path toChange = Paths.get("pathtoyourfilehere");
final Path dir = toChange.getParent();
final Path tmpfile = Files.createTempFile(dir, "foo", "bar");

try (
    final InputStream in = Files.newInputStream(toChange);
    final InputStream out = Files.newOutputStream(tmpfile);
) {
    // Work with in and out
}

// Then move!
try {
    Files.move(tmpfile, toChange, StandardCopyOption.REPLACE_EXISTING,
        StandardCopyOption.ATOMIC_MOVE);
} catch (AtomicMoveNotSupportedException ignored) {
    Files.move(tmpfile, toChange, StandardCopyOption.REPLACE_EXISTING);
}

Answer 2

You are using default constructor for FileOutputStream Try using this:

out = new FileOutputStream(fileStr,true);

So that now your data will be appended instead of being overwritten.You can go through this: doc