java - java I/O字节流类的查询

Question

下面是创建单独的字节流以读取和写入同一文件的程序。

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();
            }
        }


    }
}

以下是观察结果:

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

我知道FileInputStream和FileOutputStream是非缓冲字节流I/O类。

我希望写入同一文件后文件大小保持不变。

文件大小变为零的潜在原因是什么？

注意:正在学习 java 1.6 I/O

Answer 1

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

永远不要这样做。曾经。

结果是不可预测的。

如果您想修改文件的内容，请将新内容写入新文件中，然后自动重命名为旧文件 - 之后您已确保新内容写入成功。

此外，现在是 2014 年，所以除非您真的必须使用 Java 6，use java.nio.file instead ，特别是如果您必须重命名，因为 File will leave you stranded more often than not .

带有 java.nio.file 的示例代码:

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);
}