使用流复制 Java - 丢失 data/bytes

Question

我对这种应该使用流复制文件的方法有疑问：

public static void copyFile() {
    String[] paths = readSrcDestFromConsole(); //returns String array with two strings - source and destination file paths
    InputStream is = null; OutputStream os = null;
    File src = null; File dest = null;
    boolean error = false;
    long elapsedTimeSeconds = 0;
    double speed = 0;
    try {
        src = new File(paths[0]);
    }catch(Exception ex){
        System.err.println("File read from could not be opened.");
        error = true;
    }
    if(!error) {
        try {
            dest = new File(paths[1]);
        }catch(Exception ex){
            System.err.println("File written to could not be created.");
            error = true;
        }
        if(src.exists() && !error) {
            if(dest.exists()) {
                System.out.println("File specified already exists. Do you want to overwrite it?");
                if(askForOverwrite()) { // gets Y or N from console using scanner and returns a boolean
                    try {
                        is = new FileInputStream(src);
                        os = new FileOutputStream(dest);
                        System.out.println("Copying from: " + paths[0] + " to: " + paths[1]);
                        byte[] buffer = new byte[4096];
                        double read = 0;
                        long first = 0;
                        long second = 0;
                        long startTime = System.nanoTime();
                        while(is.read(buffer) > 0) {
                            read += buffer.length;
                            os.write(buffer);
                            first = Math.round(read / src.length() * 100);
                            if(first != second) {
                                System.out.println(first + " % copied.");
                            }
                            second = Math.round(read / src.length() * 100);
                        }
                        elapsedTimeSeconds = (System.nanoTime() - startTime) / 1_000_000_000;
                        speed = (src.length() / 1024 / 1024) / elapsedTimeSeconds;
                        is.close(); os.close();
                    }catch(Exception ex){
                        System.err.println("File is or has been corrupted.");
                        error = true;
                    }finally{
                        if(!error) {
                            if(src.length() == dest.length()) {
                                System.out.println("File copied successfully.");
                                System.out.println("Total size copied: " + ((dest.length()) / 1024 / 1024) + " MB");
                                System.out.println("Copying speed: " + speed + " MB/s" + " in " + elapsedTimeSeconds + " seconds.");
                            }else{
                                System.err.println("Error: " + "File sizes mismatch.");
                            }
                        }
                    }
                }else{
                    System.out.println("File has not been rewritten.");
                }
            }else{
                try {
                    is = new FileInputStream(src);
                    os = new FileOutputStream(dest);
                    System.out.println("Copying from: " + paths[0] + " to: " + paths[1]);
                    byte[] buffer = new byte[4096];
                    double read = 0;
                    long first = 0;
                    long second = 0;
                    long startTime = System.nanoTime();
                    while(is.read(buffer) > 0) {
                        read += buffer.length;
                        os.write(buffer);
                        first = Math.round(read / src.length() * 100);
                        if(first != second) {
                            System.out.println(first + " % copied.");
                        }
                        second = Math.round(read / src.length() * 100);
                    }
                    elapsedTimeSeconds = (System.nanoTime() - startTime) / 1_000_000_000;
                    speed = (src.length() / 1024 / 1024) / elapsedTimeSeconds;
                    is.close(); os.close();
                }catch(Exception ex){
                    System.err.println("File is or has been corrupted.");
                    error = true;
                }finally{
                    if(!error) {
                        if(src.length() == dest.length()) {
                            System.out.println("File copied successfully.");
                            System.out.println("Total size copied: " + ((dest.length()) / 1024 / 1024) + " MB");
                            System.out.println("Copying speed: " + speed + " MB/s" + " in " + elapsedTimeSeconds + " seconds.");
                        }else{
                            System.err.println("Error: " + "File sizes mismatch.");
                        }
                    }
                }
            }
        }else{
            System.err.println("File specified does not exist.");
        }
    }else{
        System.err.println("Operation could not be completed.");
    }
}

复制图像、.txt 文件或有时是视频文件时出现问题，出现错误 "File sizes mismatch."。当我比较源文件和目标文件大小时，大小确实不匹配，但打开后的图像似乎是完整的，视频也是。可见的问题是文本文件 - 在复制和丢失几个字节后，none 的内容丢失了，但在文件末尾有几行带有某种标记的 "NUL" - 使用 Notepad++查看那些。

我尝试使用 Files 中的 copy() 方法，它工作正常，但我找不到我的方法丢失字节的原因。

非常感谢您的帮助，我在任何地方都找不到解决方案。

Answer 1

读取循环是问题所在：

while(is.read(buffer) > 0) {
    read += buffer.length;
    os.write(buffer);
    first = Math.round(read / src.length() * 100);
    if(first != second) {
        System.out.println(first + " % copied.");
    }
    second = Math.round(read / src.length() * 100);
}

is.read(buffer) returns 读取的字节数。首先，您必须检查它是否大于或等于零，因为如果缓冲区大小为零，read 的约定允许它读取 0 个字节。但除此之外 - 您需要保留该值，因为您只需写入您读取的字节数。

通常文件的大小不是4096的倍数，所以最后一次读取不到4096字节。 read 合同实际上说，如果当时可用的字节更少，它甚至可以在此之前读取少于 4096。

因此，如果您没有准确写入所读取的内容，您写入的缓冲区的一部分将包含之前读取的值，或者如果文件小于 4096 字节则为零。

所以更正为：

int numRead;

while( (numRead = is.read(buffer)) >= 0) {
    read += numRead;
    os.write(buffer,0,numRead);
    first = Math.round(read / src.length() * 100);
    if(first != second) {
        System.out.println(first + " % copied.");
    }
    second = Math.round(read / src.length() * 100);
}

但实际上，只要您的副本涉及至少一个文件，建议使用 Files class，并避免使用 java.io.File 而使用 Path，同时Paths 和 Files.

中可用的操作

使用流复制 Java - 丢失 data/bytes

Copying in Java using streams - losing data/bytes

java

io

copy

file

stream