使用流在Java中復制-丟失數據/字節

Question

我對此方法有問題，該方法應該使用流復制文件：

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.");
    }
}

當復制例如圖像，.txt文件或有時復制視頻文件時，會出現問題，錯誤為“文件大小不匹配”。 發生。 當我比較源文件和目標文件的大小時，大小確實不匹配，但是打開后的圖像似乎很完整，視頻也是如此。 文本文件存在明顯的問題-復制並丟失了幾個字節后，沒有丟失任何內容，但是在文件的末尾有幾行帶有某種標記為“ NUL”的標記-使用Notepad ++查看這些內容。

我嘗試使用Files中的copy（）方法，該方法可以正常工作，但是我無法找出為什么我的方法會丟失字節。

非常感謝您的幫助，我在任何地方都找不到解決方案。

Answer 1

問題是讀取循環：

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)返回讀取的字節數。 首先，您必須檢查它是否大於或等於零，因為如果緩沖區的大小為零，則read的約定允許它讀取0個字節。 但是除此之外，您還需要保持該值不變，因為您只需寫入讀取的字節數即可。

通常，文件的大小不完全是4096的倍數。因此，最后一次讀取將讀取少於4096字節的數據。 read合約實際上說，如果當時可用的字節數更少，它甚至可以讀少於4096。

因此，如果您未完全寫入所讀取的內容，則所寫入緩沖區的一部分將包含前一次讀取的值，如果文件小於4096字節，則該值為零。

因此，將其更正為：

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);
}

但是確實，建議您每當副本涉及至少一個文件時，都使用Files類，並避免使用支持Path java.io.File ，而Paths和Files提供了操作。