// Returns the contents of the file in a byte array.
    public static byte[] getBytesFromFile(File file) throws IOException {        
        // Get the size of the file
        long length = file.length();

        // You cannot create an array using a long type.
        // It needs to be an int type.
        // Before converting to an int type, check
        // to ensure that file is not larger than Integer.MAX_VALUE.
        if (length > Integer.MAX_VALUE) {
            // File is too large
            throw new IOException("File is too large!");
        }

        // Create the byte array to hold the data
        byte[] bytes = new byte[(int)length];

        // Read in the bytes
        int offset = 0;
        int numRead = 0;

        InputStream is = new FileInputStream(file);
        try {
            while (offset < bytes.length
                   && (numRead=is.read(bytes, offset, bytes.length-offset)) >= 0) {
                offset += numRead;
            }
        } finally {
            is.close();
        }

        // Ensure all the bytes have been read in
        if (offset < bytes.length) {
            throw new IOException("Could not completely read file "+file.getName());
        }
        return bytes;
    }

让我添加另一个不使用第三方库的解决方案。它重用了Scott提出的异常处理模式(链接)。我把丑陋的部分移动到一个单独的消息(我会隐藏在一些FileUtils类;))

public void someMethod() {
    final byte[] buffer = read(new File("test.txt"));
}

private byte[] read(final File file) {
    if (file.isDirectory())
        throw new RuntimeException("Unsupported operation, file "
                + file.getAbsolutePath() + " is a directory");
    if (file.length() > Integer.MAX_VALUE)
        throw new RuntimeException("Unsupported operation, file "
                + file.getAbsolutePath() + " is too big");

    Throwable pending = null;
    FileInputStream in = null;
    final byte buffer[] = new byte[(int) file.length()];
    try {
        in = new FileInputStream(file);
        in.read(buffer);
    } catch (Exception e) {
        pending = new RuntimeException("Exception occured on reading file "
                + file.getAbsolutePath(), e);
    } finally {
        if (in != null) {
            try {
                in.close();
            } catch (Exception e) {
                if (pending == null) {
                    pending = new RuntimeException(
                        "Exception occured on closing file" 
                             + file.getAbsolutePath(), e);
                }
            }
        }
        if (pending != null) {
            throw new RuntimeException(pending);
        }
    }
    return buffer;
}

public static byte[] readBytes(InputStream inputStream) throws IOException {
    byte[] buffer = new byte[32 * 1024];
    int bufferSize = 0;
    for (;;) {
        int read = inputStream.read(buffer, bufferSize, buffer.length - bufferSize);
        if (read == -1) {
            return Arrays.copyOf(buffer, bufferSize);
        }
        bufferSize += read;
        if (bufferSize == buffer.length) {
            buffer = Arrays.copyOf(buffer, bufferSize * 2);
        }
    }
}

Guava有Files.toByteArray()可以提供给您。它有几个优点:

它涵盖了文件报告长度为0但仍然有内容的极端情况 它是高度优化的，如果在加载文件之前试图读入一个大文件，你会得到一个OutOfMemoryException。(通过巧妙地使用file.length()) 你没必要白费力气。

如果您想将字节读入预分配的字节缓冲区，这个答案可能会有所帮助。

您的第一个猜测可能是使用InputStream read(byte[])。然而，这种方法有一个缺陷，使得它难以使用:即使没有遇到EOF，也不能保证数组实际上会被完全填充。

相反，看一下DataInputStream readFully(byte[])。这是一个输入流的包装器，没有上面提到的问题。此外，该方法在遇到EOF时抛出。好得多。

这取决于什么对你来说是最好的。就生产力而言，不要重复工作，而是使用Apache Commons。FileUtils。readFileToByteArray(文件输入)。