// 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;
    }

如果你的目标版本少于26个API，试试这个

 private static byte[] readFileToBytes(String filePath) {

    File file = new File(filePath);
    byte[] bytes = new byte[(int) file.length()];

    // funny, if can use Java 7, please uses Files.readAllBytes(path)
    try(FileInputStream fis = new FileInputStream(file)){
        fis.read(bytes);
        return bytes;
    } catch (FileNotFoundException e) {
        e.printStackTrace();
    } catch (IOException e) {
        e.printStackTrace();
    }
    return null;

}

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。

// 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;
    }

基本上你必须在记忆中读它。打开文件，分配数组，并将文件中的内容读入数组。

最简单的方法是这样的:

public byte[] read(File file) throws IOException, FileTooBigException {
    if (file.length() > MAX_FILE_SIZE) {
        throw new FileTooBigException(file);
    }
    ByteArrayOutputStream ous = null;
    InputStream ios = null;
    try {
        byte[] buffer = new byte[4096];
        ous = new ByteArrayOutputStream();
        ios = new FileInputStream(file);
        int read = 0;
        while ((read = ios.read(buffer)) != -1) {
            ous.write(buffer, 0, read);
        }
    }finally {
        try {
            if (ous != null)
                ous.close();
        } catch (IOException e) {
        }

        try {
            if (ios != null)
                ios.close();
        } catch (IOException e) {
        }
    }
    return ous.toByteArray();
}

这对文件内容有一些不必要的复制(实际上数据复制了三次:从文件到缓冲区，从缓冲区到ByteArrayOutputStream，从ByteArrayOutputStream到实际的结果数组)。

你还需要确保你只在内存中读取一定大小的文件(这通常取决于应用程序):-)。

您还需要在函数外部处理IOException。

另一种方式是:

public byte[] read(File file) throws IOException, FileTooBigException {
    if (file.length() > MAX_FILE_SIZE) {
        throw new FileTooBigException(file);
    }

    byte[] buffer = new byte[(int) file.length()];
    InputStream ios = null;
    try {
        ios = new FileInputStream(file);
        if (ios.read(buffer) == -1) {
            throw new IOException(
                    "EOF reached while trying to read the whole file");
        }
    } finally {
        try {
            if (ios != null)
                ios.close();
        } catch (IOException e) {
        }
    }
    return buffer;
}

这没有不必要的复制。

FileTooBigException是自定义应用程序异常。 MAX_FILE_SIZE常量是一个应用程序参数。

对于大文件，您可能应该考虑流处理算法或使用内存映射(参见java.nio)。

ReadFully从当前文件指针开始，将b.length字节从这个文件读入字节数组。该方法从文件中重复读取，直到读取所请求的字节数。该方法将阻塞，直到读取请求的字节数、检测到流的末尾或抛出异常。

RandomAccessFile f = new RandomAccessFile(fileName, "r");
byte[] b = new byte[(int)f.length()];
f.readFully(b);