从文件中读取字节的最简单方法

import java.io.*;

class ReadBytesFromFile {
    public static void main(String args[]) throws Exception {
        // getBytes from anyWhere
        // I'm getting byte array from File
        File file = null;
        FileInputStream fileStream = new FileInputStream(file = new File("ByteArrayInputStreamClass.java"));

        // Instantiate array
        byte[] arr = new byte[(int) file.length()];

        // read All bytes of File stream
        fileStream.read(arr, 0, arr.length);

        for (int X : arr) {
            System.out.print((char) X);
        }
    }
}

//The file that you wanna convert into byte[]
File file=new File("/storage/0CE2-EA3D/DCIM/Camera/VID_20190822_205931.mp4"); 

FileInputStream fileInputStream=new FileInputStream(file);
byte[] data=new byte[(int) file.length()];
BufferedInputStream bufferedInputStream=new BufferedInputStream(fileInputStream);
bufferedInputStream.read(data,0,data.length);

//Now the bytes of the file are contain in the "byte[] data"

如果你没有Java 8，并且同意我的观点，加入一个庞大的库来避免写几行代码是一个坏主意:

public static byte[] readBytes(InputStream inputStream) throws IOException {
    byte[] b = new byte[1024];
    ByteArrayOutputStream os = new ByteArrayOutputStream();
    int c;
    while ((c = inputStream.read(b)) != -1) {
        os.write(b, 0, c);
    }
    return os.toByteArray();
}

调用者负责关闭流。

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

Java 8文档:http://docs.oracle.com/javase/8/docs/api/java/io/RandomAccessFile.html

使用与社区wiki答案相同的方法，但更干净，并编译开箱即用(如果你不想导入Apache Commons库，例如在Android上，首选的方法):

public static byte[] getFileBytes(File file) throws IOException {
    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) {
            // swallow, since not that important
        }
        try {
            if (ios != null)
                ios.close();
        } catch (IOException e) {
            // swallow, since not that important
        }
    }
    return ous.toByteArray();
}

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;

}