无法将图片正确写入外部存储中，并且NullPointerException

Question

我正在使用Android Studio构建应用程序，其中客户端从服务器接收图像，在Gallery中创建一个新目录，然后将图像保存在其中。

现在，将创建Gallery中的新文件夹以及 .jpg 文件，但是无法打开该文件。它仅在文件上显示 “！” 标记。以下是我的服务器和客户端的代码： 编辑后，即使在字节数组上执行mBitmap.compress时，我仍然得到NullPointerException，即使它打印出来并且它不为空

//Server
ContentResolver cr = getContentResolver();
try {
    InputStream is = cr.openInputStream(uri);
    System.out.println("cAME here");
    byte[] bytes = new byte[(int)uri.getPath().length()];
    is.read(bytes, 0, bytes.length);
    ObjectOutputStream oos = new ObjectOutputStream(socket.getOutputStream());
    oos.writeObject(bytes);
    oos.flush();

    socket.close();

    final String sentMsg = "File sent to: " + socket.getInetAddress();
    ServerSocketThread.this.runOnUiThread(new Runnable() {

        @Override
        public void run() {
            Toast.makeText(ServerSocketThread.this,
                sentMsg,
                Toast.LENGTH_LONG).show();
        }
    });
}

现在是客户端：

//Client
Socket socket = null;
Client.this.runOnUiThread(new Runnable() {

    @Override
    public void run() {
        Toast.makeText(Client.this,
            "connected",
            Toast.LENGTH_LONG).show();
    }
});
try {
    socket = new Socket(address, portNumber);
    File fileDir = new File(Environment
        .getExternalStoragePublicDirectory(Environment.DIRECTORY_PICTURES),
            "LetsShare");
    if(fileDir.exists() == false){
        fileDir.mkdirs();}
        File file = new File(fileDir,"test.jpg");
        ObjectInputStream ois = new ObjectInputStream(socket.getInputStream());
        byte[] bytes;
        FileOutputStream fos = null;

        try {
            bytes = (byte[])ois.readObject();
            fos = new FileOutputStream(file);
            fos.write(bytes);

            Client.this.runOnUiThread(new Runnable() {

                @Override
                public void run() {
                    Toast.makeText(Client.this,
                        "Finished",
                        Toast.LENGTH_LONG).show();
                }
            });
            scanMedia(file.toString());
            //File toSet = new File(file.toString()); 
//imageView.setImageBitmap(BitmapFactory.decodeFile(toSet.getAbsolutePath()));
        }
    }
}

有人可以解释为什么未正确保存图像并且无法打开图像吗？

已编辑的客户端：

  socket = new Socket(address, portNumber);
            File fileDir = new File(
                    Environment.getExternalStoragePublicDirectory(Environment.DIRECTORY_PICTURES),
                    "LetsShare");
            if(fileDir.exists() == false){
            fileDir.mkdirs();}
            File file = new File(fileDir,"test.jpg");
            file.createNewFile();
            InputStream is = socket.getInputStream();
            ObjectInputStream ois = new ObjectInputStream(is);
           // int length = ois.readInt();
            byte[] bytes = new byte[1024];

            try{
           bytes = (byte[])ois.readObject();}catch (ClassNotFoundException e)
            {

            }
            for (int i=0;i<bytes.length;i++)
            {
                System.out.println(bytes[i]);
            }
            FileOutputStream fos = null;
            Bitmap myBitmap = BitmapFactory.decodeByteArray(bytes,0,bytes.length);
            try {

                fos = new FileOutputStream(file);
                myBitmap.compress(Bitmap.CompressFormat.JPEG,90,fos);
                fos.flush();
                fos.close();
                Client.this.runOnUiThread(new Runnable() {

                    @Override
                    public void run() {
                        Toast.makeText(Client.this,
                                "Finished",
                                Toast.LENGTH_LONG).show();
                    }
                });
                scanMedia(file.toString());
                //File toSet = new File(file.toString());
                //imageView.setImageBitmap(BitmapFactory.decodeFile(toSet.getAbsolutePath()));
            }