I'm trying to create a very simple app - a online radio player, and I want to get the name of the song currently playing. Here is the code:

我正在尝试创建一个非常简单的应用程序-一个在线广播播放器，我想获得当前正在播放的歌曲的名称。以下是代码：

player = new ExoPlayer.Builder(this).build();
        viewPlayer.setPlayer(player);
        MediaItem mediaItem = MediaItem.fromUri(link);
        player.setMediaItem(mediaItem);
        player.prepare();
        player.play();


        player.addListener(
                new Player.Listener() {

                    @Override
                    public void onMediaMetadataChanged(MediaMetadata mediaMetadata) {
                        metadata = player.getMediaMetadata();

                        if (metadata.title!=null) {
                            MainActivity.this.title = metadata.albumTitle.toString();
                            viewSong.setText(MainActivity.this.title);
                        } else viewSong.setText("NULL");
                    }
                }
        );

I think, I have all dependencies needed in build.gradle.

我想，我有build.gradle中需要的所有依赖项。

Player works fine but getMediaMetadata() gives me this (in debug mode):

播放器工作正常，但getMediaMetadata()提供了以下内容(在调试模式下)：

metadata = {MediaMetadata@11693}
    albumArtist = null
    albumTitle = null
    artist = null
    ..............
    ..............
    genre = Rock Music
    ..............
    ..............
    station = RADIO 1 ROCK

There is some data, but what I need is 'null'.

有一些数据，但我需要的是‘空’。

I know that metadata contains the title and artist info.
VLC player gives me this:

我知道元数据包含标题和艺术家信息。VLC播放器给了我这个：

I did not find any relevant answer of this problem yet.

我还没有找到这个问题的任何相关答案。

It looks like you did not add 'title' anywhere which is why you are getting a 'NULL' value as an output. You can use the code below to add and get the title or any other information from MediaMetadata.

看起来您没有在任何地方添加‘TITLE’，这就是为什么您会得到一个‘NULL’值作为输出。您可以使用下面的代码从MediaMetadata添加和获取标题或任何其他信息。

//Add required info into mediaMetadata
val mediaMetadata = MediaMetadata.Builder()
        .setDisplayTitle("Your title here")
        .setArtist("Your artist name")
        .build()

//Build mediaItem with data added and url of media
val mediaItem = MediaItem.Builder()
        .setMediaMetadata(mediaMetadata)
        .setUri("Your track URL here")

mediaItem.build()

//Now add this mediaItem into your code like this
player.setMediaItem(mediaItem);
player.prepare();
player.play();

//Now in listener
player.addListener(new Player.Listener() {

                @Override
                public void onMediaMetadataChanged(MediaMetadata mediaMetadata) {
                    //Now get posted values like this
                    println("Title : "+mediaMetadata.displayTitle)    
                }
            }
    );

You can find more useful methods in MediaMetadata Builder like setAlbumTitle, setAlbumArtist, setSubtitle, setDescription, setUserRating, etc.

您可以在MediaMetadata Builder中找到更多有用的方法，如setAlumTitle、setAlumArtist、setSubtitle、setDescription、setUserRating等。

One more thing I think your code is in Java but my code at the beginning is in Kotlin so convert it into Java before use.

还有一件事，我认为你的代码是用Java写的，但我开头的代码是用kotlin写的，所以在使用之前把它转换成Java。