使用Android写一个简单的音乐播放器界面，播放一首歌，并给出XML和Java代码要求：上方是会转到的圆盘，中间是进度条，下方是播放，暂停，继续播放，退出四个按钮 掌握MediaPlayer、Timer、TimerTask的使用方法。 掌握Handler消息机制

以下是一个简单的音乐播放器界面的XML和Java代码：

XML代码：

<?xml version="1.0" encoding="utf-8"?>
<RelativeLayout xmlns:android="http://schemas.android.com/apk/res/android"
    android:layout_width="match_parent"
    android:layout_height="match_parent">

    <ImageView
        android:id="@+id/iv_disc"
        android:layout_width="200dp"
        android:layout_height="200dp"
        android:layout_centerInParent="true"
        android:src="@drawable/disc" />

    <ProgressBar
        android:id="@+id/progress_bar"
        android:layout_width="match_parent"
        android:layout_height="wrap_content"
        android:layout_below="@id/iv_disc"
        android:layout_margin="16dp"
        android:indeterminate="false"
        android:max="100"
        android:padding="4dp"
        android:progress="0"
        android:progressDrawable="@drawable/custom_progress_bar" />

    <LinearLayout
        android:id="@+id/buttons_layout"
        android:layout_width="match_parent"
        android:layout_height="wrap_content"
        android:layout_below="@id/progress_bar"
        android:layout_marginTop="16dp"
        android:gravity="center"
        android:orientation="horizontal"
        android:padding="16dp">

        <Button
            android:id="@+id/play_button"
            android:layout_width="wrap_content"
            android:layout_height="wrap_content"
            android:background="@drawable/play_button_background"
            android:text="Play"
            android:textColor="@android:color/white" />

        <Button
            android:id="@+id/pause_button"
            android:layout_width="wrap_content"
            android:layout_height="wrap_content"
            android:background="@drawable/pause_button_background"
            android:text="Pause"
            android:textColor="@android:color/white"
            android:visibility="gone" />

        <Button
            android:id="@+id/resume_button"
            android:layout_width="wrap_content"
            android:layout_height="wrap_content"
            android:background="@drawable/play_button_background"
            android:text="Resume"
            android:textColor="@android:color/white"
            android:visibility="gone" />

        <Button
            android:id="@+id/exit_button"
            android:layout_width="wrap_content"
            android:layout_height="wrap_content"
            android:background="@drawable/exit_button_background"
            android:text="Exit"
            android:textColor="@android:color/white" />

    </LinearLayout>

</RelativeLayout>

Java代码：

import android.media.MediaPlayer;
import android.os.Bundle;
import android.os.Handler;
import android.os.Message;
import android.view.View;
import android.widget.Button;
import android.widget.ImageView;
import android.widget.ProgressBar;
import android.widget.Toast;

import androidx.annotation.NonNull;
import androidx.appcompat.app.AppCompatActivity;

import java.util.Timer;
import java.util.TimerTask;

public class MusicPlayerActivity extends AppCompatActivity {

    private ImageView ivDisc;
    private ProgressBar progressBar;
    private Button playButton, pauseButton, resumeButton, exitButton;

    private MediaPlayer mediaPlayer;
    private Timer timer;
    private Handler handler;

    @Override
    protected void onCreate(Bundle savedInstanceState) {
        super.onCreate(savedInstanceState);
        setContentView(R.layout.activity_music_player);

        ivDisc = findViewById(R.id.iv_disc);
        progressBar = findViewById(R.id.progress_bar);
        playButton = findViewById(R.id.play_button);
        pauseButton = findViewById(R.id.pause_button);
        resumeButton = findViewById(R.id.resume_button);
        exitButton = findViewById(R.id.exit_button);

        mediaPlayer = MediaPlayer.create(this, R.raw.song);
        mediaPlayer.setOnCompletionListener(new MediaPlayer.OnCompletionListener() {
            @Override
            public void onCompletion(MediaPlayer mp) {
                playButton.setVisibility(View.VISIBLE);
                pauseButton.setVisibility(View.GONE);
                resumeButton.setVisibility(View.GONE);
                progressBar.setProgress(0);
                Toast.makeText(MusicPlayerActivity.this, "Music completed", Toast.LENGTH_SHORT).show();
            }
        });

        timer = new Timer();

        handler = new Handler(new Handler.Callback() {
            @Override
            public boolean handleMessage(@NonNull Message msg) {
                if (mediaPlayer != null) {
                    int currentPosition = mediaPlayer.getCurrentPosition();
                    int totalDuration = mediaPlayer.getDuration();
                    float progress = (float) currentPosition / (float) totalDuration * 100;
                    progressBar.setProgress((int) progress);
                }
                return true;
            }
        });

        playButton.setOnClickListener(new View.OnClickListener() {
            @Override
            public void onClick(View v) {
                mediaPlayer.start();
                playButton.setVisibility(View.GONE);
                pauseButton.setVisibility(View.VISIBLE);
                resumeButton.setVisibility(View.GONE);
                startTimer();
            }
        });

        pauseButton.setOnClickListener(new View.OnClickListener() {
            @Override
            public void onClick(View v) {
                mediaPlayer.pause();
                playButton.setVisibility(View.GONE);
                pauseButton.setVisibility(View.GONE);
                resumeButton.setVisibility(View.VISIBLE);
                stopTimer();
            }
        });

        resumeButton.setOnClickListener(new View.OnClickListener() {
            @Override
            public void onClick(View v) {
                mediaPlayer.start();
                playButton.setVisibility(View.GONE);
                pauseButton.setVisibility(View.VISIBLE);
                resumeButton.setVisibility(View.GONE);
                startTimer();
            }
        });

        exitButton.setOnClickListener(new View.OnClickListener() {
            @Override
            public void onClick(View v) {
                mediaPlayer.stop();
                stopTimer();
                finish();
            }
        });
    }

    private void startTimer() {
        TimerTask timerTask = new TimerTask() {
            @Override
            public void run() {
                handler.sendEmptyMessage(0);
            }
        };
        timer.schedule(timerTask, 0, 100);
    }

    private void stopTimer() {
        timer.cancel();
    }

    @Override
    protected void onDestroy() {
        super.onDestroy();
        if (mediaPlayer != null) {
            mediaPlayer.release();
            mediaPlayer = null;
        }
    }
}

注意：以上代码仅供参考，需要自己根据实际需求进行修改和完善。