1-首先将APK文件放到服务器上，存放位置：根目录下
2-EditTest中写入:http://39.105.59.30:80/fei.apk
3-服务器是ubuntu，这个app想要实现的功能是点击下载后，从服务器上下载指定的
APK并进行安装。
4-运行时出现解析安装包失败
代码如下：
public class MainActivity extends AppCompatActivity {

private EditText main_et_1;
private File apkFile;
@Override
protected void onCreate(Bundle savedInstanceState) {
    super.onCreate(savedInstanceState);
    setContentView(R.layout.activity_main);
    main_et_1 = findViewById(R.id.main_et_1);
}

public void downloadAPK(View view) {
    final ProgressDialog dialog = new ProgressDialog(this);
    dialog.setProgressStyle(ProgressDialog.STYLE_HORIZONTAL);
    dialog.show();
    apkFile = new File(getExternalFilesDir(null),"update.apk");
    //启动分线程
    new Thread(new Runnable() {
        @Override
        public void run() {
            try {
                //得到链接对象
                String path = main_et_1.getText().toString();
                URL url = new URL(path);
                HttpURLConnection connection = (HttpURLConnection) url.openConnection();
                //设置
                //connection.setRequestMethod("GET");   //默认为get
                connection.setConnectTimeout(5000);
                connection.setReadTimeout(10000);
                //链接
                connection.connect();
                //请求并得到响应码
                if((connection.getResponseCode())==200){
                    //设置Dialog的最大进度
                    dialog.setMax(connection.getContentLength());
                    //得到包含APK文件数据的InputStream
                    InputStream is = connection.getInputStream();
                    //创建指向apkFile的FileOutputStream
                    FileOutputStream fos = new FileOutputStream(apkFile);
                    //边读边写
                    byte[] buffer = new byte[1024];
                    int len = -1;
                    while((len=is.read(buffer))!=-1){
                        fos.write(buffer,0,len);
                        //显示下载进度
                        dialog.incrementProgressBy(len);
                    }
                    fos.close();
                    is.close();
                }
                //下载完成
                connection.disconnect();
                runOnUiThread(new Runnable() {
                    @Override
                    public void run() {
                        dialog.dismiss();
                        installAPK();
                    }
                });
            } catch (Exception e) {
                e.printStackTrace();
                dialog.dismiss();
            }
        }
    }).start();
}
/*
    启动安装APK
 */
private void installAPK(){
    Intent intent = new Intent("android.intent.action.INSTALL_PACKAGE");
    intent.setDataAndType(Uri.fromFile(apkFile),"application/vnd.android.package-archive");
    startActivity(intent);
}

}