public class WelcomeActivity extends Activity {

  @Override
  protected void onCreate(Bundle savedInstanceState) {
    super.onCreate(savedInstanceState);
    requestWindowFeature(Window.FEATURE_NO_TITLE);
    setContentView(R.layout.activity_welcome);

    new Thread(new Runnable() {
      @Override
      public void run() {
        //此处运行耗时任务
        runOnUiThread(new Runnable() {
          @Override
          public void run() {
            Intent intent = new Intent(WelcomeActivity.this, MainActivity.class);
            startActivity(intent);
            WelcomeActivity.this.finish();
          }
        });

      }
    }).start();
  }
}

public class WelcomeActivity extends Activity {

  @Override
  protected void onCreate(Bundle savedInstanceState) {
    super.onCreate(savedInstanceState);
    requestWindowFeature(Window.FEATURE_NO_TITLE);
    setContentView(R.layout.activity_welcome);
    Handler handler = new Handler();
    //当计时结束,跳转至主界面
    handler.postDelayed(new Runnable() {
      @Override
      public void run() {
        Intent intent = new Intent(WelcomeActivity.this, MainActivity.class);
        startActivity(intent);
        WelcomeActivity.this.finish();
      }
    }, 3000);
  }
}

<activity android:name=".WelcomeActivity">
      <intent-filter>
        <action android:name="android.intent.action.MAIN" />

        <category android:name="android.intent.category.LAUNCHER" />
      </intent-filter>
    </activity>