Jar文件引发异常

Question

我使用Java 1.8和IntelliJIdea。要使用该程序，我下载了字体

InputStream is1 = GameField.class.getResourceAsStream("assets/fonts/ARIALN.ttf");
private Font myArialNarrow;
    {
        try {
            myArialNarrow = Font.createFont(Font.TRUETYPE_FONT, is1);
        } catch (FontFormatException e) {
            System.out.println("error1");
            e.printStackTrace();
        } catch (IOException e) {
            System.out.println("error2");
            e.printStackTrace();
        }
    }
private Font myFont1 = myArialNarrow.deriveFont(30f);

结果是，我的字体myFont1的大小为30。在IDE中运行该程序时，一切正常，但是如果我执行jar操作，文件将引发异常：

error2
java.io.IOException: Problem reading font data.
at java.awt.Font.createFont0(Font.java:1000)
at java.awt.Font.createFont(Font.java:877)
at com.pumaex.GameField.<init>(GameField.java:57)
at com.pumaex.LinesLauncher.main(LinesLauncher.java:5)
Exception in thread "main" java.lang.NullPointerException
at com.pumaex.GameField.<init>(GameField.java:70)
at com.pumaex.LinesLauncher.main(LinesLauncher.java:5)

为什么程序找不到字体文件？我写错了路吗？告诉我，怎么了？

已添加

import java.awt.Font;
import java.awt.FontFormatException;
import java.io.BufferedInputStream;
import java.io.IOException;
import java.io.InputStream;

public class Problem {

    public static Font loadFile(String filePath) throws IOException, ClassNotFoundException, FontFormatException {
        try (final InputStream is = new BufferedInputStream(Problem.class.getClassLoader().getResourceAsStream(filePath))) {
            return Font.createFont(Font.TRUETYPE_FONT, is);
        }
    }

    public static void main(String[] args) throws Exception {
        Font font = loadFile("assets/fonts/ARIALN.ttf");
        Font myFont1 = font.deriveFont(30f);
        System.out.println(myFont1);
    }

}