java - 无法从 jar 读取输入

Question

当我运行java -jar MidiTest.jar时，输入MIDI文件，它会抛出:

Exception in thread "main" java.lang.NullPointerException
        at com.sun.media.sound.StandardMidiFileReader.getSequence(StandardMidi
leReader.java:209)
        at javax.sound.midi.MidiSystem.getSequence(MidiSystem.java:802)
        at MidiTest.playMidi(MidiTest.java:56)
        at MidiTest.(MidiTest.java:44)
        at MidiTest.main(MidiTest.java:25)

If I use java MidiTest instead it could play without issue. What wrong with the code? I have already add Main-Class: MidiTest with newline on Manifest file

My code:

        private void playMidi() {
    if(isPlaying.equals("0")) {
        try {
            song = MidiSystem.getSequence(
                getClass().getResource(filename));
            sequencer = MidiSystem.getSequencer();
            sequencer.setSequence(song);
            sequencer.open();
            sequencer.addMetaEventListener(this);
            sequencer.start();
        } catch (InvalidMidiDataException e) {
            System.out.println("Bad midi file: "
                + filename);
            System.exit(1);
        } catch (MidiUnavailableException e) {
            System.out.println("No sequencer available");
            System.exit(1);
        } catch (IOException e) {
            System.out.println("Could not read: "
                + filename);
            System.exit(1);
        }
        displayMidiInfo(filename);
        } else {
        updateTempoFactor(speed);
        }
    }

Answer 1

您似乎没有检查您尝试获取的资源是否返回非空值。具体来说:

song = MidiSystem.getSequence(
    getClass().getResource(filename));

导致了这个特殊问题。可能还有一个更深层次的问题，那就是除非 filename 表示的资源实际上位于 jar 中、类路径上或与 jar 文件 getResource()< 位于同一目录中 不会找到它。如果您尝试访问通用文件系统中任何位置的文件(而不是 jar 文件中)，那么您应该使用 File:

song = MidiSystem.getSequence(new File(filename));