The specification on javax.sound.midi.Synthesizer.unloadAllInstruments(Soundbank) says:
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void unloadAllInstruments(Soundbank soundbank)
    . . .
    Throws:
        IllegalArgumentException - thrown if the soundbank is not supported.
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The following testcase checks this behavior:
api/javax_sound/midi/Synthesizer/index.html#unload[unload001]

Here is the excerpt of the meaningful part of the code:
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        . . .
        try {
            synth = MidiSystem.getSynthesizer();
            synth.open();
            sBank = new MySoundbank();

            if (!synth.isSoundbankSupported(sBank)) {
                synth.unloadAllInstruments(sBank);
                failed = true;
                log.println("IllegalArgumentException should be thrown for "
                        + "unsupported banks!");
            }
        } catch (IllegalArgumentException ie) {
            log.println("Soundbank is not compatible. OKAY");

        }
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And here is the output:

IllegalArgumentException should be thrown for unsupported banks!
unload001: Failed.