The specification of javax.sound.midi.Synthesizer.unloadInstruments(Soundbank,Patch[]) says:
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void unloadInstruments(Soundbank soundbank,
                       Patch[] patchList)
    . . .
    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[unload006]

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)) {
                instrs = sBank.getInstruments();
                pList = new Patch[instrs.length];
                for (int i = 0; i < instrs.length; i++) {
                    pList[i] = instrs[i].getPatch();
                }
                synth.unloadInstruments(sBank, pList);
                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!
unload006: Failed.