密钥库中的 privateKey 返回 null

问：

public static PrivateKey getPrivateKey (KeyStore keystore, String keyId, char[] keyPwd) 
    throws BenefitsException {
      PrivateKey privateKey = null;
     try {
     if (keystore == null || keyPwd== null || keyId == null
     || "".equals(keyId)) {
       return privateKey;
      }
    else 
    {

    privateKey = (PrivateKey) keystore.getKey(keyId, keyPwd);
    }

    } catch (NoSuchAlgorithmException |
    UnrecoverableKeyException
    | KeyStoreException e) {
      throw new BenefitsException("Error in getPrivateKey method : ", e);
     return privateKey;

这是我的测试方法，有没有人弄清楚为什么 privatekey 返回 null

@Test public void getPrivateKey_ExceptionTest（） 抛出 Exception{

    KeyStore keystoreMock = Mockito.mock (KeyStore.class);

    FieldUtils.writeField(keystoreMock, "keyStoreSpi", Mockito.mock (KeyStoreSpi.class), true);
    FieldUtils.writeField(keystoreMock, "initialized", true, true);



   char[] pwd= {'c', 'h', 'a', 'n', 'g', 'e', 'i', 't'};

   cryptoUtils.getPrivateKey(keystoreMock, "somestring", pwd);

   }