我正在尝试显示另一个类的数据 Transaction 在我的 Application 班级。我创建了一个 Transaction 把它传给 sendUserNotification 方法，但当我对数据调用get方法时，它返回为null。 Application Class ```
public class Application {
private static final String TOPIC = "suspicious-transactions";
private static final String BOOTSTRAP_SERVERS = "localhost:9092,localhost:9093,localhost:9094";

public static void main(String[] args) {
    Application kafkaConsumerApp = new Application();

    String consumerGroup = "user-notification-service";
    if (args.length == 1) {
        consumerGroup = args[0];
    }

    System.out.println("Consumer is part of consumer group " + consumerGroup);

    Consumer<String, Transaction> kafkaConsumer = kafkaConsumerApp.createKafkaConsumer(BOOTSTRAP_SERVERS, consumerGroup);

    kafkaConsumerApp.consumeMessages(TOPIC, kafkaConsumer);

}

public static void consumeMessages(String topic, Consumer<String, Transaction> kafkaConsumer) {
    kafkaConsumer.subscribe(Collections.singletonList(topic));

    while (true) {
        ConsumerRecords<String, Transaction> consumerRecords = kafkaConsumer.poll(Duration.ofSeconds(1));

        if (consumerRecords.isEmpty()) {
            //do something
        }

        for (ConsumerRecord<String, Transaction> record : consumerRecords) {
           Transaction transaction =  new Transaction();
           sendUserNotification(transaction);

            System.out.println(String.format("Received record (key: %s, value %s, partition: %d, offset: %d",
                    record.key(), record.value(), record.partition(), record.offset()));
        }
    }
}

public static Consumer<String, Transaction> createKafkaConsumer(String bootstrapServers, String consumerGroups) {
    Properties properties = new Properties();

    properties.put(ConsumerConfig.BOOTSTRAP_SERVERS_CONFIG, bootstrapServers);
    properties.put(ConsumerConfig.KEY_DESERIALIZER_CLASS_CONFIG, StringDeserializer .class.getName());
    properties.put(ConsumerConfig.VALUE_DESERIALIZER_CLASS_CONFIG, StringDeserializer.class.getName());
    properties.put(ConsumerConfig.GROUP_ID_CONFIG, consumerGroups);
    properties.put(ConsumerConfig.ENABLE_AUTO_COMMIT_CONFIG, false);

    return new KafkaConsumer<>(properties);

}

private static void sendUserNotification(Transaction transaction) {
    // Print transaction information to the console
    String h =transaction.getUser();

    System.out.println(String.format("Sending user %s notification about a suspicious transaction of %f in their account originating in %s", h, transaction.getAmount(), transaction.getTransactionLocation()));

}

`Transaction Class`
public class Transaction {
private String user;
private double amount;
private String transactionLocation;

public String getUser() {
    return user;
}

public double getAmount() {
    return amount;
}

public String getTransactionLocation() {
    return transactionLocation;
}

public void setUser(String user) {
    this.user = user;
}

public void setAmount(double amount) {
    this.amount = amount;
}

public void setTransactionLocation(String transactionLocation) {
    this.transactionLocation = transactionLocation;
}

@Override
public String toString() {
    return "Transaction{" +
            "user='" + user + '\'' +
            ", amount=" + amount +
            ", transactionLocation='" + transactionLocation + '\'' +
            '}';
}

/**
 * Kafka Deserializer implementation.
 * Deserializes a Transaction from JSON to a {@link Transaction} object
 */
public static class TransactionDeserializer implements Deserializer<Transaction> {

    @Override
    public Transaction deserialize(String topic, byte[] data) {
        ObjectMapper mapper = new ObjectMapper();
        Transaction transaction = null;
        try {
            transaction = mapper.readValue(data, Transaction.class);
        } catch (Exception e) {
            e.printStackTrace();
        }
        return transaction;

    }
}

`Output`
Consumer is part of consumer group user-notification-service
Sending user null notification about a suspicious transaction of 0.000000 in their account originating in null
Received record (key: dkelly9283, value Transaction{user='dkelly9283', amount=1653.32, transactionLocation='China'}, partition: 0, offset: 12
Sending user null notification about a suspicious transaction of 0.000000 in their account originating in null
Received record (key: msmith2015, value Transaction{user='msmith2015', amount=50.43, transactionLocation='California'}, partition: 1, offset: 12