使用 Jackson 在 Java 中读取 yaml 的一部分

Question

if I have the following yaml (which I found online) representing a java Order class, order.yaml :

orderNo: A001
customerName: Customer, Joe
orderLines:
  - item: No. 9 Sprockets
    quantity: 12
    unitPrice: 1.23
  - item: Widget (10mm)
    quantity: 4
    unitPrice: 3.45

I was able to use

ObjectMapper objectMapper = new ObjectMapper(new YAMLFactory());
Order order = objectMapper.readValue(new File(<path_to_order>), Order.class);

But this means that I need to define orderNo and orderLines in advance... If I have a giant yaml with a bunch of nested properties this can get really annoying. What if I want a class than can read one property or a class that can read another property and "ignore" other ones? Is that even possible? That way I could just specify which java object I want without necessarily having to recursively define every property of the yaml. Thank you!

Answer 1

The Map approach will lose you the type safety. There's no need to define every single property. You can use the Json annotations just fine with YAML too, it's just a historical leftover that it is called Json. What you are looking for is @JsonIgnoreProperties(ignoreUnknown = true) .

If you don't like to specify the Annotation for every class, use objectMapper.configure(DeserializationFeature.FAIL_ON_UNKNOWN_PROPERTIES, false); .

public static void main(String[] args) throws IOException {
    ObjectMapper objectMapper = new ObjectMapper(new YAMLFactory());
    Order order = objectMapper.readValue(new File("foo.yml"), Order.class);
    System.out.println(order.getOrderLines().get(0).getItem());
}


@JsonIgnoreProperties(ignoreUnknown = true)
static class Order {
    private String orderNo;
    private List<OrderLine> orderLines;

    public String getOrderNo() {
        return orderNo;
    }

    public void setOrderNo(String orderNo) {
        this.orderNo = orderNo;
    }

    public List<OrderLine> getOrderLines() {
        return orderLines;
    }

    public void setOrderLines(List<OrderLine> orderLines) {
        this.orderLines = orderLines;
    }

    @JsonIgnoreProperties(ignoreUnknown = true)
    static class OrderLine {
        private String item;

        public String getItem() {
            return item;
        }

        public void setItem(String item) {
            this.item = item;
        }
    }
}

Answer 2

You can read the json in a Map and then retrieve whatever you want from there

ObjectMapper objectMapper = new ObjectMapper(new YAMLFactory());
Map<String,Object> jsonMap = objectMapper.readValue(new File(<path_to_order>), Map.class);