如何使用Jackson用流式API解析XML文件

Answer 1

读取树结构需要进程启动（例如，XML的 ，或者JSON的{/[)。

让根包装器和一大串汽车（为了简洁起见，我使用lombok注释）：

@Getter
@Setter
@JacksonXmlRootElement
@NoArgsConstructor
@AllArgsConstructor
static class CarBook {
    @JacksonXmlProperty(isAttribute = true)
    private int version;
    @JacksonXmlElementWrapper(localName = "cars")
    @JacksonXmlProperty(localName = "car")
    private List<Car> cars;
}

@Getter
@Setter
@ToString
@NoArgsConstructor
@AllArgsConstructor
static class Car {
    private String model;
    private String plate;
}

然后，在完全读取所有列表（可能是其他成员）之前，无法获得carbook对象。

/**
 * Method for reading a single XML value from given XML-specific input
 * source; useful for incremental data-binding, combining traversal using
 * basic Stax {@link XMLStreamReader} with data-binding by Jackson.
 * 
 * @since 2.4
 */
public <T> T readValue(XMLStreamReader r, Class<T> valueType) throws IOException {
    return readValue(r, _typeFactory.constructType(valueType));
} 

例如，让大的(1，2g)文件：

<CarBook version="1"><cars>
<car><model>Alfa Romeo Spider</model><plate>27437</plate></car>
<car><model>Almera</model><plate>6429</plate></car>
<car><model>Audi 80 and 90</model><plate>4898</plate></car>
<car><model>Audi A3</model><plate>21259</plate></car>
<car><model>Audi A4</model><plate>21056</plate></car>
<car><model>Audi Coupé</model><plate>5623</plate></car>
<car><model>Austin Metro</model><plate>26446</plate></car>
<car><model>BMW 3 Series</model><plate>16338</plate></car>
<car><model>BMW 5 Series</model><plate>29859</plate></car>
...

的，你可以懒洋洋地阅读

public static void main(String... args) throws IOException, XMLStreamException {
    XmlMapper xm = new XmlMapper();
    XMLInputFactory xif = XMLInputFactory.newInstance();
    XMLStreamReader xr = xif.createXMLStreamReader(new FileInputStream(/* 1,2G file */ "/home/josejuan/tmp/all.cars.xml"));

    // you must to read step by step
    while (xr.hasNext()) {
        xr.next();
        if (xr.getEventType() == START_ELEMENT) {
            System.out.println(xr.getLocalName());
            if ("car".equals(xr.getLocalName())) {
                Car car = xm.readValue(xr, Car.class);
                System.out.println(car);
                if ("21056".equals(car.getPlate()))
                    break;
            }
        }
    }

    System.out.println("== End Of Process ==");
}

有输出

CarBook
cars
car
WithLazyJackson.Car(model=Alfa Romeo Spider, plate=27437)
car
WithLazyJackson.Car(model=Almera, plate=6429)
car
WithLazyJackson.Car(model=Audi 80 and 90, plate=4898)
car
WithLazyJackson.Car(model=Audi A3, plate=21259)
car
WithLazyJackson.Car(model=Audi A4, plate=21056)
== End Of Process ==