Converting xml string to 'this' class using jackson

Question

I am using Jackson to work with my XML data, based on this Baeldung tutorial.

I have the following class to model some XML:

import com.fasterxml.jackson.core.JsonProcessingException;
import com.fasterxml.jackson.dataformat.xml.XmlMapper;

public class SimpleBean {

    private int x = 1;
    private int y = 2;

    public int getX() {
        return x;
    }

    public void setX(int x) {
        this.x = x;
    }

    public int getY() {
        return y;
    }

    public void setY(int y) {
        this.y = y;
    }

    public SimpleBean fromString(String xml) throws JsonProcessingException {
        XmlMapper xmlMapper = new XmlMapper();
        return xmlMapper.readValue(xml, SimpleBean.class);
    }
}

I have the following test that passes:

@Test
void testFromString() throws Exception {
    SimpleBean bean = new SimpleBean();
    String xml = "<SimpleBean><x>11</x><y>22</y></SimpleBean>";
    SimpleBean beanParsed = bean.fromString(xml);
    Assertions.assertEquals(11, beanParsed.getX());
}

What I would like to do is something like this:

@Test
void testFromStringWanted() throws Exception {
    SimpleBean bean = new SimpleBean();
    String xml = "<SimpleBean><x>11</x><y>22</y></SimpleBean>";
    bean.fromString(xml);
    Assertions.assertEquals(11, bean.getX());  // FAILS
}

I would like to parse the string into the instance bean that I already have. I do not want to have to create a second beanParsed.

I tried to change my fromString method to:

public void fromString(String xml) throws JsonProcessingException {
    XmlMapper xmlMapper = new XmlMapper();
    xmlMapper.readValue(xml, this.getClass());
}

but that did not work. No errors, it just did not produce the desired results.

Also tried this, but that does not even compile!

public void fromString(String xml) throws JsonProcessingException {
    XmlMapper xmlMapper = new XmlMapper();
    this = xmlMapper.readValue(xml, SimpleBean.class);
}

How can I do this?

How does it fail? fromString returns a Bean but you do not save/use it. bean.getX() returns a value that has not been set. — Scary Wombat
– Scary Wombat, Commented Mar 7, 2024 at 1:11
fromString is a factory method and should be static. Then you invoke it without instantiating a SimpleBean first, because it returns the new SimpleBean. — Jim Garrison
– Jim Garrison, Commented Mar 7, 2024 at 1:32
For an example of a built-in factory method, look at the reason behind things like Long.parseLong(String...) and many others in the standard library. This is what you want, not a hack to update the bean after creation. — Jim Garrison
– Jim Garrison, Commented Mar 7, 2024 at 1:39
@JimGarrison Thank you for the suggestion. In my case I will always have one instance of SimpleBean, I just wanted to be able to parse the xml string into the existing one. The answer that Nir gave, below, is exactly what I was looking for. — SiKing
– SiKing, Commented Mar 7, 2024 at 15:51

Nir Alfasi · Accepted Answer · 2024-03-07 01:17:59Z

1

Try:

public void fromString(String xml) throws JsonProcessingException {
    XmlMapper xmlMapper = new XmlMapper();
    // Use readerForUpdating() to parse into the existing instance
    xmlMapper.readerForUpdating(this).readValue(xml);
}

Jackson provides a method called readerForUpdating() specifically for parsing data into an existing object.

This method creates a reader that updates an existing object instead of creating a new one.

answered Mar 7, 2024 at 1:17

Nir Alfasi

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