How to convert XML to JSON using only Jackson?

Using Jackson 2.x

You can do that with Jackson and no POJOs are required for that:

String xml = "<?xml version=\"1.0\" encoding=\"UTF-8\"?>\n" +
             "<errors>\n" +
             "  <error>\n" +
             "    <status>400</status>\n" +
             "    <message>The field 'quantity' is invalid.</message>\n" +
             "    <details>\n" +
             "      <invalid_reason>The quantity specified is greater than the quantity of the product that is available to ship.</invalid_reason>\n" +
             "      <available_quantity>0</available_quantity>\n" +
             "      <order_product_id>12525</order_product_id>\n" +
             "    </details>\n" +
             "  </error>\n" +
             "</errors>";

XmlMapper xmlMapper = new XmlMapper();
JsonNode node = xmlMapper.readTree(xml.getBytes());

ObjectMapper jsonMapper = new ObjectMapper();
String json = jsonMapper.writeValueAsString(node);

The following dependencies are required:

<dependency>
    <groupId>com.fasterxml.jackson.core</groupId>
    <artifactId>jackson-core</artifactId>
    <version>2.8.2</version>
</dependency>
<dependency>
    <groupId>com.fasterxml.jackson.core</groupId>
    <artifactId>jackson-databind</artifactId>
    <version>2.8.2</version>
</dependency>
<dependency>
    <groupId>com.fasterxml.jackson.core</groupId>
    <artifactId>jackson-annotations</artifactId>
    <version>2.8.2</version>
</dependency>
<dependency>
    <groupId>com.fasterxml.jackson.dataformat</groupId>
    <artifactId>jackson-dataformat-xml</artifactId>
    <version>2.8.2</version>
</dependency>

Be aware of the XmlMapper limitations stated in the documentation:

Tree Model is only supported in limited fashion: specifically, Java arrays and Collections can be written, but can not be read, since it is not possible to distinguish Arrays and Objects without additional information.

As nicely highlighted by the Jackson author in the comments, Jackson 2.12 finally improved XML handling, so that duplicates are preserved if using JsonNode or Object as target types.

Using JSON.org

You also can do it with JSON.org:

String xml = "<?xml version=\"1.0\" encoding=\"UTF-8\"?>\n" +
             "<errors>\n" +
             "  <error>\n" +
             "    <status>400</status>\n" +
             "    <message>The field 'quantity' is invalid.</message>\n" +
             "    <details>\n" +
             "      <invalid_reason>The quantity specified is greater than the quantity of the product that is available to ship.</invalid_reason>\n" +
             "      <available_quantity>0</available_quantity>\n" +
             "      <order_product_id>12525</order_product_id>\n" +
             "    </details>\n" +
             "  </error>\n" +
             "</errors>";

String json = XML.toJSONObject(xml).toString();

The following dependency is required:

<dependency>
    <groupId>org.json</groupId>
    <artifactId>json</artifactId>
    <version>20160810</version>
</dependency>