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XmlUtil is a set of packages that supports multiplatform XML in Kotlin (only Javascript/JVM/Android currently).


  • Gradle wrapper validation: Validate Gradle Wrapper

This project is a cross-platform XML serialization (wrapping) library compatible with kotlin serialization. It provides capabilities for Android, JVM and JS (alpha quality)

It also provides serialization support

Help wanted: Any help with extending this project is welcome. Help is especially needed for the following aspects:

  • Documentation updates
  • Testing, in particular more extensive tests. Some tests already exist for both JVM and Android
  • Javascript support
    • Core Javascript support needs testing but should work. It is based on DOM so may be slow
    • Javascript serialization support: make serialization work on Javascript once possible
  • Native support: Currently there is no implementation for Kotlin native.

Versioning scheme

This library is based upon the unstable kotlinx.serialization library. Until that library is stable, this library will use the kotlinx.serialization library version as a prefix and append a release number. While every effort is made to limit incompatible changes, this cannot be guaranteed even in "minor" versions when the changes are due to bugs. These changes should be limited mainly to the serialization part of the library.

How to use

The library is designed as a multiplatform kotlin module, but platform-specific versions can also be used were appropriate.

Add repository

The project's maven access is hosted on OSS Sonatype (and available from Maven Central).

Releases can be added from maven central or:

repositories {
	maven {
		url  ""

Snapshots are available from:

repositories {
	maven {
		url  ""




JVM -- uses the stax API not available on Android


Android -- Uses the android streaming library


JS -- Wraps DOM













This module provides simple ktor support for xml (de)serialization. To use in your application use:

import nl.adaptivity.xmlutil.serialization.ktor.xml

fun Application.module(testing: Boolean = false) {

    install(ContentNegotiation) {
  /* ... */

Serialization help


You should be able to find examples in the Examples module


The entrypoint to the library is the XML format. There is a default, but often a child is better. A custom format is created through:

val format = XML(mySerialModule) {  
    // configuration options
    autoPolymorphism = true 

The options available are:

option description
repairNamespaces Should namespaces automatically be repaired. This option will be passed on to the XmlWriter
xmlDeclMode The mode to use for emitting XML declarations (). Replaces omitXmlDecl for more finegrained control
-omitXmlDecl- Deprecated (use xmlDeclMode). Should the generated XML contain an XML declaration or not. This is passed to the XmlWriter
indentString The indentation to use. Must be a combination of XML whitespace or comments (this is checked). This is passed to the XmlWriter
indent Deprecated for reading: The indentation level (in spaces) to use. This is backed by indentString. Reading is "invalid" for indentString values that are not purely string sequences. Writing it will set indentation as the specified amount of spaces.
-autoPolymorphic- Deprecated into policy Should polymorphic information be retrieved using SerializersModule configuration. This replaces XmlPolyChildren, but changes serialization where that annotation is not applied. This option will become the default in the future although XmlPolyChildren will retain precedence (when present)
-unknownChildHandler- Deprecated into policy A function that is called when an unknown child is found. By default an exception is thrown but the function can silently ignore it as well.
policy This is a class that can be used to define a custom policy that informs how the kotlin structure is translated to XML. It drives most complex configuration

The properties that have been moved into the policy can still be set in the builder but are no longer able to be read through the config object.


XML and Kotlin data types are not perfectly alligned. As such there are some algorithms that aim to automatically do the best thing. Most of this has been moved to the default XmlSerializationPolicy implementation, but you can customize this to change the eventual structure. This includes determining the names used.

Storage type

In the default policy, the way a field is stored is determined as follows to be one of: Element, Attribute, Text or Mixed. Mixed is a special type that allows for mixing of text and element content and needs some special treatment.:

  • If the field has an annotation such as @XmlElement or XmlValue this will take precedence. The XmlValue tag will allow the field to hold element text content (direct only).

  • If the serializer is a primitive this will normally be serialized as attribute

  • If the serializer is a list, if there is an @XmlChildrenName annotation, this will trigger named list mode where a wrapper tag is used (element). Otherwise the list elements will be written directly as tags (even primitives) using a "transparent/anonymous" list.

  • If a list has the @XmlValue tag (to actually support text content it needs to be a list of Any) this will allow the list to hold mixed content. This should also be polymorphic (but the annotation is required).

  • If a primitive is written as tag this will use the name as tag name, and value as element content.

  • A primitive written as TEXT will be text content only.

  • Polymorphic properties are treated specially in that the system does not use/require wrappers. Instead it will use the tag name to determine the type. Either specified by @XmlPolyChildren or through the serialDescriptor of the type. This also works inside lists, even transparent (invisible) lists. If multiple polymorphic properties have the same subtags this is undefined/an error (you can use @XmlPolyChildren to have different names).

    A custom policy is able to determine on individual bases whether this transparent polymorphism should be used, but the default policy provides an overall toggle (which also respects the autopolymorphic property of the configuration builder). The default will always trigger transparent mode if XmlPolyChildren is present.

  • If the serializer is polymorphic this enforces tag mode. If @XmlPolyChildren is specified or autoPolymorphic is set it triggers transparent polymorphism mode where the child name is used to look up the property it belongs to. (note that this is incorrect with multiple properties that could contain the same polymorphic value - unless @XmlPolyChildren overrides it).

  • Otherwise it will be written as a tag

Tag/attribute name

The way the name is determined is configured/implemented through the configured policy. The documentation below is for the default policy. This is designed to allow customization by users.

Based upon the storage type, the effective name for an attribute is determined as follows:

  • @XmlSerialName at property declaration site
  • @XmlSerialName at type declaration site
  • @SerialName at property declaration site
  • property name at property declaration site (note that the @SerialName annotation is invisible to the encoder)

The effective name for a regular tag is determined as follows for normal serializers:

  • @XmlSerialName at property declaration site
  • @XmlSerialName at type declaration site
  • @SerialName at type declaration site
  • type name at type declaration site. The default type declaration type name is the Kotlin/Java type name (and long). The system will try to shorten this by eliding the package name. This is configurable in the policy.

The effective name for a polymorphic child is determined as follows:

  • If the child is transparent, the annotations/serial name of the effective type is used (unless overridden by @XmlPolyChildren)
  • If the child is not transparent, the container is treated as a regular tag. It will have a type attribute to contain the serial name of the type (shortened to share the package name with the container). The value will use the default name value.

The implementation if serialization in the Kotlin compiler does not allow distinguishing between the automatic name and a @SerialName annotation. The default implementation supposes that if there is a '.' character in the name, this is a java type name and it strips the package out. (This also when it could be an attribute).

If you need to support names with dots in your format, either use the @XmlSerialName annotation, or use a different policy.


Annotation property description
@XmlSerialName Specify more detailed name information than can be provided by kotlinx.serialization.SerialName. In particular, it is not reliably possible to distinguish between @SerialName and the type name. We also need to specify namespace and prefix information.
value: String the local part of the name
namespace: String the namespace to use
val prefix: String The prefix to use
@XmlPolyChildren Mostly legacy annotation that allows specifying valid child tags for polymorphic resolution.
value: Array<String> Each string specifies a child according to the following format: childSerialName[=[prefix:]localName]. The childSerialName is the name value of the descriptor. By default that would be the class name, but @SerialName will change that. If the name is prefixed with a . the package name of the container will be prefixed. Prefix is the namespace prefix to use (the namespace will be looked up based upon this). Localname allows to specify the local name of the tag.
@XmlChildrenName Used in lists. This causes the children to be serialized as separate tags in an outer tag. The outer tag name is determined regularly.
@XmlElement Force a property to be either serialized as tag or attribute.
value: Boolean true to indicate serialization as tag, false to indicate serialization as attribute. Note that not all values can be serialized as attribute
@XmlValue Force a property to be element content. Note that only one field can be element content and tags would not be expected.
@XmlDefault Older versions of the framework do not support default values. This annotation allows a default value to be specified. The default value will not be written out if matched.
value: String The default value used if no value is specified. The value is parsed as if there was textual substitution of this value into the serialized XML.

Special type

The CompactFragment class is a special class (with supporting serializer) that will be able to capture the tag soup content of an element. Instead of using regular serialization its custom serializer will (in the case of xml serialization) directly read all the child content of the tag and store it as string content. It will also make a best effort attempt at retaining all namespace declarations necessary to understand this tag soup.

Alternatively the serialutil subproject contains the nl.adaptivity.serialutil.MixedContent type that allows for typesafe serialization/deserialization of mixed content with the proviso that the serialModule must use Any as the baseclass for the content.



Container for the core library (versions)


All code shared between Javascript and Java (either jvm or android)


All code that is common, but not shared between Jvm and Android platforms

Code specific to the Android platform (Pulls in as API dependency). This is a regular jar rather than an AAR as the only specific thing to Android is the XML library

Implementation of the shared code for Java based platforms (both Android and JVM)


Javascript based implementation


Code unique to the JVM platform (Pulls in as API dependency)


The kotlinx.serialization plugin to allow serialization to XML

The java version of the serialization plugin. Please note that it does not pull in the platform specific library. The core library is dependent on the actual platform used (JVM or Android). This library only pulls in the shared Java code.


The JVM version merely uses the jvm platform xml library but the serialization is


The Javascript version of the serialization plugin. This is not yet implemented due to missing annotation support for javascript and the 0.6.0 version of kotlinx.serialization not supporting type annotations.


An android test project to test serialization on Android.

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