Fully Native and Multiplatform Kotlin serialization library for serialization/deserialization of toml format. Uses native kotlinx.serialization, provided by Kotlin. This library contains no Java code and no Java dependencies. We believe that TOML is actually the most readable and user-friendly configuration file format. So we decided to support this format for the kotlinx serialization library.

Contribution

As this project is needed by the Kotlin community, we need your help. We will be glad if you will test ktoml or contribute to this project. In case you don't have much time for this - at least spend 5 seconds to give us a star to attract other contributors!

Thanks! 🙏 🥳

Acknowledgement

Special thanks to those awesome developers who give us great suggestions, help us to maintain and improve this project: @NightEule5, @bishiboosh, @Peanuuutz, @petertrr, @nulls, @Olivki, @edrd-f, @BOOMeranGG, @aSemy, @thomasgalvin

Supported platforms

All the code is written in Kotlin common module. This means that it can be built for each and every Kotlin native platform. However, to reduce the scope, ktoml now supports only the following platforms:

• jvm
• mingwx64
• linuxx64
• macosx64
• macosArm64 (M1)
• ios
• iosSimulatorArm64
• js (obviously only for ktoml-core!). Note, that js(LEGACY) is not supported

Other platforms could be added later on the demand (just create a corresponding issue) or easily built by users on their machines.

🌐 ktoml supports Kotlin 1.9.22

Current limitations

❗ Please note, that TOML standard does not define Java-like types: Char, Short, etc. You can check types that are supported in TOML standard here. However, in Ktoml, our goal is to comprehensively support all primitive types offered by Kotlin.

General
We are still developing and testing this library, so it has several limitations:
✅ deserialization (with some parsing limitations)
✅ serialization (with tree-related limitations)

Parsing and decoding
✅ Table sections (single and dotted)
✅ Key-value pairs (single and dotted)
✅ Long/Integer/Byte/Short types
✅ Double/Float types
✅ Basic Strings
✅ Literal Strings
✅ Char type
✅ Boolean type
✅ Simple Arrays
✅ Comments
✅ Inline Tables
✅ Offset Date-Time (to Instant of kotlinx-datetime)
✅ Local Date-Time (to LocalDateTime of kotlinx-datetime)
✅ Local Date (to LocalDate of kotlinx-datetime)
✅ Local Time (to LocalTime of kotlinx-datetime)
✅ Multiline Strings
✅ Arrays (including multiline arrays)
✅ Maps (for anonymous key-value pairs)
❌ Arrays: nested; of Different Types
❌ Nested Inline Tables
❌ Array of Tables
❌ Inline Array of Tables

Dependency

The library is hosted on the Maven Central. To import ktoml library you need to add following dependencies to your code:

Maven

<dependency>
  <groupId>com.akuleshov7</groupId>
  <artifactId>ktoml-core</artifactId>
  <version>0.5.1</version>
</dependency>
<dependency>
  <groupId>com.akuleshov7</groupId>
  <artifactId>ktoml-file</artifactId>
  <version>0.5.1</version>
</dependency>

Gradle Groovy

implementation 'com.akuleshov7:ktoml-core:0.5.1'
implementation 'com.akuleshov7:ktoml-file:0.5.1'

Gradle Kotlin

implementation("com.akuleshov7:ktoml-core:0.5.1")
implementation("com.akuleshov7:ktoml-file:0.5.1")

How to use

❗ as TOML is a foremost language for config files, we have also supported the deserialization from file. However, we are using okio to read the file, so it will be added as a dependency to your project if you will import ktoml-file. Same about okio Source (for example if you need Streaming): ktoml-source. For basic scenarios of decoding strings you can simply use ktoml-core.

❗ don't forget to add the serialization plugin kotlin("plugin.serialization") to your project. Otherwise, @Serialization annotation won't work properly.

Deserialization:

Straight-forward deserialization

// add extensions from 'kotlinx' lib to your project:
import kotlinx.serialization.decodeFromString
import kotlinx.serialization.serializer
// add com.akuleshov7:ktoml-core to your project:
import com.akuleshov7.ktoml.deserialize

@Serializable
data class MyClass(/* your fields */)

// to deserialize toml input in a string format (separated by newlines '\n')
// no need to provide serializer() explicitly if you will use extension method from
// <kotlinx.serialization.decodeFromString>
val resultFromString = Toml.decodeFromString<MyClass>(/* string with a toml input */)
val resultFromList = Toml.decodeFromString<MyClass>(serializer(), /* sequence with lines of strings with a toml input */)

Partial deserialization

Partial Deserialization can be useful when you would like to deserialize only one single table and you do not want to reproduce whole object structure in your code.

// If you need to deserialize only some part of the toml - provide the full name of the toml table. 
// The deserializer will work only with this table and it's children.
// For example if you have the following toml, but you want only to decode [c.d.e.f] table: 
// [a]
//   b = 1
// [c.d.e.f]
//   d = "5"

val result = Toml.partiallyDecodeFromString<MyClassOnlyForTable>(serializer(), /* string with a toml input */, "c.d.e.f")
val result = Toml.partiallyDecodeFromString<MyClassOnlyForTable>(serializer(), /* list with toml strings */, "c.d.e.f")

Toml File deserialization

// add com.akuleshov7:ktoml-file to your project
import com.akuleshov7.ktoml.file

val resultFromString = TomlFileReader.decodeFromFile<MyClass>(serializer(), /* file path to toml file */)
val resultFromList = TomlFileReader.partiallyDecodeFromFile<MyClass>(serializer(),  /* file path to toml file */, /* table name */)

❗ toml-file is only one of the example for reading the data from source. For your particular case you can implement your own source provider based on okio.Source. For this purpose we have prepared toml-source module and implemented an example with java streams for JVM target.

// add com.akuleshov7:ktoml-source to your project
import com.akuleshov7.ktoml.source

val resultFromString = TomlFileReader.decodeFromSource<MyClass>(serializer(), /* your source */)
val resultFromList = TomlFileReader.partiallyDecodeFromSource<MyClass>(serializer(),  /* your source */, /* table name */)

Serialization:

Straight-forward serialization

// add extensions from 'kotlinx' lib to your project:
import kotlinx.serialization.encodeToString
// add com.akuleshov7:ktoml-core to your project:
import com.akuleshov7.ktoml.Toml

@Serializable
data class MyClass(/* your fields */)

val toml = Toml.encodeToString(MyClass(/* ... */))

Toml File serialization

// add com.akuleshov7:ktoml-file to your project
import com.akuleshov7.ktoml.file.TomlFileWriter

TomlFileWriter.encodeToFile<MyClass>(serializer(), /* file path to toml file */)

Parser to AST:

Simple parser

import com.akuleshov7.ktoml.parsers.TomlParser
import com.akuleshov7.ktoml.TomlConfig
/* ========= */
var tomlAST = TomlParser(TomlInputConfig()).parseStringsToTomlTree(/* list with toml strings */)
tomlAST = TomlParser(TomlInputConfig()).parseString(/* the string that you want to parse */)
tomlAST.prettyPrint()

Configuration

Ktoml parsing and deserialization was made configurable to fit all the requirements from users. We have created a special configuration class that can be passed to the decoder method:

Toml(
    inputConfig = TomlInputConfig(
        // allow/prohibit unknown names during the deserialization, default false
        ignoreUnknownNames = false,
        // allow/prohibit empty values like "a = # comment", default true
        allowEmptyValues = true,
        // allow/prohibit null values like "a = null", default true
        allowNullValues = true,
        // allow/prohibit escaping of single quotes in literal strings, default true
        allowEscapedQuotesInLiteralStrings = true,
        // allow/prohibit processing of empty toml, if false - throws an InternalDecodingException exception, default is true
        allowEmptyToml = true,
    ),
    outputConfig = TomlOutputConfig(
        // indentation symbols for serialization, default 4 spaces
        indentation = Indentation.FOUR_SPACES,
    )
).decodeFromString<MyClass>(
    tomlString
)

How ktoml works: examples

❗ You can check how below examples work in decoding ReadMeExampleTest and encoding ReadMeExampleTest.

Deserialization

The following example:

someBooleanProperty = true
# inline tables in gradle 'libs.versions.toml' notation
gradle-libs-like-property = { id = "org.jetbrains.kotlin.jvm", version.ref = "kotlin" }

[table1]
    # null is prohibited by the TOML spec, but allowed in ktoml for nullable types
    # so for 'property1' null value is ok. Use: property1 = null  
    property1 = 100
    property2 = 6

[myMap]
    a = "b"
    c = "d"

[table2]
    someNumber = 5
[table2."akuleshov7.com"]
    name = 'this is a "literal" string'
    # empty lists are also supported
    configurationList = ["a",  "b",  "c"]

    # such redeclaration of table2
    # is prohibited in toml specification;
    # but ktoml is allowing it in non-strict mode: 
    [table2]
        otherNumber = 5.56
        # use single quotes
        charFromString = 'a'
        charFromInteger = 123

can be deserialized to MyClass:

@Serializable
data class MyClass(
    val someBooleanProperty: Boolean,
    val table1: Table1,
    val table2: Table2,
    @SerialName("gradle-libs-like-property")
    val kotlinJvm: GradlePlugin,
    val myMap: Map<String, String>
)

@Serializable
data class Table1(
    // nullable property, from toml input you can pass "null"/"nil"/"empty" value (no quotes needed) to this field
    val property1: Long?,
    // please note, that according to the specification of toml integer values should be represented with Long,
    // but we allow to use Int/Short/etc. Just be careful with overflow
    val property2: Byte,
    // no need to pass this value in the input as it has the default value and so it is NOT REQUIRED
    val property3: Short = 5
)

@Serializable
data class Table2(
    val someNumber: Long,
    @SerialName("akuleshov7.com")
    val inlineTable: NestedTable,
    val otherNumber: Double,
    // Char in a manner of Java/Kotlin is not supported in TOML, because single quotes are used for literal strings.
    // However, ktoml supports reading Char from both single-char string and from it's integer code
    val charFromString: Char,
    val charFromInteger: Char
)

@Serializable
data class NestedTable(
    val name: String,
    @SerialName("configurationList")
    val overriddenName: List<String?>
)

@Serializable
data class GradlePlugin(val id: String, val version: Version)

@Serializable
data class Version(val ref: String)

with the following code:

Toml.decodeFromString<MyClass>(/* your toml string */)

Translation of the example above to json-terminology:

{
  "someBooleanProperty": true,
  
  "gradle-libs-like-property": {
    "id": "org.jetbrains.kotlin.jvm",
    "version": {
      "ref": "kotlin"
    }
  },
  
  "table1": {
    "property1": 100,
    "property2": 5
  },
  "table2": {
    "someNumber": 5,
    
    "otherNumber": 5.56,
    "akuleshov7.com": {
      "name": "my name",
      "configurationList": [
        "a",
        "b",
        "c"
      ]
    }
  }
}

Serialization

The following example from above:

someBooleanProperty = true
# inline tables in gradle 'libs.versions.toml' notation
gradle-libs-like-property = { id = "org.jetbrains.kotlin.jvm", version.ref = "kotlin" }

[table1]
# null is prohibited by the TOML spec, but allowed in ktoml for nullable types
# so for 'property1' null value is ok. Use: property1 = null. 
# Null can also be prohibited with 'allowNullValues = false'
property1 = 100
property2 = 6

[table2]
    someNumber = 5
    [table2."akuleshov7.com"]
        name = 'this is a "literal" string'
        # empty lists are also supported
        configurationList = ["a",  "b",  "c"]

# such redeclaration of table2
# is prohibited in toml specification;
# but ktoml is allowing it in non-strict mode: 
[table2]
    otherNumber = 5.56
    # use single quotes
    charFromString = 'a'
    charFromInteger = 123

can be serialized from MyClass:

@Serializable
data class MyClass(
    val someBooleanProperty: Boolean,
    @TomlComments(
        "Comments can be added",
        "More comments can also be added"
    )
    val table1: Table1,
    val table2: Table2,
   @SerialName("gradle-libs-like-property")
   val kotlinJvm: GradlePlugin
)

@Serializable
data class Table1(
    @TomlComments(inline = "At the end of lines too")
    // nullable values, represented as "null" in toml. For more strict behavior,
    // null values can be ignored with the ignoreNullValues config property.
    val property1: Long?,
    // please note, that according to the specification of toml integer values should be represented with Long
    val property2: Long,
    // Default values can be ignored with the ignoreDefaultValues config property.
    val property3: Long = 5
)

@Serializable
data class Table2(
    // Integers can be formatted in hex, binary, etc. Currently only decimal is
    // supported.
    @TomlInteger(IntegerRepresentation.DECIMAL)
    val someNumber: Long,
    @SerialName("akuleshov7.com")
    @TomlInlineTable // Can be on the property
    val inlineTable: InlineTable,
    @TomlComments(
        "Properties always appear before sub-tables, tables aren't redeclared"
    )
    val otherNumber: Double
)

@Serializable
data class InlineTable(
    @TomlLiteral
    val name: String,
    @SerialName("configurationList")
    val overriddenName: List<String?>
)

@Serializable
@TomlInlineTable // ...or the class
data class GradlePlugin(
    val id: String,
    // version is "collapsed": single member inline tables become dotted pairs.
    val version: Version
)

@Serializable
@TomlInlineTable
data class Version(val ref: String)

with the following code:

Toml.encodeToString<MyClass>(/* your encoded object */)

Q&A

I want to catch ktoml-specific exceptions in my code, how can I do it?

Ktoml may generate various exceptions when encountering invalid input. It's important to note that certain strict checks can be enabled or disabled (refer to the Configuration section in this readme). We have intentionally exposed only two top-level exceptions, namely TomlDecodingException and TomlEncodingException, for public use. You can catch these exceptions in your code, as all other exceptions inherit from one of these two and will not be publicly accessible.

What if I do not know the names for keys and tables in my TOML, and therefore cannot specify a strict schema for decoding? Can I still decode it somehow?

Certainly. In such cases, you can decode all your key-values into a Map. However, it's important to be aware that both ktoml and kotlinx will be unable to enforce type control in this scenario. Therefore, you should not expect any "type safety." For instance, even when dealing with a mixture of types like Int, Map, String, etc., such as:

[a]
    b = 42
    c = "String"
    [a.innerTable]
        d = 5
    [a.otherInnerTable]
        d = "String"

You can still decode it using Toml.decodeFromString<MyClass>(data) where:

// MyClass(a={b=42, c=String, innerTable={d=5}, otherInnerTable={d=String}})
@Serializable
data class MyClass(
    val a: Map<String, Map<String, String>>
)

However, be aware that this may lead to unintended side effects. Our recommendation is to decode only key-values of the same type for a more predictable outcome.