isabella232 / ion-element-kotlin

IonElement is an immutable in-memory representation of the Ion data format. IonElement's API is idiomatic to Kotlin.

ion-element

ion-element is an immutable in-memory representation of Amazon Ion which has an API that is idiomatic to Kotlin and is also usable from Java. It is meant as an alternative to IonValue from ion-java but is not a complete implementation of Ion as it relies on ion-java's IonReader and IonWriter interfaces for reading and writing Ion data.

Why is IonElement Needed?

IonValue.clone()

While manipulating the IonValue instances of ion-java, they often need to be deeply copied, which can be extremely expensive for large data structures. For instance, a single IonValue instance cannot reside in multiple IonContainer instances. If this is desired, one must call IonValue.clone(). Depending on how the size of IonValue instances and how they are manipulated, the cost of calling IonValue.clone() can quickly add up causing severe performance degradation.

IonValue must be cloned before being added to a second container because they...

• ... are mutable. (Mutating a value contained within multiple containers would be very confusing and lead to countless bugs.)
• ... keep a reference to the parent container that they reside in.
• ... may contain symbol ids, which can vary depending on the current Ion symbol table.
• ... know which IonSystem they originated from.

None of the above is true about instances of IonElement. IonElement instances never need to be cloned. IonElement intentionally does not provide any method of performing a deep copy.

This example works perfectly fine with IonElement instances:

val listElement = loadSingleElement("[{ some_potentially_big_container: 42}]").asList();
val newListElement = ionListOf(listElement.values[0]) // <--  no need to call .clone().
println(newListElement)
// Output is [{ some_potentially_big_container: 42}]

But the equivalent with IonValue requires a call to .clone():

val ion = IonSystemBuilder.standard().build()
val ionList = ion.singleValue("[{ some_potentially_big_container: 42}]") as IonList
// Removal of .clone() below results in com.amazon.ion.ContainedValueException.
val newList = ion.newList(ionList[0].clone())
println(newList)
// Output is [{ some_potentially_big_container: 42}]

IonValue Down-Casting

In order to access the value of an IonValue, one must first check to make sure it is safe to down-cast, and then down-cast it to the appropriate type, but ion-element provides the interface AnyElement which conveniently performs the type before returning the down-casted value. Example:

val elem: AnyElement = loadSingleElement("\"What are you doing, Dave?\"")
println("The string was: ${elem.stringValue}")

This is contrast to the equivalent code using IonValue here:

val ion = IonSystemBuilder.standard().build()
val ionValue = ion.singleValue("\"What are you doing, Dave?\"")

// Check type of IonValue first to ensure it's safe to cast to `IonString`
if (ionValue.type == IonType.STRING) {
    val ionStr = ionValue as IonString // <-- Down cast here
    println("The string was: ${ionStr.stringValue()}")
} else {
    error("Expected an Ion string, got an IonType.${ionValue.type}");
}

Anyone may implement the IonElement interface and its sub-interfaces

Even though it is an interface, it is not possible to implement ion-java's IonValue yourself because ion-java explicitly forbids it.

This library places no such constraints on the implementations of IonElement. As long as the behavior is implemented correctly (see the documentation of each interface), the implementations of IonElement provided by this library are fully interoperable with user supplied implementations.

Inheritance Hierarchy of IonElement Sub-Interfaces

The inheritance hierarchy of IonElement and its sub-interfaces closely mirrors that of IonValue except for the addition of AnyElement, which provides value accessors and safe down-casting functions. Also, there is no sub-interface specifically for Ion null values because they require no member functions (such as value getters) in addition to those which are already defined on IonElement. A sub-interface for null values would therefore be redundant.

The IonElement and AnyElement interfaces are described as "widened" while the type specific interfaces are "narrowed". This is in reference to the range of possible data types representable by these interfaces.

• IonElement
  • AnyElement
  • BoolElement
  • IntElement
  • FloatElement
  • DecimalElement
  • TimestampElement
  • TextElement
    • StringElement
    • SymbolElement
  • LobElement
    • BlobElement
    • ClobElement
  • ContainerElement
    • SeqElement
      • ListElement
      • SexpElement
    • StructElement

AnyElement

AnyElement is a special sub-interface of IonElement that can be used by consumers of this library to reduce the overhead of accessing JVM native versions of the values represented by an IonElement. This is accomplished with two categories of accessor functions that perform type checking before returning a down-casted IonElement instance or Kotlin native value. As shown below, AnyElement.as*[OrNull]() functions perform type checking, throw an exception in the event of a type mismatch, and down-cast to the narrowed IonElement sub-interface. AnyElement.*Value[OrNull] properties are similar but return JVM native values directly.

The IonElement.asAnyElement() interface exists to convert to AnyElement. Although the implementations of IonElement included in ion-element-kotlin implement this as a simple downcast (i.e. this as AnyElement), other implementations of IonElement are free to implement this however they see fit.

An example using these functions is shown below:

val stockItemIonText = """
    stock_item::{
        name: "Fantastic Widget",
        price: 12.34,
        countInStock: 2,
        orders: [
            { customerId: 123, state: WA },
            { customerId: 456, state: "HI" }
        ]
    }
"""

val stockItem: StockItem = try {
    val stockElement: StructElement = loadSingleElement(stockItemIonText).asStruct()
    StockItem(
        stockElement["name"].textValue,
        stockElement["price"].decimalValue,
        stockElement["countInStock"].longValue,
        stockElement["orders"].listValues
            .map { order -> order.asStruct() }
            .map { order ->
                Order(
                    order["customerId"].longValue,
                    order["state"].textValue
                )
            }
    )
} catch (ex: IonElementConstraintException) {
    error("Malformed StockItem: ${ex.message}");
}

More details about these functions are included below.

AnyElement's *Value[OrNull] Properties

In order to reduce the need to downcast to the appropriate sub-interface of IonElement, AnyElement provides two read-only properties per type of Ion data: <kotlinType>Value and <kotlinType>ValueOrNull, where <jvmType> is the JVM Kotlin equivalent of the corresponding Ion type. Each of these properties will check whether the Ion type is correct for the property called, and if not, will throw IonElementConstraintException to indicate that the element is of an unexpected type.

Here is an example using longValue and stringValue

val anInteger: AnyElement = loadSingleElement("42")

// Prints 42
println(anInteger.longValue)

// throws IonElementConstraintException
anInteger.stringValue

If the AnyElement instance contains an Ion null value, an IonElementConstraintException is also thrown. These should be used when the application wishes to require elements to be non-null.

val nullElement: AnyElement = loadSingleElement("null")
val typedNullElement: AnyElement = loadSingleElement("null.int")

// Both throw IonElementConstraintException
nullElement.longValue
typedNullElement.longValue

The *Value functions include a null-check so the application doesn't have to, additionally this takes advantage of Kotlin's null safety, since the values returned from these functions are guaranteed to be non-null. For values that might legitimately be null, each of *Value properties has a corresponding *ValueOrNull, which converts Ion null values to a Kotlin null. These should be used when the application wishes to allow elements to be null.

val nullElement: AnyElement = loadSingleElement("null")
println(nullElement.longValueOrNull)
val typedNullElement: AnyElement = loadSingleElement("null.int")
println(typedNullElement.longValueOrNull)

val nullStringElement: AnyElement = loadSingleElement("null.string")
// throws IonElementConstraintException
nullStringElement.longValueOrNull

AnyElement's as*[OrNull]() Functions

Similar to AnyElement's *Value[OrNull] properties AnyElement also provides a number of as*[OrNull]() functions. These serve a similar purpose to the former, performing type checking and optionally a null-check, but the latter casts the AnyElement to a narrowed IonElement sub-interface.

Here is an example using asInt() and asString()

val anInteger: AnyElement = loadSingleElement("42")

// Prints 42
println(anInteger.longValue)

// throws IonElementConstraintException
anInteger.stringValue

As before, if the AnyElement instance contains an Ion null value, an IonElementConstraintException is also thrown. These should be used when the application wishes to require elements to be non-null.

val nullElement: AnyElement = loadSingleElement("null")
val typedNullElement: AnyElement = loadSingleElement("null.int")

// Both throw IonElementConstraintException
nullElement.asInt()
typedNullElement.asInt()

Also as before, each of as*() properties has a corresponding as*OrNull function, which converts AnyElement instance to a narrowed sub-interface of IonElement or null.

val nullElement: AnyElement = loadSingleElement("null")
// prints: null
println(nullElement.asIntOrNull()) 

val typedNullElement: AnyElement = loadSingleElement("null.int")
// prints: null
println(typedNullElement.asIntOrNull())

IonElement API Guidelines

The following guidelines are applied to the API of this package and are good practices to follow for any API accepting or returning IonElement types or its sub-types.

• Any function arguments accepting...
  • ... any type of Ion data should be of type IonElement and not of type AnyElement to avoid requiring the use of IonElement.asAnyElement() at the call-site.
  • ... a specific type of Ion data should use the narrowest possible IonElement sub-interface, for instance:
    • A function argument requiring an Ion s-expression should be of type SexpElement.
    • A function argument allowing either a list or an s-expression should be of type SeqElement, which is the narrowest type that can represent either.
• Any functions that may return...
  • ... any type of Ion data should have the AnyElement return type to avoid forcing the caller to down cast to a narrower sub-interface or AnyElement.
  • ... more than one type of Ion data should have the narrowest possible return type, for instance:
    • A function that will only ever return s-expressions should have the SexpElement return type.
    • A function that may return lists or s-expressions should have the SeqElement return type.
    • A function that may return structs, lists or s-expressions should have the ContainerElement return type.

IonElement Constructors

IonElement instances are created without any IonSystem or other factory object. Instead, IonElement instances are created by a series of top-level Kotlin functions (Ion.* static functions from Java). Not all IonElement constructor functions are shown here. Please see Ion.kt for a complete list.

Some examples are below:

val greeting = ionString("Hello world!")
println(greeting) // "Hello world!"

val ultimateAnswer = ionInt(42)
println(ultimateAnswer) // 42

val listOfInts = ionListOf((1L..10L).map { ionInt(it) })
println(listOfInts) // [1,2,3,4,5,6,7,8,9,10]

val anotherList = ionListOf(
    greeting,
    ultimateAnswer,
    listOfInts,
    annotations = listOf("yolo", "i_love_ion")
)
println(anotherList) // yolo::i_love_ion::["Hello world!",42,[1,2,3,4,5,6,7,8,9,10]]

val person = ionStructOf(
    "firstName" to ionString("Frédéric"),
    "lastName" to ionString("Chopin"),
    "birthdate" to ionTimestamp("1810-02-22")
)
println(person) // {firstName:"Frédéric",lastName:"Chopin",birthdate:1810-02-22})

IonElement and Annotations

Each IonElement constructor has an optional annotations: List<String> which may be used to specify annotations. Annotations may also be added to an existing IonElement with the IonElement.withAnnotations() and annotations removed with the IonElement.withoutAnnotations() extensions functions. Both functions treat the IonElement as a persistent data structure; that is: they return a modified shallow copy of the original.

// Creating a new `IonElement` instance with a list annotations:
val ionInt = ionInt(42, annotations = listOf("meaning_of_life"))

// Prints meaning_of_life::42
println(ionInt)

// Adding values to an existing instance
val ionInt2 = ionInt(42)
val ionInt3 = ionInt2.withAnnotations("meaning", "of", "life")

// Prints meaning::of::life::42
println(ionInt3)

// Removing annotations from an existing instance
val ionInt4 = ionInt3.withoutAnnotations()

// Prints 42
println(ionInt4) 

Loading Ion Data into IonElement

Several ways exist to load Ion data from an com.amazon.ion.IonReader instance or from a String.

loadSingleElement

Accepts either a String or IonReader, and loads the first value found into an AnyElement instance.
Throws an exception if there is more than one top level value, which is useful when a guarantee that there is only one value present is needed.

Signatures:

fun loadSingleElement(ionText: String, options: IonElementLoaderOptions = IonElementLoaderOptions())
fun loadSingleElement(ionReader: IonReader, options: IonElementLoaderOptions = IonElementLoaderOptions())

Example:

val element: AnyElement = loadSingleElement("{ some_field: 42 }")

loadCurrentElement

Loads the entire current top-level Ion value from an IonReader. Unlike loadSingleElement, does not throw an exception if another top-level value remains in the IonReader. Steps in to and out as needed for container values, but does not advance the reader to the next top level value.

Signatures:

fun loadCurrentElement(ionReader: IonReader, options: IonElementLoaderOptions = IonElementLoaderOptions()): AnyElement

Example:

IonReaderBuilder.standard().build("1 2").use { reader ->
    while (reader.next() != null) {
        println(loadCurrentElement(reader))
    }
}

loadAllElements

This is an alternative to loadCurrentElement. Returns an iterator that reads one top-level value at a time.

Signatures:

fun loadAllElements(ionText: String, options: IonElementLoaderOptions = IonElementLoaderOptions()): Iterable<AnyElement>
fun loadAllElements(
    ionReader: IonReader,
    options: IonElementLoaderOptions = IonElementLoaderOptions()
): Iterable<AnyElement>

val allElementsIterator: Iterable<AnyElement> = loadAllElements("1 2")
allElementsIterator.forEach { elem: AnyElement ->
    println(elem)
}

IonElementLoader and Dependency Injection

All the above functions are also present on the IonElementLoader interface, which can easily be mocked for the purposes of isolated unit testing. The primary difference is that the IonElementLoaderOptions instance is passed to the createIonElementLoader function, which creates a real (non-mock) implementation of IonElementLoader.

val loader: IonElementLoader = createIonElementLoader(IonElementLoaderOptions(includeLocationMeta = true));
val element: AnyElement = loader.loadSingleElement("{ some_field: 42 }")

Writing Ion Data

IonElement.toString()

The contract of IonValue.toString() does not guarantee that the method will produce valid Ion text (though in practice it does produce valid Ion text). The contract of IonElement.toString() does guarantee that the function must produce value Ion text. The string representation is produced with a standard IonTextWriter from ion-java.

As a rule of thumb, if performance or space considerations are paramount, this should be avoided. It is generally more performant to write an IonElement directly to an IonWriter, shown below.

IonElement.writeTo(IonWriter)

IonElement.writeTo(IonWriter) exists for writing Ion data to an Ion binary or text stream.

val meaningOfLife = ionInt(42)
val stringBuilder = StringBuilder()

// Construct an `IonWriter` in the usual way:
val ion = IonSystemBuilder.standard().build()
ion.newTextWriter(stringBuilder).use { writer ->
    meaningOfLife.writeTo(writer)
}

// Prints "42"
println(stringBuilder.toString())

IonElement.hashCode and IonElement.equals

IonElement.hashCode and IonElement.equals behave identically to IonValue's--the definition of equivalence is the same.

Metas

Every instance of IonElement contains a metas: HashMap<String, Any> that is useful for storing arbitrary metadata with each node. This metadata does not affect IonElement.equals or IonElement.hashCode, and it is currently up to the application to take care of persisting any metas. Future work: this library should assist with (de)serializing metas.

Loading Data With Location Metas

In certain contexts, for example, when Ion text is used as the syntax of a DSL (domain specific language) or as a configuration file, it is helpful to know the line & column number of the Ion value within a file so that semantic validation errors may be reported to the end user. This is accomplished through the use of the previously described metas.

The inclusion of location metas comes at an additional CPU cost which is non-trivial for large files, so by default is disabled. It is enabled by passing IonElementLoaderOptions(includeLocationMeta = true) along to the corresponding loader function.

val ionText = "{\n  some_field: 42 }"

val structElem: StructElement =
    loadSingleElement(ionText, IonElementLoaderOptions(includeLocationMeta = true)).asStruct()

// Struct is positioned on line 1 at character 1.
assertEquals(IonTextLocation(1, 1), structElem.metas.location)
// some_field begins on line 2 at character 3.
assertEquals(IonTextLocation(2, 3), structElem["some_field"].metas.location)

Note that .location above is an extension property provided for convenience.

Finally, this works for both Ion text and Ion binary files, however for binary files, the meta includes a byte offset instead of a line number and character offset.

Converting between IonElement and IonValue

In order to support better interoperability with existing code that uses IonValue, it is easy to convert between IonElement and IonValue instances. The IonValue.toIonElement() and IonElement.toIonValue(ValueFactory) extension functions are provided for this purpose.

val messageString = "What are you doing, Dave?"
val ion = IonSystemBuilder.standard().build()

// Converting from IonValue to IonElement
val ionValue = ion.newString(messageString)

val ionElement = ionValue.toIonElement().asString()

assertEquals(ionString(messageString), ionElement)

// Converting from IonElement to IonValue
val ionElement2 = ionString(messageString)

val ionValue2 = ionElement.toIonValue(ion)

assertEquals(ion.newString(messageString), ionValue2)

Future of IonElement

Currently, there are two possible areas of future development for IonElement.

• Path copying can be employed to aid in the transformation of nested deeply nested elements. Today, clients must provide their own solutions for transformation of deeply nested IonElement values. Ideally, this library provides such a facility out of the box.
• Lazy loading of IonElement values. An alternative to ion-java's skip-scanning abilities, this would avoid parsing the Ion text or binary values until their values are actually requested, and would provide all the benefits of skip-scanning, while also providing a rich Ion object model. By contrast, today IonElement (and alsoIonValue) eagerly read the entire top-level value and populate an entire deeply nested data structure).

Development

This repository contains the ion-tests repository as a git submodule. The easiest way to clone the ion-element-kotlin repository and initialize its submodule is to run the following command:

git clone --recursive https://github.com/amzn/ion-element-kotlin.git ion-element-kotlin

Alternatively, the submodule may be initialized independently of the clone by running the following commands:

git submodule init
git submodule update

ion-element-kotlin may now be built with the following command:

./gradlew build

Code Style Check

This project uses ktlint to enforce the official Kotlin code conventions. To automatically correct (most) style violations, run:

./gradlew ktlintFormat

Binary Compatibility Check

This project uses Kotlin/binary-compatibility-validator to prevent accidental changes to the binary API of the library. The apiCheck task will be run as part of the Gradle check task, and it will fail if the current build differs in any way from the API that is persisted in api/IonElement.api. If you are intentionally changing the API, update the persisted API definition by running:

./gradlew apiDump && git add api

When preparing a release, you can use the diff of api/IonElement.api to help determine whether a release should be a new major, minor, or patch version. If the file has no changes, it may be a patch release. If the file has only additive changes, it may be a minor version. Any other changes indicate that a new major version is required.

Pulling in Upstream Changes

To pull upstream changes into ion-element-kotlin, start with a simple git pull. This will pull in any changes to ion-element-kotlin itself (including any changes to its .gitmodules file), but not any changes to the submodules. To make sure the submodules are up-to-date, use the following command:

git submodule update --remote

For detailed walkthroughs of git submodule usage, see the Git Tools documentation.

License

This project is licensed under the Apache-2.0 License.