wrongwrong
150 lines
6.7 KiB
Markdown
150 lines
6.7 KiB
Markdown
# kotlinx-metadata-jvm
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This library provides an API to read and modify metadata of binary files generated by the Kotlin/JVM compiler, namely `.class` and `.kotlin_module` files.
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## Usage
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To use this library in your project, add a dependency on `org.jetbrains.kotlinx:kotlinx-metadata-jvm:$kotlinx_metadata_version` (where `kotlinx_metadata_version` is the version of the library).
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Example usage in Maven:
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```xml
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<project>
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<dependencies>
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<dependency>
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<groupId>org.jetbrains.kotlinx</groupId>
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<artifactId>kotlinx-metadata-jvm</artifactId>
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<version>${kotlinx_metadata_version}</version>
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</dependency>
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</dependencies>
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...
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</project>
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```
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Example usage in Gradle:
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```gradle
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repositories {
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mavenCentral()
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}
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dependencies {
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implementation "org.jetbrains.kotlinx:kotlinx-metadata-jvm:$kotlinx_metadata_version"
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}
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```
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## Overview
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The entry point for reading the Kotlin metadata of a `.class` file is [`KotlinClassMetadata.read`](src/kotlinx/metadata/jvm/KotlinClassMetadata.kt). The data it takes is encapsulated in [`KotlinClassHeader`](src/kotlinx/metadata/jvm/KotlinClassHeader.kt) which is basically what is written in the [`kotlin.Metadata`](../../stdlib/jvm/runtime/kotlin/Metadata.kt) annotation on the class file generated by the Kotlin compiler. Construct `KotlinClassHeader` by reading the values from `kotlin.Metadata` reflectively or from some other resource, and then use `KotlinClassMetadata.read` to obtain the correct instance of the class metadata.
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```kotlin
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val header = KotlinClassHeader(
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...
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/* pass Metadata.k, Metadata.d1, Metadata.d2, etc as arguments ... */
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)
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val metadata = KotlinClassMetadata.read(header)
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```
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`KotlinClassMetadata` is a sealed class, with subclasses representing all the different kinds of classes generated by the Kotlin compiler. Unless you're sure that you're reading a class of a specific kind and can do a simple cast, a `when` is a good choice to handle all the possibilities:
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```kotlin
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when (metadata) {
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is KotlinClassMetadata.Class -> ...
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is KotlinClassMetadata.FileFacade -> ...
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is KotlinClassMetadata.SyntheticClass -> ...
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is KotlinClassMetadata.MultiFileClassFacade -> ...
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is KotlinClassMetadata.MultiFileClassPart -> ...
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is KotlinClassMetadata.Unknown -> ...
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}
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```
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Let's assume we've obtained an instance of `KotlinClassMetadata.Class`; other kinds of classes are handled similarly, except some of them have metadata in a slightly different form. The main way to make sense of the underlying metadata is to invoke `toKmClass`, which returns an instance of `KmClass` (`Km` is a shorthand for “Kotlin metadata”):
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```kotlin
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val klass = metadata.toKmClass()
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println(klass.functions.map { it.name })
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println(klass.properties.map { it.name })
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```
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Please refer to [`MetadataSmokeTest.listInlineFunctions`](test/kotlinx/metadata/test/MetadataSmokeTest.kt) for an example where all inline functions are read from the class metadata along with their JVM signatures.
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## Flags
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Numerous objects have a property named `flags` of type `Flags`. These flags represent modifiers or other boolean attributes of a declaration or a type. To check if a certain flag is present, call one of the flags in [`Flag`](../src/kotlinx/metadata/Flag.kt) on the given integer value. The set of applicable flags is documented on each property or the corresponding `visit*` method. For example, for functions, this is common declaration flags (visibility, modality) plus `Flag.Function` flags:
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```kotlin
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val function: KmFunction = ...
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if (Flag.IS_PUBLIC(function.flags)) {
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println("function ${function.name} is public")
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}
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if (Flag.Function.IS_SUSPEND(function.flags)) {
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println("function ${function.name} has the 'suspend' modifier")
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}
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```
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## Writing metadata
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To create metadata of a Kotlin class file from scratch, construct an instance of `KmClass`/`KmPackage`/`KmLambda`, fill it with the data and call `accept` with the `Writer` class declared in the corresponding `KotlinClassMetadata` subclass. Finally, use `KotlinClassMetadata.header` to obtain the raw data and write it to the `kotlin.Metadata` annotation on a class file.
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When using metadata writers from Kotlin source code, it's very convenient to use Kotlin scoping functions such as `apply` to reduce boilerplate:
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```kotlin
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// Writing metadata of a class
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val klass = KmClass().apply {
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// Setting the name and the modifiers of the class.
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// Flags are constructed by invoking "flagsOf(...)"
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name = "MyClass"
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flags = flagsOf(Flag.IS_PUBLIC)
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// Adding one public primary constructor
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constructors += KmConstructor(flagsOf(Flag.IS_PUBLIC, Flag.Constructor.IS_PRIMARY)).apply {
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// Setting the JVM signature (for example, to be used by kotlin-reflect)
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signature = JvmMethodSignature("<init>", "()V")
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}
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...
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}
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// Finally writing everything to arrays of bytes
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val header = KotlinClassMetadata.Class.Writer().apply(klass::accept).write().header
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// Use header.kind, header.data1, header.data2, etc. to write values to kotlin.Metadata
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...
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```
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Please refer to [`MetadataSmokeTest.produceKotlinClassFile`](test/kotlinx/metadata/test/MetadataSmokeTest.kt) for an example where metadata of a simple Kotlin class is created, and then the class file is produced with ASM and loaded by Kotlin reflection.
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## Module metadata
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Similarly to how `KotlinClassMetadata` is used to read/write metadata of Kotlin `.class` files, [`KotlinModuleMetadata`](src/kotlinx/metadata/jvm/KotlinModuleMetadata.kt) is the entry point for reading/writing `.kotlin_module` files. Use `KotlinModuleMetadata.read` or `KotlinModuleMetadata.Writer` in very much the same fashion as with the class files. The only difference is that the source for the reader (and the result of the writer) is a simple byte array, not the structured data loaded from `kotlin.Metadata`:
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```kotlin
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// Read the module metadata
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val bytes = File("META-INF/main.kotlin_module").readBytes()
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val metadata = KotlinModuleMetadata.read(bytes)
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val module = metadata.toKmModule()
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...
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// Write the module metadata
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val bytes = KotlinModuleMetadata.Writer().apply(module::accept).write().bytes
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File("META-INF/main.kotlin_module").writeBytes(bytes)
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```
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## Laziness
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Note that until you load the actual underlying data of a `KotlinClassMetadata` or `KotlinModuleMetadata` instance by invoking `accept` or one of the `toKm...` methods, the data is not completely parsed and verified. If you need to check if the data is not horribly corrupted before proceeding, ensure that either of those is called:
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```kotlin
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val metadata: KotlinClassMetadata.Class = ...
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try {
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// Guarantees eager parsing of the underlying data
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metadata.toKmClass()
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} catch (e: Exception) {
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System.err.println("Metadata is corrupted!")
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}
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```
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