In Kotlin, accessing an abstract val in an init block causes a NullPointerException since the field is overridden by an extending class after the super class's init block executes.
The ideal solution would be a way to declare some code/function to execute after all stages of object instantiation are complete. I can only think of creating an initialize() function and manually calling it, which is bad because it's not automatic. Sticking it in init block doesn't work as shown in the below example.
As a comment pointed out below, instead of overriding fields, they can be passed in as parameters, but that doesn't work for my actual use-case. It adds a lot of clutter for object construction and is a nightmare when other classes try to extend it.
Below example shows a solution using coroutines. Waiting for a field to != null works in this case, but doesn't not when map is an open val with a default value that may or may not get overridden.
The problem is somewhat solved, but the solution is far from optimal. Any suggestions and alternative solutions would be greatly appreciated.
#Test #Suppress("ControlFlowWithEmptyBody", "SENSELESS_COMPARISON")
fun abstractValAccessInInitNPE() {
val key = "Key"
val value = "Value"
abstract class Mapper {
abstract val map: HashMap<String, String>
fun initialize() { map[key] = value }
}
// Test coroutine solution on abstract mapper
println("CoroutineMapper")
abstract class CoroutineMapper: Mapper() {
init {
GlobalScope.launch {
while (map == null) {}
initialize()
}
}
}
val coroutineMapper = object : CoroutineMapper() {
override val map = HashMap<String, String>()
}
val start = System.nanoTime()
while (coroutineMapper.map.isEmpty()) {} // For some reason map == null doesn't work
println("Overhead: ${(System.nanoTime() - start) / 1000000.0} MS")
println("Mapped: ${coroutineMapper.map[key].equals(value)}")
// Test coroutine solution on open mapper
println("\nDefaultMapper")
open class DefaultMapper: Mapper() {
override val map = HashMap<String, String>()
}
val newMap = HashMap<String, String>()
val proof = "Proof"
newMap[proof] = proof
val defaultMapper = object: DefaultMapper() {
override val map = newMap
}
Thread.sleep(1000) // Definitely finished by the end of this
println("Mapped: ${defaultMapper.map[proof].equals(proof) && defaultMapper.map[key].equals(value)}")
// Basic solution (doesn't work)
println("\nBrokenMapper")
abstract class BrokenMapper: Mapper() {
init { initialize() } // Throws NPE because map gets overridden after this
}
val brokenMapper = object: BrokenMapper() {
override val map = HashMap<String, String>()
}
println("Mapped: ${brokenMapper.map[key].equals(value)}")
}
An open (as all abstract functions are) function should never be called from a constructor because then the class's initial state cannot be guaranteed in the superclass. It can lead to all kinds of very tricky bugs.
Usually there's a good way to design around this problem if you take a step back. For instance, instead of making the map an abstract property, make it a constructor parameter in the superclass. Then you know it's already initialized before subclass constructors can try to use it.
abstract class Mapper(key: String, value: String, val map: HashMap<String, String>)
abstract class DecentMapper(key: String, value: String, map: HashMap<String, String>) : Mapper(key, value, map) {
init {
map[key] = value
}
}
val key = "Key"
val value = "Value"
val decentMapper = object : DecentMapper(key, value, HashMap()){
//...
}
how can I set properties of a dataclass by its name. For example, I have a raw HTTP GET response
propA=valueA
propB=valueB
and a data class in Kotlin
data class Test(var propA: String = "", var propB: String = ""){}
in my code i have an function that splits the response to a key value array
val test: Test = Test()
rawResp?.split('\n')?.forEach { item: String ->
run {
val keyValue = item.split('=')
TODO
}
}
In JavaScript I can do the following
response.split('\n').forEach(item => {
let keyValue = item.split('=');
this.test[keyValue[0]] = keyValue[1];
});
Is there a similar way in Kotlin?
You cannot readily do this in Kotlin the same way you would in JavaScript (unless you are prepared to handle reflection yourself), but there is a possibility of using a Kotlin feature called Delegated Properties (particularly, a use case Storing Properties in a Map of that feature).
Here is an example specific to code in your original question:
class Test(private val map: Map<String, String>) {
val propA: String by map
val propB: String by map
override fun toString() = "${javaClass.simpleName}(propA=$propA,propB=$propB)"
}
fun main() {
val rawResp: String? = """
propA=valueA
propB=valueB
""".trimIndent()
val props = rawResp?.split('\n')?.map { item ->
val (key, value) = item.split('=')
key to value
}?.toMap() ?: emptyMap()
val test = Test(props)
println("Property 'propA' of test is: ${test.propA}")
println("Or using toString: $test")
}
This outputs:
Property 'propA' of test is: valueA
Or using toString: Test(propA=valueA,propB=valueB)
Unfortunately, you cannot use data classes with property delegation the way you would expect, so you have to 'pay the price' and define the overridden methods (toString, equals, hashCode) on your own if you need them.
By the question, it was not clear for me if each line represents a Test instance or not. So
If not.
fun parse(rawResp: String): Test = rawResp.split("\n").flatMap { it.split("=") }.let { Test(it[0], it[1]) }
If yes.
fun parse(rawResp: String): List<Test> = rawResp.split("\n").map { it.split("=") }.map { Test(it[0], it[1]) }
For null safe alternative you can use nullableString.orEmpty()...
I am trying to implement a QueryBus. Basically, I want to register a list of QueryHandlers. Each QueryHandler implements a handle method defined by an interface. Each QueryHandler is associated to a Query. I want to be able to retrieve a QueryHandler using the Query and call handle on it.
The thing is the handle has to be generic because each QueryHandler handles a Query differently. They all take a dedicated Query and may return whatever they want.
interface Query<R>
interface QueryHandler<R, Q : Query<R>> {
fun handle(query: Q): R
fun listenTo(): String
}
// DTOs
data class BookDto(val name: String)
// List books query
data class ListBooksQuery(val page: Int = 1): Query<List<BookDto>>
class ListBooksQueryHandler: QueryHandler<List<BookDto>, ListBooksQuery> {
override fun handle(query: ListBooksQuery): List<BookDto> {
return listOf(BookDto("Dune"), BookDto("Dune II"))
}
override fun listenTo(): String = ListBooksQuery::class.toString()
}
// Get book query
data class GetBookQuery(val name: String): Query<BookDto?>
class GetBookQueryHandler: QueryHandler<BookDto?, GetBookQuery> {
override fun handle(query: GetBookQuery): BookDto {
return BookDto("Dune")
}
override fun listenTo(): String = GetBookQuery::class.toString()
}
// Run it!
fun main(args: Array<String>) {
// Initializing query bus
val queryHandlers = mapOf(
with(ListBooksQueryHandler()) {this.listenTo() to this},
with(GetBookQueryHandler()) {this.listenTo() to this}
)
val command = ListBooksQuery()
val result = queryHandlers[command::class.toString()].handle(command)
// Should print the list of BookDto
print(result)
}
I don't even know if its possible, to be honest.
UPDATE 1:
I changed the usage example in the main to show what I am really trying to do. The List was for (bad?) demonstration purpose. I want to store the QueryHandlers and retrieve them from a map.
Additional resources:
Here is what I really want to do:
https://gist.github.com/ValentinTrinque/76b7a32221884a46e657090b9ee60193
UPDATE I've read your gist and tried to come up with a solution that will provide a clean interface to the user of the QueryBusMiddleware.
Note that I used objects instead of classes for the QueryHandler implementations, which felt more natural to me (since there is only one possible entry in the map for each Query implementation).
interface Query<R>
interface QueryHandler<R, Q: Query<R>> {
fun handle(query: Q): R
fun listenTo(): String
}
// DTOs
data class BookDto(val name: String)
// List books query
data class ListBooksQuery(val page: Int = 1): Query<List<BookDto>>
object ListBooksQueryHandler: QueryHandler<List<BookDto>, ListBooksQuery> {
override fun handle(query: ListBooksQuery): List<BookDto> {
return listOf(BookDto("Dune"), BookDto("Dune II"))
}
override fun listenTo(): String = ListBooksQuery::class.toString()
}
// Get book query
data class GetBookQuery(val name: String): Query<BookDto?>
object GetBookQueryHandler: QueryHandler<BookDto?, GetBookQuery> {
override fun handle(query: GetBookQuery): BookDto {
return BookDto("Dune")
}
override fun listenTo(): String = GetBookQuery::class.toString()
}
// Run it!
fun main(args: Array<String>) {
// Initializing query bus
val queryHandlers = listOf(
ListBooksQueryHandler,
GetBookQueryHandler
)
val dispatcher: QueryBusMiddleware = QueryDispatcherMiddleware(queryHandlers)
// Calling query bus
val query = ListBooksQuery()
// Result should be List<BookDto>
val result = dispatcher.dispatch(query)
print(result)
}
interface QueryBusMiddleware {
fun <R, Q : Query<R>> dispatch(query: Q): R
}
class QueryDispatcherMiddleware constructor(handlers: List<QueryHandler<*, *>>) : QueryBusMiddleware {
private val handlers = HashMap<String, QueryHandler<*, *>>()
init {
handlers.forEach { handler -> this.handlers[handler.listenTo()] = handler }
}
override fun <R, Q : Query<R>> dispatch(query: Q): R {
val queryClass = query::class.toString()
val handler = handlers[queryClass] ?: throw Exception("No handler listen to the query: $queryClass")
return handler::class.members.find { it.name == "handle" }!!.call(handler, query) as R
}
}
Kotlin doesn't have the same notion of static fields as used in Java. In Java, the generally accepted way of doing logging is:
public class Foo {
private static final Logger LOG = LoggerFactory.getLogger(Foo.class);
}
Question is what is the idiomatic way of performing logging in Kotlin?
In the majority of mature Kotlin code, you will find one of these patterns below. The approach using Property Delegates takes advantage of the power of Kotlin to produce the smallest code.
Note: the code here is for java.util.Logging but the same theory applies to any logging library
Static-like (common, equivalent of your Java code in the question)
If you cannot trust in the performance of that hash lookup inside the logging system, you can get similar behavior to your Java code by using a companion object which can hold an instance and feel like a static to you.
class MyClass {
companion object {
val LOG = Logger.getLogger(MyClass::class.java.name)
}
fun foo() {
LOG.warning("Hello from MyClass")
}
}
creating output:
Dec 26, 2015 11:28:32 AM org.stackoverflow.kotlin.test.MyClass foo
INFO: Hello from MyClass
More on companion objects here: Companion Objects ... Also note that in the sample above MyClass::class.java gets the instance of type Class<MyClass> for the logger, whereas this.javaClass would get the instance of type Class<MyClass.Companion>.
Per Instance of a Class (common)
But, there is really no reason to avoid calling and getting a logger at the instance level. The idiomatic Java way you mentioned is outdated and based on fear of performance, whereas the logger per class is already cached by almost any reasonable logging system on the planet. Just create a member to hold the logger object.
class MyClass {
val LOG = Logger.getLogger(this.javaClass.name)
fun foo() {
LOG.warning("Hello from MyClass")
}
}
creating output:
Dec 26, 2015 11:28:44 AM org.stackoverflow.kotlin.test.MyClass foo
INFO: Hello from MyClass
You can performance test both per instance and per class variations and see if there is a realistic difference for most apps.
Property Delegates (common, most elegant)
Another approach, which is suggested by #Jire in another answer, is to create a property delegate, which you can then use to do the logic uniformly in any other class that you want. There is a simpler way to do this since Kotlin provides a Lazy delegate already, we can just wrap it in a function. One trick here is that if we want to know the type of the class currently using the delegate, we make it an extension function on any class:
fun <R : Any> R.logger(): Lazy<Logger> {
return lazy { Logger.getLogger(unwrapCompanionClass(this.javaClass).name) }
}
// see code for unwrapCompanionClass() below in "Putting it all Together section"
This code also makes sure that if you use it in a Companion Object that the logger name will be the same as if you used it on the class itself. Now you can simply:
class Something {
val LOG by logger()
fun foo() {
LOG.info("Hello from Something")
}
}
for per class instance, or if you want it to be more static with one instance per class:
class SomethingElse {
companion object {
val LOG by logger()
}
fun foo() {
LOG.info("Hello from SomethingElse")
}
}
And your output from calling foo() on both of these classes would be:
Dec 26, 2015 11:30:55 AM org.stackoverflow.kotlin.test.Something foo
INFO: Hello from Something
Dec 26, 2015 11:30:55 AM org.stackoverflow.kotlin.test.SomethingElse foo
INFO: Hello from SomethingElse
Extension Functions (uncommon in this case because of "pollution" of Any namespace)
Kotlin has a few hidden tricks that let you make some of this code even smaller. You can create extension functions on classes and therefore give them additional functionality. One suggestion in the comments above was to extend Any with a logger function. This can create noise anytime someone uses code-completion in their IDE in any class. But there is a secret benefit to extending Any or some other marker interface: you can imply that you are extending your own class and therefore detect the class you are within. Huh? To be less confusing, here is the code:
// extend any class with the ability to get a logger
fun <T: Any> T.logger(): Logger {
return Logger.getLogger(unwrapCompanionClass(this.javaClass).name)
}
Now within a class (or companion object), I can simply call this extension on my own class:
class SomethingDifferent {
val LOG = logger()
fun foo() {
LOG.info("Hello from SomethingDifferent")
}
}
Producing output:
Dec 26, 2015 11:29:12 AM org.stackoverflow.kotlin.test.SomethingDifferent foo
INFO: Hello from SomethingDifferent
Basically, the code is seen as a call to extension Something.logger(). The problem is that the following could also be true creating "pollution" on other classes:
val LOG1 = "".logger()
val LOG2 = Date().logger()
val LOG3 = 123.logger()
Extension Functions on Marker Interface (not sure how common, but common model for "traits")
To make the use of extensions cleaner and reduce "pollution", you could use a marker interface to extend:
interface Loggable {}
fun Loggable.logger(): Logger {
return Logger.getLogger(unwrapCompanionClass(this.javaClass).name)
}
Or even make the method part of the interface with a default implementation:
interface Loggable {
public fun logger(): Logger {
return Logger.getLogger(unwrapCompanionClass(this.javaClass).name)
}
}
And use either of these variations in your class:
class MarkedClass: Loggable {
val LOG = logger()
}
Producing output:
Dec 26, 2015 11:41:01 AM org.stackoverflow.kotlin.test.MarkedClass foo
INFO: Hello from MarkedClass
If you wanted to force the creation of a uniform field to hold the logger, then while using this interface you could easily require the implementer to have a field such as LOG:
interface Loggable {
val LOG: Logger // abstract required field
public fun logger(): Logger {
return Logger.getLogger(unwrapCompanionClass(this.javaClass).name)
}
}
Now the implementer of the interface must look like this:
class MarkedClass: Loggable {
override val LOG: Logger = logger()
}
Of course, an abstract base class can do the same, having the option of both the interface and an abstract class implementing that interface allows flexibility and uniformity:
abstract class WithLogging: Loggable {
override val LOG: Logger = logger()
}
// using the logging from the base class
class MyClass1: WithLogging() {
// ... already has logging!
}
// providing own logging compatible with marker interface
class MyClass2: ImportantBaseClass(), Loggable {
// ... has logging that we can understand, but doesn't change my hierarchy
override val LOG: Logger = logger()
}
// providing logging from the base class via a companion object so our class hierarchy is not affected
class MyClass3: ImportantBaseClass() {
companion object : WithLogging() {
// we have the LOG property now!
}
}
Putting it All Together (A small helper library)
Here is a small helper library to make any of the options above easy to use. It is common in Kotlin to extend API's to make them more to your liking. Either in extension or top-level functions. Here is a mix to give you options for how to create loggers, and a sample showing all variations:
// Return logger for Java class, if companion object fix the name
fun <T: Any> logger(forClass: Class<T>): Logger {
return Logger.getLogger(unwrapCompanionClass(forClass).name)
}
// unwrap companion class to enclosing class given a Java Class
fun <T : Any> unwrapCompanionClass(ofClass: Class<T>): Class<*> {
return ofClass.enclosingClass?.takeIf {
ofClass.enclosingClass.kotlin.companionObject?.java == ofClass
} ?: ofClass
}
// unwrap companion class to enclosing class given a Kotlin Class
fun <T: Any> unwrapCompanionClass(ofClass: KClass<T>): KClass<*> {
return unwrapCompanionClass(ofClass.java).kotlin
}
// Return logger for Kotlin class
fun <T: Any> logger(forClass: KClass<T>): Logger {
return logger(forClass.java)
}
// return logger from extended class (or the enclosing class)
fun <T: Any> T.logger(): Logger {
return logger(this.javaClass)
}
// return a lazy logger property delegate for enclosing class
fun <R : Any> R.lazyLogger(): Lazy<Logger> {
return lazy { logger(this.javaClass) }
}
// return a logger property delegate for enclosing class
fun <R : Any> R.injectLogger(): Lazy<Logger> {
return lazyOf(logger(this.javaClass))
}
// marker interface and related extension (remove extension for Any.logger() in favour of this)
interface Loggable {}
fun Loggable.logger(): Logger = logger(this.javaClass)
// abstract base class to provide logging, intended for companion objects more than classes but works for either
abstract class WithLogging: Loggable {
val LOG = logger()
}
Pick whichever of those you want to keep, and here are all of the options in use:
class MixedBagOfTricks {
companion object {
val LOG1 by lazyLogger() // lazy delegate, 1 instance per class
val LOG2 by injectLogger() // immediate, 1 instance per class
val LOG3 = logger() // immediate, 1 instance per class
val LOG4 = logger(this.javaClass) // immediate, 1 instance per class
}
val LOG5 by lazyLogger() // lazy delegate, 1 per instance of class
val LOG6 by injectLogger() // immediate, 1 per instance of class
val LOG7 = logger() // immediate, 1 per instance of class
val LOG8 = logger(this.javaClass) // immediate, 1 instance per class
}
val LOG9 = logger(MixedBagOfTricks::class) // top level variable in package
// or alternative for marker interface in class
class MixedBagOfTricks : Loggable {
val LOG10 = logger()
}
// or alternative for marker interface in companion object of class
class MixedBagOfTricks {
companion object : Loggable {
val LOG11 = logger()
}
}
// or alternative for abstract base class for companion object of class
class MixedBagOfTricks {
companion object: WithLogging() {} // instance 12
fun foo() {
LOG.info("Hello from MixedBagOfTricks")
}
}
// or alternative for abstract base class for our actual class
class MixedBagOfTricks : WithLogging() { // instance 13
fun foo() {
LOG.info("Hello from MixedBagOfTricks")
}
}
All 13 instances of the loggers created in this sample will produce the same logger name, and output:
Dec 26, 2015 11:39:00 AM org.stackoverflow.kotlin.test.MixedBagOfTricks foo
INFO: Hello from MixedBagOfTricks
Note: The unwrapCompanionClass() method ensures that we do not generate a logger named after the companion object but rather the enclosing class. This is the current recommended way to find the class containing the companion object. Stripping "$Companion" from the name using removeSuffix() does not work since companion objects can be given custom names.
Have a look at the kotlin-logging library.
It allows logging like that:
private val logger = KotlinLogging.logger {}
class Foo {
logger.info{"wohoooo $wohoooo"}
}
Or like that:
class FooWithLogging {
companion object: KLogging()
fun bar() {
logger.info{"wohoooo $wohoooo"}
}
}
I also wrote a blog post comparing it to AnkoLogger: Logging in Kotlin & Android: AnkoLogger vs kotlin-logging
Disclaimer: I am the maintainer of that library.
Edit: kotlin-logging now has multiplatform support: https://github.com/MicroUtils/kotlin-logging/wiki/Multiplatform-support
KISS: For Java Teams Migrating to Kotlin
If you don't mind providing the class name on each instantiation of the logger (just like java), you can keep it simple by defining this as a top-level function somewhere in your project:
import org.slf4j.LoggerFactory
inline fun <reified T:Any> logger() = LoggerFactory.getLogger(T::class.java)
This uses a Kotlin reified type parameter.
Now, you can use this as follows:
class SomeClass {
// or within a companion object for one-instance-per-class
val log = logger<SomeClass>()
...
}
This approach is super-simple and close to the java equivalent, but just adds some syntactical sugar.
Next Step: Extensions or Delegates
I personally prefer going one step further and using the extensions or delegates approach. This is nicely summarized in #JaysonMinard's answer, but here is the TL;DR for the "Delegate" approach with the log4j2 API (UPDATE: no need to write this code manually any more, as it has been released as an official module of the log4j2 project, see below). Since log4j2, unlike slf4j, supports logging with Supplier's, I've also added a delegate to make using these methods simpler.
import org.apache.logging.log4j.LogManager
import org.apache.logging.log4j.Logger
import org.apache.logging.log4j.util.Supplier
import kotlin.reflect.companionObject
/**
* An adapter to allow cleaner syntax when calling a logger with a Kotlin lambda. Otherwise calling the
* method with a lambda logs the lambda itself, and not its evaluation. We specify the Lambda SAM type as a log4j2 `Supplier`
* to avoid this. Since we are using the log4j2 api here, this does not evaluate the lambda if the level
* is not enabled.
*/
class FunctionalLogger(val log: Logger): Logger by log {
inline fun debug(crossinline supplier: () -> String) {
log.debug(Supplier { supplier.invoke() })
}
inline fun debug(t: Throwable, crossinline supplier: () -> String) {
log.debug(Supplier { supplier.invoke() }, t)
}
inline fun info(crossinline supplier: () -> String) {
log.info(Supplier { supplier.invoke() })
}
inline fun info(t: Throwable, crossinline supplier: () -> String) {
log.info(Supplier { supplier.invoke() }, t)
}
inline fun warn(crossinline supplier: () -> String) {
log.warn(Supplier { supplier.invoke() })
}
inline fun warn(t: Throwable, crossinline supplier: () -> String) {
log.warn(Supplier { supplier.invoke() }, t)
}
inline fun error(crossinline supplier: () -> String) {
log.error(Supplier { supplier.invoke() })
}
inline fun error(t: Throwable, crossinline supplier: () -> String) {
log.error(Supplier { supplier.invoke() }, t)
}
}
/**
* A delegate-based lazy logger instantiation. Use: `val log by logger()`.
*/
#Suppress("unused")
inline fun <reified T : Any> T.logger(): Lazy<FunctionalLogger> =
lazy { FunctionalLogger(LogManager.getLogger(unwrapCompanionClass(T::class.java))) }
// unwrap companion class to enclosing class given a Java Class
fun <T : Any> unwrapCompanionClass(ofClass: Class<T>): Class<*> {
return if (ofClass.enclosingClass != null && ofClass.enclosingClass.kotlin.companionObject?.java == ofClass) {
ofClass.enclosingClass
} else {
ofClass
}
}
Log4j2 Kotlin Logging API
Most of the previous section has been directly adapted to produce the Kotlin Logging API module, which is now an official part of Log4j2 (disclaimer: I am the primary author). You can download this directly from Apache, or via Maven Central.
Usage is basically as describe above, but the module supports both interface-based logger access, a logger extension function on Any for use where this is defined, and a named logger function for use where no this is defined (such as top-level functions).
As a good example of logging implementation I'd like to mention Anko which uses a special interface AnkoLogger which a class that needs logging should implement. Inside the interface there's code that generates a logging tag for the class. Logging is then done via extension functions which can be called inside the interace implementation without prefixes or even logger instance creation.
I don't think this is idiomatic, but it seems a good approach as it requires minimum code, just adding the interface to a class declaration, and you get logging with different tags for different classes.
The code below is basically AnkoLogger, simplified and rewritten for Android-agnostic usage.
First, there's an interface which behaves like a marker interface:
interface MyLogger {
val tag: String get() = javaClass.simpleName
}
It lets its implementation use the extensions functions for MyLogger inside their code just calling them on this. And it also contains logging tag.
Next, there is a general entry point for different logging methods:
private inline fun log(logger: MyLogger,
message: Any?,
throwable: Throwable?,
level: Int,
handler: (String, String) -> Unit,
throwableHandler: (String, String, Throwable) -> Unit
) {
val tag = logger.tag
if (isLoggingEnabled(tag, level)) {
val messageString = message?.toString() ?: "null"
if (throwable != null)
throwableHandler(tag, messageString, throwable)
else
handler(tag, messageString)
}
}
It will be called by logging methods. It gets a tag from MyLogger implementation, checks logging settings and then calls one of two handlers, the one with Throwable argument and the one without.
Then you can define as many logging methods as you like, in this way:
fun MyLogger.info(message: Any?, throwable: Throwable? = null) =
log(this, message, throwable, LoggingLevels.INFO,
{ tag, message -> println("INFO: $tag # $message") },
{ tag, message, thr ->
println("INFO: $tag # $message # $throwable");
thr.printStackTrace()
})
These are defined once for both logging just a message and logging a Throwable as well, this is done with optional throwable parameter.
The functions that are passed as handler and throwableHandler can be different for different logging methods, for example, they can write the log to file or upload it somewhere. isLoggingEnabled and LoggingLevels are omitted for brevity, but using them provides even more flexibility.
It allows for the following usage:
class MyClass : MyLogger {
fun myFun() {
info("Info message")
}
}
There is a small drawback: a logger object will be needed for logging in package-level functions:
private object MyPackageLog : MyLogger
fun myFun() {
MyPackageLog.info("Info message")
}
Would something like this work for you?
class LoggerDelegate {
private var logger: Logger? = null
operator fun getValue(thisRef: Any?, property: KProperty<*>): Logger {
if (logger == null) logger = Logger.getLogger(thisRef!!.javaClass.name)
return logger!!
}
}
fun logger() = LoggerDelegate()
class Foo { // (by the way, everything in Kotlin is public by default)
companion object { val logger by logger() }
}
Anko
You can use Anko library to do it. You would have code like below:
class MyActivity : Activity(), AnkoLogger {
private fun someMethod() {
info("This is my first app and it's awesome")
debug(1234)
warn("Warning")
}
}
kotlin-logging
kotlin-logging(Github project - kotlin-logging ) library allows you to write logging code like below:
class FooWithLogging {
companion object: KLogging()
fun bar() {
logger.info{"Item $item"}
}
}
StaticLog
or you can also use this small written in Kotlin library called StaticLog then your code would looks like:
Log.info("This is an info message")
Log.debug("This is a debug message")
Log.warn("This is a warning message","WithACustomTag")
Log.error("This is an error message with an additional Exception for output", "AndACustomTag", exception )
Log.logLevel = LogLevel.WARN
Log.info("This message will not be shown")\
The second solution might better if you would like to define an output format for logging method like:
Log.newFormat {
line(date("yyyy-MM-dd HH:mm:ss"), space, level, text("/"), tag, space(2), message, space(2), occurrence)
}
or use filters, for example:
Log.filterTag = "filterTag"
Log.info("This log will be filtered out", "otherTag")
Log.info("This log has the right tag", "filterTag")
timberkt
If you'd already used Jake Wharton's Timber logging library check timberkt.
This library builds on Timber with an API that's easier to use from Kotlin. Instead of using formatting parameters, you pass a lambda that is only evaluated if the message is logged.
Code example:
// Standard timber
Timber.d("%d %s", intVar + 3, stringFun())
// Kotlin extensions
Timber.d { "${intVar + 3} ${stringFun()}" }
// or
d { "${intVar + 3} ${stringFun()}" }
Check also: Logging in Kotlin & Android: AnkoLogger vs kotlin-logging
Hope it will help
That's what companion objects are for, in general: replacing static stuff.
What about an extension function on Class instead? That way you end up with:
public fun KClass.logger(): Logger = LoggerFactory.getLogger(this.java)
class SomeClass {
val LOG = SomeClass::class.logger()
}
Note - I've not tested this at all, so it might not be quite right.
First, you can add extension functions for logger creation.
inline fun <reified T : Any> getLogger() = LoggerFactory.getLogger(T::class.java)
fun <T : Any> T.getLogger() = LoggerFactory.getLogger(javaClass)
Then you will be able to create a logger using the following code.
private val logger1 = getLogger<SomeClass>()
private val logger2 = getLogger()
Second, you can define an interface that provides a logger and its mixin implementation.
interface LoggerAware {
val logger: Logger
}
class LoggerAwareMixin(containerClass: Class<*>) : LoggerAware {
override val logger: Logger = LoggerFactory.getLogger(containerClass)
}
inline fun <reified T : Any> loggerAware() = LoggerAwareMixin(T::class.java)
This interface can be used in the following way.
class SomeClass : LoggerAware by loggerAware<SomeClass>() {
// Now you can use a logger here.
}
create companion object and mark the appropriate fields with #JvmStatic annotation
There are many great answers here already, but all of them concern adding a logger to a class, but how would you do that to do logging in Top Level Functions?
This approach is generic and simple enough to work well in both classes, companion objects and Top Level Functions:
package nieldw.test
import org.apache.logging.log4j.LogManager
import org.apache.logging.log4j.Logger
import org.junit.jupiter.api.Test
fun logger(lambda: () -> Unit): Lazy<Logger> = lazy { LogManager.getLogger(getClassName(lambda.javaClass)) }
private fun <T : Any> getClassName(clazz: Class<T>): String = clazz.name.replace(Regex("""\$.*$"""), "")
val topLog by logger { }
class TopLevelLoggingTest {
val classLog by logger { }
#Test
fun `What is the javaClass?`() {
topLog.info("THIS IS IT")
classLog.info("THIS IS IT")
}
}
I have heard of no idiom in this regard.
The simpler the better, so I would use a top-level property
val logger = Logger.getLogger("package_name")
This practice serves well in Python, and as different as Kotlin and Python might appear, I believe they are quite similar in their "spirit" (speaking of idioms).
Slf4j example, same for others. This even works for creating package level logger
/**
* Get logger by current class name.
*/
fun getLogger(c: () -> Unit): Logger =
LoggerFactory.getLogger(c.javaClass.enclosingClass)
Usage:
val logger = getLogger { }
fun <R : Any> R.logger(): Lazy<Logger> = lazy {
LoggerFactory.getLogger((if (javaClass.kotlin.isCompanion) javaClass.enclosingClass else javaClass).name)
}
class Foo {
val logger by logger()
}
class Foo {
companion object {
val logger by logger()
}
}
This is still WIP (almost finished) so I'd like to share it:
https://github.com/leandronunes85/log-format-enforcer#kotlin-soon-to-come-in-version-14
The main goal of this library is to enforce a certain log style across a project. By having it generate Kotlin code I'm trying to address some of the issues mentioned in this question. With regards to the original question what I usually tend to do is to simply:
private val LOG = LogFormatEnforcer.loggerFor<Foo>()
class Foo {
}
You can simply build your own "library" of utilities. You don't need a large library for this task which will make your project heavier and complex.
For instance, you can use Kotlin Reflection to get the name, type and value of any class property.
First of all, make sure you have the meta-dependency settled in your build.gradle:
dependencies {
implementation "org.jetbrains.kotlin:kotlin-reflect:$kotlin_version"
}
Afterwards, you can simply copy and paste this code into your project:
import kotlin.reflect.full.declaredMemberProperties
class LogUtil {
companion object {
/**
* Receives an [instance] of a class.
* #return the name and value of any member property.
*/
fun classToString(instance: Any): String {
val sb = StringBuilder()
val clazz = instance.javaClass.kotlin
clazz.declaredMemberProperties.forEach {
sb.append("${it.name}: (${it.returnType}) ${it.get(instance)}, ")
}
return marshalObj(sb)
}
private fun marshalObj(sb: StringBuilder): String {
sb.insert(0, "{ ")
sb.setLength(sb.length - 2)
sb.append(" }")
return sb.toString()
}
}
}
Example of usage:
data class Actor(val id: Int, val name: String) {
override fun toString(): String {
return classToString(this)
}
}
For Kotlin Multiplaform logging I could not find a library that had all the features I needed so I ended up writing one. Please check out KmLogging. The features it implements is:
Uses platform specific logging on each platform: Log on Android, os_log on iOS, and console on JavaScript.
High performance. Only 1 boolean check when disabled. I like to put in lots of logging and want all of it turned off when release and do not want to pay much overhead for having lots of logging. Also, when logging is on it needs to be really performant.
Extensible. Need to be able add other loggers such as logging to Crashlytics, etc.
Each logger can log at a different level. For example, you may only want info and above going to Crashlytics and all other loggers disabled in production.
To use:
val log = logging()
log.i { "some message" }
I have a framework written in Java that, using reflection, get the fields on an annotation and make some decisions based on them. At some point I am also able to create an ad-hoc instance of the annotation and set the fields myself. This part looks something like this:
public #interface ThirdPartyAnnotation{
String foo();
}
class MyApp{
ThirdPartyAnnotation getInstanceOfAnnotation(final String foo)
{
ThirdPartyAnnotation annotation = new ThirdPartyAnnotation()
{
#Override
public String foo()
{
return foo;
}
};
return annotation;
}
}
Now I am trying to do the exact thing in Kotlin. Bear in mind that the annotation is in a third party jar.
Anyway, here is how I tried it in Kotlin:
class MyApp{
fun getAnnotationInstance(fooString:String):ThirdPartyAnnotation{
return ThirdPartyAnnotation(){
override fun foo=fooString
}
}
But the compiler complains about: Annotation class cannot be instantiated
So the question is: how should I do this in Kotlin?
You can do this with Kotlin reflection:
val annotation = ThirdPartyAnnotation::class.constructors.first().call("fooValue")
In the case of annotation having no-arg constructor (e.g. each annotation field has a default value), you can use following approach:
annotation class SomeAnnotation(
val someField: Boolean = false,
)
val annotation = SomeAnnotation::class.createInstance()
This is the solution I might have found but feels like a hack to me and I would prefer to be able to solve it within the language.
Anyway, for what is worth,it goes like this:
class MyApp {
fun getInstanceOfAnnotation(foo: String): ThirdPartyAnnotation {
val annotationListener = object : InvocationHandler {
override fun invoke(proxy: Any?, method: Method?, args: Array<out Any>?): Any? {
return when (method?.name) {
"foo" -> foo
else -> FindBy::class.java
}
}
}
return Proxy.newProxyInstance(ThirdPartyAnnotation::class.java.classLoader, arrayOf(ThirdPartyAnnotation::class.java), annotationListener) as ThirdPartyAnnotation
}
}