A Logging API package for Kotlin. Version 1.4.0 is the first release version and is a port of SwiftLog version 1.4.0.
The primary goal of this project is to promote code portability between Swift and Kotlin. It is useful for organizations want to implement native mobile apps but have not found or cannot adopt a cross-platform mobile framework. By using common libraries with exact API, business logics written in Swift or Kotlin can be easily ported to the other platform.
PromiseKt is another example of this effort.
Similar to SwiftLog, KotlinLog is an API Package which tries to establish a common API the ecosystem can use. To make logging really work for real-world workloads, we need KotlinLog-compatible logging backends which then either persist the log messages in files, write to Logcat, or send them over to cloud services.
If you have a JVM application written in Kotlin, or an Android app, and would like to log, using this API package can get you started quickly.
repositories {
jcenter()
maven { url "https://jitpack.io" }
}
dependencies {
implementation "com.github.inmotionsoftware:kotlin-log:1.4.0"
}
// 1) let's import the logging API package
import com.inmotionsoftware.logging.Logger
// 2) we need to create a logger, the label works similarly to a DispatchQueue label
val logger = Logger(label= "com.example.BestExampleApp.main")
// 3) we're now ready to use it
logger.info("Hello World!")2020-10-09 14:55:51.611 Info com.example.BestExampleApp.main: Hello World!
// log with file, method, and line information
logger.debug("Should not occur here", location=__location())
// log with throwable
try {
...
} catch (e: Throwable) {
logger.error(e, location=__location(error=e))
}KotlintLog provides for very basic console logging out-of-the-box by way of StreamLogHandler. It is possible to switch the default output to System.err like so:
LoggingSystem.bootstrap { StreamLogHandler.standardError(it) }StreamLogHandler is primarily a convenience only and does not provide any substantial customization. Library maintainers who aim to build their own logging backends for integration and consumption should implement the LogHandler protocol directly.
As the API has just launched, not many implementations exist yet. If you are interested in implementing one see the "Implementation considerations" section below explaining how to do so. List of existing SwiftLog API compatible libraries:
| Repository | Handler Description |
|---|---|
| inmotionsoftware/kotlin-log-android | A logging backend for KotlinLog that sends log messages to Logcat |
| inmotionsoftware/kotlin-log-analytics-firebase | A logging backend for KotlinLog that sends analytics log messages to Firebase |
Please see SwiftLog's What is an API package section.
Loggers are used to emit log messages and therefore the most important type in KotlinLog, so their use should be as simple as possible. Most commonly, they are used to emit log messages in a certain log level. For example:
// logging an informational message
logger.info("Hello World!")
// ouch, something went wrong
logger.error("Houston, we have a problem: $problem")The following log levels are supported:
TraceDebugInfoNoticeWarningErrorCritical
The log level of a given logger can be changed, but the change will only affect the specific logger you changed it on.
Logging metadata is metadata that can be attached to loggers to add information that is crucial when debugging a problem. In servers, the usual example is attaching a request UUID to a logger that will then be present on all log messages logged with that logger. Example:
val logger = Logger(label="Test logger")
logger["request-uuid"] = "${UUID.randomUUID()}".asLoggerMetadataValue()
logger.info("hello world")will print
2020-10-09 15:11:56.810 Info Test logger: request-uuid=String(value=e8733b5d-e5d9-4b74-9872-8f850edc7711) hello world
with the default logging backend implementation that ships with KotlinLog. Needless to say, the format is fully defined by the logging backend you choose.
Note: If you don't want to implement a custom logging backend, everything in this section is probably not very relevant, so please feel free to skip.
To become a compatible logging backend that all KotlinLog consumers can use, you need to do two things: 1) Implement a class implements LogHandler, an interface provided by KotlinLog and 2) instruct KotlinLog to use your logging backend implementation.
A LogHandler or logging backend implementation is anything that conforms to the following interface
interface LogHandler {
fun log(level: LoggerLevel,
message: LoggerMessage,
metadata: LoggerMetadata?,
source: String?,
file: String?,
function: String?,
line: Int?)
operator fun get(metadataKey: String): LoggerMetadataValue?
operator fun set(metadataKey: String, value: LoggerMetadataValue)
var metadata: LoggerMetadata
var logLevel: LoggerLevel
}Instructing KotlinLog to use your logging backend as the one the whole application (including all libraries) should use is very simple:
LoggingSystem.bootstrap { MyLogHandler() }LogHandlers control most parts of the logging system:
LogHandlers control the two crucial pieces of Logger configuration, namely:
- log level (
logger.logLevelproperty) - logging metadata (
logger[metadataKey:]andlogger.metadata)
For the system to work, however, it is important that LogHandler treat the configuration as value types. This means that LogHandlers should NOT be shared or create a new instance as needed.
However, in special cases, it is acceptable that a LogHandler provides some global log level override that may affect all LogHandlers created.
- emitting the log message itself
LogHandlers do not control if a message should be logged or not. Logger will only invoke the log function of a LogHandler if Logger determines that a log message should be emitted given the configured log level.
A Logger carries a label and each log message may carry a source parameter. The Logger's label
identifies the creator of the Logger. If you are using structured logging by preserving metadata across multiple modules, the Logger's
label is not a good way to identify where a log message originated from as it identifies the creator of a Logger which is often passed
around between libraries to preserve metadata and the like.
If you want to filter all log messages originating from a certain subsystem, filter by source which defaults to the module that is emitting the
log message.
MIT