DEV Community: Adam Cervenka

Quickly read file or any url in Scala/Java

Adam Cervenka — Thu, 18 Nov 2021 16:07:10 +0000

One thing that is missing in Scala/Java standard library is a simple function loading a text file and returning it as a String. Of course this is not a way to deal with big files, but when you just need to load small and simple config file it would be useful.

Luckily, Apache commons provides such a function:

import org.apache.commons.io.IOUtils
import java.net.URL

def loadFromUrl(urlString: String): String = {
  IOUtils.toString(new URL(urlString))
}

And as a bonus, you can use it for both local files and resources on the web as well.

loadFromUrl("file:///Users/foo/conf.json")
loadFromUrl("http://example.com/")

If you inspect the code of IOUtils you will find that there is proper error handling, that means resources are closed in case of an exception. That part is usually missing in similar quick and simple solutions.

To use this, you need to add the following dependency:

<dependency>
    <groupId>commons-io</groupId>
    <artifactId>commons-io</artifactId>
    <version>2.4</version>
</dependency>

Scala Tip: mapValues may not do what you expect

Adam Cervenka — Fri, 15 Oct 2021 08:03:32 +0000

Often in Scala you may need to map over values of Map data structure. You can use the usual map function, but there is also more concise mapValues:

Map("a" -> 2).map{ case (k,v) => k -> (v + 2) }

Map("a" -> 2).mapValues(_ + 2)

In many situations the result of these two will be identical, but there is one important difference.

mapValues doesn't apply the provided function on the value right away. Instead, it creates a view (wrapper) over the data and only evaluates the function when the value is accessed.

This is OK if your code doesn't have any side effects, but if it has, it may cause hard to find issues. For example leaking of exceptions:

scala.util.Try(
  Map("a" -> 2).mapValues(v => (throw new RuntimeException))
).isSuccess
// returns true

The newly created map will happily leave the Try wrapper as success, and the exception will be thrown only when the value is accessed later.

In my opinion, mapValues name was a poor choice because it seems to suggest the same functionality as map, but only for values. Maybe wrapValues would be better?

If you want to be more concise, but still have the same semantics as a map function, you can use transform:

Map("a" -> 2).transform((k,v) => v + 2)

Set logback to different level for specific package

Adam Cervenka — Thu, 14 Oct 2021 12:54:10 +0000

If I want to set log level for package com.foo.bar to TRACE I can use a system property logback.configurationFile to define path to logback config.

-Dlogback.configurationFile=/Users/foo/logback.xml

In the Logback config I just need to set a new logger like this:

<configuration>
    <appender name="STDOUT" class="ch.qos.logback.core.ConsoleAppender">
        <encoder>
            <pattern>%d{HH:mm:ss.SSS} [%thread] %-5level %logger{36} - %msg%n</pattern>
        </encoder>
    </appender>

    <logger name="com.foo.bar" level="TRACE"/>

    <root level="INFO">
        <appender-ref ref="STDOUT"/>
    </root>
</configuration>

Scala Tip: appending a sequence optionally

Adam Cervenka — Tue, 31 Aug 2021 08:21:14 +0000

If you want to add option's content to a sequence, you can easily do that using ++ or appendAll method.

This is possible because ++ expects collection and there is an implicit conversion from an option to collection in Scala.

But be careful with :+ because that expects just one element and no conversion will be applied and this happens: