DEV Community: Isaias Velasquez

How to free up disk space used by Docker

Isaias Velasquez — Sat, 04 Jul 2026 07:14:25 +0000

How to free up disk space used by Docker

When you use Docker on a daily basis, your disk can get full of unnamed images, stopped containers, dangling volumes and build cache. Here is how to clean all that up.

First, check how much space Docker is using:

docker system df

This will show you images, containers, volumes and build cache with their sizes.

To remove all stopped containers, dangling images and unused networks in one go:

docker system prune -f

If you also want to remove dangling volumes:

docker system prune -af --volumes

Just be careful with the --volumes flag. Once they are gone, any data stored in anonymous volumes is lost.

Now if you want to remove only what is old:

docker system prune -af --filter "until=24h"

The --filter flag removes everything created or used more than 24 hours ago.

That's all for now.
Thanks for reading!

How to deal with empty lists that defaults to null in Jackson and Kotlin

Isaias Velasquez — Sat, 04 Jul 2026 07:04:39 +0000

How to deal with empty lists that defaults to null in Jackson and Kotlin

Most times using Jackson will be straightforward and we won’t have to fiddle, but in the case you need to deserialize a data class that can have empty lists in Kotlin you can run into a null assertion error.
Here’s a data class:


    @JsonInclude(JsonInclude.Include.NON_NULL)
    data class SomeDataClass(
      val otherFieldA: String? = null
      val someOtherDataClassList: List<SomeOtherDataClass> = arrayListOf()
    )

In the previous code we have a potential error if payload received is an empty list in the field someOtherDataClassList

The simplest to this, using Jackson 2.17 (most recent version to the date this article was written) was to declare the Jackson Kotlin Module manually, and instead of using ObjectMapper.registerKotlinModule(), use this:

 object JacksonProvider{

      private val kotlinModule = KotlinModule.Builder()
        .configure(KotlinFeature.NullIsSameAsDefault,true)
        .build()

      val mapper: ObjectMapper = ObjectMapper()
        .registerModule(kotlinModule)
      ...(your other configuration options)...
    }

And that’s it! now you have a working empty list that does not throws an error while using Jackson and Kotlin.

The needed dependency is:

  implementation("com.fasterxml.jackson.module:jackson-module-kotlin:2.17.+")

Thank you for reading!

How to copy files from a docker container to host

Isaias Velasquez — Sat, 04 Jul 2026 07:02:35 +0000

How to copy files from a docker container to host

Sometimes we just want to know the content of a generated file into some container. And this is the way to do:

First of all, make sure the data you want to copy is in container:

 docker exec -it <container reference> /bin/bash

I strongly suggest you to use /bin/bash instead of sh because sh lacks lots of useful features, like autocomplete, command history.

Then, assured what you need is there, just do it almost like a normal copy:

 sudo docker cp -a <container reference>:<container path> <host path>
    Example:
    sudo docker cp -a 3bbef5e21df:/home/fls/ .

The difference is an ‘-a’ flag which means archive mode. It’ll help you copying with GID|UID information.

That’s all for now.
Thanks for reading!