[Spring Boot] Send HTML email with attachment using Thymeleaf template

Thymeleaf provides a powerful and easy to use template engine that is effortlessly integrated in Spring and Spring Boot projects.

In this example we look at how to send HTML emails with attachments using a Thymeleaf template from a Spring Boot 2 project.

First of all these are the imports we need to add:

org.springframework.boot:spring-boot-starter-mail
org.springframework.boot:spring-boot-starter-thymeleaf

[Ubuntu] Install Microsoft fonts

Most of the world uses Microsoft Windows and Office, therefore it might be sometimes necessary to generate documents using Microsoft fonts, which of course are only available on Windows.

For Ubuntu, the ttf-mscorefonts-installer package contains SOME of those fonts, simply install it and you can start using them AFTER regenerating the font cache with fc-cache

[Linux] Bind special key press to ALSA control action

If using ALSA, it's possible to bind special keyboard keys to action controls with a few configuration lines.

First of all, to find the available action controls, run in a terminal: amixer scontrols

Then, find the key event we need to bind to the desired action, and add it as an event,action pair in a new configuration file under /etc/acpi/events for example:

event=button/mute MUTE 00000080 00000000 K
action=/usr/bin/amixer sset 'Master',0 toggle

which in this case for my lapotop binds the mute button to the mute action. It is also possible to execute a script instead, for example:

event=ibm/hotkey HKEY 00000080 00001009
action=/etc/acpi/undock.sh

which runs some actions when the laptop is undocked.

[Linux] Find which event is generated on special keypress

To determine which ACPI event is generated when a special key is pressed, simply open a terminal and run: acpi_listen

Then press any special key (media keys and such, normal keys won't generate an event) and you will see the output eg:

button/volumedown VOLDN 00000080 00000000 K
button/mute MUTE 00000080 00000000 K
button/volumeup VOLUP 00000080 00000000 K

[Ubuntu] Boot USB after error mmx64.efi not found

UEFI, great leap into the future and also great pain.

There is a funny error that can happen after creating a UEFI bootable USB to install a linux distro for example. If the installation is not completed immediately and the system is restarted, some files are left in some area of the EFI partition (I guess) and subsequent attempts to boot the exact same device will fail with an error:

Failed to open \EFI\BOOT\mmx64.efi - Not Found
Failed to load image \EFI\BOOT\mmx64.efi: Not Found
Failed to start MokManager: Not Fond
Something has gone seriously wrong: import_mok_state() failed

The easiest fix, is to simply browse the USB drive and go into the efi/boot folder, then copy the grubx64.efi file and rename it to mmx64.efi

The next boot will work again as intended. Magic of the machines

[Java] Useful keytool commands

Keytool is a tool to manage Java keystores. Here are some useful commands to:

• generate a new key and place it in a keystore:

keytool -genkey -keystore myKeystore.jks -storepass myPass -alias myKey -keyalg RSA -sigalg SHA256withRSA -keysize 2048 -validity 3650 -ext ku=digitalSignature

• verify the content of a keystore:

keytool -list -keystore myKeystore.jks -storepass myPass -v

• export the public key:

keytool -exportcert -keystore myKeystore.jks -storepass myPass -alias myAlias -rfc -file myPubKey.cer

• delete a key:

keytool -delete -alias myAlias -keystore myKeystore.jks -storepass myPass

• add a private key:

keytool -importkeystore -srckeystore myKeystore.jks -srcstorepass myPass -srcalias myAlias -destkeystore myNewKeystore.jks -deststorepass myNewPass -deststoretype JKS -destalias myNewAlias

• add a public key:

keytool -import -noprompt -alias myAlias -keystore myKeystore.jks -file myPubKey.cer -storepass myPass

[Ubuntu] Revert kernel back to older version

Intro story: So much luck in a single day.

I always keep only 2 kernel versions after every update, so that I can boot back if something goes wrong. This usually helps, except today I decided to upgrade my Ubuntu from 16.04 to 18.04 since I like spending my nights fixing stuff after unnecessary upgrades, but hey, the future is there.

So I first fully updated my current installation (1st kernel bump), then migrated to the new one (2nd kernel bump) and it turns out I got two 4.15 kernels, with only different minor version.

Double interesting, my wireless card:

02:00.0 Network controller: Realtek Semiconductor Co., Ltd. RTL8821AE 802.11ac PCIe Wireless Network Adapter

apparently has issues (read: bug) with kernel 4.15 preventing it from working at all (not even connected to the router), and even more interesting, on one version it can actually only connect to websites under google.com domain..

Anyway, quick fix was to revert back my kernel, here is how for future me to remember:

1. go to http://kernel.ubuntu.com/~kernel-ppa/mainline/ and download in the same folder: linux-headers-VERSION-generic-XXX_SYSTEM.deb, linux-headers-VERSION_all.deb, linux-image-VERSION-generic-XXX_SYSTEM.deb for your system, eg I have 64 bit system and got amd64 files
2. install them by going into the folder where you downloaded them, make sure there are only those three .deb files and run: sudo dpkg -i *.deb
3. restart the system, then press SHIFT at boot to enter the GRUB choices, then check from the top line (number 0) to the line of your kernel version. If your installation has the "Advanced" menu entry and does not display all kernels, go there and do the same counting. We need this to set a default GRUB entry to always boot in this kernel
4. edit /etc/default/grub and change the value of GRUB_DEFAULT to the number you counted OR to (exactly as shown) "1>N" if you have the "Advanced" menu option. This means you want to boot into the Nth entry (starting from 0) under the "Advanced" menu. Also set GRUB_TIMEOUT to some value in seconds that's more than 1, to have some time to block the boot process in case of mistakes
5. run sudo update-grub and you are set

Note that this is a TEMPORARY fix, you would not want to have a fully updated system run on an older version of the kernel, mainly because some obscure dependency or feature might be missing/not working, therefore check back periodically to see if your issue has been addressed and try the latest kernel version out!