Max Rohde

Cannot Install VirtualBox Guest Additions: Installer Hangs

Problem

For a CentOS based guest, the update of the VirtualBox Guest Additions hangs after the step: “Removing exiting VirtualBox non-DKMS kernel modules”

install

Solutions

  • Check the log file in /var/log/VBoxGuestAdditions-uninstall.log. See if there is anything obvious reported there you can fix.
  • Check if your RAM disks are valid by running the command `sudo lsinitrd`.
    • If they are not, try rebuilding broken images with `sudo dracut -f [broken-image].img [kernel version]`
  • Verify you have all dependencies installed:
    • yum install dkms
    • yum groupinstall “Development Tools”
    • yum install kernel-devel
  • Reinstall your Kernel image
  • Verify that you have the correct version of kernel-devel installed
    • yum remove kernel-devel
    • yum install kernel-devel-`uname -r`
    • Restart system!
  • Wait!
    • Let the installation run for a while, at least 15-20 min. It might do something!
Max Rohde

Java: Find all instances of System.out.println()

A good Java application should print only the absolute necessary to standard out. Not to do so can mean a serious hit in performance and can make it difficult to debug issues.

Unfortunately either we ourselves or our colleagues put System.out.println() statements in the code and then forget to remove them.

Here are two solutions how to find all those nasty statements.

Solution 1: Do a Full Text Search

Use your IDE and search for all occurrences of the string “System.out.println()” in your code. In eclipse this can be done through Menu / Search / File …

Make sure to use *.java for the file name pattern.

search

Solution 2: Put a Breakpoint in PrintStream

The above solution might not be practical if there are many System.out statements in the code which are legitimately there. In that case, you can also put a break point into the Java standard class PrintStream.

In eclipse, open the search for class dialog (Ctrl+Shift+t) and type in the class name PrintStream from the package java.io.

printstream.PNG

In this class, find the print(x) method and add a breakpoint:

breakpoint

Then run your program in debug mode. You can then find the places where System.out is called from the stack trace.

 

Max Rohde

Route 53 Cannot Find CloudFront Distribution

Problem

You have create a CloudFront distribution with a custom domain name (such as yourdomain.com).

Now if you try to link this distribution to your domain using Route 53, you get the following error message:

`No AWS resource exists with the Alias Target that you specified.`

error_message

Solution

Try the following to solve this problem:

  • Make sure that the CNAME you specified for the CloudFront distribution matches your domain name exactly. For instance, if your domain name is http://www.yourdomain.com, make sure that this is also the CNAME.
  • When creating the record set in Route 53, make sure to select the record type `A – IPv4 Address` and not CNAME.

ipv4

 

 

Max Rohde

Solving ‘One or more of your origins do not exist’ for Cloud Front

Problem

You are trying to create a CloudFront distribution using Amazon’s API.

You get the error:

“One or more of your origins do not exist”

Solution

In my case, I provided a different value for these two properties:

DistributionConfig.DefaultCacheBehavior.TargetOriginId

and

DistributionConfig.Origins.Items[0].Id

Just make sure that the Id for one of your origins matches the TargetOriginId of the DefaultCacheBehavior and the error should disappear.

 

 

Max Rohde

Sharing Folders with VirtualBox

Problem

You would like to share a folder between a VirtualBox Linux Guest and a Windows Host.

Solution

  • Start your VM
  • Switch to Windowed Mode
  • Select Devices / Shared Folders / Shared Folder Settings …
  • Create a New Folder on your Linux Guest. e.g.:
    • /home/[Your User]/WinDocuments
  • Add a new Shared folder there, e.g.
    • Share Name WinDocuments
    • Windows Folder: C:\Users\[Your User]\Documents
  • Open a Terminal and run the following:
sudo mount -t vboxsf -o rw,uid=1000,gid=1000 Documents /home/[Your User]/WinDocuments

Now you should be able to copy files to and from Windows from your Linux host.

windocs

Permission Problems

There are often tricky permission problems that prevent you from copying folders to Windows. You will get messages like ‘Access denied’ or ‘Error while copying’ or ‘You do not have permissions for this operation’.

There are many ways to fix this, but the easiest is to start your the file browser of your Linux system with sudo or, if your Linux doesn’t come with an UI, copy the files using sudo.

For instance, if you have the Nautilus file browser installed, run the following on the command line:

sudo nautilus

Then navigate to your shared folder and copy the files there.

Otherwise, copy files as follows:

sudo cp /local/file /home/[Your User]/WinDocuments

Unmounting the Folder

When you are done working with the files, simply run the following from the command line:

sudo umount /home/[Your User]/WinDocuments

References

Max Rohde

Fix PhoneGap Shows Black Screen after Splash

In order to test how an ObjectHub Micro App can be packaged as a PhoneGap application, I used PhoneGap Build to package up a simple application.

Unfortunately, the application that I developed started off with a big, ugly black screen instead of a splash screen.

It took me quite some digging around to find out what was going on there. So here I have collected some potential solutions for the problem.

  • Do the src attributes in the splash elements point to valid files?
  • Are you using PhoneGap Build? In that case, follow the instructions on the PhoneGap Build documentation.
  • Did you include the following in your config.xml:
<preference name="SplashScreen" value="screen" />
<preference name="SplashScreenDelay" value="50000" />
  • You can try this workaround to have your app load quicker.
  • You can try to manually handle the splash screen lifecyle as shown here.

Further Links

 

Max Rohde

Test if Firebase is Initialized on Node.JS / Lambda

Firebase is build on the assumption that it will only be initialized once.

This can be a problem in Node.JS applications sometimes, especially if they are run as part of an Amazon Lambda function.

This can lead to errors as the following:

Firebase App named '[DEFAULT]' already exists.

Thankfully, there is an easy way to check if Firebase has already been initialized (firebase.initializeApp). Just wrap your call to initializeApp in the following:

if (firebase.apps.length === 0) {
    firebase.initializeApp({
        serviceAccount: {
            ...
        },
        databaseURL: ...
    });
}

Sources