Error : java.lang.SecurityException: Can not initialize cryptographic mechanism

During certificate import using InstallCert.Java, I faced this issue. This can cause if you don't have required restriction policy file or the key size is greater than the once deployed by Java by default which is 128 bit. 

This can be solved by downloading the Java Cryptography Extension (JCE) Unlimited Strength Jurisdiction Policy Files from Java Cryptography Extension (JCE) Unlimited Strength Jurisdiction Policy Files  and copying them in your JRE.

Error : unable to find valid certification path to requested target

I was trying to run Java application, which was using REST Template to implement rest client. The application was connecting to REST service (REST API's) over SSL. The certificate used for SSL was self signed certificate.

The self signed certificate was causing error "unable to find valid certification path to requested target" every time I tried to connect to REST Service. 

In order to resolve this error, I had to add the certificate to the JVM key store. I tried multiple steps, even corrupted the keystore but finally was able to run application successfully. 

You would need InstallCert.Java to successfully import the certificate which can be found at InstallCert.Java

Above git repo has ReadMe which details out all steps required for importing the certificate. 
If you happen to corrupt the keystore you can follow below link to restore it.

http://architecturalatrocities.com/post/19073788679/fixing-the-trustanchors-problem-when-running

Error: Cannot connect to the Docker daemon. Is the docker daemon running on this host?

If you are trying to start docker first time, you might get an error "Cannot connect to the Docker daemon. Is the docker daemon running on this host?"

This might happen because of multiple reasons.

1. You might not have permissions to to access docker

You should grant permission to use docker for specific user if he is not  root user.

In order to grant permission execute below command:

$ sudo usermod -aG docker <username>

replace <username> with actual user name. Also make sure you log off/restart machine before using docker, after executing above command.

2. Docker service might not be running

In order to re-start the docker service execute below commands

$ sudo service docker stop

$ sudo service docker start

Now you should be able to use docker.