When downloading the content.jar file you're redirected to another mirror site.
In my case i was redirected to "https://mirrors.shu.edu.cn/eclipse/datatools/updates/content.jar"
This website expose an ssl certificate to you but you don't have any certificate authority able to validate this certificate.
What can you do?
- download keystore explorer
- In Internet explorer open the developper tools and start network recording in the network view
- In the address bar copy/paste "http://download.eclipse.org/datatools/updates/content.jar"
- Get the https redirected url from the network view
- Launch Keystore explorer and open your cacerts file (JRE_OR_JDK_HOME/jre/lib/security/cacerts). The access password is "changeit"
- I don't know the exact translation but in keystore explorer menu bar you will find a menu item something along the line "get certificate from TLS/SSL", click on it and provide the secured domain name (for me : mirrors.shu.edu.cn)
- Import the certificate in cacerts and save cacerts
- restart eclipse and test again
Some important points:
. cacerts is the default repository for certificate authorities used by java
. it is better to import the certificate chain (all the certificates shown by keystore explorer except the website's one)
. Take a good look to the certificates you're importing in cacerts because java will trust them