Liferay installation problem: Could not find the main class: . Program will exit.

A few weeks ago I was installing Liferay at a client and ran into a very weird installation problem. I was under the impression that I'd done everything correctly, but when I tried to start Liferay I got a baffling error message almost immediatly:

"Could not find the main class: . Program will exit."

As Eddie Izzard would say: Quod The Fuck! How can a simple Liferay install go this wrong? I followed some simple steps to install it:

• download Liferay
• transfer ZIP file to server
• unzip the downloaded file
• check installed Java version
• check DB connection
• configure DB connection in portal-ext.properties
• start Liferay

By now I've done this simple procedure more than a few times and it never failed, ... until now. So I was kinda gobsmacked. I started digging around, but couldn't find an obvious cause. Also Googling didn't turn up any leads. So I was on my own and decided to start from the beginning and check every line in the Tomcat start script and quickly found that strange things were happening with the startup classpath, hence the empty main class name. A JAR file seemed to be missing: bootstrap.jar.

How could that be possible. I downloaded a fresh Liferay ZIP that unzipped without problems. Then I remembered a long pause during the download around the same time the datacenter people were fiddling with the firewall. A second fresh download later, this time without hickups, I was able to run a file size check against the old download and what do you think: the first download was 20Mb smaller.

How it was able to unzip with throwing obvious errors I still don't know, but after deleting the previous install, unzipping the new one, Liferay started without any problem.