VMware on Monday agreed to acquire SpringSource for $420 million in a deal that promises to push the maker of virtualization software into Java development tools.

Palo Alto, California, VMware, a unit of storage giant EMC, agreed to pay $362 million in cash and stock and pick up the tab for some $58 million in unvested stock and options at SpringSource.

“SpringSource is only five years old and has a track record of innovation and expects to be cash flow positive in the first half of 2010,” VMware CFO Mark Peek said on the company’s conference call.

Virtualization software allows a single computer to run multiple operating systems simultaneously, turning one PC into many. What this does is let, for example, companies to run Linux, Microsoft, and Mac servers all at once, on one box, allowing companies with data centers to save resources and money.

San Mateo, California, SpringSource develops subscription-based open-source software. SpringSource’s Spring Framework is the leader in extending the Java programming language and application platform and is used by more than 2 million developers.

“Spring has become the de facto standard programming model for modern enterprise Java, rich web, and enterprise integration applications,” VMware CTO Steve Herrod wrote on the company’s blog.

The VMware-SpringSource marriage promises to boost the combined company’s position in the so-called platform-as-a-service market. The nascent platform as a service, or PaaS, model provides a complete computing platform for others to build Web-based businesses upon. The market for platform as a service is inhabited by the likes of such companies as Amazon and Google. Forrester Research forecasts the market for PaaS to reach $15 billion by 2016.

“We think the deal was motivated by the need to add capabilities for cloud computing that are unique to the cloud computing approach,” Benchmark analyst Brent Williams wrote in a note to clients. Benchmark does not make a market in VMware shares.

VMware is already prepared to sell SpringSource developer tools. That’s because the company has recently added executives from Microsoft and Borland who have a track record in marketing developer tools, Mr. Williams wrote.

For VMware, the acquisition marks its largest such transaction to date. The virtualization software company said it doesn’t expect the acquisition to have an impact on its near-term earnings guidance. Before the SpringSource deal, expected to close in September, VMware had $2.1 billion in cash reserves.