Users last week expressed some skepticism about Sun Microsystems Inc."s plan to offer its application server software under an open-source license.

The so-called GlassFish project to offer the Java System Application Server Platform source code on an open basis will be formally announced this week at the vendor"s annual JavaOne conference in San Francisco. Sun will also use the JavaOne stage to unveil an open-source enterprise service bus (ESB).

Arnaud Lucas, a senior integration architect at Houston-based online consumer network WhiteFence Inc., said he already uses JBoss Inc."s namesake open-source application server and wouldn"t consider switching to Sun"s alternative offering.

Lucas, who is also president of the Houston Java User Group, described the move as Sun"s attempt to boost its application server market share against encroaching open-source products such as JBoss and The Apache Software Foundation"s Geronimo.

Mark Johnson, a senior consultant at Montreal-based CGI Group Inc., said that JBoss has a command of the open-source application server market today.

"JBoss has a lot of mind space there, and it will take some effort to throw JBoss off its perch," said Johnson, a member of the New England Java Users Group.

Anne Thomas Manes, an analyst at Burton Group in Midvale, Utah, agreed that the market for open-source application servers likely isn"t large enough to support another vendor.

Joe Keller, Sun"s vice president of marketing for application and developer platforms, said the move is designed to lower the barrier of entry for developers to use the application server software.

"This is a community around the implementation of Java from the source of Java," Keller said. "It is not a competing community to things like Eclipse. It has the ability of having community developer participation, which [other application server vendors] don"t have."

Frank Greco, who founded the New York Java Special Interest Group in 1995, said his group"s members would likely consider using Sun"s open-source application server because the software is now becoming a commodity.

"People are tired of paying overly expensive licenses for a technology that doesn"t scale too well and is complex to develop and manage," said Greco, CEO of Java engineering firm Crossroads Technologies Inc. in New York.

Greco and others are less enthusiastic about Sun"s ability to tap the burgeoning ESB market with its unveiling of an offering based on the newly approved Java Business Integration specification. The spec is designed to help Java developers build service-oriented architectures.

Greco noted that many developers today are already struggling to use Enterprise JavaBeans - the Sun specification for how Java objects will interact - and don"t need the additional complexity of Web services. In addition, integration vendor Tibco Software Inc. already has a strong hold on New York-based financial firms, he added.

Thomas Manes agreed, saying that Sun will have a hard time competing with established ESB vendors. "The ESB market is already confused and overcrowded, and I don"t see Sun effectively competing with folks like IBM, BEA, Tibco, Sonic [Software] or even Cape Clear," she said.