you are saying that you are using different technologies, so what the part of the esb solution  is not suitable? the web service  is providing synchronous communication in a vendor neutral way, you might want to consider asynch connection if possible. for connectivity to legacy application depending on asynch / sych connection you probably need to provide an adapter if its not provided one already

Current open source  ESB offerings include Mule, Synapse, Celtix, ServiceMix, JBossESB, and Open ESB. Among the considerations you¢ll want to take into account when evaluating an ESB are these:
Level of commitment to standards: certain projects are built from the ground up in order to conform to and support certain standards (e.g., ServiceMix with JBI and Synapse with the WS-* specifications). Others, like Mule, are more agnostic with respect to standards, supporting them but not dogmatically. You may want to choose your ESB based on your own level of commitment to standards.

Platform support: you¢ll of course want to consider whether the ESB you¢re thinking of using supports the platforms (application servers, web  servers, messaging middleware, application frameworks) you¢re already using.



Regarding requirements that you mentioned ,I think the best solution  is Mule.

Currently, the best ESB is Windows Azure Service Bus, which supports multiple enterprise programming language, including Java. Even better, the source code of its Java SDK is open so you can get many insights of how it works if needed.

The source code of Windows Azure SDK for Java is available at

https://github.com/WindowsAzure/azure-sdk-for-java

There are tutorials available online at
www.windowsazure.com/.../

and

www.windowsazure.com/.../

The nice part of Windows Azure Service Bus is that it is a service so you do not need to install/manage it as much as local ESB instances.