каким образом можно сделать возможным выбор нескольких элементов в древе на java

в каком дереве? телепатов не держим

GWT виджет для выбора элементов из бд(от google)

а названием виджет обделен?

GWT-Google Web Toolkit

Есть и описания, правда на англ, но что то лучше чем ничего=))

Google Web Toolkit (GWT) makes it easier to write high-performance AJAX applications. You write your front end in the Java programming language and GWT compiles your source into highly optimized JavaScript. Writing web apps today is a tedious and error-prone process. You spend 90% of your time working around browser quirks, and JavaScript's lack of modularity makes sharing, testing, and reusing AJAX components difficult and fragile. It doesn't have to be that way.

High performance JavaScript. GWT produces AJAX apps that:

* Load faster than hand-written JavaScript apps
* Use smaller, more compact, cacheable code
* Automatically support IE, Firefox, Mozilla, Safari, and Opera
* Use the browser's "back" button correctly

Better development tools. Since you're writing in Java, you can use:

* IDEs that you love like Eclipse, IntelliJ, and NetBeans
* Full-featured debugging, with variable watches and breakpoints
* Unit tests (based on JUnit) both in a debugger and in a browser

Google APIs and reusable UI components. GWT comes equipped with useful libraries:

* Built-in UI components serve as cross-browser building blocks for your app
* RPC helps with client-server interaction
* JavaScript Native Interface (JSNI) simplifies integrating GWT code with existing JavaScript code