The first question is that
what is the major advantage in Eclipse?
Eclipse is widely used and the community is eager to help. It's open source. It has a great plugin framework and active plugin developer community. There are some big companies using it (IBM comes to mind, and Rational but now they're the same). SWT makes the Eclipse UI a more responsive and native look'n'feel.
Second question is that if I use Eclipse compares with oracle 9i developer suite,what is the major difference?
JDeveloper provides you built-in integrations to, for example, Oracle databases, TopLink object-relational mapping tool, Oracle's application server, etc. It's more like an all-in-one shop -- kind of like MS Visual Studio, which has built-in integrations with other Microsoft products. Eclipse is more open and does not rely on a single commercial entity (Oracle).