Is it necessary for me to learn WSDL to create webservice ?
Since there are many tools like Axis which can auto-generate the WSDL required for us, I dont think its necessary to learn WSDL.
Is it possible to create a webservice by writing WSDL file manually ?
Does anyone write WSDL file manually ?
Not neccessarily!. BUT tweaking the auto generated WSDL may help you to talk with non-axis clients and vice versa. In a long run webservices tool will become fully automatable and interoperable with lesser efforts. Axis 2 has promises to fully support doc/literal encodings. At this moment it is better to learn the basic concepts of WSDL like what is operations, bindings, encoding and types. If you are going to transfer complex objects/security stuff then it is better to learn WSDL, because WSDL is like a CV(resume) for a given webservices.
Answer for #2
No. IMHO it doesn't serve the purpose very well. People use to do it when the tools were not strong enough, now we have tools which avoids such writings.
Java-to-WSDL tools are great for prototyping, especially if you need to quickly bring up a proof-of-concept for existing functionality - however when designing and implementing something for production it�s best to start designing a proper XML Schema (especially if your data is complex) for the data to be exchanged, then devising WSDL operations that will be used to exchange the data. That doesn't mean you have to do it manually - you can and should use (visual) tools to design your XML Schemas and WSDL documents - however you need to be thinking in terms of the strengths and weaknesses of XML Schema and WSDL and not about Java objects (or worse object graphs).
If your data exchange is simple enough you could start by designing Transfer Objects that follow the guidelines of JAX-RPC Value Types (Java API for XML-Based RPC (JAX-RPC) Specification 1.1: 5.4 JAX-RPC Value Type) and then use those as parameters and return values in your endpoint interface. Finally you would use Java-to-WSDL to come up with you first pass at the WSDL (making sure that you specify WS-I compatibility for your tool).
Java-to-WSDL tends to give developers the illusion that everything is transparent - and that they don't need to deal with XML Schema and WSDL - sometimes with the effect that in the generated output there are some "little details" they are unable to discern - details that can have undesirable consequences down the road.
The convenience of Java-to-WSDL also may lead some developers to "forget" the cost of crossing that great divide. The blueprints clearly recommends the separation of a "service interaction" layer and a "processing layer" (3.3 Key Web Services Design Decisions). The section elaborates that the service interaction layer also contains the developer supplied logic (i.e. in addition to the generated code) to map business objects:
This mapping of incoming XML documents to business objects is not the same as the JAX-RPC runtime mapping between XML documents and Java objects. Although the container performs the JAX-RPC runtime mapping for all requests and responses, the developer chooses the mapping of incoming XML documents to business objects.
This expresses the expectation that business objects will be converted to XML Documents, JAX-RPC Value types (Transfer Objects) or JAX-RPC mapped standard Java types, then sent over the wire and then reconstituted on the other side; it also indicates that those mappings are the developer�s job. The existence of Java-to-WSDL tools has created the expectation in some people that you should be able to simply stuff an arbitrary custom object down the pipe.
Java-to-WSDL tools also make it all too easy to expose fine-grained object methods (not that you could'nt do the same with WSDL-to-Java tools - but at least WSDL serves as a reminder that "you are not in Kansas anymore") - tools that make it too easy to abuse a technology can end up giving the technology a bad name.