You can't set or get user context for your service using web services.
You'll have to,
1. Create an instance of ServiceClientFactory.
2. Authenticate using AuthenticationManager Service and get the AuthResult.
3. Initialize the Context by passing the AuthResult as shown below,
ServiceClientFactory factory = ServiceClientFactory.createInstance(properties);
AuthenticationManager authenticationManager = new AuthenticationManagerServiceClient(factory);
AuthResult authResult = null;
try {
authResult = authenticationManager.authenticate("administrator", "password".getBytes());
} catch (UMException e) {
throw new RuntimeException(e);
}
com.adobe.idp.Context context = new com.adobe.idp.Context();
context.initPrincipal(authResult);