1. A system for implementing dynamic Web service invocations, the system having a processor and a memory coupled to the processor, the system comprising: a control module, stored in memory and executed by a processor, for controlling operations of components in a service broker according to customized invocation criteria contained in a Web service request received at said service broker from a Web service requester, said invocation criteria comprising one or more service discovery rules, one or more service selection rules, and one or more invocation result comparison rules, said control module comprising: a route service module for receiving a Web service request from a Web service requester and returning response messages of the service broker to the requester; a Web service request parser for parsing the Web service request from the route service module and extracting various rules for the discovery module, selection module or invocation and comparison module, respectively; a session manager for maintaining session information, wherein the session information contains correspondence relation between a session identifier for identifying a service request and the service implementation candidates obtained during the session indicated by the session identifier; and a response handler for constructing a service response message based on the processing results by said discovery module, selection module or invocation and comparison module and sending the message to the requester through said route service module; a discovery module for discovering Web service implementation candidates that meet said invocation criteria from a source of Web service implementations, said discovery module comprising: a discovery executor for parsing said one or more service discovery rules, and for each service discovery rule invoking a corresponding discoverer module in a discoverer container for enforcing the rules, and obtaining Web service implementation candidates that meet said invocation criteria based on the results of enforcing the various service discovery rules; and a discoverer container that contains one or more discoverer modules, each of which implements a discovery mechanism for discovering Web service implementation candidates that match the service discovery rule, wherein said discovery mechanism is one of UDDI, WSIL and USML; a selection module for further selecting Web service implementation candidates that meet said invocation criteria from the Web service implementation candidates discovered by said discovery module according to said invocation criteria, said selection module comprising: a selection executor for parsing said one or more service selection rules, and for each service selection rule invoking a corresponding selector module in a selector container for enforcing the rules, and selecting Web service implementation candidates that meet said invocation criteria based on results of enforcing the various service selection rules; and a selector container that contains one or more selector modules, each of which is used to select selected Web service implementation candidates that match a service selection rule from discovered Web service implementation candidates discovered by the discovery module; and an invocation and comparison module for invoking each one of said Web service implementation candidates and comparing invocation results returned by the Web service implementation candidates according to said invocation criteria, so as to select the best Web service implementations, wherein said invocation and comparison module further comprises: an invoker that invokes each of said Web service implementation candidates; a comparison executor for parsing said one or more invocation result comparison rules, and for each invocation result comparison rule invoking a corresponding comparator module in comparator container for enforcing the rule, and obtaining the best Web service implementations that meet said invocation criteria based on the enforcing results of the various invocation result comparison rules; and a comparator container that contains one or more comparator modules, each of which is used to compare invocation results of various Web service implementation candidates returned from said invoker and select the best Web service implementations that match the invocation result comparison rule.