# Guide to OpenIG
Chapter 12. Customizing OpenIG

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## 12.2. Implementing a Filter

The `Filter` interface exposes a filter method, which takes an Exchange object and the Chain of remaining filters and handler to dispatch to. Initially, exchange.request contains the request to be filtered. To pass the request to the next filter or handler in the chain, the filter calls chain.handle(exchange). After this call, exchange.response contains the response that can be filtered.

A filter might elect not to pass the request to the next filter or handler, and instead handle the request itself. It can achieve this by merely avoiding a call to `chain.handle(exchange)` and creating its own response object in the exchange. The filter is also at liberty to replace a response with another of its own. A filter can exist in more than one chain, therefore should make no assumptions or correlations using the chain it is supplied. The only valid use of a chain by a filter is to call its `handle` method to dispatch the exchange to the rest of the chain.

| Note
If an existing response exists in the exchange object and the filter intends to replace it with its own, it must first check to see if the existing response has an entity, and if it does, must call its close method in order to signal that the processing of the response from a remote server is complete.