# Class Filter

java.lang.Object
com.sun.net.httpserver.Filter

public abstract class Filter extends Object
A filter used to pre- and post-process incoming requests. Pre-processing occurs before the application's exchange handler is invoked, and post-processing occurs after the exchange handler returns. Filters are organised in chains, and are associated with `HttpContext` instances.

Each `Filter` in the chain, invokes the next filter within its own `doFilter(HttpExchange, Chain)` implementation. The final `Filter` in the chain invokes the applications exchange handler.

Since:
  1.6

- ## Nested Class Summary

  Nested Classes
  Modifier and Type
  Class
  Description
  `static class`
  `Filter.Chain`
  A chain of filters associated with a `HttpServer`.

- ## Constructor Summary

  Constructors
  Modifier
  Constructor
  Description
  `protected`
  `Filter()`
  Constructor for subclasses to call.

- ## Method Summary

  Modifier and Type
  Method
  Description
  `static Filter`
  `adaptRequest(String description, UnaryOperator<Request> requestOperator)`
  Returns a pre-processing Filter that inspects and possibly adapts the request state.
  `static Filter`
  `afterHandler(String description, Consumer<HttpExchange> operation)`
  Returns a post-processing `Filter` with the given description and operation.
  `static Filter`
  `beforeHandler(String description, Consumer<HttpExchange> operation)`
  Returns a pre-processing `Filter` with the given description and operation.
  `abstract String`
  `description()`
  Returns a short description of this `Filter`.
  `abstract void`
  `doFilter(HttpExchange exchange, Filter.Chain chain)`
  Asks this filter to pre/post-process the given exchange.

  ### Methods declared in class Object

  `clone, equals, finalize, getClass, hashCode, notify, notifyAll, toString, wait, wait, wait`

- ## Constructor Details

  - ### Filter

    protected Filter()
    Constructor for subclasses to call.

- ## Method Details

  - ### doFilter

    public abstract void doFilter(HttpExchange exchange, Filter.Chain chain) throws IOException
    Asks this filter to pre/post-process the given exchange. The filter can:
    - Examine or modify the request headers.
    - Filter the request body or the response body, by creating suitable filter streams and calling `HttpExchange.setStreams(InputStream, OutputStream)`.
    - Set attribute objects in the exchange, which other filters or the exchange handler can access.
    - Decide to either:
      1. Invoke the next filter in the chain, by calling `Filter.Chain.doFilter(HttpExchange)`.
      2. Terminate the chain of invocation, by not calling `Filter.Chain.doFilter(HttpExchange)`.
    - If option 1. above is taken, then when doFilter() returns all subsequent filters in the Chain have been called, and the response headers can be examined or modified.
    - If option 2. above is taken, then this Filter must use the HttpExchange to send back an appropriate response.
    Parameters:
      `exchange` - the `HttpExchange` to be filtered
      `chain` - the `Chain` which allows the next filter to be invoked
    Throws:
      `IOException` - may be thrown by any filter module, and if caught, must be rethrown again
      `NullPointerException` - if either exchange or chain are `null`

  - ### description

    public abstract String description()
    Returns a short description of this `Filter`.
    Returns:
      a `String` describing the `Filter`

  - ### beforeHandler

    public static Filter beforeHandler(String description, Consumer<HttpExchange> operation)
    Returns a pre-processing `Filter` with the given description and operation.

    The `operation` is the effective implementation of the filter. It is executed for each `HttpExchange` before invoking either the next filter in the chain or the exchange handler (if this is the final filter in the chain). Exceptions thrown by the `operation` are not handled by the filter.

    API Note:
      A beforeHandler filter is typically used to examine or modify the exchange state before it is handled. The filter `operation` is executed before `Filter.Chain.doFilter(HttpExchange)` is invoked, so before any subsequent filters in the chain and the exchange handler are executed. The filter `operation` is not expected to handle the request or send response headers, since this is commonly done by the exchange handler.

      Example of adding the `"Foo"` response header to all responses:

              var filter = Filter.beforeHandler("Add response header Foo",
                          e -> e.getResponseHeaders().set("Foo", "Bar"));
              httpContext.getFilters().add(filter);

    Parameters:
      `description` - the string to be returned from `description()`
      `operation` - the operation of the returned filter
    Returns:
      a filter whose operation is invoked before the exchange is handled
    Throws:
      `NullPointerException` - if any argument is null
    Since:
      17

  - ### afterHandler

    public static Filter afterHandler(String description, Consumer<HttpExchange> operation)
    Returns a post-processing `Filter` with the given description and operation.

    The `operation` is the effective implementation of the filter. It is executed for each `HttpExchange` after invoking either the next filter in the chain or the exchange handler (if this filter is the final filter in the chain). Exceptions thrown by the `operation` are not handled by the filter.

    API Note:
      An afterHandler filter is typically used to examine the exchange state rather than modifying it. The filter `operation` is executed after `Filter.Chain.doFilter(HttpExchange)` is invoked, this means any subsequent filters in the chain and the exchange handler have been executed. The filter `operation` is not expected to handle the exchange or send the response headers. Doing so is likely to fail, since the exchange has commonly been handled before the `operation` is invoked. More specifically, the response may be sent before the filter `operation` is executed.

      Example of adding a filter that logs the response code of all exchanges:

              var filter = Filter.afterHandler("Log response code", e -> log(e.getResponseCode());
              httpContext.getFilters().add(filter);

      Example of adding a sequence of afterHandler filters to a context:
      The order in which the filter operations are invoked is reverse to the order in which the filters are added to the context's filter-list.

              var a1Set = Filter.afterHandler("Set a1", e -> e.setAttribute("a1", "some value"));
              var a1Get = Filter.afterHandler("Get a1", e -> doSomething(e.getAttribute("a1")));
              httpContext.getFilters().addAll(List.of(a1Get, a1Set));

      The operation of `a1Get` will be invoked after the operation of `a1Set` because `a1Get` was added before `a1Set`.

    Parameters:
      `description` - the string to be returned from `description()`
      `operation` - the operation of the returned filter
    Returns:
      a filter whose operation is invoked after the exchange is handled
    Throws:
      `NullPointerException` - if any argument is null
    Since:
      17

  - ### adaptRequest

    public static Filter adaptRequest(String description, UnaryOperator<Request> requestOperator)
    Returns a pre-processing Filter that inspects and possibly adapts the request state. The `Request` returned by the `requestOperator` will be the effective request state of the exchange. It is executed for each `HttpExchange` before invoking either the next filter in the chain or the exchange handler (if this is the final filter in the chain). Exceptions thrown by the `requestOperator` are not handled by the filter.
    API Note:
      When the returned filter is invoked, it first invokes the `requestOperator` with the given exchange, `ex`, in order to retrieve the adapted request state. It then invokes the next filter in the chain or the exchange handler, passing an exchange equivalent to `ex` with the adapted request state set as the effective request state.

      Example of adding the `"Foo"` request header to all requests:

              var filter = Filter.adaptRequest("Add Foo header", r -> r.with("Foo", List.of("Bar")));
              httpContext.getFilters().add(filter);

    Parameters:
      `description` - the string to be returned from `description()`
      `requestOperator` - the request operator
    Returns:
      a filter that adapts the request state before the exchange is handled
    Throws:
      `NullPointerException` - if any argument is null
    Since:
      18