A comprehensive filtering system.

A filter is a strategy to decide whether a certain object should be accepted or not; the type of objects a filter considers is called the base type of the filter. In most cases, the base type is going to be a URL or a fetched page. More precisely, a prefetch filter is one that has BURL as its base type (typically: to decide whether a URL should be scheduled for later visit, or should be fetched); a postfetch filter is one that has FetchedResponse as base type and decides whether to do something with that response (typically: to parse it, to store it, etc.).

Various kinds of filters are available, and moreover they can be composed with boolean operators using the static methods specified in the Filters class. Additionally, a filter parser is provided in the package; since the parser itself is written using JavaCC, we provide a description of it here.

Two filters are called homogeneous if they filter the same kind of objects, heterogeneous otherwise.

A filter parser is instantiated on the basis of the kind of filters it will actually return; more precisely a FilterParser<T> is a filter parser that will return a Filter<T>; for technical reasons, the class T must be provided as unique parameter when the parser is constructed. A parser can be used many times. Every time a filter is sought, the parse(String x) method of the parser is called, which returns a filter of the correct kind, or throws a parse exception.

The syntax used by the filter parser is available. Basically, it is a propositional calculus, with and (denoted by infix and or &), or (denoted by infix or or |) and not (denoted by prefix not or !), whose ground terms have the same form as returned by the toString() method of the Filter class.

Here are some examples:

  • HostEquals(
  • (HostEndsWith( and not ForbiddenHost(http://xxx.yyy.zzz/list-of-forbidden-hosts)) or NoMoreSlashThan(10)

Usually, an expression should only contain references to homogeneous filters of type T, where T is the type used to instantiate the parser. Nonetheless, if some ground term refers to a filter of some other type D, the parser will try to find a static method in the Filters class having the following signature:

        public static Filter<T> adaptFilterD2T( Filter<D> f ) 

that adapts the given filter f to a filter of the correct type. If this method is missing, the parser will itself throw an exception.

  • Interface Summary
    Interface Description
    A filter is a strategy to decide whether to accept a given object or not.
  • Class Summary
    Class Description
    An abstract implementation of a Filter providing a method that helps in implementing properly Object.toString() for atomic (i.e., class-based) filters.
    Accepts only fetched response whose content type starts with a given string.
    Accepts only records of given digest, specified as a hexadecimal string.
    Accepts only URIs whose path does not contain too many duplicate segments.
    A collection of static methods to deal with filters.
    Accepts only URIs whose host ends with (case-insensitively) a certain suffix.
    Accepts only URIs whose host equals (case-insensitively) a certain string.
    Accepts only records that are http/https responses.
    Accepts only http responses whose content stream appears to be binary.
    Accepts only URIs whose path ends (case-insensitively) with one of a given set of suffixes.
    Accepts only URIs whose scheme equals a certain string (typically, http).
    Accepts only fetched response whose status category (status/100) has a certain value.
    Accepts only a given URIs.
    Accepts only URIs that match a certain regular expression.
    Accepts only URIs whose overall length is below a given threshold.