Package opendap.servers

Class FunctionLibrary

  • public class FunctionLibrary
extends Object
    Represents a library of available server-side functions for use in evaluating constraint expressions.

    When created, a FunctionLibrary is empty. There are two ways to populate it, described below. Once the FunctionLibrary has been created and populated, it should be used to create a ClauseFactory, which can then be given to the CEEvaluator for use in parsing.

    The most straightforward is to pass an instance of each ServerSideFunction class to the add() method. If you have a large number of ServerSideFunctions, or if you want to have choose class names for your functions independent of the actual function name, this is the approach to use.

    A second, more complex method resolves server-side function names at runtime. The function library automatically looks for a class whose name matches the name requested (if a prefix is set, it will look for a class whose name is prefix + name requested). If such a class exists, and it implements the ServerSideFunction interface, a new instance of the class is created using the default constructor, added to the list of available functions, and returned to the caller.

    This is not quite as complicated as it sounds. For instance, say the FunctionLibrary's prefix is set to "opendap.servers.test.SSF". Then the first time the server gets a constraint expression containing the function "dummy()", the FunctionLibrary will automatically load the class "opendap.servers.test.SSFdummy", and return a new instance using the class's default constructor.

    This avoids the need to hardcode a list of server-side functions, and allows you to create new functions at any time. For instance to create a function "newfn()" you can simply create the class "opendap.servers.test.SSFnewfn" and put it somewhere in the server's classpath . Then the first CE containing this function will cause that class to be automatically loaded, just the like the dummy() function was.

    • Field Detail

      • boolFunctions

        protected Map boolFunctions

      • btFunctions

        protected Map btFunctions

      • prefix

        protected String prefix

    • Constructor Detail

      • FunctionLibrary

        public FunctionLibrary()
        Creates a new FunctionLibrary with no prefix set.

      • FunctionLibrary

        public FunctionLibrary​(String prefix)
        Creates a new FunctionLibrary.
        Parameters:
        prefix - A string that will be prepended to function names in order to create a classname for that function. For example,

    • Method Detail

      • setPrefix

        public void setPrefix​(String prefix)
        Sets the prefix to use for classname lookup. For instance, if the prefix is "myserver.SSF", then when the library doesn't have an entry for function "xyz" it will look for a class "myserver.SSFxyz".

      • getPrefix

        public String getPrefix()
        Returns the prefix being used for classname lookup.

      • add

        public void add​(ServerSideFunction function)
        Adds a function to the library. The function will be inspected to determine whether it is a boolean or BaseType function.

      • getBoolFunction

        public BoolFunction getBoolFunction​(String name)
                             throws opendap.dap.NoSuchFunctionException
        Retrieves a boolean function from the library. If the function is not found the library will attempt to load it using the mechanism described in the class documentation.
        Parameters:
        name - The name of the function being requested.
        Returns:
        Null if the function is not in the library, and the attempt to load it fails.
        Throws:
        opendap.dap.NoSuchFunctionException

      • getBTFunction

        public BTFunction getBTFunction​(String name)
                         throws opendap.dap.NoSuchFunctionException
        Retrieves a BaseType function from the library. If the function is not found the library will attempt to load it using the mechanism described in the class documentation.
        Parameters:
        name - The name of the function being requested.
        Returns:
        Null if the function is not in the library, and the attempt to load it fails.
        Throws:
        opendap.dap.NoSuchFunctionException

      • loadNewFunction

        protected void loadNewFunction​(String name)
        Tries to load a function with the given name.