For convenience, there are several forms of the parse method. Most of these take two arguments, a base, and some specification of the MoML to parse (a stream or the text itself). The base is used to interpret relative URLs that might be present in the MoML. For example, the base might be the document base of an applet. An applet might use this class as follows:
MoMLParser parser = new MoMLParser(); URL docBase = getDocumentBase(); URL xmlFile = new URL(docBase, modelURL); NamedObj toplevel = parser.parse(docBase, xmlFile);If the first argument to parse() is null, then it is assumed that all URLs in the MoML file are absolute.
It can be difficult to create an appropriate URL to give as a base, particularly if what you have is a file or file name in the directory that you want to use as a base. The easiest technique is to use the toURL() method of the File class. Some of the URL constructors, for reasons we don't understand, create URLs that do not work.
The MoML code given to a parse() method may be a fragment, and does not need to include the "<?xml ... >" element nor the DOCTYPE specification. However, if the DOCTYPE specification is not given, then the DTD will not be read. The main consequence of this, given the parser we are using, is that default values for attributes will not be set. This could cause errors. The parser itself is not a validating parser, however, so it makes very limited use of the DTD. This may change in the future, so it is best to give the DOCTYPE element.
The parse() methods can be used for incremental parsing. After creating an initial model using a call to parse(), further MoML fragments without top-level entity or derived objects can be evaluated to modify the model. You can specify the context in which the MoML to be interpreted by calling setContext(). However, the XML parser limits each fragment to one element. So there always has to be one top-level element. If you wish to evaluate a group of MoML elements in some context, set the context and then place your MoML elements within a group element, as follows:
<group> ... sequence of MoML elements ... </group>The group element is ignored, and just serves to aggregate the MoML elements, unless it has a name attribute. If it has a name attribute, then the name becomes a prefix (separated by a colon) of all the names of items immediately in the group element. If the value of the name attribute is "auto", then the group is treated specially. Each item immediately contained by the group (i.e. not deeply contained) will be created with its specified name or a modified version of that name that does not match a pre-existing object already contained by the container. That is, when name="auto" is specified, each item is forced to be created with unique name, rather than possibly matching a pre-existing item.
The parse methods throw a variety of exceptions if the parsed data does not represent a valid MoML file or if the stream cannot be read for some reason.
This parser supports the way Ptolemy II handles hierarchical models, where components are instances cloned from reference models called "classes." A model (a composite entity) is a "class" in Ptolemy II if the elementName field of its MoMLInfo object is the string "class". If a component is cloned from a class, then when that component exports MoML, it references the class from which it was cloned and exports only differences from that class. I.e., if further changes are made to the component, it is important that when the component exports MoML, that those changes are represented in the exported MoML. This effectively implements an inheritance mechanism, where a component inherits all the features of the master from which it is cloned, but then extends the model with its own changes.
This class always processes MoML commands in the following order within a "class" or "entity" element, irrespective of the order in which they appear:
This class works closely with MoMLChangeRequest to implement another feature of Ptolemy II hierarchy. In particular, if an entity is cloned from another that identifies itself as a "class", then any changes that are made to the class via a MoMLChangeRequest are also made to the clone. This parser ensures that those changes are not exported when MoML is exported by the clone, because they will be exported when the master exports MoML. @see MoMLChangeRequest @author Edward A. Lee, Steve Neuendorffer, John Reekie, Contributor: Christopher Hylands @version $Id: MoMLParser.java,v 1.343 2007/12/07 06:28:36 cxh Exp $ @since Ptolemy II 0.4 @Pt.ProposedRating Red (eal) @Pt.AcceptedRating Red (johnr)
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