[GitHub] jena issue #139: JSON-LD output

[afs <gi...@git.apache.org>]

Github user afs commented on the issue:

    https://github.com/apache/jena/pull/139
  
    Apologies that this has taken so long.
    
    > TODO some documentation. Where should it be added? The best form would be small sample code. 
    
    Examples can go in `jena-arq/src-examples/arq/examples/riot/`. A new section about JSON-LD output for https://jena.apache.org/documentation/io/rdf-output.html, and use code links to apache/jena github, master branch. 
    
    Symbols are fine but URI's please! (changing `RIOT.riotIRI` to "http://jena.apache.org/riot"):
    
    e.g.  in JsonLDWriter
    ```
        public static final Symbol JSONLD_CONTEXT = Symbol.create("JSONLD_CONTEXT");
    ```
    
    **RDFDataMgr**
    
    The way to set things specially for a writing is to do:
    
    ```
        WriterDatasetRIOT w = RDFDataMgr.createDatasetWriter(RDFFormat.JSONLD_COMPACT_FLAT) ;
        w.write(System.out,  dataset.asDatasetGraph(), null, "http://base", RIOT.getContext()) ;
    ```
    rather than add `RDFDataMgr.write(..., Context cxt)`
    
    See [examples/riot/ExRIOT_2](https://github.com/apache/jena/blob/master/jena-arq/src-examples/arq/examples/riot/ExRIOT_2.java) for an example for a reader.
    
    If you want, add a new API class specific to JSON output `JsonLdMgr` (to go in the same package as `RDFDataMgr`) with JSON-LD-specific functions to wrap common ways to call JSON-LD output. 
    
    **Constants**
    
    This could be tidied out with a subclass of `RDFFormatVariant` that carries JSON-LD information:
    
    ```
        // From JSON writer
        public static enum JSONLD_FORMAT {
            COMPACT, FLATTEN, EXPAND, FRAME
        }
        
        static class JSONLDFormatVariant extends RDFFormatVariant {
            private JSONLD_FORMAT option ;
            private boolean prettyJson ;
    
            public JSONLDFormatVariant(String name, boolean prettyJson, JSONLD_FORMAT option) { 
                super(name) ;
                this.options = option ;
                this.prettyJson = prettyJson ;
            }
            
            public boolean prettyJson() { return prettyJson ; }
            
            public boolean option(JSONLD_FORMAT fmt) {
                for ( JSONLD_FORMAT f : options ) {
                    if ( fmt == f )
                        return true ; 
                }
                return false ;
            }
        }
    
    
        private static final RDFFormatVariant JSONLD_EXPAND_PRETTY      = new JSONLDFormatVariant("expand pretty", true, JSONLD_FORMAT.EXPAND) ;
        private static final RDFFormatVariant JSONLD_EXPAND_FLAT        = new JSONLDFormatVariant("expand flat", false, JSONLD_FORMAT.COMPACT) ;
        ...
    ```
    then use in JsonLDWriter by casting to and JSONLDFormatVariant
    ```
            JSONLDFormatVariant format = (JSONLDFormatVariant)format ;
    ```
    ```        
            if ( format.option(COMPACT) ) {
              ...
            }
    ```
    
    **Tests**
    
    I ran the tests for JSONLD but got:
    
    ```
    10:13:04 WARN  JsonLDWriter              :: JSONLD_CONTEXT value is not a String. Assuming a context object expected by JSON-LD JsonLdProcessor.compact or flatten
    10:13:04 INFO  TestJsonLDWriter          :: Sorry to get this exception
    org.apache.jena.riot.RiotException: com.github.jsonldjava.core.JsonLdError: loading remote context failed: http://schema.org/
        at org.apache.jena.riot.out.JsonLDWriter.serialize(JsonLDWriter.java:218)
        at org.apache.jena.riot.out.JsonLDWriter.write(JsonLDWriter.java:123)
        at org.apache.jena.riot.out.JsonLDWriter.write(JsonLDWriter.java:129)
        at org.apache.jena.riot.system.RiotLib$WriterAdapter.write(RiotLib.java:376)
        at org.apache.jena.riot.RDFDataMgr.write$(RDFDataMgr.java:1232)
        ...
    ```
    
    We need to be able to run the tests without assuing that external network resources are availanle and functioning.
    
    The tests and build should work if there is no extenral connectivity.
    
    **Code organisation**
    
    If you feel there classes, fell free to create a package org.apache.jena.riot.out.jsonld to put them together.
    
    **Other**
    
    Indentation: spaces, not tabs please.



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