Jackan organization logo

This release allows to search Ckan, write into it and convert CKAN metadata into DCAT format. If you are upgrading from previous version, see Release notes.

Getting started

With Maven: If you use Maven as build system, put this in the dependencies section of your pom.xml:


Without Maven: you can download Jackan jar and its dependencies from here, then copy the jars to your project classpath.

In case updates are available, version numbers follow semantic versioning rules.

Search ckan

Get the dataset list of dati.trentino.it:

Code can be found in TestApp1.java

import eu.trentorise.opendata.jackan.CkanClient; public class TestApp1 { public static void main(String[] args) { CkanClient cc = new CkanClient("http://dati.trentino.it"); System.out.println(cc.getDatasetList()); } }

Get list of first 10 datasets of dati.trentino.it and print their resources:

Code can be found in TestApp2.java

import eu.trentorise.opendata.jackan.CkanClient; import eu.trentorise.opendata.jackan.model.CkanDataset; import eu.trentorise.opendata.jackan.model.CkanResource; import java.util.List; public class TestApp2 { public static void main(String[] args) { CkanClient cc = new CkanClient("http://dati.trentino.it"); List<String> ds = cc.getDatasetList(10, 0); for (String s : ds) { System.out.println(); System.out.println("DATASET: " + s); CkanDataset d = cc.getDataset(s); System.out.println(" RESOURCES:"); for (CkanResource r : d.getResources()) { System.out.println(" " + r.getName()); System.out.println(" FORMAT: " + r.getFormat()); System.out.println(" URL: " + r.getUrl()); } } } }

Should give something like this:

DATASET: abitazioni RESOURCES: abitazioni FORMAT: JSON URL: http://www.statweb.provincia.tn.it/INDICATORISTRUTTURALISubPro/exp.aspx?idind=133&info=d&fmt=json abitazioni FORMAT: CSV URL: http://dati.trentino.it/storage/f/2013-06-16T113651/_lcmGkp.csv numero-di-abitazioni FORMAT: JSON URL: http://www.statweb.provincia.tn.it/INDICATORISTRUTTURALISubPro/exp.aspx?ntab=Sub_Numero_Abitazioni&info=d&fmt=json numero-di-abitazioni FORMAT: CSV URL: http://dati.trentino.it/storage/f/2013-06-16T113652/_yWBmJG.csv DATASET: abitazioni-occupate RESOURCES: abitazioni-occupate FORMAT: JSON URL: http://www.statweb.provincia.tn.it/INDICATORISTRUTTURALISubPro/exp.aspx?idind=134&info=d&fmt=json abitazioni-occupate FORMAT: CSV URL: http://dati.trentino.it/storage/f/2013-06-16T113653/_iaMMc2.csv numero-di-abitazioni-occupate FORMAT: JSON URL: http://www.statweb.provincia.tn.it/INDICATORISTRUTTURALISubPro/exp.aspx?ntab=Sub_Numero_Abitazioni_Occupate&info=d&fmt=json numero-di-abitazioni-occupate FORMAT: CSV URL: http://dati.trentino.it/storage/f/2013-06-16T113654/__lLACk.csv ...

Search datasets filtering by tags and groups:

Code can be found in TestApp3.java

import eu.trentorise.opendata.jackan.CkanClient; import eu.trentorise.opendata.jackan.CkanQuery; import eu.trentorise.opendata.jackan.model.CkanDataset; import java.util.List; public class TestApp3 { public static void main(String[] args) { CkanClient cc = new CkanClient("http://dati.trentino.it"); CkanQuery query = CkanQuery.filter().byGroupNames("turismo").byTagNames("ristoranti"); List<CkanDataset> filteredDatasets = cc.searchDatasets(query, 10, 0).getResults(); for (CkanDataset d : filteredDatasets) { System.out.println(); System.out.println("DATASET: " + d.getName()); } } }

Should give something like this:

DATASET: osterie-tipiche-trentine DATASET: poi-trento DATASET: punti-di-interesse-valsugana DATASET: poi-altopiano-di-pine-e-valle-di-cembra DATASET: punti-di-ristoro-vivifiemme-2013

Write in Ckan

Supported operations

First, a brief recap of operations for writing offered by ckan:

  • create: in ckan creation often acts more as upsert, that is, if object with existing id/name already exists it is updated
  • update: update completely replaces stuff on server, and if you don't send a list or set it to null it gets emptied on the server. This can problematic for example when updating datasets containg a list of resources.
  • patch: added in Ckan 2.3 for less destructive updates. Jackan does not implement patch operations so far and instead offers so-called patch-update operations that emulate patch but only by calling update (so they work also in ckan < 2.3)
  • delete: marks objects as as non-visible in the website, but they will still be retrievable with the webapi if you know the ids. To really delete things purge operations would need to be implemented.
  • purge: this one really deletes stuff

Currently Jackan supports:

create update patch patch update delete purge
Resource X* X* X* X
Dataset X X X X
Group X
Organization X
User X
Tag X
Vocabulary X

*Resource create and update change only metadata, the don't allow uploading/modifying files.

Data validation

Sometimes Ckan forgets to properly validate input. For example, at least with Ckan 2.2 we have been able to create resources with empty id :-/ To prevent writing such garbage we extended default CkanClient with CheckedCkanClient, which is more picky about possibly inconsistent input. If you also care about data integrity you might want to use the Checked client or extend it with your own validation rules when writing into Ckan. To try how different clients behave against the extensive Jackan test suite when running tests we set the client client class to use as parameter jackan.test.ckan.client-class=eu.trentorise.opendata.jackan.CheckedCkanClient in conf/jackan.test.properties
Maybe in the future we will implement also java.validation api support.

What we POST

All writable classes have an ancestor with "Base" appended to the Ckan object name, like CkanDatasetBase.
When writing Jackan sends to Ckan only the non-null fields of such base classes (except for patch-update, which is more sophisticated). Notice CKAN instances might have custom data schemas that force presence of custom properties among ‘regular’ ones. In this case, they go to java others hashmap and when serialized are put into the main json body (Note that to further complicate things there is also an extrasfield).

Examples for writing

Many test cases for writing can be found in WriteCkan*IT.java files. Here we just report a couple of them.

Write a dataset
 	  // here we use CheckedCkanClient for extra safety
        CkanClient myClient = new CheckedCkanClient("http://put-your-catalog.org", "put your ckan api key token");

        CkanDatasetBase dataset = new CkanDatasetBase();
        dataset.setName("my-cool-dataset-" + new Random().nextLong());
        // notice Jackan will only send field 'name' as it is non-null
        CkanDataset createdDataset = myClient.createDataset(dataset);

        checkNotEmpty(createdDataset.getId(), "Invalid dataset id!");
        assertEquals(dataset.getName(), createdDataset.getName());
        System.out.println("Dataset is available online at " + CkanClient.makeDatasetURL(myClient.getCatalogURL(), dataset.getName()));

Patch update a dataset

Shows Jackan-specific patch-update functionality, in this case for changing tags assigned to a dataset (and also shows that new free tags can be created at dataset creation)

        // here we use CheckedCkanClient for extra safety
        CkanClient myClient = new CheckedCkanClient("http://put-your-catalog.org", "put your ckan api key token");

		// we create a dataset with one tag 'cool'
        CkanDatasetBase dataset = new CkanDatasetBase("my-dataset-" + new Random().nextLong());
        List<CkanTag> tags_1 = new ArrayList();
        tags_1.add(new CkanTag("cool"));
        CkanDataset createdDataset = myClient.createDataset(dataset);

        // now we assign a new array with one tag ["amazing"] 
        List<CkanTag> tags_2 = new ArrayList();
        tags_2.add(new CkanTag("amazing"));

        // let's patch-update, jackan will take care of merging tags to prevent erasure of 'cool'
        CkanDataset updatedDataset = myClient.patchUpdateDataset(createdDataset);

        assert 2 == updatedDataset.getTags().size(); //  'amazing' has been added to ['cool']
        System.out.println("Merged tags = "
                + updatedDataset.getTags().get(0).getName()
                + ", " + updatedDataset.getTags().get(1).getName());

        System.out.println("Updated dataset is available online at " + CkanClient.makeDatasetURL(myClient.getCatalogURL(), dataset.getName()));

JSON Serialization

For ser/deserializing JSON there are two kinds of configurations, a default one for reading from ckan and one for writing (that is, POSTing). Most probably you are interested in the default one.

Jackson library annotations are used to automatically convert to/from JSON using Jackson's ObjectMapper object. Notice that although field names of Java objects are camelcase (like authorEmail), serialized fields follows CKAN API stlye and use underscores (like author_email).

Default JSON Ser/deserialization

Here is an example of serialization/deserialization:

		// your Jackson ObjectMapper
        ObjectMapper objectMapper = new ObjectMapper();
        CkanDataset dataset = new CkanDataset();
        String json = objectMapper.writeValueAsString(dataset);

        System.out.println("json = " +  json);

        CkanDataset reconstructed = objectMapper.readValue(json, CkanDataset.class);
        assert "hello".equals(reconstructed.getName());

For more fine-grained control you can just register JackanModule into your Jackson object mapper:

        ObjectMapper objectMapper = new ObjectMapper();
        objectMapper.registerModule(new JackanModule());

Posting JSON

This more advanced usage is for the case you want to do your own POST operations (create/update/delete/purge) to ckan (or maybe extend Jackan :-) ...

Notice for this you might need a different object mapper for each class you intend to post, so to be able to configure each mapper in a fine-grained way. You can find an example for datasets in method CkanClient.configureObjectMapperForPosting:

        ObjectMapper mapperForDatasetPosting = new ObjectMapper();
        CkanClient.configureObjectMapperForPosting(mapperForDatasetPosting, CkanDatasetBase.class);
        CkanDataset dataset = new CkanDataset("random-name-" + new Random().nextLong());
        // this would be the POST body. 
        String json = mapperForDatasetPosting.writeValueAsString(dataset);


CKAN uses timestamps in a format like 1970-01-01T01:00:00.000010. In the client we store them as java.sql.Timestamp so to be able to preserve the microseconds. To parse/format Ckan timestamps, use

    CkanClient.formatTimestamp(new Timestamp(123));


DCAT is an emerging W3C standard for representing catalog metadata. For this reason, when we use Jackan we usually convert Ckan objects to their DCAT representation, which gives us a consistent well defined view of open data catalogs.

There has long been a plugin for ckan to serve metadata as rdf in DCAT format, but according to maintainer (July 2015):
Historically you have been able to access an RDF representation of a CKAN dataset metadata by navigating to /dataset/{id}.rdf or /dataset/{id}.n3. These were rendered using templates, and were outdated, incomplete and broken [1].
Situation on ckan side is getting much better with the new version of the plugin in progress, but we cannot expect all CKAN instances around the world to adopt it now. So currently we provide a class to convert from CKAN objects to their DCAT equivalent called DcatFactory. It will convert a CkanDataset to a DcatDataset and a CkanResource to a DcatDistribution according to this mapping.

Examples code:

        DcatFactory dcatFactory = new DcatFactory();

        CkanDataset ckanDataset = new CkanDataset("my-dataset");
        DcatDataset dcatDataset
                = dcatFactory.makeDataset(
                        Locale.ITALIAN); // default locale of metadata

        CkanResource ckanResource = new CkanResource(

        DcatDistribution dcatDistribution
                = dcatFactory.makeDistribution(
                        "my-dataset", // owner dataset id
                        "cc-zero", // license id
                        Locale.ITALIAN); // default locale of metadata

To extract more stuff during conversion, you can use GreedyDcatFactory or extend DcatFactory and override the extract* and/or postProcess* methods.


Jackan uses native Java logging system (JUL). If you also use JUL in your application and want to see Jackan logs, you can take inspiration from jackan test logging properties. If you have an application which uses SLF4J logging system, you can route logging with JUL to SLF4J bridge, just remember to programmatically install it first.