Question

EDIT: by request of some of you I’m posting my real data structures and classes. I failed in trying to make it easier for you to understand the problem.

I have a problem in resolving with Jackson null JSON array elements, ending up with an exception that I’ll report later in this post.

I have a long-winded JSON string coming from a 3rd party provider:

[
  null,
  null,
  [
    {
      "t" : {
        "id" : 0,
        "it" : "Attuazione"
      },
      "c" : "DM.10.0.2",
      "l" : "Macro 2",
      "v0" : 0,
      "scid" : "1",
      "m" : "10",
      "sam" : 0,
      "v1" : 100,
      "smcm" : true,
      "w" : "OD.10.4",
      "st" : {
        "auto_formula" : {
          "ia" : "($x > 0 ? 1 : 0)",
          "id" : "($x > 0 ? 1 : 0)",
          "bit" : "($x > 0 ? 1 : 0)",
          "oa" : "($x > 0 ? 1 : 0)",
          "od" : "($x > 0 ? 1 : 0)",
          "dm" : "($x > 0 ? 1 : 0)"
        },
        "id" : 0,
        "css_rule" : "containerTypeOnOff",
        "default_measure_unit" : null,
        "it" : "On \/ Off",
        "widget" : {
          "id" : 0,
          "caption" : {
            "it" : "Etichetta di testo"
          }
        }
      }
    }
  ],
  null,
  null,
  null,
  null,
  null
]

My Java classes ensemble is the following.

This is the outmost container:

public class TinyMap {
    ArrayList<TinyMapModule> modules;

    public TinyMap() { ...  }

    public ArrayList<TinyMapModule> getModules() { ... }

    /**
     * @param modules the modules to set
     */
    @SuppressWarnings("unchecked")
    public void setModules(ArrayList<TinyMapModule> modules) { ... }

    public void appendNewMacro(TinyMapModule m) { ... }

}

And this is the first-level container:

public class TinyMapModule {
    private ArrayList<TinyMapMacro> macros;

    /**
     * C'tor 
     */
    public TinyMapModule() { ... }

    /**
     * @return ArrayList<TinyMapMacro> 
     */
    public ArrayList<TinyMapMacro> getMacros() { ... }

    /**
     * @param macros the macros to set
     */
    @SuppressWarnings("unchecked")
    public void setMacros(ArrayList<TinyMapMacro> macros) { ... }

}

And this is the innermost level:

public class TinyMapMacro {
    private String c; 
    private String l;
    private String m;
    private MacroType t;
    private String w;

    private static Logger logger;

    static {
        logger = Logger.getLogger(TinyMapMacro.class);
        DOMConfigurator.configure("log4j.xml");
    }

    /**
     * C'tor 
     */
    public TinyMapMacro() { ... }

    public static class MacroType {
        private int id;
        private String it; // Italian

        public int getId() { ... }
        public void setId(int id) { ... }
        public String getIt() { ... }
        public void setIt(String it) { ... }
    }

    public String getC() { ... }

    public void setC(String c) { ... }

    public String getL() { ... }

    public void setL(String l) { ... }

    public String getM() { ... }

    public void setM(String m) { ... }

    public MacroType getT() { ... }

    public void setT(MacroType t) { ... }

    public String getW() { ... }

    public void setW(String w) { ... }

    public int getTypeId() { ... }

    public String getLocalizedTypeString() { ... }

}

Finally, this is my deserialization code:

ObjectMapper mapper = new ObjectMapper();
mapper.disable(DeserializationFeature.FAIL_ON_UNKNOWN_PROPERTIES);
mapper.disable(DeserializationFeature.FAIL_ON_NULL_FOR_PRIMITIVES);
ObjectReader reader = mapper.reader(TinyMapModule.class);
try {
    logger.info("Instantiating ObjectMapper");
    TinyMap tm = new TinyMap();
    tm.buildTinyMap(reader.readValues(jsonString.replaceAll("null", "[{\"name\":\"\"}]")));
    for (int i = 0; i < tm.getModules().size(); i++) {
        if (tm.getModules().get(i) == null) 
            continue;
        for (int j = 0; j < tm.getModules().get(i).getMacros().size(); j++) {
            if (tm.getModules().get(i).getMacros().get(j) == null) 
                continue;
            System.out.println(String.format("c: %s", tm.getModules().get(i).getMacros().get(i).getC()));
            System.out.println(String.format("l: %s", tm.getModules().get(i).getMacros().get(i).getL()));
            System.out.println(String.format("m: %s", tm.getModules().get(i).getMacros().get(i).getM()));
            System.out.println(String.format("t.id: %d", tm.getModules().get(i).getMacros().get(i).getTypeId()));
            System.out.println(String.format("t.it: %s", tm.getModules().get(i).getMacros().get(i).getLocalizedTypeString()));
            System.out.println(String.format("w: %s", tm.getModules().get(i).getMacros().get(i).getW()));
        }
    }
    logger.info("Execution terminated");
} 
... 

In extreme synthesis, I tried to schematize the problem as follows:

1. TinyMap class has a modules member, which is an ArrayList of TinyMapModule objects
2. TinyMapModule class has a macros member, which is an ArrayList of TinyMapMacro objects
3. TinyMapMacro class maps objects with the only attributes I strictly need.

Now, when I try to deserialize the above JSON string into my objects I get an exception telling me that JSON can’t be deserialized:

Exception in thread "main" com.fasterxml.jackson.databind.RuntimeJsonMappingException: Can not deserialize instance of it.asystelsrl.hcsbridge.communication.TinyMapModule out of START_ARRAY token
 at [Source: java.io.StringReader@3c50507; line: 1, column: 2]
    at com.fasterxml.jackson.databind.MappingIterator.next(MappingIterator.java:122)
    at it.asystelsrl.hcsbridge.communication.TinyMap.buildTinyMap(TinyMap.java:47)
    at it.asystelsrl.hcsbridge.tests.TestTinyMap.main(TestTinyMap.java:69)
Caused by: com.fasterxml.jackson.databind.JsonMappingException: Can not deserialize instance of it.asystelsrl.hcsbridge.communication.TinyMapModule out of START_ARRAY token
 at [Source: java.io.StringReader@3c50507; line: 1, column: 2]
    at com.fasterxml.jackson.databind.JsonMappingException.from(JsonMappingException.java:164)
    at com.fasterxml.jackson.databind.DeserializationContext.mappingException(DeserializationContext.java:599)
    at com.fasterxml.jackson.databind.DeserializationContext.mappingException(DeserializationContext.java:593)
    at com.fasterxml.jackson.databind.deser.BeanDeserializer.deserializeFromArray(BeanDeserializer.java:531)
    at com.fasterxml.jackson.databind.deser.BeanDeserializer.deserialize(BeanDeserializer.java:141)
    at com.fasterxml.jackson.databind.MappingIterator.nextValue(MappingIterator.java:188)
    at com.fasterxml.jackson.databind.MappingIterator.next(MappingIterator.java:120)
    ... 2 more

I already read a lot about Jackson annotations, custom deserializers, etc. but the only solution I seem to come up with is to convert the TinyMap.modules member from ArrayList<TinyMapModules> to ArrayList<Object> and use a custom deserializer to handle NullType objects.

By this choice (if feasible, because I have not tested it yet) I would lose semantic value of the TinyMap.modules member, and I wouldn’t be forced to.

Is there another way I can resolve the null elements issue?

EDIT:

It is important I can preserve ordinal position of non-null objects within this structure, so I should definitely at least replace null elements with something usable as a placeholder.

Answer 1

The problem here is that I made a lot of confusion using Jackson.

I came to understand it reading this article by ProgrammerBruce.

I was trying to deserialize a Collection into something which it is not, as StaxMan suggested in his comment.

Thanks to ProgrammerBruce for his useful and clear article.