Cut the json string in to two different json string in java

Create a class to hold your JSON, like below:

public class Bar {

    private String var;

    @JsonProperty("_env")
    private Object env;

    public String getVar() {
        return var;
    }

    public void setVar(String var) {
        this.var = var;
    }

    public Object getEnv() {
        return env;
    }

    public void setEnv(Object env) {
        this.env = env;
    }
}

And set the mapper to not fail on unknown properties before deserializing;

public static void main(String[] argv) throws Exception {
    ObjectMapper mapper = new ObjectMapper();
    mapper.configure(DeserializationFeature.FAIL_ON_UNKNOWN_PROPERTIES, false);

    Bar bar = mapper.readValue(JSON, Bar.class);

}

This way, both when the _env is present or not, it will work.

If you need to "split" the json by each node, you can do something like this:

JsonNode node = mapper.readTree(JSON);
List<String> nodeJsons = new ArrayList<>();
Iterator<Entry<String, JsonNode>> nodeIterator = node.fields();
while (nodeIterator.hasNext()) {
    Map.Entry<String, JsonNode> entry = nodeIterator.next();
    nodeJsons.add(mapper.writeValueAsString(entry));
}

This way you will have a list of json strings in the end with every node serialized by itself, instead of one "big" json with everything.

with dynamic input, it is best to load json into a Map:

public static void main(String[] args)
{
    ObjectMapper mapper = new ObjectMapper();
    try (Reader reader = new FileReader("C://Temp/xx.json")) {
        // load whatever json into Map
        @SuppressWarnings("unchecked")
        Map<String, Object> inMap = mapper.readValue(reader, Map.class);
        System.out.println(inMap);
        // exmaple for generating json for each key found 
        for (String key : inMap.keySet()) {
            mapper.writeValue(System.out, Collections.singletonMap(key, inMap.get(key)));
        }
    } catch (Exception e) {
        e.printStackTrace();
    }
}

No need for custom class and obscure jackson configuration. Whatever is in the input gets loaded into the Map. You can produce a json for every separate key, or interogate the map contents any way yyou like