Update Solr index by http request

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SOLR

Let's assume we have the following index structure:

<field name="id" type="string" indexed="true" stored="true"
required="true" />
<field name="user" type="string" indexed="true" stored="true"/>

And we would like to add a single value to this index. In cookbook it's done by such command

curl 'localhost:8983/solr/update?commit=true' -H 'Contenttype:
application/json' -d '[{"id":"1","user":{"add":"jack"}}]'

How can I add value to index from java code by passing http request?