QueryBuilderVisitorTests.java

    public static class QueryShapeVisitor implements QueryBuilderVisitor {
        private final SetOnce<String> queryType = new SetOnce<>();
        private final Map<BooleanClause.Occur, List<QueryShapeVisitor>> childVisitors = new EnumMap<>(BooleanClause.Occur.class);

        @Override
        public void accept(QueryBuilder qb) {
            queryType.set(qb.getName());
        }

        @Override
        public QueryBuilderVisitor getChildVisitor(BooleanClause.Occur occur) {
            // Should get called once per Occur value
            if (childVisitors.containsKey(occur)) {
                throw new IllegalStateException("getChildVisitor already called for " + occur);
            }
            final List<QueryShapeVisitor> childVisitorList = new ArrayList<>();
            QueryBuilderVisitor childVisitorWrapper = new QueryBuilderVisitor() {
                QueryShapeVisitor currentChild;
                @Override
                public void accept(QueryBuilder qb) {
                    currentChild = new QueryShapeVisitor();
                    childVisitorList.add(currentChild);
                    currentChild.accept(qb);
                }

                @Override
                public QueryBuilderVisitor getChildVisitor(BooleanClause.Occur occur) {
                    return currentChild.getChildVisitor(occur);
                }
            };
            childVisitors.put(occur, childVisitorList);
            return childVisitorWrapper;
        }

        public String toJson() {
            StringBuilder outputBuilder = new StringBuilder("{\"type\":\"").append(queryType.get()).append("\"");
            for (Map.Entry<BooleanClause.Occur, List<QueryShapeVisitor>> entry : childVisitors.entrySet()) {
                outputBuilder.append(",\"").append(entry.getKey().name().toLowerCase(Locale.ROOT)).append("\"[");
                boolean first = true;
                for (QueryShapeVisitor child : entry.getValue()) {
                    if (!first) {
                        outputBuilder.append(",");
                    }
                    outputBuilder.append(child.toJson());
                    first = false;
                }
                outputBuilder.append("]");
            }
            outputBuilder.append("}");
            return outputBuilder.toString();
        }
    }


    public void testQueryShapeVisitor() {
        QueryBuilder builder = new BoolQueryBuilder()
            .must(new TermQueryBuilder("foo", "bar"))
            .filter(new ConstantScoreQueryBuilder(new RangeQueryBuilder("timestamp").from("12345677").to("2345678")))
            .should(new BoolQueryBuilder()
                .must(new MatchQueryBuilder("text", "this is some text"))
                .mustNot(new RegexpQueryBuilder("color", "red.*"))
            )
            .must(new TermsQueryBuilder("genre", "action", "drama", "romance"));
        QueryShapeVisitor shapeVisitor = new QueryShapeVisitor();
        builder.visit(shapeVisitor);
        // {"type":"bool","must"[{"type":"term"},{"type":"terms"}],"filter"[{"type":"constant_score","filter"[{"type":"range"}]}],"should"[{"type":"bool","must"[{"type":"match"}],"must_not"[{"type":"regexp"}]}]}
        assertEquals(
            "{\"type\":\"bool\",\"must\"[{\"type\":\"term\"},{\"type\":\"terms\"}],\"filter\"[{\"type\":\"constant_score\",\"filter\"[{\"type\":\"range\"}]}],\"should\"[{\"type\":\"bool\",\"must\"[{\"type\":\"match\"}],\"must_not\"[{\"type\":\"regexp\"}]}]}",
            shapeVisitor.toJson());
    }

We can also do pretty-printing output:

        public String prettyPrintTree(String indent) {
            StringBuilder outputBuilder = new StringBuilder(indent).append(queryType.get()).append("\n");
            for (Map.Entry<BooleanClause.Occur, List<QueryShapeVisitor>> entry : childVisitors.entrySet()) {
                outputBuilder.append(indent).append("  ").append(entry.getKey().name().toLowerCase(Locale.ROOT)).append(":\n");
                for (QueryShapeVisitor child : entry.getValue()) {
                    outputBuilder.append(child.prettyPrintTree(indent + "    "));
                }
            }
            return outputBuilder.toString();
        }

That outputs:

bool
  must:
    term
    terms
  filter:
    constant_score
      filter:
        range
  should:
    bool
      must:
        match
      must_not:
        regexp

Great presentation of the solution for the problem.

Thanks @msfroh this is super helpful