How to use the Java API to build an Aggregation from JSON in 6.2.4?

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I found the same question here. But this is not working in ES 6.2.4.

In old version, we can do this:
SearchRequestBuilder builder = client.prepareSearch(index);
QueryBuilder queryBuilder = getQueryBuilder(query);
builder.setQuery(queryBuilder); // set query
builder.setAggregations(agg.toString().getBytes()); // set aggregation

But in 6.2.4, we only have these 2 APIs:
builder.addAggregation(AggregationBuilder aggregation)
builder.addAggregation(PipelineAggregationBuilder aggregation)

How to make the "set aggregation" from JSON string work correctly?