JanusGraph Error: Unknown external index backend [How to Solve]

Use java to connect janusgraph to create a graph. The error is the code in line 50 of the program, which is used to generate a mixed index. The screenshot is as follows:

Later, a screenshot is posted in the official document of version 0.6:

the example about definitions a mixed index containing the property keys name and age. The definition references to the indexing backend name search so that janusgraph knows which configured indexing backend it should use for this particular index The search parameter specified in the buildmixedindex call must match the second claim in the janusgraph configuration definition like this: index.search.backend if the index was named solrsearch then the configuration definition would appear like this: index.solrsearch.backend.
this means that the parameters of the buildmixedindex method should be the same as those specified The written index configuration of the configuration file is consistent with the second sub sentence. For example, my is: index. Search. Backend = elasticsearch
then the parameter of buildmixedindex should be search. After the code in line 50 is modified, no error will be reported

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