Must build project before running benchmarks (#9454)

* Must build project before running benchmarks

That is, it does not test the version downloaded from Maven central.

* Update README.md

benchmarks/README.md

@@ -87,7 +87,17 @@ To run all the benchmark dataset:
 
 ### Java:
 
+First build the Java binary in the usual way with Maven:
+
+```
+$ cd java
+$ mvn install
+```
+
+Assuming that completes successfully,
+
 ```
+$ cd ../benchmarks
 $ make java
 ```