if I do:

db.requestStart();
db.setWriteConcern(WriteConcern.STRICT);
String colName = "testCollection";
DBCollection collection = db.getCollection(colName);
//mongodb.save(getTestObject(), colName);
//long size = collection.getCount();
//assert size == 1;
String colNewName = "testCollection2";
collection.rename(colNewName);
System.out.println("collection name = " + collection.getName());
assert collection.getName().equals(colNewName);
db.requestDone();

then I get:

com.mongodb.MongoException: rename failed:

{ "errmsg" : "assertion: source namespace does not exist" , "ok" : 0.0}

at com.mongodb.DBCollection.rename(DBCollection.java:499)
at com.refineddata.MongoDatabaseTest.testCreateExistsRenameDeleteCollection(MongoDatabaseTest.java:70)

which doesn't seem appropriate as namespace should exist, the collection itself exists.

If I save something in the code:

db.requestStart();
db.setWriteConcern(WriteConcern.STRICT);
String colName = "testCollection";
DBCollection collection = db.getCollection(colName);
mongodb.save(getTestObject(), colName);
long size = collection.getCount();
assert size == 1;
String colNewName = "testCollection2";
collection.rename(colNewName);
System.out.println("collection name = " + collection.getName());
assert collection.getName().equals(colNewName);
db.requestDone();

the following is printed:

collection name = testCollection

the following assertion fails (which shouldn't happen as no exception is thrown)
assert collection.getName().equals(colNewName);