presto-hive/src/main/java/io/prestosql/plugin/hive/metastore/UserDatabaseKey.java/UserDatabaseKey/UserDatabaseKey
class UserDatabaseKey:    @JsonCreator
    public UserDatabaseKey(@JsonProperty("user") String user, @JsonProperty("database") String database)
    {
        this.user = requireNonNull(user, "principalName is null");
        this.database = requireNonNull(database, "database is null");
    }