public ParserBuilder exec(ConfigSource execConfig) {
            if (execConfig == null) {
                throw new NullPointerException("execConfig");
            }
            this.execConfig = execConfig.deepCopy();
            return this;
        }
This method returns PreviewResult.

@return PreviewResult returns the result by PreviewExecutor
@throws IOException in case of failures in file operations
embulk-junit4/src/main/java/org/embulk/test/TestingEmbulk.java