public EmbulkEmbed initialize() {
            if (this.started) {
                throw new IllegalStateException("System already initialized");
            }
            this.started = true;

            final EmbulkSystemProperties embulkSystemProperties = EmbulkSystemProperties.of(this.embulkSystemPropertiesBuilt);
            final BufferAllocator bufferAllocator = createBufferAllocatorFromSystemConfig(embulkSystemProperties);
            final TempFileSpaceAllocator tempFileSpaceAllocator = new SimpleTempFileSpaceAllocator();

            return new EmbulkEmbed(
                    decoderPlugins,
                    encoderPlugins,
                    executorPlugins,
                    fileInputPlugins,
                    fileOutputPlugins,
                    filterPlugins,
                    formatterPlugins,
                    guessPlugins,
                    inputPlugins,
                    outputPlugins,
                    parserPlugins,
                    embulkSystemProperties,
                    bufferAllocator,
                    tempFileSpaceAllocator,
                    alternativeBulkLoader);
        }
Sets an alternative {@code BulkLoader} instance mainly for testing-purpose.
embulk-core/src/main/java/org/embulk/EmbulkEmbed.java