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Supported Data Formats

DataWeave can read and write many types of data formats, such as JSON, XML, and many others. Before you begin, note that DataWeave version 2 is for Mule 4 apps. For a Mule 3 app, refer to the DataWeave version 1 documentation set in the Mule 3.9 documentation. For other Mule versions, you can use the version selector for the Mule Runtime table of contents.

DataWeave supports the following formats as input and output:

MIME Type ID Supported Formats



Avro Format



CSV Format



DataWeave Format (dw) for testing a DataWeave expression



Flat File Format, COBOL Copybook Format, Fixed Width Format



Java Format, Enum Format for Java



JSON Format



Binary Format



Excel Format



XML Format, CData Custom Type



Newline Delimited JSON Format (ndjson)



URL Encoded Format



YAML Format



Multipart Format



Text Plain Format



Text Java Properties

DataWeave Readers

DataWeave can read input data as a whole in-memory, in indexed fashion, and for some data formats, part-by-part by streaming the input. When attempting to read a large file, it is possible to run out of memory or to impact performance negatively. Streaming can improve performance but impacts access to file.

  • Indexed and In-Memory:
    Allow for random access to data because both strategies parse the entire document. When using these strategies, a DataWeave script can access any part of the resulting value at any time.

    • Indexed:
      Uses indexes over the disk.

      DataWeave can only read the following formats in the indexed mode:

      CSV, JSON, XML (starting in Mule 4.3.0)

      Note that DataWeave does not support reading Excel (XLSX) input in the indexed mode.

    • In-Memory:
      Parses the entire document in memory.

      DataWeave can read all supported formats using the in-memory mode.

  • Streaming:
    Allows for sequential access to the file. This strategy partitions the input document into smaller items and accesses its data sequentially, storing the current item in memory. A DataWeave selector can access the portion of the file that is getting read.

    DataWeave supports streaming only for the following formats:

    CSV, JSON, Excel (XLSX, starting in Mule 4.2.2), XML (starting in Mule 4.3.0)

For details, see Streaming in DataWeave.

Using Reader and Writer Properties

In some cases, it is necessary to modify or specify aspects of the format through format-specific properties. For example, you can specify CSV input and output properties, such as the separator (or delimiter) to use in the CSV file. For Cobol copybook, you need to specify the path to a schema file using the schemaPath property.

You can append reader properties to the MIME type (outputMimeType) attribute for certain components in your Mule app. Listeners and Read operations accept these settings. For example, this On New File listener example identifies the , separator for a CSV input file:

Example: Properties for the CSV Reader
<file:listener doc:name="On New File" config-ref="File_Config" outputMimeType='application/csv; separator=","'>
  <scheduling-strategy >
    <fixed-frequency frequency="45" timeUnit="SECONDS"/>
  <file:matcher filenamePattern="comma_separated.csv" />

Note that the outputMimeType setting above helps the CSV reader interpret the format and delimiter of the input comma_separated.csv file, not the writer.

To specify the output format, you can provide the MIME type and any writer properties for the writer, such as the CSV or JSON writer used by a File Write operation. For example, you might need to write a pipe (|) delimiter in your CSV output payload, instead of some other delimiter used in the input. To do this, you append the property and its value to the output directive of a DataWeave expression. For example, this Write operation specifies the pipe as a separator:

Example: output Directive for the CSV Writer
<file:write doc:name="Write" config-ref="File_Config" path="my_transform">
  <file:content ><![CDATA[#[output application/csv separator="|" --- payload]]]></file:content>

The following example shows a DataWeave script in which the output directive specifies two writer properties, one for the CSV field separator, another to indicate that there is no header.

%dw 2.0
output application/csv separator=";", header=false

Writer properties depend on the format. The following example uses the JSON writer property, skipNullOn:

%dw 2.0
output application/json skipNullOn="everywhere"

The following example shows a DataWeave script that uses an invisible ASCII character as a separator for the CSV output. The separator property value is \u001E. \u is the escape sequence representation and 001E is the hexadecimal representation of the record separator (RS) ASCII character.

%dw 2.0
output application/csv separator = "\u001E"
		"Name" : "Tom",
		"UID" : 2569,
		"ShipOrderID" : "ui-288188"
		"Name" : "Alan",
		"UID" : 7756,
		"ShipOrderID" : "xj-232142"
		"Name" : "Dan",
		"UID" : 7821,
		"ShipOrderID" : "uk-259459"

The CSV example produces the following output:


Scripts that use the read or readUrl methods can also specify reader properties through the readerProperties parameter. See examples in read. However, note that use of reader properties is more common in message sources, such as the HTTP listener, as shown in the CSV reader example.

It is also possible to use a Mule variable as a value of a configuration property. For more complete examples, see Set Reader and Writer Configuration Properties.

Setting MIME Types

You can specify the MIME type for the input and output data that flows through a Mule app.

For DataWeave transformations, you can specify the MIME type for the output data. For example, you might set the output header directive of an expression in the Transform Message component or a Write operation to output application/json or output application/csv.

This example sets the MIME type through a File Write operation to ensure that a format-specific writer, the CSV writer, outputs the payload in CSV format:

Example: MIME Type for the CSV Writer
<file:write doc:name="Write" config-ref="File_Config" path="my_transform">
  <file:content ><![CDATA[#[output application/csv --- payload]]]></file:content>

Starting in Mule 4.3.0, you can set the output directive using the format ID alone, instead of using the MIME type. For example, you might set the output header directive of an expression in the Transform Message component or a Write operation to output json or output csv. You can also use the format ID to differentiate the format of the output data from the MIME type in the output header, and you can use a custom MIME type. For example, you can write JSON data but customize the MIME type to application/problem+json by using the DataWeave directive output application/problem+json with json. See Change a Script’s MIME Type Output for examples that customize the MIME type output of a script.

The with keyword separates the output MIME type from the DataWeave writer for the script’s output. For example, you can specify 'output applicatio:page-aliases:

For input data, format-specific readers for Mule sources (such as the On New File listener), Mule operations (such as Read and HTTP Request operations), and DataWeave expressions attempt to infer the MIME type from metadata that is associated with input payloads, attributes, and variables in the Mule event. When the MIME type cannot be inferred from the metadata (and when that metadata is not static), Mule sources and operations allow you to specify the MIME type for the reader. For example, you might set the MIME type for the On New File listener to outputMimeType='application/csv' for CSV file input. This setting provides information about the file format to the CSV reader.

Example: MIME Type for the CSV Reader
<file:listener doc:name="On New File"

Note that reader settings are not used to perform a transformation from one format to another. They simply help the reader interpret the format of the input.

You can also set special reader and writer properties for use by the format-specific reader or writer of a source, operation, or component. See Using Reader and Writer Properties.