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Java Transformer Reference

When Mule does not provide an out-of-the-box transformer to meet your application integration needs, the Java transformer enables you to package custom-coded Java logic (that is, a Java class) that is triggered when the transformer processes the message. In most cases, this Mule processor transforms a message from its original format to a new, modified format.

To configure a Java transformer, you need to specify the following:

  • The Java class your transformer calls.

  • An encoding format the transformer applies to messages.

  • A MIME type for your data.

The Java transformer applies these configuration settings to each message, then sends the transformed message to the next processor in your flow. Additionally, if you need to execute logic on a particular property within your Java class, the Java transformer allows you to configure Spring beans, which are containers that include developer-defined logic that executes on a specific property within your Java class. The modified Spring data can be saved in persistent storage, then recalled later in the flow. If you choose to add a property to a class, it is possible to inject additional values into the Spring property by configuring the property sub-elements. This allows you to reference arrays, maps, beans, etc. within the defined bean factory (class) or an external bean factory.

In general, MuleSoft recommends that you use a Java transformer if your transformation logic is complex or if message transformation requires the use of multiple out-of-the-box transformers. For example, when you need to create an application that returns all acceptable payment methods a customer can apply when an order is received, you can use a custom Java transformer to host a script that executes the application and returns the information to the client (below).

If rather than just changing the payload, you want to trigger a more complex set of processes coded in Java, you might be better off using a Java Componentrather than a Java Transformer.

General Tab

This Properties pane tab lets you change the default display name from Java to a name that closely describes the transformation being implemented. It also lets you configure the transformer’s class settings, which expose the collection of properties, attributes and methods referenced by the transformer. (At minimum, you must select a transformer class.) Finally, use the General tab to configure Spring bean settings for the transformer.

Field Description

Display Name

This field defaults to the generic transformer name. Change the Display Name, which must be alpha-numeric, to reflect the transformer’s specific role, such as, EnglishToMetric Transformer.

Transformer Class

To select a Java class for this transformer instance, click the magnifying glass icon to the right of the Transformer Class text box. After the pop-up appears, type the first few characters of a class name, then scroll to select the class you want. This is a mandatory setting.

Return Class

The class of the message generated by the transformer. This is used whenever transformers are auto-selected to verify that the transformer returns the correct class type.

Ignore Bad Input

If this Java transformer is part of a chain, and message class is not supported by the Java transformer, Check the box to continue processing the message through the flow. If unchecked, the chain ends at this point, and the message is stored.


Select the character encoding format for messages as they flow out of the Java transformer. As an example UTF-8 is the most common encoding for Web pages.


Select the file format for messages emerging from the Java transformer (such as, text/plain or application/json).

To configure Spring bean settings, click the Add Property (plus) button, then enter a name and its associated value or reference. Spring bean configuration data can be saved in a persistent store and restored at a later time.

Field Description


Enter the property name of the Spring element.


Enter a string to specify a Spring element.


Enter a string that specifies the Spring property you want to reference (that is, ref="MyFooSpringBean" or src/reference/path).

Spring bean sub-elements are used to define a bean value inline, instead of referring to a bean defined elsewhere in the class. To configure Spring sub-elements, complete the following steps:

  1. Click the Add Property (plus) button.

  2. Select the Advanced tab.

  3. Click the Add Sub-elements (plus) button.

  4. Choose a property element to configure.

Basic Hello World Java Transformer Class

To create a brand new Java Transformer Class, follow these steps:

  1. In the package explorer right click your project and select New > Class.

  2. Name your class, and make the package org.mule.transformers.

  3. Insert the basic class skeleton provided below to construct a simple transformer:

    package org.mule.transformers;
    import org.mule.api.MuleMessage;
    import org.mule.api.transformer.TransformerException;
    import org.mule.transformer.AbstractMessageTransformer;
    public class helloWorldTransformer extends AbstractMessageTransformer{
         * @param args
        public Object transformMessage(MuleMessage message, String outputEncoding) throws TransformerException {
            return "Hello World!";
    Note that the return argument becomes the output payload of your transformer.
  4. Add a new Java Transformer into your flow, and set the Transformer Class field to reference your newly created class.

Configuring an HTTP Connector for the Java Transformer

In an HTTP client and server model, a client requests information from an external or internal server. Unless an error occurs, the server returns the requested data to the user’s browser. The data sent back to the client, called the message payload, contains a header, certain data used to serve up the Web page to the client, and the message payload.

For example, suppose you need a transformation process that converts HTTP data into a map so that the Java class can:

  1. Parse the data

  2. Look up a specific attribute

  3. Return the attribute’s value to the client

To implement this scenario in a flow, use the following processors:

  • HTTP connector

  • Java transformer

The transport used to connect to the web resources is the HTTP connector, which contains specific parameters you enter during configuration. If the HTTP request contains form parameters, these are passed forward to the next processor as a key:value map in the message payload.

Finally, the Java transformer modifies the message by invoking the MapLookup class and all defined attributes then retrieving specific parameters out of the map. Once this transformation process completes, the flow returns the data to the client.


Java Transformer Configuration

In our example flow, a Java transformer is used to reference the data from the map. Remember, our example also wants to use the MapLookup class to reference a parameter and retrieve its value before returning the transformed message to the client. Before you configure the Java transformer, first, you need to either create a new Java class that references the MapLookup class or create a Global Java transformer that can be used to extend the MapLookup class to other Mule environments.

To select the Java transformer class double-click the icon to open the Properties Editor. Click the magnifying glass icon, and type the first few characters of a class name to narrow your search. After selecting a class, you can switch to the Configuration XML editor to enter any additional code that needs to execute.

<mule xmlns="" xmlns:http="" xmlns:doc="" xmlns:spring="" xmlns:core="" xmlns:jdbc="" xmlns:vm="" xmlns:xsi="" xsi:schemaLocation=" ">

        <custom-transformer encoding="UTF-8" mimeType="text/html" class="org.mule.transformer.simple.MapLookup" doc:name="Java"/>

Spring Configuration

Configure Spring properties to assign a value or reference to a specific property, attribute, or element. If you need to assign a value or reference to a defined property, you can extend the class by configuring the specific property subelement. In our example, after the HTTP request was converted to a map, a value is assigned to a Spring property before the response message is returned to the client.

<mule xmlns="" xmlns:http="" xmlns:doc="" xmlns:spring="" xmlns:core="" xmlns:jdbc="" xmlns:vm="" xmlns:xsi="" xsi:schemaLocation=" ">

        <custom-transformer encoding="UTF-8" mimeType="text/html" class="org.mule.transformer.simple.MapLookup" doc:name="Java"/>
                 <spring:property name="homeLocation" value="SanFrancisco"/>
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