Type Converter

Converting body payloads from one type to another is common when routing messages between endpoints. Conversions regularly occur between the following types:

File
String
byte[] and ByteBuffer
InputStream and OutputStream
Reader and Writer
Document and Source

To convert the body of a message to another type:

Message message = exchange.getIn();
Document document = message.getBody(Document.class); (1)

1	where `Message#getBody(Class)` performs the conversion.

How Type Conversion works

The type conversion strategy is defined by the TypeConverter interface that can be customized on a CamelContext.

Camel provides a default type converter registry. From Camel 3 onwards, the type converter registry implementation generates source code using the Camel Maven Package Plugin, allowing Camel, at runtime, to load and invoke these type converters via quick Java method invocations. In Camel 2, type converters are registered using annotation based discovery, invoking the type converters via the Java Reflection API.

TypeConverterRegistry

Add a TypeConverter to the TypeConverterRegistry at runtime using the CamelContext:

CamelContext context = ...;
context.getTypeConverterRegistry();

This is useful on platforms where the automatic type converters discovery fails due to classloading issues.

TypeConverterRegistry utilization statistics

Camel can gather utilization statistics of the runtime usage of type converters. These statistics are available in JMX as well as from TypeConverterRegistry#getStatistics().

These statistics are turned off by default as there is some performance overhead under very high concurrent load.

Enabling statistics in Java:

CamelContext context = ...;
context.setTypeConverterStatisticsEnabled(true);

Enabling statistics in XML DSL:

<camelContext xmlns="http://camel.apache.org/schema/spring" typeConverterStatisticsEnabled="true">
    ...
</camelContext>

Add type converter classes at runtime

Classes implementing TypeConverters are added to the type converter registry at runtime. Use @Converter to mark each type converter.

private class MyOrderTypeConverters implements TypeConverters {
    @Converter
    public MyOrder toMyOrder(String orderId) {
        MyOrder order = new MyOrder();
        order.setId(Integer.parseInt(orderId));
        return order;
    }
}

Then, add these converters to the registry:

MyOrderTypeConverters myClass = ...;
context.getTypeConverterRegistry().addTypeConverters(myClass);

If you are using Spring or Blueprint, then you can just declare a <bean> then CamelContext will automatic discover and add the converters.

<bean id="myOrderTypeConverters" class="..."/>
    <camelContext ...>
        ...
    </camelContext>

You can declare multiple `<bean>`s if you have more classes.

Discovering Type Converters

Camel automatically discovers and loads the type converters from all JARs on the classpath at startup.

Camel searches the classpath for a file called META-INF/services/org/apache/camel/TypeConverterLoader which lists all type converter loader classes. These are automatically generated by the Camel Maven Package Plugin. These loader classes will load the type converters into the Camel type converter registry and invoke them in a fast way using standard Java method calls.

Discovering Camel 2.x based type converters (not loader)

Camel will not perform additional package scanning for type converters which have no source code generated for loader classes.

To enable additional package scanning in Java:

camelContext.setLoadTypeConverters(true);

In XML DSL:

<camelContext loadTypeConverters="true">
...
</camelContext>

In Spring Boot application.properties:

camel.springboot.load-type-converters=true

Camel will discover Camel 2.x compatible type converters by searching the classpath for a file called META-INF/services/org/apache/camel/TypeConverter which lists all type converter classes. These classes are automatically registered in the type converter registry. However, invoking these type converters does not happen in a fast way and uses the Java Reflection API. It is therefore recommended to upgrade any type converters to use the faster way. See below for more details.

You can turn off the fallback of discovering Camel 2.x compatible type converters by setting CamelContext.setLoadTypeConverters(false).

@Converter must appear at the class and method level for each type converter.

@Converter
public class IOConverter {
    @Converter
    public static InputStream toInputStream(File file) throws FileNotFoundException {
        return new BufferedInputStream(new FileInputStream(file));
    }
}

Discovering Type Converters in the fast way

To enable the fast type converter way, you should enable generateLoader = true on the class level annotation as shown:

@Converter(generateLoader = true)
public class IOConverter {
    @Converter
    public static InputStream toInputStream(File file) throws FileNotFoundException {
        return new BufferedInputStream(new FileInputStream(file));
    }
}

And then you should have the Camel Maven Package Plugin in as build plugin when compiling the project. Also add the build helper plugin which ensures the generated source code in src/generated will be included in the source path.

When using Maven you add:

      <plugin>
        <groupId>org.apache.camel</groupId>
        <artifactId>camel-package-maven-plugin</artifactId>
        <version>${camel-version}</version>
        <executions>
          <execution>
            <id>generate</id>
            <goals>
              <goal>generate-component</goal>
            </goals>
            <phase>process-classes</phase>
          </execution>
        </executions>
      </plugin>
      <plugin>
        <groupId>org.codehaus.mojo</groupId>
        <artifactId>build-helper-maven-plugin</artifactId>
        <executions>
          <execution>
            <phase>initialize</phase>
            <goals>
              <goal>add-source</goal>
              <goal>add-resource</goal>
            </goals>
            <configuration>
              <sources>
                <source>src/generated/java</source>
              </sources>
              <resources>
                <resource>
                  <directory>src/generated/resources</directory>
                </resource>
              </resources>
            </configuration>
          </execution>
        </executions>
      </plugin>

Returning null values

By default when using a method in a POJO annotation with @Converter returning null is not a valid response. If null is returned, then Camel will regard that type converter as a miss, and prevent from using it in the future. If null should be allowed as a valid response, then from Camel 2.11.2/2.12 onwards you can specify this in the annotation as shown:

@Converter(allowNull = true)
public static InputStream toInputStream(File file) throws IOException {
    if (file.exist()) {
        return new BufferedInputStream(new FileInputStream(file));
    } else {
        return null;
    }
}

Discovering Type Converters in the fastest way

Available in Camel 3.7

In Camel 3.7 we optimized the type converter system for optimal performance when using the built-in converters. This was done by bulking together all the converters in the same Maven module into a single class. The class has a single convert method where all the supported converters are available and discovered in a fast way using Java primitives.

To enable this then set generateBulkLoader=true in the class @Converter annotation. You should do this for all the converter classes within the same Maven artifact. Then they will be bulked together into a single class.

@Converter(generateBulkLoader = true)
public class IOConverter {
    @Converter
    public static InputStream toInputStream(File file) throws FileNotFoundException {
        return new BufferedInputStream(new FileInputStream(file));
    }
}

There is a few limitations:

fallback converters is not supported
the order of the @Converter methods matters. If you have multiple @Converter methods that accept as from type types which are from the same class hierachy then put the methods first that are the most concrete.

For example in camel-xml-jaxp we have in the XmlConverter multiple @Converter methods which can convert to DomSource. We had to put the method that takes org.w3c.dom.Document before the method that takes org.w3c.dom.Node as Document extends Node.

The following code shows snippet of the source code generated bulk class. As you can see we have the Document method before the Node method below:

        } else if (to == javax.xml.transform.dom.DOMSource.class) {
            if (value instanceof org.w3c.dom.Document) {
                return getXmlConverter().toDOMSource((org.w3c.dom.Document) value);
            }
            if (value instanceof org.w3c.dom.Node) {
                return getXmlConverter().toDOMSource((org.w3c.dom.Node) value);
            }

Discovering Fallback Type Converters

Available in Camel 2.0

The AnnotationTypeConverterLoader has been enhanced to also look for methods defined with a @FallbackConverter annotation, and register it as a fallback type converter.

Fallback type converters are used as a last resort for converting a given value to another type. Its used when the regular type converters give up. The fallback converters is also meant for a broader scope, so its method signature is a bit different:

@FallbackConverter
public static <T> T convertTo(Class<T> type, Exchange exchange, Object value, TypeConverterRegistry registry)

Or you can use the non generic signature.

@FallbackConverter
public static Object convertTo(Class type, Exchange exchange, Object value, TypeConverterRegistry registry)

And the method name can be anything (convertTo is not required as a name), so it can be named convertMySpecialTypes if you like.
The Exchange parameter is optional, just as its with the regular @Converter methods.

The purpose with this broad scope method signature is allowing you to control if you can convert the given type or not. The type parameter holds the type we want the value converted to. Its used internally in Camel for wrapper objects so we can delegate the type convertions to the body that is wrapped.

For instance in the method below we will handle all type conversions that is based on the wrapper class GenericFile and we let Camel do the type conversions on its body instead.

@FallbackConverter
public static <T> T convertTo(Class<T> type, Exchange exchange, Object value, TypeConverterRegistry registry) {
    // use a fallback type converter so we can convert the embedded body
    // if the value is GenericFile
    if (GenericFile.class.isAssignableFrom(value.getClass())) {
        GenericFile file = (GenericFile) value;
        Class from = file.getBody().getClass();
        TypeConverter tc = registry.lookup(type, from);
        if (tc != null) {
            Object body = file.getBody();
            return tc.convertTo(type, exchange, body);
        }
    }
    return null;
}

Writing your own Type Converters

You are welcome to write your own converters. Remember to use the @Converter annotations on the classes and methods you wish to use. And on the top-level class add Converter(loader = true) to support the fast way of using type converters.

static methods are encouraged to reduce caching, but instance methods are fine, particularly if you want to allow optional dependency injection to customize the converter
converter methods should be thread safe and reentrant

Exchange parameter

The type converter accepts the Exchange as an optional 2nd parameter. This is usable if the type converter for instance needs information from the current exchange. For instance combined with the encoding support its possible for type converters to convert with the configured encoding. An example from camel-core for the byte[] → String converter:

@Converter
public static String toString(byte[] data, Exchange exchange) {
    String charsetName = exchange.getProperty(Exchange.CHARSET_NAME, String.class);
    if (charsetName != null) {
        try {
            return new String(data, charsetName);
        } catch (UnsupportedEncodingException e) {
            // ignore
        }
        return new String(data);
    }
}