[[TypeConverter-TypeConverter]]
=== Type Converter

It is very common when routing messages from one endpoint to another to
need to convert the body payloads from one type to another such as to
convert to and from the following common types:

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

The
http://camel.apache.org/maven/current/camel-core/apidocs/org/apache/camel/Message.html[Message
interface] defines a helper method to allow conversions to be done via
the
http://camel.apache.org/maven/current/camel-core/apidocs/org/apache/camel/Message.html#getBody(java.lang.Class)[getBody(Class)]
method.

So in an endpoint you can convert a body to another type via:

[source,java]
----
Message message = exchange.getIn();
Document document = message.getBody(Document.class);
----

[[TypeConverter-HowTypeConversionworks]]
==== How Type Conversion works

The type conversion strategy is defined by the
http://camel.apache.org/maven/current/camel-core/apidocs/org/apache/camel/TypeConverter.html[TypeConverter]
interface that can be customized on a
http://camel.apache.org/maven/current/camel-core/apidocs/org/apache/camel/CamelContext.html[CamelContext].

Camel provides a default type converter registy.
From Camel 3 onwards there type converter registry implementation uses source code generation via the `camel-apt` compiler plugin to generate source code, that allows Camel at runtime to load and invoke these type converters via quick java method invocations. The older implementation in Camel 2 uses a annotation based discover to load and register type converters and invokes the type converters via Java method call reflections.


[[TypeConverter-TypeConverterRegistry]]
==== TypeConverterRegistry

Exposed the
http://camel.apache.org/maven/current/camel-core/apidocs/org/apache/camel/spi/TypeConverterRegistry.html[TypeConverterRegistry]
from link:camelcontext.adoc[CamelContext] so end users more easily will
be able to add type converters at runtime. This is also usable in
situations where the default discovering of type converters fails on
platforms with classloading issues.

To access the registry, you get it from the `CamelContext`:

[source,java]
----
CamelContext context = ...
context.getTypeConverterRegistry()
----

[[TypeConverter-TypeConverterRegistryutilizationstatistics]]
===== TypeConverterRegistry utilization statistics

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

These statistics are turned off by
default as there is some performance overhead under very high concurrent
load. To enable the statistics in Java, do the following:

[source,java]
----
CamelContext context = ...
context.setTypeConverterStatisticsEnabled(true);
----

Or in the XML DSL with:

[source,xml]
----
<camelContext xmlns="http://camel.apache.org/schema/spring" typeConverterStatisticsEnabled="true">
    ...
</camelContext>
----

[[TypeConverter-Addtypeconverterclassesatruntime]]
===== Add type converter classes at runtime

You can add new type converters at runtime, by having your classes
implement `org.apache.camel.TypeConverters` which is an marker
interface. Then for each type converter you want use
the `@Converter` annotation.

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

Then you can add these converters to the registry using:

[source,xml]
----
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.

[source,xml]
----
<bean id="myOrderTypeConverters" class="..."/>
    <camelContext ...>
        ...
    </camelContext>
----

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

[[TypeConverter-DiscoveringTypeConverters]]
==== Discovering Type Converters

Camel will automatic discover and load type converters from all the JARs in the classpath on startup.

Camel will search the classpath for a file called
`META-INF/services/org/apache/camel/TypeConverterLoader`, which lists
all type converter loader classes (they are automatic source code generated by the `camel-apt` compilter plugin).
These _loader_ classes will load the type converters into the Camel type converter registry
and invoking these type converters is done in a _fast way_ using standard java method calls.

The discover will fallback and discover Camel 2.x compatible type converters by
search the classpath for a file called `META-INF/services/org/apache/camel/TypeConverter`,
which lists all the type converter classes. These classes will then automatic
be registered in the type converter registry. However invoking these type converters
does **not** happen in a _fast way_ but uses Java method call reflection. It is therefore
recommended to migrate your type converters to use the faster way. See further below.

TIP: You can turn off the fallback of discovering Camel 2.x compatible type converters by
setting the `loadTypeConverters` option to `false` on `CamelContext`.

The converter classes must be annotated on the top class level with the `@Converter` annotation,
and each converter method as well.

E.g. the following shows how to register a converter from `File` to
`InputStream`:

[source,java]
----
@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 `loader = true`
on the class level annotation as shown:

[source,java]
----
@Converter(loader = 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-apt` JAR as dependency when compiling the project.
When using Maven you add:

[source,xml]
----
<dependency>
  <groupId>org.apache.camel</groupId>
  <artifactId>apt</artifactId>
  <scope>provided</scope>
</dependency>
----

[[TypeConverter-Returningnullvalues]]
===== 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:

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

[[TypeConverter-DiscoveringFallbackTypeConverters]]
==== Discovering Fallback Type Converters

*Available in Camel 2.0*

The
http://camel.apache.org/maven/current/camel-core/apidocs/org/apache/camel/impl/converter/AnnotationTypeConverterLoader.html[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:

[source,java]
----
@FallbackConverter
public static <T> T convertTo(Class<T> type, Exchange exchange, Object value, TypeConverterRegistry registry)
----

Or you can use the non generic signature.

[source,java]
----
@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.

[source,java]
----
@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;
}
----

[[TypeConverter-WritingyourownTypeConverters]]
==== 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

[[TypeConverter-Exchangeparameter]]
==== 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:

[source,java]
----
@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);
    }
}
----
