This document details the changes that you need to make to your code when upgrading from one version to another.

Upgrading From 0.9 to 1.0

Custom Handlers

The interfaces SerializationHandlerInterface, and DeserializationHandlerInterface have been removed. Instead, you can now use either an event listener, or the new handler concept. As a general rule, if your handler was registered for a specific type, you would use the new handler system, if you instead were handling an arbitrary number of possibly unknown types, you would use the event system.

Please see the documentation for how to set-up one of these.
Objects implementing Traversable

Objects that implement the Traversable interface are not automatically treated specially anymore, but are serialized just like any regular object. If you would like to restore the previous behavior, you can either add a custom handler, or force the serialization type to array using the @Type annotation (or its equivalent in XML/YML):
```
/** @Type("array") */
private $myTraversableObject;
```
Configuration

Most of the configuration under jms_serializer.handlers is gone. The order is not important anymore as a handler can only be registered for one specific type.

You can still configure the built-in datetime handler though:
```
jms_serializer:
    handlers:
        datetime:
            default_format: DateTime::ISO8601
            default_timzone: UTC
```
This is not necessary anymore though as you can now specify the format each time when you use a DateTime by using the @Type annotation:
```
/** @Type("DateTime<'Y-m-d', 'UTC'>") */
private $createdAt;
```

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