This solution is based on Spring FactoryBean interface and the way how Spring container uses it.

refresh-aware-factory-bean

Refresh Aware Factory Bean is a Spring Boot module to help with Java objects which content should be updated at some intervals from file or other sources. Implementation is based on the FactoryBean interface and AOP Framework provided by Spring.

The main idea is borrowed from Spring AbstractFactoryBean<T> and then extended with refresh support in form of RefreshAwareFactoryBean<T> as base class and ProxyBasedRefreshAwareFactoryBean<T> which should be used to create custom RefreshAwareFactoryBean-s.

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Problem

Let's imagine that we have some job which creates/updates file content every hour. In that file there is data which we want to use in our application and once file content is updated, we want those changes reflected in our application.

To keep things simple, let's imagine that file content is an array of numbers (numbers.txt). In our application we could use this data in List<Integer> form. So the idea is to create some kind of List implementation that 'can' track file changes and update itself. The idea is not to update the content of the list, but to create a new list and then start using that new instance. This is useful when new data is corrupted and should not be used.

Solution

Split implementation into two parts. RefreshAwareFactoryBean which will provide a life-cycle for some instance and Dynamic Proxy which knows how to delegate all calls to 'real' instance currently held by that RefreshAwareFactoryBean. RefreshAwareFactoryBean life-cycle is described below in implementation details.

Users should only implement custom RefreshAwareFactoryBean by extending ProxyRefreshAwareFactoryBean<T> base class and dynamic proxy will be automatically created based on instance T type.

This approach can be used for any Java type (provided or newly created) which is not final!

For example:

• Both, List and ArrayList can be used with this solution.
• String class can't be used because it's final and can't be proxied, but CharSequence interface can.

Benefits from this approach are clean and unit testable code and simple usage. Disadvantages are usage of Dynamic Proxies and it's a little harder to understand the logic behind this solution.

Example

For given file (numbers.txt) content:

1, 2, 3, 4, 5, 6, 7, 8, 9

We just need to implement an appropriate RefreshAwareFactoryBean which will track file and 'update' List content. In this case we can extend FileProxyRefreshAwareFactoryBean to create a List of Integers from file. Here we are using FileProxyRefreshAwareFactoryBean because it knows how to check if file content is modified and when to call refreshInstance (delegated to createInstance by default) to create a new numbers List with new data.

@Component("numbers")
public class NumbersFactoryBean extends FileProxyRefreshAwareFactoryBean<List<Integer>> {

   @Autowired
   public NumbersFactoryBean(@Value("${filepath-from-properties}") Path filepath) {
      super(filepath);
   }

   @NonNull
   @Override
   protected List<Integer> createInstance(@NonNull Path filepath) throws Exception {
      String content = Files.readString(filepath);
      String[] numbers = content.split(", *");
      
      return Stream.of(numbers).map(Integer::parseInt).collect(toList());
   }
   
}

and then in our NumbersService we can autowire numbers list and use it:

@Service
public class NumbersService {

   private final List<Integer> numbers;

   @Autowired
   public NumbersService(@Qualifier("numbers") List<Integer> numbers) {
      this.numbers = numbers;
   }

   public void doSomething() {
     // use numbers here...
   }
 
}

Once the file content is updated, those data will be reflected to numbers list without any additional effort.

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Implementation details

RefreshAwareFactoryBean implements the RefresableBean interface in order to be notified for refresh events. RefresableBean interface provides a single refresh method which is called by different thread managed by Spring TaskScheduler at intervals specified in application properties. You must use @EnableScheduling annotation in order to enable beans refreshing. This is implemented using Spring auto configuration approach, so all you need to do is include this module in your project and start using it.

This are all properties that you can configure:

• enabled - Enable beans refreshing. Default is true.
• initial-delay - Delay before the first refresh. Default is 1 minute. Ignored in case of cron usage.
• fixed-rate - Call refresh with a fixed period between invocations. Default is 1 minute.
• fixed-delay - Call refresh with a fixed period between the end of the last invocation and the start of the next. If specified, it has priority over fixedRate property.
• cron - Call refresh with specified cron. If specified, it has priority over fixed-delay and fixed-rate properties.

Lifecycle

RefreshAwareFactoryBean lifecycle is managed by five methods: afterPropertiesSet, getObjectType, getObject, refresh and destroy. These methods are all called by Spring Container. Beside these five methods, there are five template methods createInstance, createDummyInstance, shouldRefresh, refreshInstance, destroyInstance which end user has to override (some are optional) in order to implement specific RefreshAwareFactoryBean behaviour.

On application startup, Spring IoC Container will create RefreshAwareFactoryBean autowiring all required dependencies. After this, afterPropertiesSet method is called. This method will first try to call createInstance in order to create an initial instance to be autowired where it is required. If createInstance for some reason fails (exception occurred), then createDummyInstance is called. By default createDummyInstance will return null and this means that application is in an illegal state because there is no initial instance that can be autowired and application won't start (IllegalStateException will be thrown). But, if you want an application to continue to work, you can provide some dummy instance by overriding createDummyInstance (e.g. empty list) and that instance will be used as an initial instance. This is useful when creating instance from file content but the file is missing or is corrupted. Also, in this step instance type resolution is performed because it is needed for Dynamic Proxy which is also created.

(Once all RefreshableBeans are created, they are collected into List and autowired to RefreshableBeanAutoConfiguration for refresh scheduling.)

Now that RefreshAwareFactoryBean is initialized, Spring IoC Container will detect that the created bean is of FactoryBean type which means it needs to call getObjectType method in order to receive information about the object type this factory creates. With this information Spring IoC knows which beans depend on instance kept by RefreshAwareFactoryBean. By calling getObject, Spring IoC will obtain proxy created by this factory bean. In every moment, proxy knows how to delegate method calls to "real" instance kept by this RefreshAwareFactoryBean.

After initial-delay, first refresh is performed by calling the refresh method from RefreshAwareFactoryBean (refresh is called by scheduled thread in RefreshableBeanAutoConfiguration of this module). Inside this method, the first is called the shouldRefresh method. It should signal the new instance should be created (e.g. file content is updated). In case shouldRefresh returns true, refreshInstance is called. By default, refreshInstance will call createInstance. If you need some custom refresh logic, you can override refreshInstance. After a successful refresh, a new instance is created, and the current instance is replaced with a newly created instance. After that, destroyInstance is called with an old instance as an argument in order to close opened resources. All calls through Dynamic Proxy now delegate method calls to newly created instance. This process is repeated for every refresh event. Note that you can pass TaskScheduler as a constructor parameter if you want refreshInstance and destroyInstance to be called in seperate threads with configurable delays (e.g. wait 10 seconds to make sure old instance is not used by anyone). Also, once an old instance is replaced with a new one, the switch is instant in all parts of the application.

At the end, on application shutdown, Spring IoC Container will call destroy method. In this case destroyInstance will be called with the currently held instance by RefreshAwareFactoryBean.

API

• afterPropertiesSet - Called at application startup. Calls createInstance and createDummyInstance.
• createInstance - Creates instance.
• createDummyInstance - Creates a dummy instance to be used in case when createInstance fails. This is optional.
• refresh - Called at predefined intervals. Default is 1 minute and it can be set in application properties. Calls shouldRefresh, refreshInstance and destroyInstance. Supports async refresh by providing TaskScheduler as constructor parameter.
• shouldRefresh - Signals when the instance should be refreshed (recreated).
• refreshInstance - Recreates instance based on new updated content. Default implementation will call createInstance.
• destroyInstance - Destroys previously created instance. Default implementation will try to call close from AutoCloseable.
• destroy - Called at application shutdown. Calls destroyInstance with the currently held instance.
• getObject - Return an instance of the object managed by this factory.
• getObjectType - Return the type of object that this FactoryBean creates.

Provided implementations

There are several specific implementations of RefreshAwareFactoryBean:

• ProxyRefreshAwareFactoryBean<T> - Generic base class for others to extend from. Specific implementation is required to override createInstance and shouldRefresh methods in order to create java objects and to signal when it should be updated (recreated) respectively. Methods refreshInstance and destroyInstance are optional to override. By default refreshInstance will call createInstance and destroyInstance method will call close if the object implements AutoCloseable interface.
• FileProxyRefreshAwareFactoryBean<T> - Creates Java object based on file content and recreates it once the file content is updated, knows when file content is updated. Specific implementation is required to override createInstance method in order to parse file content into Java objects.
• JsonFileProxyRefreshAwareFactoryBean<T> - Similar as previous, but knows how to parse a json file content
• YamlFileProxyRefreshAwareFactoryBean<T> - Similar as previous, but knows how to parse a yaml file content

Additional information

Java CDI

This implementation is specific to Spring Framework. If you need Java CDI implementation of this, you can contact me for more information. I have it implemented and it uses a slightly different approach and requires one additional @Producer method in every RefreshAwareFactoryBean.

Performance

Application performance can depend on the number of used dynamic proxies in it. If you want to improve performance of Dynamic Proxies created by this module, you could switch to ByteBuddy and you will be able to create Dynamic Proxies with a direct call to method, eliminating any additional performance overhead. Contact me for more information.