Can I replace a Spring bean definition at runtime?

Consider the following scenario. I have a Spring application context with a bean whose properties should be configurable, think DataSource or MailSender. The mutable application configuration is managed by a separate bean, let's call it configuration.

An administrator can now change the configuration values, like email address or database URL, and I would like to re-initialize the configured bean at runtime.

Assume that I can't just simply modify the property of the configurable bean above (e.g. created by FactoryBean or constructor injection) but have to recreate the bean itself.

Any thoughts on how to achieve this? I'd be glad to receive advice on how to organize the whole configuration thing as well. Nothing is fixed. :-)

EDIT

To clarify things a bit: I am not asking how to update the configuration or how to inject static configuration values. I'll try an example:

<beans>
    <util:map id="configuration">
        <!-- initial configuration -->
    </util:map>

    <bean id="constructorInjectedBean" class="Foo">
        <constructor-arg value="#{configuration['foobar']}" />
    </bean>

    <bean id="configurationService" class="ConfigurationService">
        <property name="configuration" ref="configuration" />
    </bean>
</beans>

So there's a bean constructorInjectedBean that uses constructor injection. Imagine the construction of the bean is very expensive so using a prototype scope or a factory proxy is not an option, think DataSource.

What I want to do is that every time the configuration is being updated (via configurationService the bean constructorInjectedBean is being recreated and re-injected into the application context and dependent beans.

We can safely assume that constructorInjectedBean is using an interface so proxy magic is indeed an option.

I hope to have made the question a little bit clearer.

Here is how I have done it in the past: running services which depend on configuration which can be changed on the fly implement a lifecycle interface: IRefreshable:

public interface IRefreshable {
  // Refresh the service having it apply its new values.
  public void refresh(String filter);
	
  // The service must decide if it wants a cache refresh based on the refresh message filter.
  public boolean requiresRefresh(String filter);
}

Controllers (or services) which can modify a piece of configuration broadcast to a JMS topic that the configuration has changed (supplying the name of the configuration object). A message driven bean then invokes the IRefreshable interface contract on all beans which implement IRefreshable.

The nice thing with spring is that you can automatically detect any service in your application context that needs to be refreshed, removing the need to explicitly configure them:

public class MyCacheSynchService implements InitializingBean, ApplicationContextAware {
 public void afterPropertiesSet() throws Exception {
  Map<String, ?> refreshableServices = m_appCtx.getBeansOfType(IRefreshable.class);
  for (Map.Entry<String, ?> entry : refreshableServices.entrySet() ) {
   Object beanRef = entry.getValue();
   if (beanRef instanceof IRefreshable) {
    m_refreshableServices.add((IRefreshable)beanRef);
   }
  }
 }
}

This approach works particularly well in a clustered application where one of many app servers might change the configuration, which all then need to be aware of. If you want to use JMX as the mechanism for triggering the changes, your JMX bean can then broadcast to the JMS topic when any of its attributes are changed.

I can think of a 'holder bean' approach (essentially a decorator), where the holder bean delegates to holdee, and it's the holder bean which is injected as a dependency into other beans. Nobody else has a reference to holdee but the holder. Now, when the holder bean's config is changed, it recreates the holdee with this new config and starts delegating to it.

You should have a look at JMX. Spring also provides support for this.

• Spring 2.0.x
• Spring 2.5.x
• Spring 3.0.x

Further updated answer to cover scripted bean

Another approach supported by spring 2.5.x+ is that of the scripted bean. You can use a variety of languages for your script - BeanShell is probably the most intuitive given that it has the same syntax as Java, but it does require some external dependencies. However, the examples are in Groovy.

Section 24.3.1.2 of the Spring Documentation covers how to configure this, but here are some salient excerpts illustrating the approach which I've edited to make them more applicable to your situation:

<beans>

    <!-- This bean is now 'refreshable' due to the presence of the 'refresh-check-delay' attribute -->
    <lang:groovy id="messenger"
          refresh-check-delay="5000" <!-- switches refreshing on with 5 seconds between checks -->
          script-source="classpath:Messenger.groovy">
        <lang:property name="message" value="defaultMessage" />
    </lang:groovy>

    <bean id="service" class="org.example.DefaultService">
        <property name="messenger" ref="messenger" />
    </bean>

</beans>

With the Groovy script looking like this:

package org.example

class GroovyMessenger implements Messenger {

    private String message = "anotherProperty";

    public String getMessage() {
        return message;
    }

    public void setMessage(String message) {
        this.message = message
    }
}

As the system administrator wants to make changes then they (or you) can edit the contents of the script appropriately. The script is not part of the deployed application and can reference a known file location (or one that is configured through a standard PropertyPlaceholderConfigurer during startup).

Although the example uses a Groovy class, you could have the class execute code that reads a simple properties file. In that manner, you never edit the script directly, just touch it to change the timestamp. That action then triggers the reload, which in turn triggers the refresh of properties from the (updated) properties file, which finally updates the values within the Spring context and off you go.

The documentation does point out that this technique doesn't work for constructor-injection, but maybe you can work around that.

Updated answer to cover dynamic property changes

Quoting from this article, which provides full source code, one approach is:

> * a factory bean that detects file system changes > * an observer pattern for Properties, so that file system changes can be propagated > * a property placeholder configurer that remembers where which placeholders were used, and updates singleton beans’ properties > * a timer that triggers the regular check for changed files > > The observer pattern is implemented by > the interfaces and classes > ReloadableProperties, > ReloadablePropertiesListener, > PropertiesReloadedEvent, and > ReloadablePropertiesBase. None of them > are especially exciting, just normal > listener handling. The class > DelegatingProperties serves to > transparently exchange the current > properties when properties are > updated. We only update the whole > property map at once, so that the > application can avoid inconsistent > intermediate states (more on this > later). > > Now the > ReloadablePropertiesFactoryBean can be > written to create a > ReloadableProperties instance (instead > of a Properties instance, as the > PropertiesFactoryBean does). When > prompted to do so, the RPFB checks > file modification times, and if > necessary, updates its > ReloadableProperties. This triggers > the observer pattern machinery. > > In our case, the only listener is the > ReloadingPropertyPlaceholderConfigurer. > It behaves just like a standard spring > PropertyPlaceholderConfigurer, except > that it tracks all usages of > placeholders. Now when properties are > reloaded, all usages of each modified > property are found, and the properties > of those singleton beans are assigned > again.

Original answer below covering static property changes:

Sounds like you just want to inject external properties into your Spring context. The PropertyPlaceholderConfigurer is designed for this purpose:

  <!-- Property configuration (if required) -->
  <bean id="serverProperties" class="org.springframework.beans.factory.config.PropertyPlaceholderConfigurer">
    <property name="locations">
      <list>
        <!-- Identical properties in later files overwrite earlier ones in this list -->
        <value>file:/some/admin/location/application.properties</value>
      </list>
    </property>
  </bean>

you then reference the external properties with Ant syntax placeholders (that can be nested if you want from Spring 2.5.5 onwards)

  <bean id="example" class="org.example.DataSource">
    <property name="password" value="${password}"/>
  </bean>

You then ensure that the application.properties file is only accessible to the admin user and the user running the application.

Example application.properties:

password=Aardvark

Or you could use the approach from [this similar question][1] and hence also [my solution][2]:

The approach is to have beans that are configured via property files and the solution is to either

• refresh the entire applicationContext (automatically using a scheduled task or manually using JMX) when properties have changed or
• use a dedicated property provider object to access all properties. This property provider will keep checking the properties files for modification. For beans where prototype-based property lookup is impossible, [register a custom event][3] that your property provider will fire when it finds an updated property file. Your beans with complicated lifecycles will need to listen for that event and refresh themselves.

[1]: https://stackoverflow.com/q/4084890/342852 "Spring - Replacing the bean property values with new Property File values." [2]: https://stackoverflow.com/questions/4084890/spring-replacing-the-bean-property-values-with-new-property-file-values/4085110#4085110 [3]: http://static.springsource.org/spring/docs/3.0.x/spring-framework-reference/html/beans.html#context-functionality-events

This is not something I tried, I am trying to provide pointers.

Assuming your application context is a subclass of AbstractRefreshableApplicationContext(example XmlWebApplicationContext, ClassPathXmlApplicationContext). AbstractRefreshableApplicationContext.getBeanFactory() will give you instance of ConfigurableListableBeanFactory. Check if it is instance of BeanDefinitionRegistry. If so you can call 'registerBeanDefinition' method. This approach will be tightly coupled with Spring implementation,

Check the code of AbstractRefreshableApplicationContext and DefaultListableBeanFactory(this is the implementation you get when you call 'AbstractRefreshableApplicationContext getBeanFactory()')

You can create a custom scope called "reconfigurable" into the ApplicationContext. It creates and caches instances of all beans in this scope. On a configuration change it clears the cache and re-creates the beans on first access with the new configuration. For this to work you need to wrap all instances of reconfigurable beans into an AOP scoped proxy, and access the configuration values with Spring-EL: put a map called config into the ApplicationContext and access the configuration like #{ config['key'] }.

Option 1 :

1. Inject the configurable bean into the DataSource or MailSender. Always get the configurable values from the configuration bean from within these beans.
2. Inside the configurable bean run a thread to read the externally configurable properties (file etc..) periodically. This way the configurable bean will refresh itself after the admin had changed the properties and so the DataSource will get the updated values automatically.

• You need not actually implement the "thread" - read : http://commons.apache.org/configuration/userguide/howto_filebased.html#Automatic_Reloading

---

Option 2 (bad, i think, but maybe not - depends on use case) :

1. Always create new beans for beans of type DataSource / MailSender - using prototype scope. In the init of the bean, read the properties afresh.

---

Option 3 : I think, @mR_fr0g suggestion on using JMX might not be a bad idea. What you could do is :

1. expose your configuration bean as a MBean (read http://static.springsource.org/spring/docs/2.5.x/reference/jmx.html)
2. Ask your admin to change the configuration properties on the MBean (or provide an interface in the bean to trigger property updates from their source)
3. This MBean (a new piece of java code that you will need to write), MUST keep references of Beans (the ones that you want to change / inject the changed properties into). This should be simple (via setter injection or runtime fetch of bean names / classes)
4. When the property on the MBean is changed (or triggered), it must call the appropriate setters on the respective beans. That way, your legacy code does not change, you can still manage runtime property changes.

HTH!

You may want to have a look at the Spring Inspector a plug-gable component that provides programmatic access to any Spring based application at run-time. You can use Javascript to change configurations or manage the application behaviour at run-time.

Here is the nice idea of writing your own PlaceholderConfigurer that tracks the usage of properties and changes them whenever a configuration change occurs. This has two disadvantages, though:

1. It does not work with constructor injection of property values.
2. You can get race conditions if the reconfigured bean receives a changed configuration while it is processing some stuff.

My solution was to copy the original object. Fist i created an interface

/**
 * Allows updating data to some object.
 * Its an alternative to {@link Cloneable} when you cannot 
 * replace the original pointer. Ex.: Beans 
 * @param <T> Type of Object
 */
public interface Updateable<T>
{
    /**
     * Import data from another object
     * @param originalObject Object with the original data
     */
    public void copyObject(T originalObject);
}

For easing the implementation of the function fist create a constructor with all fields, so the IDE could help me a bit. Then you can make a copy constructor that uses the same function Updateable#copyObject(T originalObject). You can also profit of the code of the constructor created by the IDE to create the function to implement:

public class SettingsDTO implements Cloneable, Updateable<SettingsDTO>
{
    private static final Logger LOG = LoggerFactory.getLogger(SettingsDTO.class);
  
    @Size(min = 3, max = 30)  
    private String id;
 
    @Size(min = 3, max = 30)
    @NotNull 
    private String name;
 
    @Size(min = 3, max = 100)
    @NotNull 
    private String description;
    
    @Max(100)
    @Min(5) 
    @NotNull
    private Integer pageSize;
    
    @NotNull 
    private String dateFormat; 

    public SettingsDTO()
    { 
    }   

    public SettingsDTO(String id, String name, String description, Integer pageSize, String dateFormat)
    {
        this.id = id;
        this.name = name;
        this.description = description;
        this.pageSize = pageSize;
        this.dateFormat = dateFormat;
    }
  
    public SettingsDTO(SettingsDTO original)
    {
        copyObject(original);
    }

    @Override
    public void copyObject(SettingsDTO originalObject)
    {
        this.id = originalObject.id;
        this.name = originalObject.name;
        this.description = originalObject.description;
        this.pageSize = originalObject.pageSize;
        this.dateFormat = originalObject.dateFormat;
    } 
}

I used it in a Controller for updating the current settings for the app:

        if (bindingResult.hasErrors())
        {
            model.addAttribute("settingsData", newSettingsData);
            model.addAttribute(Templates.MSG_ERROR, "The entered data has errors");
        }
        else
        {
            synchronized (settingsData)
            {
                currentSettingData.copyObject(newSettingsData);
                redirectAttributes.addFlashAttribute(Templates.MSG_SUCCESS, "The system configuration has been updated successfully");
                return String.format("redirect:/%s", getDao().getPath());
            }
        }

So the currentSettingsData which has the configuration of the application gonna have the updated values, located in newSettingsData. These method allows updating any bean without high complexity.