Question

I am using prototype scoped bean definitions in my Spring XML descriptors to configure the default properties (these beans have many properties) and then I call the application context with a getBean(beanName, ctorArgs[]) to create instances.

The bean definitions require 2-3 constructor arguments that supply logically unique keys used for things like key properties for the JMX ObjectName etc. Additionally, the variables that the constructor arguments are written to are final.

What I am seeing is that when the application context refreshes, it attempts to instantiate these prototypes, which seems completely the opposite of what you want prototypes to do. They’re templates, not actual instances. To work around this, I have been configuring the prototypes with bogus ctor values so these bogus bean instances are instantiated and I simply filter out the created MBeans later in the code.

My question is, how do I configure the application context to register these prototype bean definitions, but not instantiate them until I make a getBean call ?

Answer 1

UPDATE:

The problem is a bit more complicated than I initialy thought. In fact, lazy is the default behaviour for prototype-scoped beans. I digged a bit and I managed to reproduce your problem and find the solution. So what is the problem?

You probably have <aop:scoped-proxy/> enabled or (@ComponentScan(scopedProxy=...) equivalent). During context refresh Spring wraps your prototype bean (ClosedMetricSubscriberFeed) with scoped proxy. It uses class proxy because (a) class proxies are chosen or (b) the class has no interfaces.

The class-based proxy is basically a CGLIB subclass of your bean that must call (due to JVM rules) a base class’s constructor. And CGLIB generated class always calls no-arg constructor.

I know this sounds complicated, here’s what you can do:

1. Disable <aop:scoped-proxy/>. Just like that.

2. Provide a dummy no-arg constructor and deprecate it just in case. Unfortunately you will have to discover such bogus instances manunally. Note that in this case the class will be of type: “.

3. Extract an interface from your class and use interfaces for scoped proxies:

.

@Scope(
  value = ConfigurableBeanFactory.SCOPE_PROTOTYPE,
  proxyMode = ScopedProxyMode.INTERFACES)

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Old answer:

Use lazy initialization with @Lazy annotation or lazy-init="true" (see 4.4.4 Lazy-initialized beans in reference documentation) configuration attribute.

<bean id="proto" class="MyPrototype" scope="prototype" lazy-init="true"/>

or:

@Service
@Scope("prototype")
@Lazy
public class MyPrototype {/*...*/}