Spring The resolvedConstructorArguments field in BeanDefiniton is always null or EMPTY_ARGS

I found that during Bean instantiation, the resolvedConstructorArguments are always empty or null.

The purpose of using the cache parameter is to quickly find the corresponding method when the Bean is instantiated the second time. However, in order to avoid creating multiple prototype beans whose properties reference the same object, we cannot directly cache the parameters to be used by the method, so the preparedConstructorArguments parameter appears.

As far as I can tell, resolvedConstructorArguments is either null or an empty array, and it's only going to be an empty array if the following if-condition is satisfied. The rest of the time it is null.

In ConstructorResolver, its instantiateUsingFactoryMethod and autowireConstructor methods contain the following code:

if (candidates.length == 1 && explicitArgs == null && !mbd.hasConstructorArgumentValues()) {
    Constructor<?> uniqueCandidate = candidates[0];
    if (uniqueCandidate.getParameterCount() == 0) {
        synchronized (mbd.constructorArgumentLock) {
            mbd.resolvedConstructorOrFactoryMethod = uniqueCandidate;
            mbd.constructorArgumentsResolved = true;
            mbd.resolvedConstructorArguments = EMPTY_ARGS;
        }
        bw.setBeanInstance(instantiate(beanName, mbd, uniqueCandidate, EMPTY_ARGS));
        return bw;
    }
}

When resolvedConstructorArguments is an empty array, it looks like resolvedConstructorArguments's job is to make sure that no arguments are needed when the method is called.

I don't understand why it's designed this way.

The resolvedConstructorArguments field doesn't seem necessary.

Comment From: JulianHang

I also want to know the explanation of this question，thanks! @spring-issuemaster

Comment From: daimingzhi

My guess is that in order to expand, we can through some way directly to assign some value to "resolvedConstructorArguments"。 but I can not sure, hope someone can give the answer

Comment From: snicoll

resolvedConstructorArguments is a package-visible field that's access by ConstructorResolver and is necessary.