3. Register BeanPostProcessor

1. Code to Study

• AbstractApplicationContext.registerBeanPostProcessors

protected void registerBeanPostProcessors(ConfigurableListableBeanFactory beanFactory) {
    // Pass in the current Bean factory, register BeanPostProcessors,
    // and invoke their postProcess methods.
    PostProcessorRegistrationDelegate.registerBeanPostProcessors(beanFactory, this);
}

The purpose of PostProcessorRegistrationDelegate.registerBeanPostProcessors(beanFactory, this); is to pass in the current Bean factory, register BeanPostProcessor instances, and invoke their postProcess methods.

• registerBeanPostProcessors (PostProcessorRegistrationDelegate)

public static void registerBeanPostProcessors(
        ConfigurableListableBeanFactory beanFactory, AbstractApplicationContext applicationContext) {

    // Obtain PostProcessors of type BeanPostProcessor from the Bean factory.
    String[] postProcessorNames = beanFactory.getBeanNamesForType(BeanPostProcessor.class, true, false);

    // Register BeanPostProcessorChecker that logs an info message when
    // a bean is
    // a bean is not eligible for getting processed by all BeanPostProcessors.
    int beanProcessorTargetCount = beanFactory.getBeanPostProcessorCount() + 1 + postProcessorNames.length;
    beanFactory.addBeanPostProcessor(new BeanPostProcessorChecker(beanFactory, beanProcessorTargetCount));

    // Separate between BeanPostProcessors that implement PriorityOrdered,
    // Ordered, and the rest.
    List<BeanPostProcessor> priorityOrderedPostProcessors = new ArrayList<BeanPostProcessor>();
    List<BeanPostProcessor> internalPostProcessors = new ArrayList<BeanPostProcessor>();
    List<String> orderedPostProcessorNames = new ArrayList<String>();
    List<String> nonOrderedPostProcessorNames = new ArrayList<String>();
    // Iterate over all PostProcessors and divide them into four groups:
    // PriorityOrdered, MergedBeanDefinitionPostProcessor, Ordered, and ordinary.
    for (String ppName : postProcessorNames) {
        if (beanFactory.isTypeMatch(ppName, PriorityOrdered.class)) {
            BeanPostProcessor pp = beanFactory.getBean(ppName, BeanPostProcessor.class);
            priorityOrderedPostProcessors.add(pp);
            if (pp instanceof MergedBeanDefinitionPostProcessor) {
                internalPostProcessors.add(pp);
            }
        }
        else if (beanFactory.isTypeMatch(ppName, Ordered.class)) {
            orderedPostProcessorNames.add(ppName);
        }
        else {
            nonOrderedPostProcessorNames.add(ppName);
        }
    }

    // First, register the BeanPostProcessors that implement PriorityOrdered.
    // Sort and add BeanPostProcessors implementing PriorityOrdered to the Bean factory.
    sortPostProcessors(priorityOrderedPostProcessors, beanFactory);
    registerBeanPostProcessors(beanFactory, priorityOrderedPostProcessors);

    // Next, register the BeanPostProcessors that implement Ordered.
    // Sort and add BeanPostProcessors implementing Ordered to the Bean factory.
    List<BeanPostProcessor> orderedPostProcessors = new ArrayList<BeanPostProcessor>();
    for (String ppName : orderedPostProcessorNames) {
        BeanPostProcessor pp = beanFactory.getBean(ppName, BeanPostProcessor.class);
        orderedPostProcessors.add(pp);
        if (pp instanceof MergedBeanDefinitionPostProcessor) {
            internalPostProcessors.add(pp);
        }
    }
    sortPostProcessors(orderedPostProcessors, beanFactory);
    registerBeanPostProcessors(beanFactory, orderedPostProcessors);

    // Now, register all regular BeanPostProcessors.
    // Sort and add ordinary BeanPostProcessors to the Bean factory.
    List<BeanPostProcessor> nonOrderedPostProcessors = new ArrayList<BeanPostProcessor>();
    for (String ppName : nonOrderedPostProcessorNames) {
        BeanPostProcessor pp = beanFactory.getBean(ppName, BeanPostProcessor.class);
        nonOrderedPostProcessors.add(pp);
        if (pp instanceof MergedBeanDefinitionPostProcessor) {
            internalPostProcessors.add(pp);
        }
    }
    registerBeanPostProcessors(beanFactory, nonOrderedPostProcessors);

    // Finally, re-register all internal BeanPostProcessors.
    // Sort and add BeanPostProcessors of type MergedBeanDefinitionPostProcessor to the Bean factory.
    sortPostProcessors(internalPostProcessors, beanFactory);
    registerBeanPostProcessors(beanFactory, internalPostProcessors);

    // Re-register post-processor for detecting inner beans as ApplicationListeners,
    // moving it to the end of the processor chain (for picking up proxies etc).
    // Finally add an ApplicationListenerDetector.
    beanFactory.addBeanPostProcessor(new ApplicationListenerDetector(applicationContext));
}

2. Obtain All PostProcessors of Type BeanPostProcessor

// Obtain PostProcessors of type BeanPostProcessor from the Bean factory.
String[] postProcessorNames = beanFactory.getBeanNamesForType(BeanPostProcessor.class, true, false);

3. Iterate and Divide Them into PriorityOrdered, MergedBeanDefinitionPostProcessor, Ordered, and Ordinary Groups

List<BeanPostProcessor> priorityOrderedPostProcessors = new ArrayList<BeanPostProcessor>();
List<BeanPostProcessor> internalPostProcessors = new ArrayList<BeanPostProcessor>();
List<String> orderedPostProcessorNames = new ArrayList<String>();
List<String> nonOrderedPostProcessorNames = new ArrayList<String>();
// Iterate over all PostProcessors and divide them into four groups:
// PriorityOrdered, MergedBeanDefinitionPostProcessor, Ordered, and ordinary.
for (String ppName : postProcessorNames) {
    if (beanFactory.isTypeMatch(ppName, PriorityOrdered.class)) {
        BeanPostProcessor pp = beanFactory.getBean(ppName, BeanPostProcessor.class);
        priorityOrderedPostProcessors.add(pp);
        if (pp instanceof MergedBeanDefinitionPostProcessor) {
            internalPostProcessors.add(pp);
        }
    }
    else if (beanFactory.isTypeMatch(ppName, Ordered.class)) {
        orderedPostProcessorNames.add(ppName);
    }
    else {
        nonOrderedPostProcessorNames.add(ppName);
    }
}

4. Sort and Add BeanPostProcessors Implementing PriorityOrdered to the Bean Factory

// First, register the BeanPostProcessors that implement PriorityOrdered.
// Sort and add BeanPostProcessors implementing PriorityOrdered to the Bean factory.
sortPostProcessors(priorityOrderedPostProcessors, beanFactory);
registerBeanPostProcessors(beanFactory, priorityOrderedPostProcessors);

5. Sort and Add BeanPostProcessors Implementing Ordered to the Bean Factory

// Next, register the BeanPostProcessors that implement Ordered.
// Sort and add BeanPostProcessors implementing Ordered to the Bean factory.
List<BeanPostProcessor> orderedPostProcessors = new ArrayList<BeanPostProcessor>();
for (String ppName : orderedPostProcessorNames) {
    BeanPostProcessor pp = beanFactory.getBean(ppName, BeanPostProcessor.class);
    orderedPostProcessors.add(pp);
    if (pp instanceof MergedBeanDefinitionPostProcessor) {
        internalPostProcessors.add(pp);
    }
}
sortPostProcessors(orderedPostProcessors, beanFactory);
registerBeanPostProcessors(beanFactory, orderedPostProcessors);

6. Sort and Add Ordinary BeanPostProcessors to the Bean Factory

// Now, register all regular BeanPostProcessors.
// Sort and add ordinary BeanPostProcessors to the Bean factory.
List<BeanPostProcessor> nonOrderedPostProcessors = new ArrayList<BeanPostProcessor>();
for (String ppName : nonOrderedPostProcessorNames) {
    BeanPostProcessor pp = beanFactory.getBean(ppName, BeanPostProcessor.class);
    nonOrderedPostProcessors.add(pp);
    if (pp instanceof MergedBeanDefinitionPostProcessor) {
        internalPostProcessors.add(pp);
    }
}
registerBeanPostProcessors(beanFactory, nonOrderedPostProcessors);

7. Sort and Add BeanPostProcessors of Type MergedBeanDefinitionPostProcessor to the Bean Factory

// Finally, re-register all internal BeanPostProcessors.
// Sort and add BeanPostProcessors of type MergedBeanDefinitionPostProcessor to the Bean factory.
sortPostProcessors(internalPostProcessors, beanFactory);
registerBeanPostProcessors(beanFactory, internalPostProcessors);