bean post processor

There is an interface in spring named BeanPostProcessor. It has 2 functions:

public interface BeanPostProcessor
{

    public abstract Object postProcessBeforeInitialization(Object obj, String s)
        throws BeansException;

    public abstract Object postProcessAfterInitialization(Object obj, String s)
        throws BeansException;
}

We can know it from the name of its function:

1. postProcessBeforeInitialization: Operate before init
2. postProcessAfterInitialization: Operate after init

We find its source from the AbstractAutowireCapableBeanFactory class:

protected Object initializeBean(final String beanName, final Object bean, RootBeanDefinition mbd) {
    if (System.getSecurityManager() != null) {
        AccessController.doPrivileged(new PrivilegedAction<Object>() {
            @Override
            public Object run() {
                invokeAwareMethods(beanName, bean);
                return null;
            }
        }, getAccessControlContext());
    }
    else {
        invokeAwareMethods(beanName, bean);
    }

    Object wrappedBean = bean;
    if (mbd == null || !mbd.isSynthetic()) {
        wrappedBean = applyBeanPostProcessorsBeforeInitialization(wrappedBean, beanName);
    }

    try {
        invokeInitMethods(beanName, wrappedBean, mbd);
    }
    catch (Throwable ex) {
        throw new BeanCreationException(
                (mbd != null ? mbd.getResourceDescription() : null),
                beanName, "Invocation of init method failed", ex);
    }

    if (mbd == null || !mbd.isSynthetic()) {
        wrappedBean = applyBeanPostProcessorsAfterInitialization(wrappedBean, beanName);
    }
    return wrappedBean;
}

And the source code of applyBeanPostProcessorsBeforeInitialization and applyBeanPostProcessorsAfterInitialization is:　　

public Object applyBeanPostProcessorsBeforeInitialization(Object existingBean, String beanName)
        throws BeansException {

    Object result = existingBean;
    for (BeanPostProcessor beanProcessor : getBeanPostProcessors()) {
        result = beanProcessor.postProcessBeforeInitialization(result, beanName);
        if (result == null) {
            return result;
        }
    }
    return result;
}

public Object applyBeanPostProcessorsAfterInitialization(Object existingBean, String beanName)
        throws BeansException {

    Object result = existingBean;
    for (BeanPostProcessor beanProcessor : getBeanPostProcessors()) {
        result = beanProcessor.postProcessAfterInitialization(result, beanName);
        if (result == null) {
            return result;
        }
    }
    return result;
}

From the code we know around invokeInitMethods, spring will invoke BeanPostProcessor and do its functions. In invokeInitMethods, there are 2 important points:

1. Judge whether extends InitializingBean, if so then invoke afterPropertiesSet().
2. Judge whether set the init-method, if so then do the function which is setted.

protected void invokeInitMethods(String beanName, final Object bean, RootBeanDefinition mbd)
        throws Throwable {

    boolean isInitializingBean = (bean instanceof InitializingBean);
    if (isInitializingBean && (mbd == null || !mbd.isExternallyManagedInitMethod("afterPropertiesSet"))) {
        if (logger.isDebugEnabled()) {
            logger.debug("Invoking afterPropertiesSet() on bean with name '" + beanName + "'");
        }
        if (System.getSecurityManager() != null) {
            try {
                AccessController.doPrivileged(new PrivilegedExceptionAction<Object>() {
                    @Override
                    public Object run() throws Exception {
                        ((InitializingBean) bean).afterPropertiesSet();
                        return null;
                    }
                }, getAccessControlContext());
            }
            catch (PrivilegedActionException pae) {
                throw pae.getException();
            }
        }
        else {
            ((InitializingBean) bean).afterPropertiesSet();
        }
    }

    if (mbd != null) {
        String initMethodName = mbd.getInitMethodName();
        if (initMethodName != null && !(isInitializingBean && "afterPropertiesSet".equals(initMethodName)) &&
                !mbd.isExternallyManagedInitMethod(initMethodName)) {
            invokeCustomInitMethod(beanName, bean, mbd);
        }
    }
}

DemoService interface

public interface DemoService {
    public void sayHello();
}

DemoServiceImpl

public class DemoServiceImpl implements DemoService,NameInit {
    String name;

    @Override
    public void sayHello() {
        System.out.println("hello "+name);
    }

    @Override
    public void setName(String name) {
        this.name=name;
    }
}

<bean id="demoService" class="com.zjl.beanpostprocessor.DemoServiceImpl">
</bean>

public interface NameInit {
    public void setName(String name);
}

public class NameBeanPostProcessor implements BeanPostProcessor {
    String name;

    public String getName() {
        return name;
    }

    public void setName(String name) {
        this.name = name;
    }

    @Override
    public Object postProcessBeforeInitialization(Object bean, String beanName) throws BeansException {
        if(bean instanceof NameInit){
            ((NameInit)bean).setName(name);
        }
        return bean;
    }

    @Override
    public Object postProcessAfterInitialization(Object bean, String beanName) throws BeansException {
        return bean;
    }
}

<bean id="nameBeanPostProcessor" class="com.zjl.beanpostprocessor.NameBeanPostProcessor">
    <property name="name" value="kincolle"></property>
</bean>

public class LogBeanPostProcessor implements BeanPostProcessor {

    @Override
    public Object postProcessBeforeInitialization(Object bean, String beanName) throws BeansException {
        System.out.println("running:"+beanName);
        return bean;
    }

    @Override
    public Object postProcessAfterInitialization(Object bean, String beanName) throws BeansException {
        System.out.println("running done:"+beanName);
        return bean;
    }
}

<bean id="logBeanPostProcessor" class="com.zjl.beanpostprocessor.LogBeanPostProcessor">
</bean>

public class BeanPostProcessorTest {
    public static void main(String[] args) {
        ApplicationContext context=new FileSystemXmlApplicationContext("beanpostprocessor.xml");
        DemoService demoService=(DemoService) context.getBean("demoService");
        demoService.sayHello();
    }
}

running:demoService
running done:demoService
hello kincolle