BeanPostProcessor NullPointerException 问题纪要

项目中应用到须要使用BeanPostProcessor用来作当bean加载之后出现一些初始化工做的处理，当我继承 BeanPostProcessor 而且实现改方法的时候，却在启动项目的时候出现以下错误：

java.lang.NullPointerException
	at org.springframework.beans.factory.support.AbstractBeanFactory.getObjectForBeanInstance(AbstractBeanFactory.java:1501)
	at org.springframework.beans.factory.support.AbstractBeanFactory.doGetBean(AbstractBeanFactory.java:314)
	at org.springframework.beans.factory.support.AbstractBeanFactory.getBean(AbstractBeanFactory.java:194)
	at org.springframework.beans.factory.support.DefaultListableBeanFactory.preInstantiateSingletons(DefaultListableBeanFactory.java:736)
	at org.springframework.context.support.AbstractApplicationContext.finishBeanFactoryInitialization(AbstractApplicationContext.java:757)
	at org.springframework.context.support.AbstractApplicationContext.refresh(AbstractApplicationContext.java:480)
	at org.springframework.web.servlet.FrameworkServlet.configureAndRefreshWebApplicationContext(FrameworkServlet.java:664)
	at org.springframework.web.servlet.FrameworkServlet.createWebApplicationContext(FrameworkServlet.java:630)
	at org.springframework.web.servlet.FrameworkServlet.createWebApplicationContext(FrameworkServlet.java:678)
	at org.springframework.web.servlet.FrameworkServlet.initWebApplicationContext(FrameworkServlet.java:549)
	at org.springframework.web.servlet.FrameworkServlet.initServletBean(FrameworkServlet.java:490)
	at org.springframework.web.servlet.HttpServletBean.init(HttpServletBean.java:136)
	at javax.servlet.GenericServlet.init(GenericServlet.java:158)
	at org.apache.catalina.core.StandardWrapper.initServlet(StandardWrapper.java:1282)
	at org.apache.catalina.core.StandardWrapper.loadServlet(StandardWrapper.java:1195)
	at org.apache.catalina.core.StandardWrapper.allocate(StandardWrapper.java:866)
	at org.apache.catalina.core.StandardWrapperValve.invoke(StandardWrapperValve.java:134)
	at org.apache.catalina.core.StandardContextValve.invoke(StandardContextValve.java:122)
	at org.apache.catalina.authenticator.AuthenticatorBase.invoke(AuthenticatorBase.java:505)
	at org.apache.catalina.core.StandardHostValve.invoke(StandardHostValve.java:169)
	at org.apache.catalina.valves.ErrorReportValve.invoke(ErrorReportValve.java:103)
	at org.apache.catalina.valves.AccessLogValve.invoke(AccessLogValve.java:956)
	at org.apache.catalina.core.StandardEngineValve.invoke(StandardEngineValve.java:116)
	at org.apache.catalina.connector.CoyoteAdapter.service(CoyoteAdapter.java:442)
	at org.apache.coyote.http11.AbstractHttp11Processor.process(AbstractHttp11Processor.java:1082)
	at org.apache.coyote.AbstractProtocol$AbstractConnectionHandler.process(AbstractProtocol.java:623)
	at org.apache.tomcat.util.net.JIoEndpoint$SocketProcessor.run(JIoEndpoint.java:316)
	at java.util.concurrent.ThreadPoolExecutor.runWorker(ThreadPoolExecutor.java:1145)
	at java.util.concurrent.ThreadPoolExecutor$Worker.run(ThreadPoolExecutor.java:615)
	at org.apache.tomcat.util.threads.TaskThread$WrappingRunnable.run(TaskThread.java:61)
	at java.lang.Thread.run(Thread.java:745)

继承实现源码为:

public class DefinedBeanPostProcessor implements BeanPostProcessor {

    @Override
    public Object postProcessAfterInitialization(Object bean, String beanName) throws BeansException {
        if (bean instanceof StringHttpMessageConverter) {
            MediaType mediaType = new MediaType("text", "plain", Charset.forName("UTF-8"));
            List<MediaType> types = new ArrayList<MediaType>();
            types.add(mediaType);
            ((StringHttpMessageConverter) bean).setSupportedMediaTypes(types);
        }
        return bean;
    }

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

}

上述源码为给springmvc 返回 responseBody的时候转化为 utf-8 格式 ; 由于没有用到postProcessBeforeInitialization，因此代码自动生成的时候没有去管他，结果发现就报错了。经过检查代码，发现时 执行 postProcessBeforeInitialization 方法的时候将 参数对象设置为空了，最后再传回 BeanFactory 的时候丢失了该bean对象，因此报了空指针异常； 最后结果代码为:

public class DefinedBeanPostProcessor implements BeanPostProcessor {

    @Override
    public Object postProcessAfterInitialization(Object bean, String beanName) throws BeansException {
        if (bean instanceof StringHttpMessageConverter) {
            MediaType mediaType = new MediaType("text", "plain", Charset.forName("UTF-8"));
            List<MediaType> types = new ArrayList<MediaType>();
            types.add(mediaType);
            ((StringHttpMessageConverter) bean).setSupportedMediaTypes(types);
        }
        return bean;
    }

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

}