BeanPostProcessor NullPointerException 問題紀要

項目中應用到須要使用BeanPostProcessor用來作當bean加載之後出現一些初始化工做的處理，當我繼承 BeanPostProcessor 而且實現改方法的時候，卻在啓動項目的時候出現以下錯誤：

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)

繼承實現源碼爲:

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;
    }

}

上述源碼爲給springmvc 返回 responseBody的時候轉化爲 utf-8 格式 ; 由於沒有用到postProcessBeforeInitialization，因此代碼自動生成的時候沒有去管他，結果發現就報錯了。經過檢查代碼，發現時 執行 postProcessBeforeInitialization 方法的時候將 參數對象設置爲空了，最後再傳回 BeanFactory 的時候丟失了該bean對象，因此報了空指針異常； 最後結果代碼爲:

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;
    }

}