DefaultSpringEncoderParser自定义valueEncoder、valueDecoder时，报空指针异常

在查看了DefaultSpringEncoderParser类中的parseDecoder方法，发现可以通过在配置中以"bean:"为前缀来指定自定义的ValueEncoder和ValueDecoder，但发现该类运行到Object bean = applicationContext.getBean(beanName);这一行代码时，由于applicationContext未完成初始化，会报空指针异常，完法实现自定义valueEncoder、valueDecoder。原因是parseDecoder方法会早于该类的setApplicationContext方法之前运行。

Read DefaultSpringEncoderParser.parseDecoder and setApplicationContext first, focusing on their execution order and the applicationContext access in parseDecoder. Reproduce the configuration using a "bean:"-prefixed custom ValueEncoder or ValueDecoder, then verify that both can be resolved without a null-pointer exception.

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