hdfs中ReflectionUtils在实例化ObjectWritable内部类NullIn...

hadoop版本：1.0.3

问题描述：

在研究DataNode启动代码的时候遇到这么一个问题，通过Hadoop工具类ReflectionUtils反射生成NullInstance实例时，NullInstance父类Configured的成员变量conf未正确赋值，由于未正确赋值，在随后的代码中会抛出空指针异常。

问题分析：

Hadoop在ObjectWritable对象的反序列化过程中，对于非基本类型都会通过反射生成对应的类实例，在这个过程中还会根据类是不是接口Configurable的实现类，进行成员变量conf的赋值操作。相关代码如下：

ObjectWritable:
public static Object readObject(DataInput in, ObjectWritable objectWritable, Configuration conf)
    throws IOException {
      ... 
      Writable writable = WritableFactories.newInstance(instanceClass, conf);
      writable.readFields(in);
      instance = writable;

      ...
      
  }

WritableFactories:
public static Writable newInstance(Class<? extends Writable> c, Configuration conf) {
    WritableFactory factory = WritableFactories.getFactory(c);
    if (factory != null) {
      Writable result = factory.newInstance();
      if (result instanceof Configurable) {
        ((Configurable) result).setConf(conf);
      }
      return result;
    } else {
      return ReflectionUtils.newInstance(c, conf);
    }
  }

ReflectionUtils:
public static <T> T newInstance(Class<T> theClass, Configuration conf) {
		T result;
		try {
			Constructor<T> meth = (Constructor<T>) CONSTRUCTOR_CACHE
					.get(theClass);
			if (meth == null) {
				meth = theClass.getDeclaredConstructor(EMPTY_ARRAY);
				meth.setAccessible(true);
				CONSTRUCTOR_CACHE.put(theClass, meth);
			}
			result = meth.newInstance();
		} catch (Exception e) {
			throw new RuntimeException(e);
		}
		setConf(result, conf);
		return result;
	}
public static void setConf(Object theObject, Configuration conf) {
 if (conf != null) {
      if (theObject instanceof Configurable) {
            ((Configurable) theObject).setConf(conf);
      }
      setJobConf(theObject, conf);
 }
} 

回到NullInstance的类定义，由于继承了类Configured，意味着NullInstance需要在实例化的时候，初始化conf对象。如果conf对象未被正确初始化，可能会出现程序异常，如空指针错误，conf为空，在执行下面代码的时候，会抛空指针异常：

NullInstance：
public void readFields(DataInput in) throws IOException {
      String className = Text.readString(in);
      declaredClass = PRIMITIVE_NAMES.get(className);
      if (declaredClass == null) {
        try {
          //此处调用getConf()可能拿到null
          declaredClass = getConf().getClassByName(className); 
        } catch (ClassNotFoundException e) {
          throw new RuntimeException(e.toString());
        }
      }
    }

问题解决：

在ReflectionUtils的方法setConf中增加代码：

if(Configured.class.isAssignableFrom(theObject.getClass())){
	((Configured)theObject).setConf(conf);
}

来源：oschina

链接：https://my.oschina.net/u/781254/blog/102035