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());
}
}
}在ReflectionUtils的方法setConf中增加代码：
if(Configured.class.isAssignableFrom(theObject.getClass())){
((Configured)theObject).setConf(conf);
}
