Hadoop-common之WritableFactories

重温战场

　　WritableFactories是个工厂类，通过它可以创建实例对象。原文中对此对象的注释是Factories for non-public writables，我的理解是处理那些不确定类型的对象进行实例化。例如在ObjectWritable的readObject方法中就调用了WritableFactories     

public static Object readObject(DataInput in, ObjectWritable objectWritable, Configuration conf)
throws IOException {
.......
// Writable
Class instanceClass = null;
String str = UTF8.readString(in);
instanceClass = loadClass(conf, str);
Writable writable = WritableFactories.newInstance(instanceClass, conf);
writable.readFields(in);
instance = writable;
.....
return instance;
}
　　WritableFactories提供了注册机制，用户可以对某种类型自定义一个WritableFactory，然后调用WritableFactories的setFactory方法进行注册。WritableFactories.setFactory() 需要两个参数，分别是 
　　注册类对应的类对象和能够构造注册类的WritableFactory 接口的实现     

private static final Map<Class, WritableFactory> CLASS_TO_FACTORY =
new ConcurrentHashMap<Class, WritableFactory>();
private WritableFactories() {}                  // singleton
/** Define a factory for a class. */
public static void setFactory(Class c, WritableFactory factory) {
CLASS_TO_FACTORY.put(c, factory);
}
　　在HDFS的Block类中，就调用了此方法    

public class Block implements Writable, Comparable<Block> {
public static final String BLOCK_FILE_PREFIX = "blk_";
public static final String METADATA_EXTENSION = ".meta";
static {                                      // register a ctor
WritableFactories.setFactory
(Block.class,
new WritableFactory() {
@Override
public Writable newInstance() { return new Block(); }
});
}
　　WritableFactories的核心方法当然是创建对接咯：newInstance。它的处理逻辑很简单，首先通过类型找到对应的工厂类，然后用工厂类创建实例对象，如果没有工厂类，则用Java的反射机制创建   

/** Create a new instance of a class with a defined factory. */
public static Writable newInstance(Class<? extends Writable> c, Configuration conf) {
//根据类型获取对应的工厂实现类
WritableFactory factory = WritableFactories.getFactory(c);
if (factory != null) {
//通过实现工厂创建实例
Writable result = factory.newInstance();
//如果实例是可配置的，注入Configuration对象
if (result instanceof Configurable) {
((Configurable) result).setConf(conf);
}
return result;
} else {
//如果没有对应实现类，则用反射创建实例
return ReflectionUtils.newInstance(c, conf);
}
}