ObjectWritable类主要方法
public void write(DataOutput out) throws IOException {
writeObject(out, instance, declaredClass, conf);
}
public void readFields(DataInput in) throws IOException {
readObject(in, this, this.conf);
}
write是把ObjectWritable对象中的instance写入out，那么instance是从哪里来的呢？
一种方式是通过new时设置的：
public ObjectWritable(Object instance) {
set(instance);
}
另一种方式可以通过readFields方法读取的。
然后看看readObject方法是怎么反序列化一个object的：
public static Object readObject(DataInput in, ObjectWritable objectWritable, Configuration conf)
throws IOException {
String className = UTF8.readString(in);
Class<?> declaredClass = PRIMITIVE_NAMES.get(className);
if (declaredClass == null) {
try {
declaredClass = conf.getClassByName(className);
} catch (ClassNotFoundException e) {
throw new RuntimeException("readObject can't find class " + className, e);
}
}
Object instance;
if (declaredClass.isPrimitive()) { // primitive types
if (declaredClass == Boolean.TYPE) { // boolean
instance = Boolean.valueOf(in.readBoolean());
} else if (declaredClass == Character.TYPE) { // char
instance = Character.valueOf(in.readChar());
} else if (declaredClass == Byte.TYPE) { // byte
instance = Byte.valueOf(in.readByte());
} else if (declaredClass == Short.TYPE) { // short
instance = Short.valueOf(in.readShort());
} else if (declaredClass == Integer.TYPE) { // int
instance = Integer.valueOf(in.readInt());
} else if (declaredClass == Long.TYPE) { // long
instance = Long.valueOf(in.readLong());
} else if (declaredClass == Float.TYPE) { // float
instance = Float.valueOf(in.readFloat());
} else if (declaredClass == Double.TYPE) { // double
instance = Double.valueOf(in.readDouble());
} else if (declaredClass == Void.TYPE) { // void
instance = null;
} else {
throw new IllegalArgumentException("Not a primitive: "+declaredClass);
}
} else if (declaredClass.isArray()) { // array
int length = in.readInt();
instance = Array.newInstance(declaredClass.getComponentType(), length);
for (int i = 0; i < length; i++) {
Array.set(instance, i, readObject(in, conf));
}
} else if (declaredClass == String.class) { // String
instance = UTF8.readString(in);
} else if (declaredClass.isEnum()) { // enum
instance = Enum.valueOf((Class<? extends Enum>) declaredClass, UTF8.readString(in));
} else { // Writable
Class instanceClass = null;
String str = "";
try {
str = UTF8.readString(in);
instanceClass = conf.getClassByName(str);
} catch (ClassNotFoundException e) {
throw new RuntimeException("readObject can't find class " + str, e);
}
Writable writable = WritableFactories.newInstance(instanceClass, conf);
writable.readFields(in);
instance = writable;
if (instanceClass == NullInstance.class) { // null
declaredClass = ((NullInstance)instance).declaredClass;
instance = null;
}
}
if (objectWritable != null) { // store values
objectWritable.declaredClass = declaredClass;
objectWritable.instance = instance;
}
return instance;
}
可以看出 instance 指向的是java基本类型，或者Array，Enum，或者Writable 。而如果DataInput中传过来的是Writable 类型，则会在readObject再去调用readFields方法（writable.readFields(in)），直到DataInput中传递的是非Writable 类型，就这样递归的反序列化DataInput中的Writable对象。
再看看writeObject方法是如何序列化Writable对象的：
/** Write a {@link Writable}, {@link String}, primitive type, or an array of
* the preceding. */
public static void writeObject(DataOutput out, Object instance,
Class declaredClass,
Configuration conf) throws IOException {
if (instance == null) { // null
instance = new NullInstance(declaredClass, conf);
declaredClass = Writable.class;
}
UTF8.writeString(out, declaredClass.getName()); // always write declared
if (declaredClass.isArray()) { // array
int length = Array.getLength(instance);
out.writeInt(length);
for (int i = 0; i < length; i++) {
writeObject(out, Array.get(instance, i),
declaredClass.getComponentType(), conf);
}
} else if (declaredClass == String.class) { // String
UTF8.writeString(out, (String)instance);
} else if (declaredClass.isPrimitive()) { // primitive type
if (declaredClass == Boolean.TYPE) { // boolean
out.writeBoolean(((Boolean)instance).booleanValue());
} else if (declaredClass == Character.TYPE) { // char
out.writeChar(((Character)instance).charValue());
} else if (declaredClass == Byte.TYPE) { // byte
out.writeByte(((Byte)instance).byteValue());
} else if (declaredClass == Short.TYPE) { // short
out.writeShort(((Short)instance).shortValue());
} else if (declaredClass == Integer.TYPE) { // int
out.writeInt(((Integer)instance).intValue());
} else if (declaredClass == Long.TYPE) { // long
out.writeLong(((Long)instance).longValue());
} else if (declaredClass == Float.TYPE) { // float
out.writeFloat(((Float)instance).floatValue());
} else if (declaredClass == Double.TYPE) { // double
out.writeDouble(((Double)instance).doubleValue());
} else if (declaredClass == Void.TYPE) { // void
} else {
throw new IllegalArgumentException("Not a primitive: "+declaredClass);
}
} else if (declaredClass.isEnum()) { // enum
UTF8.writeString(out, ((Enum)instance).name());
} else if (Writable.class.isAssignableFrom(declaredClass)) { // Writable
UTF8.writeString(out, instance.getClass().getName());
((Writable)instance).write(out);
} else {
throw new IOException("Can't write: "+instance+" as "+declaredClass);
}
}
在这两个方法中，向数据流中写数据都是用UTF8类，UTF8类相当于一个工具类。