实现Hadoop的Writable接口的枚举值 [英] Enum value implementing Writable interface of Hadoop

问题描述

假设我有一个枚举：

public enum SomeEnumType implements Writable {
  A(0), B(1);

  private int value;

  private SomeEnumType(int value) {
    this.value = value;
  }

  @Override
  public void write(final DataOutput dataOutput) throws IOException {
    dataOutput.writeInt(this.value);
  }

  @Override
  public void readFields(final DataInput dataInput) throws IOException {
    this.value = dataInput.readInt();
  }
}

我想将其实例作为一部分传递

I want to pass an instance of it as a part of some other class instance.

等于将不起作用，因为它将不考虑枚举的内部变量，而且所有枚举实例都在编译时固定，因此无法在其他地方创建。

The equals would not work, because it will not consider the inner variable of enumeration, plus all enum instances are fixed at compile time and could not be created elsewhere.

这是否意味着我无法通过Hadoop在网络上发送枚举或有解决方案？

Does it mean I could not send enums over the wire in Hadoop or there's a solution?

推荐答案

我对Hadoop中枚举的常规和首选解决方案是通过枚举的序号序列化枚举。

My normal and preferred solution for enums in Hadoop is serializing the enums through their ordinal value.

public class EnumWritable implements Writable {

    static enum EnumName {
        ENUM_1, ENUM_2, ENUM_3
    }

    private int enumOrdinal;

    // never forget your default constructor in Hadoop Writables
    public EnumWritable() {
    }

    public EnumWritable(Enum<?> arbitraryEnum) {
        this.enumOrdinal = arbitraryEnum.ordinal();
    }

    public int getEnumOrdinal() {
        return enumOrdinal;
    }

    @Override
    public void readFields(DataInput in) throws IOException {
        enumOrdinal = in.readInt();
    }

    @Override
    public void write(DataOutput out) throws IOException {
        out.writeInt(enumOrdinal);
    }

    public static void main(String[] args) {
        // use it like this:
        EnumWritable enumWritable = new EnumWritable(EnumName.ENUM_1);
        // let Hadoop do the write and read stuff
        EnumName yourDeserializedEnum = EnumName.values()[enumWritable.getEnumOrdinal()];
    }

}

显然它有缺点：普通人可以进行更改，因此如果将 ENUM_2 与 ENUM_3 交换并读取以前的序列化文件，这将返回另一个错误的枚举。

Obviously it has drawbacks: Ordinals can change, so if you exchange ENUM_2 with ENUM_3 and read a previously serialized file, this will return the other wrong enum.

因此，如果您事先知道枚举类，则可以编写枚举的名称并像这样使用它：

So if you know the enum class beforehand, you can write the name of your enum and use it like this:

 enumInstance = EnumName.valueOf(in.readUTF());

这将使用更多的空间，但更节省了对枚举名称的更改。

This will use slightly more space, but it is more save to changes to your enum names.

完整的示例如下：

public class EnumWritable implements Writable {

    static enum EnumName {
        ENUM_1, ENUM_2, ENUM_3
    }

    private EnumName enumInstance;

    // never forget your default constructor in Hadoop Writables
    public EnumWritable() {
    }

    public EnumWritable(EnumName e) {
        this.enumInstance = e;
    }

    public EnumName getEnum() {
        return enumInstance;
    }

    @Override
    public void write(DataOutput out) throws IOException {
        out.writeUTF(enumInstance.name());
    }

    @Override
    public void readFields(DataInput in) throws IOException {
        enumInstance = EnumName.valueOf(in.readUTF());
    }

    public static void main(String[] args) {
        // use it like this:
        EnumWritable enumWritable = new EnumWritable(EnumName.ENUM_1);
        // let Hadoop do the write and read stuff
        EnumName yourDeserializedEnum = enumWritable.getEnum();

    }

}

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