Android枚舉字段序列化

實體類實現Parcelable接口後枚舉字段該怎麼寫？正好碰到這個問題，記錄一下。

枚舉類

public enum EnumBean {
    STEAM,
    EPIC
}

枚舉字段序列化，這裏直接上寫法。實際上用了AS插件Android Parcelable Code Generator生成，當然還是本着知其所以然的態度，思考一下爲啥是這個寫法。

public class Test implements Parcelable {
    public String name;

    public int age;

    public EnumBean e;

    @Override
    public int describeContents() {
        return 0;
    }

    @Override
    public void writeToParcel(Parcel dest, int flags) {
        dest.writeString(this.name);
        dest.writeInt(this.age);
        dest.writeInt(this.e == null ? -1 : this.e.ordinal());
    }

    public void readFromParcel(Parcel source) {
        this.name = source.readString();
        this.age = source.readInt();
        int tmpE = source.readInt();
        this.e = tmpE == -1 ? null : EnumBean.values()[tmpE];
    }

    public Test() {
    }

    protected Test(Parcel in) {
        this.name = in.readString();
        this.age = in.readInt();
        int tmpE = in.readInt();
        this.e = tmpE == -1 ? null : EnumBean.values()[tmpE];
    }

    public static final Parcelable.Creator<Test> CREATOR = new Parcelable.Creator<Test>() {
        @Override
        public Test createFromParcel(Parcel source) {
            return new Test(source);
        }

        @Override
        public Test[] newArray(int size) {
            return new Test[size];
        }
    };
}

其實從write和read字段e就可以看出，這裏寫入的是枚舉類數組下標，讀取時根據下標取出枚舉類實體。

dest.writeInt(this.e == null ? -1 : this.e.ordinal());

this.e = tmpE == -1 ? null : EnumBean.values()[tmpE];

衆所周知枚舉中的每個聲明都會創建枚舉類對象，ordinal()也就是枚舉中聲明的順序，對應到枚舉類數組values()的下標。

看一下編譯後的EnumBean.class文件字節碼

  //枚舉類默認繼承Enum
  public final enum com/chenxuan/kotlin/EnumBean extends java/lang/Enum

  public final static enum Lcom/chenxuan/kotlin/EnumBean; STEAM

  public final static enum Lcom/chenxuan/kotlin/EnumBean; EPIC

  private final static synthetic [Lcom/chenxuan/kotlin/EnumBean; $VALUES

  //構造方法
  private <init>(Ljava/lang/String;I)V
    L0
    LINENUMBER 3 L0
    ALOAD 0
    ALOAD 1
    ILOAD 2
    INVOKESPECIAL java/lang/Enum.<init> (Ljava/lang/String;I)V
    RETURN

  //static塊，創建聲明的枚舉類對象，枚舉類數組。
  static <clinit>()V
   L0
    LINENUMBER 4 L0
    NEW com/chenxuan/kotlin/EnumBean
    DUP
    LDC "STEAM"
    ICONST_0
    INVOKESPECIAL com/chenxuan/kotlin/EnumBean.<init> (Ljava/lang/String;I)V
    PUTSTATIC com/chenxuan/kotlin/EnumBean.STEAM : Lcom/chenxuan/kotlin/EnumBean;
   L1
    LINENUMBER 5 L1
    NEW com/chenxuan/kotlin/EnumBean
    DUP
    LDC "EPIC"
    ICONST_1
    INVOKESPECIAL com/chenxuan/kotlin/EnumBean.<init> (Ljava/lang/String;I)V
    PUTSTATIC com/chenxuan/kotlin/EnumBean.EPIC : Lcom/chenxuan/kotlin/EnumBean;
   L2
    LINENUMBER 3 L2
    ICONST_2
    ANEWARRAY com/chenxuan/kotlin/EnumBean
    DUP
    ICONST_0
    GETSTATIC com/chenxuan/kotlin/EnumBean.STEAM : Lcom/chenxuan/kotlin/EnumBean;
    AASTORE
    DUP
    ICONST_1
    GETSTATIC com/chenxuan/kotlin/EnumBean.EPIC : Lcom/chenxuan/kotlin/EnumBean;
    AASTORE
    PUTSTATIC com/chenxuan/kotlin/EnumBean.$VALUES : [Lcom/chenxuan/kotlin/EnumBean;
    RETURN

  public static values()[Lcom/chenxuan/kotlin/EnumBean;
   L0
    LINENUMBER 3 L0
    GETSTATIC com/chenxuan/kotlin/EnumBean.$VALUES : [Lcom/chenxuan/kotlin/EnumBean;
    INVOKEVIRTUAL [Lcom/chenxuan/kotlin/EnumBean;.clone ()Ljava/lang/Object;
    CHECKCAST [Lcom/chenxuan/kotlin/EnumBean;
    ARETURN

  public static valueOf(Ljava/lang/String;)Lcom/chenxuan/kotlin/EnumBean;
   L0
    LINENUMBER 3 L0
    LDC Lcom/chenxuan/kotlin/EnumBean;.class
    ALOAD 0
    INVOKESTATIC java/lang/Enum.valueOf (Ljava/lang/Class;Ljava/lang/String;)Ljava/lang/Enum;
    CHECKCAST com/chenxuan/kotlin/EnumBean
    ARETURN

很好理解，聲明瞭兩個EnumBean類型的變量STEAM和EPIC並在static塊中初始化；同時聲明瞭EnumBean數組$VALUES，按照枚舉聲明順序添加到數組。

需注意枚舉變量初始化時調用的構造方法傳入了兩個參數：聲明的枚舉變量對應的字符串STEAM、下標0。

    LDC "STEAM"
    ICONST_0
    INVOKESPECIAL com/chenxuan/kotlin/EnumBean.<init> (Ljava/lang/String;I)V

EnumBean構造方法調用父類Enum的構造方法

  private <init>(Ljava/lang/String;I)V
    L0
    LINENUMBER 3 L0
    ALOAD 0
    ALOAD 1
    ILOAD 2
    INVOKESPECIAL java/lang/Enum.<init> (Ljava/lang/String;I)V
    RETURN

繼續看Enum的構造方法

    private final String name;
    private final int ordinal;

    protected Enum(String name, int ordinal) {
        this.name = name;
        this.ordinal = ordinal;
    }

    public final int ordinal() {
        return ordinal;
    }

如此一來，ordinal就和枚舉聲明順序或者說枚舉數組下標聯繫起來了。

回看readFromParcel中EnumBean.values()[tmpE]，由上述字節碼可知values()方法調用$VALUES.clone()也就是枚舉類數組，然後根據數組下標取出枚舉類實體。

還有我們常用的valueOf()方法，調用父類Enum.valueOf()傳入了EnumBean.class。

  public static valueOf(Ljava/lang/String;)Lcom/chenxuan/kotlin/EnumBean;
   L0
    LINENUMBER 3 L0
    LDC Lcom/chenxuan/kotlin/EnumBean;.class
    ALOAD 0
    INVOKESTATIC java/lang/Enum.valueOf (Ljava/lang/Class;Ljava/lang/String;)Ljava/lang/Enum;
    CHECKCAST com/chenxuan/kotlin/EnumBean
    ARETURN

Enum.valueOf()

    public static <T extends Enum<T>> T valueOf(Class<T> enumType,
                                                String name) {
        Objects.requireNonNull(enumType, "enumType == null");
        Objects.requireNonNull(enumType, "name == null");
        T[] values = getSharedConstants(enumType);
        if (values == null) {
            throw new IllegalArgumentException(enumType.toString() + " is not an enum type.");
        }

        for (int i = values.length - 1; i >= 0; --i) {
            T value = values[i];
            if (name.equals(value.name())) {
                return value;
            }
        }
        throw new IllegalArgumentException(
                "No enum constant " + enumType.getCanonicalName() + "." + name);
    }

Enum.getSharedConstants()根據傳入的class獲取枚舉類數組values

    public static <T extends Enum<T>> T[] getSharedConstants(Class<T> enumType) {
        return (T[]) sharedConstantsCache.get(enumType);
    }

    private static final BasicLruCache<Class<? extends Enum>, Object[]> sharedConstantsCache
            = new BasicLruCache<Class<? extends Enum>, Object[]>(64) {
        @Override protected Object[] create(Class<? extends Enum> enumType) {
            return enumValues(enumType);
        }
    };

    private static Object[] enumValues(Class<? extends Enum> clazz) {
        if (!clazz.isEnum()) {
            return null;
        }
        try {
            Method valueMethod = clazz.getDeclaredMethod("values");
            return (Object[]) valueMethod.invoke(null);
        } catch (NoSuchMethodException | IllegalAccessException | InvocationTargetException e) {
            throw new RuntimeException(e);
        }
    }

這裏通過反射獲取到枚舉類數組，然後迭代數組找到name相同的枚舉類。而name其實就是聲明的枚舉變量對應的字符串，在static塊中初始化時調用到父類Enum構造方法賦值。

    NEW com/chenxuan/kotlin/EnumBean
    DUP
    LDC "STEAM"
    ICONST_0
    INVOKESPECIAL com/chenxuan/kotlin/EnumBean.<init> (Ljava/lang/String;I)V
    PUTSTATIC com/chenxuan/kotlin/EnumBean.STEAM : Lcom/chenxuan/kotlin/EnumBean;