java中枚举实现原理

介绍

枚举是java5新增的特性，我们可以用来替代以前用常量完成的功能。java对枚举的支持还有EnumSet,EnumMap等工具类。

使用

public enum UserGenderEnum {
  MALE("0", "男"),
  FEMALE("1", "女"),
  NOT_KNOWN("2", "未知");

  private final String code;
  private final String info;

  UserGenderEnum(String code, String info) {
    this.code = code;
    this.info = info;
  }

  public String getCode() {
    return code;
  }

  public String getInfo() {
    return info;
  }
}

public class Client {
  public static void main(String[] args) {
    UserGenderEnum[] userGenderEnums = UserGenderEnum.values();
    for (UserGenderEnum genderEnum : userGenderEnums) {
      System.out.println("enum:" + genderEnum + "," + genderEnum.name() + "," + genderEnum.ordinal());
    }
    System.out.println(UserGenderEnum.valueOf("MALE"));
  }
}

输出结果为

enum:MALE,MALE,0
enum:FEMALE,FEMALE,1
enum:NOT_KNOWN,NOT_KNOWN,2
MALE

我们看一下枚举反编译后的结果，如何反编译看上一篇文章

public final class UserGenderEnum extends Enum<UserGenderEnum> {
    public static final /* enum */ UserGenderEnum MALE = new UserGenderEnum("MALE", 0, "0", "\u7537");
    public static final /* enum */ UserGenderEnum FEMALE = new UserGenderEnum("FEMALE", 1, "1", "\u5973");
    public static final /* enum */ UserGenderEnum NOT_KNOWN = new UserGenderEnum("NOT_KNOWN", 2, "2", "\u672a\u77e5");
    private final String code;
    private final String info;
    private static final /* synthetic */ UserGenderEnum[] $VALUES;

    public static UserGenderEnum[] values() {
        return (UserGenderEnum[])$VALUES.clone();
    }

    public static UserGenderEnum valueOf(String name) {
        return Enum.valueOf(UserGenderEnum.class, name);
    }

    private UserGenderEnum(String string, int n, String code, String info) {
        super(string, n);
        this.code = code;
        this.info = info;
    }

    public String getCode() {
        return this.code;
    }

    public String getInfo() {
        return this.info;
    }

    static {
        $VALUES = new UserGenderEnum[]{MALE, FEMALE, NOT_KNOWN};
    }
}

枚举类默认继承Enum抽象类，这也是枚举不能继承其他类的原因。编译器帮我们创建了3个静态实例，并将构造器设为私有，在我们的构造参数基础上增加了name和ordinal两个参数，indinal可以看做实例的索引。接下来我们看一下valueOf方法的实现原理，

这个方法是编译器帮我们创建的，反编译后才可以看到，

实际上调用的是Enum内部的方法，

继续跟到Class类


终于看到了最终实现，可以看到Class还是调用了UserGenderEnum的values方法，上面可以看到values方法返回的是实例的数组，兜兜转转有回到了UserGenderEnum。
接下来看一下switch对枚举的支持，

public class Client {
  public static void main(String[] args) {
    UserGenderEnum male = UserGenderEnum.MALE;
    switch (male) {
      case MALE:
        System.out.println("male");
        break;
      case FEMALE:
        System.out.println("female");
        break;
      case NOT_KNOWN:
        System.out.println("not_known");
        break;
    }
  }
}

反编译结果为

public class Client {
    public static void main(String[] args) {
        UserGenderEnum male = UserGenderEnum.MALE;
        switch (male.ordinal()) {
            case 1: {
                System.out.println("male");
                break;
            }
            case 2: {
                System.out.println("female");
                break;
            }
            case 3: {
                System.out.println("not_known");
            }
        }
    }
}

可以看到最终还是转成了int类型。