Java中的枚举类型存储在Jvm运行期的那个区域？为什么单例模式用枚举就完美解决了？

Question

Java中的枚举类型是存储在Java运行期的那个区域？为什么单例模式用枚举就完美解决了？

枚举的内存分配是发生在什么时候？初始化又是啥发生在什么时候？代码中使用枚举时候，发生了什么事情？

Answer 1

Enumerations in Java exist in Method Area(method area)

public enum T {
    E1, E2
}

The above code is compiled as follows:

$ javap T.class
Compiled from "T.java"
public final class io.zhudy.web.T extends java.lang.Enum<io.zhudy.web.T> {
  public static final io.zhudy.web.T E1;
  public static final io.zhudy.web.T E2;
  public static io.zhudy.web.T[] values();
  public static io.zhudy.web.T valueOf(java.lang.String);
  static {};
}

You can find that constants are actually compiled into static variables in the end. Static variables in Java are stored in Method Area.

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单例模式的目的是为了保证在内存中只存在唯一一个实例，而枚举值 is fixed just enough to achieve the purpose of controlling the number of instances

The traditional method of bypassing access controlclass实现单例与enum不同之处呢，在于使用使用class需要将constructor访问级别设置为private如果还要防止reflectcreating objects requires additional processing as follows:

public class T2 {

    public static final T2 INSTANCE = new T2();

    private T2() {
        if (INSTANCE != null) {
            throw new AssertionError("实例已存在");
        }
    }

    public static void main(String[] args) throws Exception {
        Constructor c = T2.class.getDeclaredConstructor();
        Object o = c.newInstance();
        System.out.println(o);
    }
}