Java 枚举实现原理

Posted 2021-02-07 fengda666

枚举 反编译
示意枚举类如下：

package com.jiankunking.test;

public enum ApplicationInterfaceTypeEnum {

dubbo("dubbo", 1), webapp("webapp", 2), custom("custom", 3);
private String name;
private int index;

//私有构造,防止被外部调用
private ApplicationInterfaceTypeEnum(String name, int index) {
this.name = name;
this.index = index;
}

public String getName() {
return name;
}

public void setName(String name) {
this.name = name;
}

public int getIndex() {
return index;
}

public void setIndex(int index) {
this.index = index;
}
}
使用：

ApplicationInterfaceTypeEnum.webapp.getName()
ApplicationInterfaceTypeEnum.webapp.getIndex()
编译：

反编译（这里使用的工具是：jad） 

将编译好的ApplicationInterfaceTypeEnum.class，拷贝到jad所在的文件夹，执行

jad ApplicationInterfaceTypeEnum.class

使用编辑器打开ApplicationInterfaceTypeEnum.jad文件如下：

// Decompiled by Jad v1.5.8g. Copyright 2001 Pavel Kouznetsov.
// Jad home page: http://www.kpdus.com/jad.html
// Decompiler options: packimports(3)
// Source File Name: ApplicationInterfaceTypeEnum.java

package com.jiankunking.test;

public final class ApplicationInterfaceTypeEnum extends Enum
{

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

public static ApplicationInterfaceTypeEnum valueOf(String s)
{
return (ApplicationInterfaceTypeEnum)Enum.valueOf(com/jiankunking/test/ApplicationInterfaceTypeEnum, s);
}

private ApplicationInterfaceTypeEnum(String s, int i, String s1, int j)
{
super(s, i);
name = s1;
index = j;
}

public String getName()
{
return name;
}

public void setName(String s)
{
name = s;
}

public int getIndex()
{
return index;
}

public void setIndex(int i)
{
index = i;
}

public static final ApplicationInterfaceTypeEnum dubbo;
public static final ApplicationInterfaceTypeEnum webapp;
public static final ApplicationInterfaceTypeEnum custom;
private String name;
private int index;
private static final ApplicationInterfaceTypeEnum $VALUES[];

static
{
dubbo = new ApplicationInterfaceTypeEnum("dubbo", 0, "dubbo", 1);
webapp = new ApplicationInterfaceTypeEnum("webapp", 1, "webapp", 2);
custom = new ApplicationInterfaceTypeEnum("custom", 2, "custom", 3);
$VALUES = (new ApplicationInterfaceTypeEnum[] {
dubbo, webapp, custom
});
      从反编译的代码可以看出编译器帮助我们生成了一个ApplicationInterfaceTypeEnum类(注意该类是final类型的，将无法被继承)而且该类继承自java.lang.Enum类，该类是一个抽象类。
      除此之外，编译器还帮助我们生成了3个ApplicationInterfaceTypeEnum类型的实例对象分别对应枚举中定义的3中类型。
      编译器还为我们生成了两个静态方法，分别是values()和 valueOf()。
      使用关键字enum定义的枚举类型，在编译期后，也将转换成为一个实实在在的类，而在该类中，会存在每个在枚举类型中定义好变量的对应实例对象，如上述的ApplicationInterfaceTypeEnum枚举类型对应

public static final ApplicationInterfaceTypeEnum dubbo;
public static final ApplicationInterfaceTypeEnum webapp;
public static final ApplicationInterfaceTypeEnum custom;
同时编译器会为该类创建两个方法，分别是values()和valueOf()。

抽象类 Enum
代码结构

源码
open jdk 1.8

/*
* Copyright (c) 2003, 2011, Oracle and/or its affiliates. All rights reserved.
* DO NOT ALTER OR REMOVE COPYRIGHT NOTICES OR THIS FILE HEADER.
*
* This code is free software; you can redistribute it and/or modify it
* under the terms of the GNU General Public License version 2 only, as
* published by the Free Software Foundation. Oracle designates this
* particular file as subject to the "Classpath" exception as provided
* by Oracle in the LICENSE file that accompanied this code.
*
* This code is distributed in the hope that it will be useful, but WITHOUT
* ANY WARRANTY; without even the implied warranty of MERCHANTABILITY or
* FITNESS FOR A PARTICULAR PURPOSE. See the GNU General Public License
* version 2 for more details (a copy is included in the LICENSE file that
* accompanied this code).
*
* You should have received a copy of the GNU General Public License version
* 2 along with this work; if not, write to the Free Software Foundation,
* Inc., 51 Franklin St, Fifth Floor, Boston, MA 02110-1301 USA.
*
* Please contact Oracle, 500 Oracle Parkway, Redwood Shores, CA 94065 USA
* or visit www.oracle.com if you need additional information or have any
* questions.
*/

package java.lang;

import java.io.Serializable;
import java.io.IOException;
import java.io.InvalidObjectException;
import java.io.ObjectInputStream;
import java.io.ObjectStreamException;

/**
* This is the common base class of all Java language enumeration types.
*
* More information about enums, including descriptions of the
* implicitly declared methods synthesized by the compiler, can be
* found in section 8.9 of
* <cite>The Java&trade; Language Specification</cite>.
*
* <p> Note that when using an enumeration type as the type of a set
* or as the type of the keys in a map, specialized and efficient
* {@linkplain java.util.EnumSet set} and {@linkplain
* java.util.EnumMap map} implementations are available.
*
* @param <E> The enum type subclass
* @author Josh Bloch
* @author Neal Gafter
* @see Class#getEnumConstants()
* @see java.util.EnumSet
* @see java.util.EnumMap
* @since 1.5
*/
public abstract class Enum<E extends Enum<E>>
implements Comparable<E>, Serializable {
/**
* The name of this enum constant, as declared in the enum declaration.
* Most programmers should use the {@link #toString} method rather than
* accessing this field.
*/
private final String name;

/**
* Returns the name of this enum constant, exactly as declared in its
* enum declaration.
*
* <b>Most programmers should use the {@link #toString} method in
* preference to this one, as the toString method may return
* a more user-friendly name.</b> This method is designed primarily for
* use in specialized situations where correctness depends on getting the
* exact name, which will not vary from release to release.
*
* @return the name of this enum constant
*/
public final String name() {
return name;
}

/**
* The ordinal of this enumeration constant (its position
* in the enum declaration, where the initial constant is assigned
* an ordinal of zero).
*
* Most programmers will have no use for this field. It is designed
* for use by sophisticated enum-based data structures, such as
* {@link java.util.EnumSet} and {@link java.util.EnumMap}.
*/
private final int ordinal;

/**
* Returns the ordinal of this enumeration constant (its position
* in its enum declaration, where the initial constant is assigned
* an ordinal of zero).
*
* Most programmers will have no use for this method. It is
* designed for use by sophisticated enum-based data structures, such
* as {@link java.util.EnumSet} and {@link java.util.EnumMap}.
*
* @return the ordinal of this enumeration constant
*/
public final int ordinal() {
return ordinal;
}

/**
* Sole constructor. Programmers cannot invoke this constructor.
* It is for use by code emitted by the compiler in response to
* enum type declarations.
*
* @param name - The name of this enum constant, which is the identifier
* used to declare it.
* @param ordinal - The ordinal of this enumeration constant (its position
* in the enum declaration, where the initial constant is assigned
* an ordinal of zero).
*/
protected Enum(String name, int ordinal) {
this.name = name;
this.ordinal = ordinal;
}

/**
* Returns the name of this enum constant, as contained in the
* declaration. This method may be overridden, though it typically
* isn‘t necessary or desirable. An enum type should override this
* method when a more "programmer-friendly" string form exists.
*
* @return the name of this enum constant
*/
public String toString() {
return name;
}

/**
* Returns true if the specified object is equal to this
* enum constant.
*
* @param other the object to be compared for equality with this object.
* @return true if the specified object is equal to this
* enum constant.
*/
public final boolean equals(Object other) {
return this==other;
}

/**
* Returns a hash code for this enum constant.
*
* @return a hash code for this enum constant.
*/
public final int hashCode() {
return super.hashCode();
}

/**
* Throws CloneNotSupportedException. This guarantees that enums
* are never cloned, which is necessary to preserve their "singleton"
* status.
*
* @return (never returns)
*/
protected final Object clone() throws CloneNotSupportedException {
throw new CloneNotSupportedException();
}

/**
* Compares this enum with the specified object for order. Returns a
* negative integer, zero, or a positive integer as this object is less
* than, equal to, or greater than the specified object.
*
* Enum constants are only comparable to other enum constants of the
* same enum type. The natural order implemented by this
* method is the order in which the constants are declared.
*/
public final int compareTo(E o) {
Enum<?> other = (Enum<?>)o;
Enum<E> self = this;
if (self.getClass() != other.getClass() && // optimization
self.getDeclaringClass() != other.getDeclaringClass())
throw new ClassCastException();
return self.ordinal - other.ordinal;
}

/**
* Returns the Class object corresponding to this enum constant‘s
* enum type. Two enum constants e1 and e2 are of the
* same enum type if and only if
* e1.getDeclaringClass() == e2.getDeclaringClass().
* (The value returned by this method may differ from the one returned
* by the {@link Object#getClass} method for enum constants with
* constant-specific class bodies.)
*
* @return the Class object corresponding to this enum constant‘s
* enum type
*/
@SuppressWarnings("unchecked")
public final Class<E> getDeclaringClass() {
Class<?> clazz = getClass();
Class<?> zuper = clazz.getSuperclass();
return (zuper == Enum.class) ? (Class<E>)clazz : (Class<E>)zuper;
}

/**
* Returns the enum constant of the specified enum type with the
* specified name. The name must match exactly an identifier used
* to declare an enum constant in this type. (Extraneous whitespace
* characters are not permitted.)
*
* <p>Note that for a particular enum type {@code T}, the
* implicitly declared {@code public static T valueOf(String)}
* method on that enum may be used instead of this method to map
* from a name to the corresponding enum constant. All the
* constants of an enum type can be obtained by calling the
* implicit {@code public static T[] values()} method of that
* type.
*
* @param <T> The enum type whose constant is to be returned
* @param enumType the {@code Class} object of the enum type from which
* to return a constant
* @param name the name of the constant to return
* @return the enum constant of the specified enum type with the
* specified name
* @throws IllegalArgumentException if the specified enum type has
* no constant with the specified name, or the specified
* class object does not represent an enum type
* @throws NullPointerException if {@code enumType} or {@code name}
* is null
* @since 1.5
*/
public static <T extends Enum<T>> T valueOf(Class<T> enumType,
String name) {
T result = enumType.enumConstantDirectory().get(name);
if (result != null)
return result;
if (name == null)
throw new NullPointerException("Name is null");
throw new IllegalArgumentException(
"No enum constant " + enumType.getCanonicalName() + "." + name);
}

/**
* enum classes cannot have finalize methods.
*/
protected final void finalize() { }

/**
* prevent default deserialization
*/
private void readObject(ObjectInputStream in) throws IOException,
ClassNotFoundException {
throw new InvalidObjectException("can‘t deserialize enum");
}

private void readObjectNoData() throws ObjectStreamException {
throw new InvalidObjectException("can‘t deserialize enum");
}
}