优雅管理代码中的常量

Posted 2021-06-14 菜鸟修仙传

tags:

篇首语：本文由小常识网(cha138.com)小编为大家整理，主要介绍了优雅管理代码中的常量相关的知识，希望对你有一定的参考价值。

先看看前辈们是怎么做的

从事Web开发，有个绕不开的常量就是HTTP的状态码，不如以此为起点

使用枚举

org.springframework.http.HttpStatus

public enum HttpStatus {
    CONTINUE(100, "Continue"),
    SWITCHING_PROTOCOLS(101, "Switching Protocols"),
    PROCESSING(102, "Processing"),
    CHECKPOINT(103, "Checkpoint"),
    OK(200, "OK"),
    CREATED(201, "Created"),
    ACCEPTED(202, "Accepted"),
    NON_AUTHORITATIVE_INFORMATION(203, "Non-Authoritative Information"),
    NO_CONTENT(204, "No Content"),
    ...

类中定义常量

org.eclipse.jetty.http.HttpStatus

public class HttpStatus {
    public static final int CONTINUE_100 = 100;
    public static final int SWITCHING_PROTOCOLS_101 = 101;
    public static final int PROCESSING_102 = 102;
    public static final int OK_200 = 200;
    public static final int CREATED_201 = 201;
    public static final int ACCEPTED_202 = 202;
    public static final int NON_AUTHORITATIVE_INFORMATION_203 = 203;
    public static final int NO_CONTENT_204 = 204;
    ...

final类中定义常量

org.apache.hc.core5.http.HttpStatus

public final class HttpStatus {
    public static final int SC_INFORMATIONAL = 100;
    public static final int SC_CONTINUE = 100;
    public static final int SC_SWITCHING_PROTOCOLS = 101;
    public static final int SC_PROCESSING = 102;
    public static final int SC_EARLY_HINTS = 103;
    public static final int SC_SUCCESS = 200;
    public static final int SC_OK = 200;
    public static final int SC_CREATED = 201;
    public static final int SC_ACCEPTED = 202;
    public static final int SC_NON_AUTHORITATIVE_INFORMATION = 203;
    public static final int SC_NO_CONTENT = 204;
    ...

以上都是在常用开源框架中的定义，在业务中还曾见过一种用法，是

在接口中定义常量

public interface HttpStatus {
    public static final int CONTINUE_100 = 100;
    public static final int SWITCHING_PROTOCOLS_101 = 101;
    public static final int PROCESSING_102 = 102;
    public static final int OK_200 = 200;
    public static final int CREATED_201 = 201;
    public static final int ACCEPTED_202 = 202;
    public static final int NON_AUTHORITATIVE_INFORMATION_203 = 203;
    public static final int NO_CONTENT_204 = 204;
    ...

这种接口被称为常量接口，不过在《Effective Java》中的“接口只用于定义类型”这条建议中，特别批评了这种常量接口，认为“常量接口模式是对接口的不良使用”。原文叙述如下：

The constant interface pattern is a poor use of interfaces. That a class uses some constants internally is an implementation detail. Implementing a constant interface causes this implementation detail to leak into the class’s exported API. It is of no consequence to the users of a class that the class implements a constant interface. In fact, it may even confuse them. Worse, it represents a commitment: if in a future release the class is modified so that it no longer needs to use the constants, it still must implement the interface to ensure binary compatibility. If a nonfinal class implements a constant interface, all of its subclasses will have their namespaces polluted by the constants in the interface.

常量接口模式是使用接口的糟糕方式。类内部会使用一些常量，这是实现细节。然而，实现常量接口会导致这个实现细节泄漏到类的导出 API 中。对于类的用户来说，类实现一个常量接口没有什么价值。事实上，这甚至会让他们感到困惑。更糟糕的是，它代表了一种承诺：如果在将来的版本中修改了类，使其不再需要使用常量，那么它仍然必须实现接口以确保二进制兼容性。如果一个非 final 类实现了一个常量接口，那么它的所有子类的命名空间都会被接口中的常量所污染。

考虑到常量类不应该被继承，使用final要比没有合适。

此外考虑到常量类应该不需要被实例化，将构造函数设置为private也是种额外的保护。

综上，使用枚举和final常量类就是首选了，那么这两者之间又如何抉择呢？我们还是能从《Effective Java》得到有效的建议，第34条中就是在建议我们用枚举替代int常量。