java - Java文件中import语句的含义

Question

谁能清楚地向我解释当我们在 Java 文件中使用 import 语句时到底发生了什么？如果我们添加越来越多的 java 类，它会增加文件的大小吗？为什么我们不使用类加载器呢？进口有什么限制？

Answer 1

import declarations(不是 statements)本质上是源代码级别的简写使能器:它允许您引用一个类型或使用单个标识符(例如 List、min)而不是完全限定名称(例如 java.util)的 static 成员.List, Math.min).

import 声明部分是源代码的编译时元素，在运行时不存在。在 JVM 字节码中，类型名称始终是完全限定的，除非您使用编写不佳的编译器，否则二进制文件应该只包含实际使用的类型的名称。

类加载器用于完全不同的概念，与 import 功能完全无关。

JLS 7.5 Import Declarations

An import declaration allows a static member or a named type to be referred to by a simple name that consists of a single identifier. Without the use of an appropriate import declaration, the only way to refer to a type declared in another package, or a static member of another type, is to use a fully qualified name.

A single-type-import declaration imports a single named type, by mentioning its canonical name.

A type-import-on-demand declaration imports all the accessible types of a named type or package as needed. It is a compile time error to import a type from the unnamed package.

A single static import declaration imports all accessible static members with a given name from a type, by giving its canonical name.

A static-import-on-demand declaration imports all accessible static members of a named type as needed.

引用文献

• JLS 7.5.1 Single-Type-Import Declaration
• JLS 7.5.2 Type-Import-on-Demand Declaration
• JLS 7.5.3 Single Static Import Declaration
• JLS 7.5.4 Static-Import-on-Demand Declaration

另见

• Java Tutorials/Using package members
• Java Language Guide/static import

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各种import相关问题

import的语法作用:

• What is an import called? - 这是一个声明，而不是一个声明

按需与单一类型:

• Import package.* vs import package.SpecificType
• Why is using a wild card with a Java import statement bad?
• What’s the difference between import java.util.*; and import java.util.Date;?

关于导入静态:

• What does the static modifier after import mean?
• What is a good use case for static import of methods?
• Should I use static import?

性能相关问题:

• Does importing of packages change visibility of classes? - 绝对不会!
• Do multiple import statements in a program affect performance? - 不!
• Any reason to clean up unused imports in Java, other than reducing clutter?