Java 中的私有内部类有性能开销吗？

发布于 2024-10-16 11:37:53 字数 316 浏览 10 评论 0原文

当我有带有私有方法或字段的内部类时，编译器必须创建合成的受包保护的访问器方法，以允许外部类访问这些私有元素（反之亦然）。

为了避免这种情况，我通常将所有字段、方法和构造函数设置为包保护而不是私有的。

但是类本身的可见性怎么样？？

 private static class A {
      A(){}
 }

注意构造

 static class A {
      A(){}
 }

函数在这两种情况下都是受包保护的，或者使类私有会改变这一点吗

原文

When I have inner classes with private methods or fields the compiler has to create synthetic package-protected accessor methods to allow the outer class to access those private elements (and vice-versa).

To avoid that, I usually make all fields and methods and constructors package-protected instead of private.

But how about the visibility of the class itself? Is there an overhead to

 private static class A {
      A(){}
 }

versus

 static class A {
      A(){}
 }

Note that the constructor is package-protected in both cases, or does making the class private change that?

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那片花海 2024-10-23 11:37:53

您是否尝试过编译它并比较字节码？这是我的结果。对于：

public class Example {
  public static void main(String[] args) {
    System.out.println("Hello world!");
  }
  private static class A {
    A(){}
  }
}

上面生成以下 *.class 文件：

-rw-r--r--    1 michaelsafyan  staff   238 Feb 10 00:11 Example$A.class
-rw-r--r--    1 michaelsafyan  staff   474 Feb 10 00:11 Example.class

现在，如果我移动类文件，删除 private 修饰符，然后重新编译，我得到：

 -rw-r--r--    1 michaelsafyan  staff   238 Feb 10 00:15 Example$A.class
 -rw-r--r--    1 michaelsafyan  staff   474 Feb 10 00:15 Example.class

如果您查看 VM Spec on class files，您会看到有一个常量- 大小的位字段用于指定访问修饰符，因此生成的文件大小相同也就不足为奇了。

简而言之，您的访问修饰符不会影响生成的字节代码的大小（也不应该对性能产生任何影响）。您应该使用最有意义的访问修饰符。

我还应该补充一点，如果将内部类从声明 static 更改为不声明 static，会有细微的差别，因为这意味着引用外部类的附加字段。与声明内部类 static 相比，这会占用稍多的内存，但是对此进行优化会很疯狂（在有意义的地方使用 static ，在哪里使用 static ）你需要它是非静态的，让它成为非静态的，但不要仅仅为了在这里或那里保存内存指针而使你的设计复杂化）。

Have you tried compiling it and comparing the byte code? Here are my results. For:

public class Example {
  public static void main(String[] args) {
    System.out.println("Hello world!");
  }
  private static class A {
    A(){}
  }
}

The above yields the following *.class files:

-rw-r--r--    1 michaelsafyan  staff   238 Feb 10 00:11 Example$A.class
-rw-r--r--    1 michaelsafyan  staff   474 Feb 10 00:11 Example.class

Now, if I move the class files, delete the private modifier, and recompile, I get:

 -rw-r--r--    1 michaelsafyan  staff   238 Feb 10 00:15 Example$A.class
 -rw-r--r--    1 michaelsafyan  staff   474 Feb 10 00:15 Example.class

If you look at the VM Spec on class files, you'll see that there is a constant-sized bit field for specifying the access modifiers, so it should not be any surprise that the generated files are the same size.

In short, your access modifiers won't affect the size of the generated byte code (it also should not have any performance impact, either). You should use the access modifier that makes the most sense.

I should also add that there is a slight difference if you change the inner class from being declared static to not being declared static, as it implies an additional field referencing the outer class. This will take up slightly more memory than if you declared the inner class static, but you'd be insane to optimize for this (use static where it makes sense, and where you need it to be non-static, make it non-static, but don't convolute your design just to save a pointer of memory here or there).

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