“变量示例可能尚未初始化”在匿名课堂上 [英] "Variable example might not have been initialized" in anonymous class

问题描述

此自行回答的问题受到变量'snackbar'可能尚未初始化。我觉得有更多细节可以更好地与特定问题分开添加。

This self-answered question was inspired by Variable 'snackbar' might not have been initialized. I felt that there was more detail which would be better added separate from that specific question.

为什么下面的代码不能被编译？

Why can the following code not be compiled?

public class Example {
  public static void main(String[] args) {
    final Runnable example = new Runnable() {
      @Override
      public void run() {
        System.out.println(example);  // Error on this line
      }
    };
  }
}

编译错误：

error: variable example might not have been initialized

推荐答案

这是因为匿名类的实现方式。你可以看到这个，如果你对代码稍作修改然后反编译：

This occurs because of the way that anonymous classes are implemented. You can see this if you make a slight change to the code and then decompile:

    final Runnable other = null;
    final Runnable example = new Runnable() {
      @Override
      public void run() {
        System.out.println(other);
      }
    };

即。使匿名类引用不同的局部变量。现在这将编译;我们可以使用 javap 进行反编译，并查看匿名类的界面：

i.e. make the anonymous class refer to a different local variable. This now will compile; we can decompile using javap and see the interface of the anonymous class:

final class Example$1 implements java.lang.Runnable {
  final java.lang.Runnable val$other;
  Example$1(java.lang.Runnable);
  public void run();
}

（示例$ 1 是Java内部引用匿名类的名称。）

(Example$1 is the name by which Java internally refers to the anonymous class).

这表明编译器已经为匿名类添加了一个构造函数，该构造函数需要 Runnable 参数;它还有一个名为 val $ other 的字段。此字段的此名称应提示此字段与其他本地变量相关。

This shows that the compiler has added a constructor to the anonymous class which takes a Runnable parameter; it also has a field called val$other. This name of this field should hint that this field is related to the other local variable.

您可以深入研究字节码进一步，并看到此参数被分配给 val $ other ：

You can dig into the bytecode further, and see that this parameter is assigned to val$other:

  Example$1(java.lang.Runnable);
    Code:
       0: aload_0
       // This gets the parameter...
       1: aload_1  
       // ...and this assigns it to the field val$other
       2: putfield      #1                  // Field val$other:Ljava/lang/Runnable;
       5: aload_0
       6: invokespecial #2                  // Method java/lang/Object."<init>":()V
       9: return

因此，这表明匿名类从其封闭范围访问变量的方式：它们只是在构造时传递值。

So, what this shows is the way that anonymous classes access the variables from their enclosing scope: they are simply passed the value at construction time.

这应该有希望说明为什么编译器会阻止你编写问题中的代码：它需要能够将引用传递给 Runnable 到匿名类以构造它。但是，Java评估以下代码的方式：

This should hopefully show why the compiler stops you from writing code such as that in the question: it needs to be able to pass the reference to the Runnable to the anonymous class in order to construct it. However, the way that Java evaluates the following code:

final Runnable example = new Runnable() { ... }

首先完全评估右侧，然后将其分配给左侧的变量侧。但是，它需要右侧的变量值才能传递给生成的构造函数 Runnable $ 1 ：

is to fully evaluate the right-hand side first, and then assign it to the variable on the left-hand side. However, it needs the value of the variable on the right-hand side in order to pass into the generated constructor of Runnable$1:

final Runnable example = new Example$1(example);

先前未声明示例这不是问题，因为此代码在语义上与以下内容相同：

That example hasn't been previously declared is not a problem, since this code is semantically identical to:

final Runnable example;
example = new Example$1(example);

所以你得到的错误并不是无法解析变量 - 但是， example 在用作构造函数的参数之前尚未赋值，因此编译错误。

so the error that you get isn't that the variable cannot be resolved - however, example hasn't been assigned a value before it is used as an argument to the constructor, hence the compiler error.

可能有人认为这只是一个实现细节：参数必须传递给构造函数并不重要，因为 run（）方法可以在赋值之前调用。

It might be argued that this is simply an implementation detail: it shouldn't matter that the argument has to be passed into the constructor, as there is no way that the run() method can be invoked prior to the assignment.

实际上，这不是真的：你可以调用在赋值之前运行（），如下所示：

Actually, that's not true: you can invoke run() before the assignment, as follows:

final Runnable example = new Runnable() {
  Runnable runAndReturn() {
    run();
    return this;
  }

  @Override public void run() {
    System.out.println(example);
  }
}.runAndReturn();

如果在匿名类中引用 example 被允许，你可以写这个。因此，不允许参考该变量。

If referring to example inside the anonymous class were allowed, you would be able to write this. Hence, referring to that variable is disallowed.

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