静态初始化程序在定义之前无法引用字段 [英] Static initializer cannot reference a field before it is defined

问题描述

我有以下代码，错误已注释

I have the following code with the error commented

public final class MyStaticClass {

    private MyStaticClass() {}

    static {

        a = new A();
        b = new B(a);    // Cannot access a field before it is defined
    }

    private static final A a;
    private static final B b;
}

我对使用静态初始化器相当新，但我不知道为什么这不会编译。我查看了有关此主题的一些帖子，并看到了初始化运行的顺序，但这似乎并没有违反规则。当b被初始化时，应该已经有了。

I'm fairly new to using static initializers, but I have no idea why this will not compile. I've looked around a few of the posts on this topic, and have seen the order that initialisation runs, but this doesn't seem to violate the rule. By the time b is being initialized, a should already have been.

我有一个解决方法，就是将这个类设置为单例，但这样做会使代码的可读性降低一些。我很想知道这里出了什么问题。

I have a work around, which would be to set up this class as a singleton, but doing so would make the code a little less readable. I'm keen to know what is going wrong here.

推荐答案

这在 JLS 8.3.3 。事实上，有几种方法可以解决它。

This is explained in JLS 8.3.3. In fact, there are a few ways to fix it.

使用限定名称 a ：

// #1
public final class MyStaticClass {
    static {
        a = new A();
        b = new B(MyStaticClass.a);
    }

    private static final A a;
    private static final B b;
}

如果 a 和 b 是在实例初始化程序中初始化的实例字段， a 可以限定为。 a 。

If a and b were instance fields being initialized in an instance initializer, a could be qualified as this.a.

将前向引用放在 a 的左侧赋值：

Put the forward reference to a on the left-hand of an assignment:

// #2
public final class MyStaticClass {
    static {
        b = new B(a = new A());
    }

    private static final A a;
    private static final B b;
}

当然，在参考文献之前以文字方式提交声明：

And of course put the declaration textually before the reference:

// #3
public final class MyStaticClass {
    private static final A a;
    private static final B b;

    static {
        a = new A();
        b = new B(a);
    }
}

根据JLS，＃3在技术上不是必需的（这些类变量在范围内），而这是为了捕获特定类型的错误，其中字段按顺序初始化：

According to the JLS, #3 is not technically necessary ("these class variables are in scope"), rather this is designed to catch a particular kind of error where fields are initialized out of order:

public final class MyStaticClass {
    private static final B b = new B(a); // a is null
    private static final A a = new A();
}

（虽然我只是告诉你两种方法来阻止它并且无论如何都要犯错误。）

(Though I just showed you two ways to thwart it and make the error anyway.)

我建议＃1或＃3，因为＃2有点深奥。您似乎没有犯这个规则旨在捕获的错误。

I'd recommend #1 or #3 since #2 is a bit esoteric. You don't seem to be making the error this rule is designed to catch.

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