类中的静态块和静态变量以什么顺序执行?

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问题描述

为什么在初始化块中更新的是字符串变量而不是整数(即使块是先写入的)

Why is the string variable updated in the initialization block and not the integer(even though the block is written first)

class NewClass
{
    static
    {
       System.out.println(NewClass.string+" "+NewClass.integer);
    }

    final static String string="static";
    final static Integer integer=1;

    public static void main(String [] args)//throws Exception
    {
    }
}

我的输出是

static null

P.S:还注意到只有当我插入最终修饰符时，字符串变量初始化才会在块之前发生.为什么会这样?为什么不是整数?我也将其声明为最终静态

P.S:Also noticed that string variable initialization happens before the block only when i insert the final modifier. why is that?why not for integer as well?I have declared it as final static too

推荐答案

来自 JLS 的第 12.4.2 节，适当剪下:

初始化C的过程如下:

然后，初始化其值为编译时常量表达式的最终类变量和接口字段(第 8.3.2.1 节、第 9.3.1 节、第 13.4.9 节、第 15.28 节).

Then, initialize the final class variables and fields of interfaces whose values are compile-time constant expressions (§8.3.2.1, §9.3.1, §13.4.9, §15.28).

接下来，按文本顺序执行类的类变量初始值设定项和静态初始值设定项，或接口的字段初始值设定项，就好像它们是单个块一样.

Next, execute either the class variable initializers and static initializers of the class, or the field initializers of the interface, in textual order, as though they were a single block.

因此对于非编译时常量，它不是所有变量"的情况；然后是所有静态初始值设定项"反之亦然 - 所有这些都按文本顺序放在一起.所以如果你有:

So for non-compile-time-constants, it's not a case of "all variables" and then "all static initializers" or vice versa - it's all of them together, in textual order. So if you had:

static int x = method("x");

static {
    System.out.println("init 1");
}

static int y = method("y");

static {
    System.out.println("init 2");
}

static int method(String name) {
    System.out.println(name);
    return 0;
}

那么输出将是:

x
init 1
y
init 2

即使将 x 或 y 设为 final 也不会对这里产生影响，因为它们仍然不是编译时常量.

Even making x or y final wouldn't affect this here, as they still wouldn't be compile-time constants.

P.S:还注意到只有当我插入最终修饰符时，字符串变量初始化才会在块之前发生.

在这一点上，它是一个编译时常量，它的任何使用基本上都是内联的.此外，变量值在其余初始化器之前分配，如上.

At that point, it's a compile-time constant, and any uses of it basically inlined. Additionally, the variable value is assigned before the rest of the initializers, as above.

第 15.28 节JLS 定义了编译时常量 - 它包括所有原始值和 String，但不包装器类型，例如 Integer.

Section 15.28 of the JLS defines compile-time constants - it includes all primitive values and String, but not the wrapper types such as Integer.

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