java - 如果放置在声明之前，则静态初始化程序错误

Question

我注意到静态初始化器中的某些东西可能是 javac 中的错误。我构建了一个场景，我可以在其中为变量赋值但不读回该值。

下面是两个示例，第一个编译正常，第二个在尝试从 tmp 读取值时出错，但出于某种原因允许将值分配给 tmp。我可以理解它是否既不能读取也不能写入变量，因为 tmp 是在静态初始化程序之后声明的，但是只有其中一个错误对我来说没有意义。

//Compiles Successfully:
public class Script
{
    public static Object tmp;
    static
    {
        tmp = new Object();
        System.out.println(tmp);
    }

}

//error only on the read but not the assignment
public class Script
{

    static
    {
        tmp = new Object();
        System.out.println(tmp);
    }
    public static Object tmp;
}

进一步强调这一点，这确实编译成功。

public class Script
{

    static
    {
        tmp = new Object();
    }
    public static Object tmp;
}

Answer 1

这似乎是在规范中定义的(参见 JLS 8.3.2.3 ):

The declaration of a member needs to appear textually before it is used only if the member is an instance (respectively static) field of a class or interface C and all of the following conditions hold:

• The usage occurs in an instance (respectively static) variable initializer of C or in an instance (respectively static) initializer
  of C.

• The usage is not on the left hand side of an assignment.

• The usage is via a simple name.

• C is the innermost class or interface enclosing the usage.

因此，如果用法在赋值的左侧，那么它是合法的，因为第二个不再成立。