java - 静态类什么时候初始化？

Question

考虑一个 Java class with static fields only并且没有构造函数:

public class OnlyStatic {
   static O1 o1 = new o1();
   static O2 o2 = new o2();

   public static int compute(int whatever) {
       return o1.foo+o2.bar+whatever;
   }
}

在另一个类中，compute 方法被使用，或者通过static import:

static import OnlyStatic.compute
int a = OnlyStatic.compute(3);

或者直接假设调用者在同一个包中:

int a = OnlyStatic.compute(3);

o1 和 o2 什么时候初始化？在导入时，还是第一次调用 compute() 时？

Answer 1

对象 o1 和 o2 在您的 static 上下文中不可用，除非它们也成为 static。

JVMS指出

Any static initializers declared in a class are executed when the class is initialized

Further

A class or interface type T will be initialized immediately before the first occurrence of any one of the following:

• T is a class and an instance of T is created.
• T is a class and a static method declared by T is invoked.
• A static field declared by T is assigned.
• A static field declared by T is used and the field is not a constant variable (§4.12.4).
• T is a top-level class, and an assert statement (§14.10) lexically nested within T is executed.

所以在您的情况下，当静态方法 compute() 第一次执行时。