关于“Inside the Java Virtual Machine – Chapter 7 The Lifetime of a Type – Initialization”的疑问
在Inside the Java Virtual Machine – Chapter 7 The Lifetime of a Type – Initialization中，有这么一段：
A use of a non-constant static field is an active use of only the class or interface that actually declares the field. For example, a field declared in a class may be referred to via a subclass. A field declared in an interface may be referred to via a subinterface or class that implements the interface. These are passive uses of the subclass, subinterface, or class that implements the interface–uses that won’t trigger their initialization. They are an active use only of the class or interface in which the field is actually declared. Here’s an example that illustrates this principle:
// On CD-ROM in file classlife/ex2/NewParent.java class NewParent { static int hoursOfSleep = (int) (Math.random() * 3.0); static { System.out.println("NewParent was initialized."); } } // On CD-ROM in file classlife/ex2/NewbornBaby.java class NewbornBaby extends NewParent { static int hoursOfCrying = 6 + (int) (Math.random() * 2.0); static { System.out.println("NewbornBaby was initialized."); } } // On CD-ROM in file classlife/ex2/Example2.java class Example2 { // Invoking main() is an active use of Example2 public static void main(String[] args) { // Using hoursOfSleep is an active use of NewParent, // but a passive use of NewbornBaby int hours = NewbornBaby.hoursOfSleep; System.out.println(hours); } static { System.out.println("Example2 was initialized."); } }
然后它说：
In the above example, executing main() of Example2 causes only Example2 and NewParent to be initialized. NewbornBaby is not initialized and need not be loaded.
我明白“NewbornBaby is not initialized”，但是为什么“NewbornBaby need not be loaded”呢？如果 NewbornBaby 都没有被 loaded，那么 JVM 怎么可能知道 hoursOfSleep 是来源于 NewParent 的呢？
java -verbose:class Example2的输出片段如下：
[Loaded Example2 from file:/Users/jason/trivial/] [Loaded sun.launcher.LauncherHelper$FXHelper from /Library/Java/JavaVirtualMachines/jdk1.8.0_181.jdk/Contents/Home/jre/lib/rt.jar] [Loaded java.lang.Class$MethodArray from /Library/Java/JavaVirtualMachines/jdk1.8.0_181.jdk/Contents/Home/jre/lib/rt.jar] [Loaded java.lang.Void from /Library/Java/JavaVirtualMachines/jdk1.8.0_181.jdk/Contents/Home/jre/lib/rt.jar] Example2 was initialized. [Loaded NewParent from file:/Users/jason/trivial/] [Loaded NewbornBaby from file:/Users/jason/trivial/] [Loaded java.lang.Math$RandomNumberGeneratorHolder from /Library/Java/JavaVirtualMachines/jdk1.8.0_181.jdk/Contents/Home/jre/lib/rt.jar] [Loaded java.util.Random from /Library/Java/JavaVirtualMachines/jdk1.8.0_181.jdk/Contents/Home/jre/lib/rt.jar] NewParent was initialized. 1 虽然结果显示 NewbornBaby 被 loaded 了，但是为什么 NewParent 先于 NewbornBaby 被 loaded 呢？