“Inside the Java Virtual Machine - Chapter 8 The Linking Model - Resolution of CONSTANT_Fieldref_info Entries”的一个错误

[JasonLaw]

在Inside the Java Virtual Machine - Chapter 8 The Linking Model - Resolution of CONSTANT_Fieldref_info Entries中，它说：

If resolution to the CONSTANT_Class_info completes successfully, the virtual machine performs the field lookup process using these steps:

1. The virtual machine checks the referenced type for a field of the specified name and type. If the virtual machine discovers such a field, that field is the result of the successful field lookup.
2. Otherwise, the virtual machine checks any interfaces directly implemented or extended by the type, and recursively, any superinterfaces of interfaces directly implemented or extended by the type, for a field of the specified name and type. If the virtual machine discovers such a field, that field is the result of the successful field lookup.
3. Otherwise, if the type has a direct superclass, the virtual machine checks the type's direct superclass, and recursively all the superclasses of the type, for a field of the specified name and type. If the virtual machine discovers such a field, that field is the result of the successful field lookup.
4. Otherwise, field lookup fails.

关于第 3 点，它说“if the type has a direct superclass, the virtual machine checks the type's direct superclass, and recursively all the superclasses of the type, for a field of the specified name and type”，这里遗漏了“对 superclass 的 superinterfaces 的检查”。

下面的例子可以证明，JVM 会检查 superclass 的 superinterfaces 。

import java.lang.Math;

public class Test {
    public static void main(String[] args) {
        System.out.println(A.i);
    }
}

class A extends AParent {
}

class AParent implements B {
}

interface B {
    int i = (int) (Math.random() * 100);
}

javac Test.java之后，javap -c Test.class的输出如下：

Compiled from "Test.java"
public class Test {
  public Test();
    Code:
       0: aload_0
       1: invokespecial #1                  // Method java/lang/Object."<init>":()V
       4: return

  public static void main(java.lang.String[]);
    Code:
       0: getstatic     #2                  // Field java/lang/System.out:Ljava/io/PrintStream;
       3: getstatic     #3                  // Field A.i:I
       6: invokevirtual #4                  // Method java/io/PrintStream.println:(I)V
       9: return
}

运行java Test没有问题，可以看出 JVM 解析下标为 3 的 constant pool entry 时，最后会找到class A，通过class A会找到class AParent，最后会找class AParent的 superinterface interface B。