java - java类文件加载/链​​接/初始化阶段的查询

Question

下面是代码:

package packagename;

import java.lang.reflect.Method;

class Super{
    static{
        System.out.println("Super");
    }
    public void superMethod(){

    }
}

class Sub extends Super{
    static{
        System.out.println("Sub");
    }
    public void subMethod(){

    }
}

public class Example {
     static{
        System.out.println("In Example");
      }
     public static void main(String[] args){
        Class myClass = Sub.class;
        Method[] methods = myClass.getMethods();
        for(Method eachMethod :  methods){
            System.out.println(eachMethod.getName() + " with " + eachMethod.getParameterCount() + " parameters");
        }
    }
}

编译此代码并执行时，

> java 示例,

类示例被加载\链接\初始化。当 Java 解释器开始解释 class Example 的 main() 方法时，在计算表达式 Sub.class 并分配给 Class myClass 变量、class Super 和 class Sub 已加载和链接，但未初始化>。这就是 class Super 和 class Sub 的静态初始化 block 未执行的原因，如下输出所示。

In Example
subMethod with 0 parameters
superMethod with 0 parameters
wait with 0 parameters
wait with 2 parameters
wait with 1 parameters
equals with 1 parameters
toString with 0 parameters
hashCode with 0 parameters
getClass with 0 parameters
notify with 0 parameters
notifyAll with 0 parameters

我对类Example、Super和Sub的加载/链接/初始化阶段的理解正确吗？

Answer 1

参见JVMS §5.5

5.5. Initialization

Initialization of a class or interface consists of executing its class or interface initialization method (§2.9).

A class or interface C may be initialized only as a result of:

• The execution of any one of the Java Virtual Machine instructions new, getstatic, putstatic, or invokestatic that references C (§new, §getstatic, §putstatic, §invokestatic). These instructions reference a class or interface directly or indirectly through either a field reference or a method reference.

  Upon execution of a new instruction, the referenced class is initialized if it has not been initialized already.

  Upon execution of a getstatic, putstatic, or invokestatic instruction, the class or interface that declared the resolved field or method is initialized if it has not been initialized already.

• The first invocation of a java.lang.invoke.MethodHandle instance which was the result of method handle resolution (§5.4.3.5) for a method handle of kind 2 (REF_getStatic), 4 (REF_putStatic), 6 (REF_invokeStatic), or 8 (REF_newInvokeSpecial).This implies that the class of a bootstrap method is initialized when the bootstrap method is invoked for an invokedynamic instruction (§invokedynamic), as part of the continuing resolution of the call site specifier.

• Invocation of certain reflective methods in the class library (§2.12), for example, in class Class or in package java.lang.reflect.

• If C is a class, the initialization of one of its subclasses.

• If C is an interface that declares a non-abstract, non-static method, the initialization of a class that implements C directly or indirectly.

• If C is a class, its designation as the initial class at Java Virtual Machine startup (§5.2).

Prior to initialization, a class or interface must be linked, that is, verified, prepared, and optionally resolved.

一般来说，行为都有规范。如果想了解详情，可以考虑学习The Java® Language Specification和 The Java® Virtual Machine Specification首先提出问题，如果您不理解某个部分，然后指向提出问题的部分。