为什么来自一个类的Java字节码来了新的staic内部类的代码出现jvm指令ACONST_NULL

问题描述

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我尝试创建一个内部staic类，但是我发现字节码出现在jvm指令 ACONST_NULL 和 NEW ， DUP 和 INVOKE_SPECIAL ，但是我知道新的一类是

I try to new a inner staic class, But I find that bytecode appear the jvm instruction ACONST_NULL bwteen NEW,DUP and INVOKE_SPECIAL,But I know about a class new is

• 新
• DUP
• INVOKE_SPECIAL

package com.hoho.api;

/**
 * @author linuxea
 */
public class Main {

    private static class InnerMain {
        // no field
    }

    public static void main(String[] args) {
        InnerMain innerMain = new InnerMain();
    }

}

// class version 52.0 (52)
// access flags 0x21
public class com/hoho/api/Main {

  // compiled from: Main.java
  // access flags 0xA
  private static INNERCLASS com/hoho/api/Main$InnerMain com/hoho/api/Main InnerMain
  // access flags 0x1008
  static synthetic INNERCLASS com/hoho/api/Main$1 null null

  // access flags 0x1
  public <init>()V
   L0
    LINENUMBER 6 L0
    ALOAD 0
    INVOKESPECIAL java/lang/Object.<init> ()V
    RETURN
   L1
    LOCALVARIABLE this Lcom/hoho/api/Main; L0 L1 0
    MAXSTACK = 1
    MAXLOCALS = 1

  // access flags 0x9
  public static main([Ljava/lang/String;)V
    // parameter  args
   L0
    LINENUMBER 13 L0
    NEW com/hoho/api/Main$InnerMain
    DUP
    ACONST_NULL
    INVOKESPECIAL com/hoho/api/Main$InnerMain.<init> (Lcom/hoho/api/Main$1;)V
    ASTORE 1
   L1
    LINENUMBER 14 L1
    RETURN
   L2
    LOCALVARIABLE args [Ljava/lang/String; L0 L2 0
    LOCALVARIABLE innerMain Lcom/hoho/api/Main$InnerMain; L1 L2 1
    MAXSTACK = 3
    MAXLOCALS = 2
}

为什么

 L0
    LINENUMBER 13 L0
    NEW com/hoho/api/Main$InnerMain
    DUP
    ACONST_NULL
    INVOKESPECIAL com/hoho/api/Main$InnerMain.<init> (Lcom/hoho/api/Main$1;)V
    ASTORE 1

什么是 ACONST_NULL ?

推荐答案

这是 javac 解决JDK 11之前的私有成员访问问题的方式.

This is the way how javac solves private member access problem before JDK 11.

InnerMain 被声明为私有，并且具有私有默认构造函数:

InnerMain is declared private, and it has private default constructor:

private InnerMain() {
}

根据JVM规范，任何类都不能访问其他类的私有成员，但是根据Java语言规则， Main 应该能够访问 InnerMain 的私有成员..为了解决这个问题， javac 生成了一个综合包专用桥构造函数.为了确保不直接从用户代码中调用此构造函数，编译器在签名中添加了一个伪类 Main $ 1 :

According to the JVM specification, no class may access private members of other classes, but according to Java language rules, Main should be able to access private members of InnerMain. To solve this problem, javac generates a synthetic package private bridge constructor. To ensure this constructor is not called directly from user code, compiler adds a dummy class Main$1 in the signature:

// Real constructor
private InnerMain() {
}


// Synthetic bridge constructor (package private, so Main can call it)
InnerMain(Main$1 dummy) {
    this();
}

当您编写 new InnerMain()时，编译器实际上会调用此桥构造函数.dummy参数的实际值并不重要，因此将其设置为 null -因此，使用 ACONST_NULL 指令:

When you write new InnerMain(), the compiler actually calls this bridge constructor. The actual value of the dummy argument does not matter, so it's just set to null - hence ACONST_NULL instruction:

public static void main(String[] args) {
    InnerMain innerMain = new InnerMain(null);
}

请注意，当内部类是public或package private时，不需要桥接方法.

Note that bridge methods are not needed when the inner class is public or package private.

自JDK 11以来，引入了一种全新的机制，请参见 JEP 181:基于嵌套的访问控制.现在，如果您使用JDK 11或更高版本编译Main类，则 Main 和 InnerMain 将成为嵌套对象，并且无需桥接方法和合成即可访问彼此的私有成员.课.

Since JDK 11, a fundamentally new mechanism was introduced, see JEP 181: Nest-Based Access Control. Now, if you compile your Main class with JDK 11 or newer, Main and InnerMain will become nestmates and will be able to access private members of each other without bridge methods and synthetic classes.

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