Java 7 to 17 Nest-Based Access Control

一、前言

返回目錄

Java 從 7 版本到 17 版本，在語言和虛擬機層面不斷演進，其中一個重要的改進是針對巢狀類別 (nested classes) 的存取控制。在 Java 11 之前，巢狀類別的存取控制在某些情況下略顯不足，導致編譯器需要產生「橋接方法」（bridge methods）來允許內部類別存取外部類別的私有成員。Java 11 引入了「巢狀式存取控制」 (Nest-Based Access Control)，通過在類別檔案中引入 NestHost 和 NestMembers 屬性，提升了巢狀類別的存取效率和可維護性。這種改進簡化了虛擬機的行為，避免了編譯器合成橋接方法，使得程式碼更加清晰且執行效率更高。本文將深入探討 Java 7 至 17 關於巢狀存取控制的變化，分析其架構，工具與範例，並做出總結。

目錄

• 一、前言
  • 目錄
• 二、舊有巢狀類別存取機制的挑戰 (Java 11 之前)
• 三、巢狀存取控制 (Nest-Based Access Control) 的引入 (Java 11)
  • 架構分析
  • 工具與實例
    • 觀察 OuterClassAfter11.class 檔案：
    • 觀察 OuterClassAfter11$InnerClass.class 檔案：
    • 觀察 OuterClassAfter11$StaticNested.class 檔案：
    • Nest:
• 四、Java 12 至 17 的相關演進
• 五、總結

二、舊有巢狀類別存取機制的挑戰 (Java 11 之前)

返回目錄

在 Java 11 之前，巢狀類別（包括內部類別和靜態巢狀類別）存取外部類別的私有成員時，會遇到一些挑戰：

• 編譯器合成橋接方法: 當內部類別需要存取外部類別的私有成員時，編譯器會自動生成橋接方法 (bridge method) ，這些橋接方法實際上是包裝器，用於存取外部類別的私有成員。這些方法在類別檔案中是可見的，但它們並不是程式設計師直接寫的，因此會造成混亂和額外的運行時開銷。
• 存取效能較差: 由於需要經過橋接方法的中轉，巢狀類別存取外部類別私有成員的效率會相對較低。
• 類別檔案膨脹: 編譯器合成的橋接方法會增加類別檔案的大小，影響載入速度和記憶體使用。

以下範例說明了橋接方法的存在：

public class OuterClass { private int privateField = 10; public class InnerClass { public void accessOuterPrivateField() { System.out.println(privateField); } } public static void main(String[] args) { OuterClass outer = new OuterClass(); OuterClass.InnerClass inner = outer.new InnerClass(); inner.accessOuterPrivateField(); } }

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public class OuterClass {

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    private int privateField = 10;

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​

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    public class InnerClass {

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        public void accessOuterPrivateField() {

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            System.out.println(privateField);

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        }

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    }

9

​

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    public static void main(String[] args) {

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        OuterClass outer = new OuterClass();

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        OuterClass.InnerClass inner = outer.new InnerClass();

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        inner.accessOuterPrivateField();

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    }

15

}

16

​

編譯 OuterClass.java 檔案，使用 Java 11 以前版本進行編譯：

$ /d/jdk/zulu8.46.0.19-ca-jdk8.0.252-win_x64/bin/javac.exe -encoding utf-8 OuterClass.java

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$ /d/jdk/zulu8.46.0.19-ca-jdk8.0.252-win_x64/bin/javac.exe -encoding utf-8 OuterClass.java

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​

在 Java 11 之前，上述程式碼會生成一個包含橋接方法的類別檔案 OuterClass$InnerClass.class 。可以使用 javap -p 工具查看編譯後的 bytecode。

$ /d/jdk/zulu8.46.0.19-ca-jdk8.0.252-win_x64/bin/javap.exe -v OuterClass\$InnerClass.class Classfile /D:/Ethan/workspace/lab/example/text/java-newfeatures/src/OuterClass$InnerClass.class Last modified 2025/1/20; size 567 bytes MD5 checksum dc6b564ec623dfd8d92b68d922eb298e Compiled from "OuterClass.java" public class OuterClass$InnerClass minor version: 0 major version: 52 flags: ACC_PUBLIC, ACC_SUPER Constant pool: #1 = Fieldref #6.#18 // OuterClass$InnerClass.this$0:LOuterClass; #2 = Methodref #7.#19 // java/lang/Object."<init>":()V #3 = Fieldref #20.#21 // java/lang/System.out:Ljava/io/PrintStream; #4 = Methodref #22.#23 // OuterClass.access$000:(LOuterClass;)I #5 = Methodref #24.#25 // java/io/PrintStream.println:(I)V #6 = Class #26 // OuterClass$InnerClass #7 = Class #29 // java/lang/Object #8 = Utf8 this$0 #9 = Utf8 LOuterClass; #10 = Utf8 <init> #11 = Utf8 (LOuterClass;)V #12 = Utf8 Code #13 = Utf8 LineNumberTable #14 = Utf8 accessOuterPrivateField #15 = Utf8 ()V #16 = Utf8 SourceFile #17 = Utf8 OuterClass.java #18 = NameAndType #8:#9 // this$0:LOuterClass; #19 = NameAndType #10:#15 // "<init>":()V #20 = Class #30 // java/lang/System #21 = NameAndType #31:#32 // out:Ljava/io/PrintStream; #22 = Class #33 // OuterClass #23 = NameAndType #34:#35 // access$000:(LOuterClass;)I #24 = Class #36 // java/io/PrintStream #25 = NameAndType #37:#38 // println:(I)V #26 = Utf8 OuterClass$InnerClass #27 = Utf8 InnerClass #28 = Utf8 InnerClasses #29 = Utf8 java/lang/Object #30 = Utf8 java/lang/System #31 = Utf8 out #32 = Utf8 Ljava/io/PrintStream; #33 = Utf8 OuterClass #34 = Utf8 access$000 #35 = Utf8 (LOuterClass;)I #36 = Utf8 java/io/PrintStream #37 = Utf8 println #38 = Utf8 (I)V { final OuterClass this$0; descriptor: LOuterClass; flags: ACC_FINAL, ACC_SYNTHETIC public OuterClass$InnerClass(OuterClass); descriptor: (LOuterClass;)V flags: ACC_PUBLIC Code: stack=2, locals=2, args_size=2 0: aload_0 1: aload_1 2: putfield #1 // Field this$0:LOuterClass; 5: aload_0 6: invokespecial #2 // Method java/lang/Object."<init>":()V 9: return LineNumberTable: line 4: 0 public void accessOuterPrivateField(); descriptor: ()V flags: ACC_PUBLIC Code: stack=2, locals=1, args_size=1 0: getstatic #3 // Field java/lang/System.out:Ljava/io/PrintStream; 3: aload_0 4: getfield #1 // Field this$0:LOuterClass; 7: invokestatic #4 // Method OuterClass.access$000:(LOuterClass;)I 10: invokevirtual #5 // Method java/io/PrintStream.println:(I)V 13: return LineNumberTable: line 6: 0 line 7: 13 } SourceFile: "OuterClass.java" InnerClasses: public #27= #6 of #22; //InnerClass=class OuterClass$InnerClass of class OuterClass

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$ /d/jdk/zulu8.46.0.19-ca-jdk8.0.252-win_x64/bin/javap.exe -v OuterClass\$InnerClass.class

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Classfile /D:/Ethan/workspace/lab/example/text/java-newfeatures/src/OuterClass$InnerClass.class

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  Last modified 2025/1/20; size 567 bytes

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  MD5 checksum dc6b564ec623dfd8d92b68d922eb298e

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  Compiled from "OuterClass.java"

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public class OuterClass$InnerClass

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  minor version: 0

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  major version: 52

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  flags: ACC_PUBLIC, ACC_SUPER

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Constant pool:

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   #1 = Fieldref           #6.#18         // OuterClass$InnerClass.this$0:LOuterClass;

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   #2 = Methodref          #7.#19         // java/lang/Object."<init>":()V

13

   #3 = Fieldref           #20.#21        // java/lang/System.out:Ljava/io/PrintStream;

14

   #4 = Methodref          #22.#23        // OuterClass.access$000:(LOuterClass;)I

15

   #5 = Methodref          #24.#25        // java/io/PrintStream.println:(I)V

16

   #6 = Class              #26            // OuterClass$InnerClass

17

   #7 = Class              #29            // java/lang/Object

18

   #8 = Utf8               this$0

19

   #9 = Utf8               LOuterClass;

20

  #10 = Utf8               <init>

21

  #11 = Utf8               (LOuterClass;)V

22

  #12 = Utf8               Code

23

  #13 = Utf8               LineNumberTable

24

  #14 = Utf8               accessOuterPrivateField

25

  #15 = Utf8               ()V

26

  #16 = Utf8               SourceFile

27

  #17 = Utf8               OuterClass.java

28

  #18 = NameAndType        #8:#9          // this$0:LOuterClass;

29

  #19 = NameAndType        #10:#15        // "<init>":()V

30

  #20 = Class              #30            // java/lang/System

31

  #21 = NameAndType        #31:#32        // out:Ljava/io/PrintStream;

32

  #22 = Class              #33            // OuterClass

33

  #23 = NameAndType        #34:#35        // access$000:(LOuterClass;)I

34

  #24 = Class              #36            // java/io/PrintStream

35

  #25 = NameAndType        #37:#38        // println:(I)V

36

  #26 = Utf8               OuterClass$InnerClass

37

  #27 = Utf8               InnerClass

38

  #28 = Utf8               InnerClasses

39

  #29 = Utf8               java/lang/Object

40

  #30 = Utf8               java/lang/System

41

  #31 = Utf8               out

42

  #32 = Utf8               Ljava/io/PrintStream;

43

  #33 = Utf8               OuterClass

44

  #34 = Utf8               access$000

45

  #35 = Utf8               (LOuterClass;)I

46

  #36 = Utf8               java/io/PrintStream

47

  #37 = Utf8               println

48

  #38 = Utf8               (I)V

49

{

50

  final OuterClass this$0;

51

    descriptor: LOuterClass;

52

    flags: ACC_FINAL, ACC_SYNTHETIC

53

​

54

  public OuterClass$InnerClass(OuterClass);

55

    descriptor: (LOuterClass;)V

56

    flags: ACC_PUBLIC

57

    Code:

58

      stack=2, locals=2, args_size=2

59

         0: aload_0

60

         1: aload_1

61

         2: putfield      #1                  // Field this$0:LOuterClass;

62

         5: aload_0

63

         6: invokespecial #2                  // Method java/lang/Object."<init>":()V

64

         9: return

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      LineNumberTable:

66

        line 4: 0

67

​

68

  public void accessOuterPrivateField();

69

    descriptor: ()V

70

    flags: ACC_PUBLIC

71

    Code:

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      stack=2, locals=1, args_size=1

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         0: getstatic     #3                  // Field java/lang/System.out:Ljava/io/PrintStream;

74

         3: aload_0

75

         4: getfield      #1                  // Field this$0:LOuterClass;

76

         7: invokestatic  #4                  // Method OuterClass.access$000:(LOuterClass;)I

77

        10: invokevirtual #5                  // Method java/io/PrintStream.println:(I)V

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        13: return

79

      LineNumberTable:

80

        line 6: 0

81

        line 7: 13

82

}

83

SourceFile: "OuterClass.java"

84

InnerClasses:

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     public #27= #6 of #22; //InnerClass=class OuterClass$InnerClass of class OuterClass

86

​

橋接方法分析:

• Constant pool 中的 #4 = Methodref #22.#23: 這個常數池項目表示 OuterClass$InnerClass 類別引用了 OuterClass.access$000:(LOuterClass;)I 這個方法。
• accessOuterPrivateField() 方法中的 7: invokestatic #4: 在 accessOuterPrivateField() 方法的 bytecode 中，可以看到它使用 invokestatic 指令來呼叫 OuterClass.access$000 方法。
• access$000 方法: 這個 access$000 方法是一個由編譯器自動生成的橋接方法，它存在於 OuterClass 中，目的是讓 InnerClass 可以存取 OuterClass 的私有成員 privateField。

小結

在 Java 11 之前，當內部類別 InnerClass 需要存取外部類別 OuterClass 的私有成員 privateField 時，編譯器會做以下事情：

• 自動在 OuterClass 中生成一個 access$000 的靜態方法。這個方法的簽名通常是 static int access$000(OuterClass outer)，它會存取 outer.privateField 並返回。
• InnerClass 的 accessOuterPrivateField 方法會呼叫 OuterClass.access$000 來間接存取 privateField。

這個 access$000 方法就是橋接方法。它在程式碼中是不可見的，由編譯器自動生成，增加了類別檔案的大小和執行時的開銷。

三、巢狀存取控制 (Nest-Based Access Control) 的引入 (Java 11)

返回目錄

Java 11 引入了巢狀式存取控制，旨在解決舊有巢狀類別存取機制的問題。巢狀存取控制的核心概念是 巢 (nest) 。一個巢由一個外部類別 (nest host) 和所有在語法上屬於這個巢的類別 (nest members) 所組成。 巢內的類別可以自然存取巢內其他類別的私有成員，無需橋接方法。

架構分析

返回目錄

• NestHost 屬性: 在類別檔案中，每個巢成員都會有一個 NestHost 屬性，指向它的巢的主體。這個屬性明確標示了該類別屬於哪個巢。
• NestMembers 屬性: 巢主體類別會有一個 NestMembers 屬性，列出屬於這個巢的所有成員類別。
• 存取規則: 屬於同一個巢的類別，彼此可以自由存取對方所有層次的成員，包括私有成員，無需產生橋接方法。

工具與實例

返回目錄

Java 11 及之後的版本，可以使用 javap -v 工具查看類別檔案的詳細資訊，包括 NestHost 和 NestMembers 屬性。

修改先前範例，使其符合巢狀存取控制的設計：

public class OuterClassAfter11 { private int privateField = 10; public class InnerClass { public void accessOuterPrivateField() { System.out.println(privateField); } } public static class StaticNested { public void accessOuterPrivateField(OuterClassAfter11 outer) { System.out.println(outer.privateField); } } public static void main(String[] args) { OuterClassAfter11 outer = new OuterClassAfter11(); OuterClassAfter11.InnerClass inner = outer.new InnerClass(); inner.accessOuterPrivateField(); OuterClassAfter11.StaticNested staticNested = new StaticNested(); staticNested.accessOuterPrivateField(outer); } }

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public class OuterClassAfter11 {

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  private int privateField = 10;

3

​

4

  public class InnerClass {

5

    public void accessOuterPrivateField() {

6

      System.out.println(privateField);

7

    }

8

  }

9

​

10

  public static class StaticNested {

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    public void accessOuterPrivateField(OuterClassAfter11 outer) {

12

      System.out.println(outer.privateField);

13

    }

14

  }

15

​

16

  public static void main(String[] args) {

17

    OuterClassAfter11 outer = new OuterClassAfter11();

18

    OuterClassAfter11.InnerClass inner = outer.new InnerClass();

19

    inner.accessOuterPrivateField();

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    OuterClassAfter11.StaticNested staticNested = new StaticNested();

21

    staticNested.accessOuterPrivateField(outer);

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  }

23

}

24

​

編譯 OuterClassAfter11.java 檔案：

$ javac -encoding utf-8 OuterClassAfter11.java

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$ javac -encoding utf-8 OuterClassAfter11.java

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​

執行結果：

$ java OuterClassAfter11.java 10 10

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$ java OuterClassAfter11.java

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10

3

10

4

​

觀察 OuterClassAfter11.class 檔案：

返回目錄

• NestMembers 屬性:
  • 從 javap -v OuterClassAfter11.class 的輸出中，我們看到 NestMembers: 屬性包含 OuterClassAfter11$StaticNested 和 OuterClassAfter11$InnerClass。這明確指出 OuterClassAfter11 是 Nest Host，而 StaticNested 和 InnerClass 都是這個 Nest 的成員。
• InnerClasses 屬性:
  • InnerClasses: 屬性列出所有內部類別，public #41= #14 of #8; // InnerClass=class OuterClassAfter11$InnerClass of class OuterClassAfter11 和 public static #42= #28 of #8; // StaticNested=class OuterClassAfter11$StaticNested of class OuterClassAfter11 確認了它們是 OuterClassAfter11 的內部類別。

$ javap -v OuterClassAfter11.class Classfile /D:/Ethan/workspace/lab/example/text/java-newfeatures/src/OuterClassAfter11.class Last modified 2025年1月20日; size 730 bytes SHA-256 checksum fda0dbffcb4d714172e20c1dce54e80a9ebd13ff08e01b48ee3be130f100dbdd Compiled from "OuterClassAfter11.java" public class OuterClassAfter11 minor version: 0 major version: 61 flags: (0x0021) ACC_PUBLIC, ACC_SUPER this_class: #8 // OuterClassAfter11 super_class: #2 // java/lang/Object interfaces: 0, fields: 1, methods: 2, attributes: 3 Constant pool: #1 = Methodref #2.#3 // java/lang/Object."<init>":()V #2 = Class #4 // java/lang/Object #3 = NameAndType #5:#6 // "<init>":()V #4 = Utf8 java/lang/Object #5 = Utf8 <init> #6 = Utf8 ()V #7 = Fieldref #8.#9 // OuterClassAfter11.privateField:I #8 = Class #10 // OuterClassAfter11 #9 = NameAndType #11:#12 // privateField:I #10 = Utf8 OuterClassAfter11 #11 = Utf8 privateField #12 = Utf8 I #13 = Methodref #8.#3 // OuterClassAfter11."<init>":()V #14 = Class #15 // OuterClassAfter11$InnerClass #15 = Utf8 OuterClassAfter11$InnerClass #16 = Methodref #17.#18 // java/util/Objects.requireNonNull:(Ljava/lang/Object;)Ljava/lang/Object; #17 = Class #19 // java/util/Objects #18 = NameAndType #20:#21 // requireNonNull:(Ljava/lang/Object;)Ljava/lang/Object; #19 = Utf8 java/util/Objects #20 = Utf8 requireNonNull #21 = Utf8 (Ljava/lang/Object;)Ljava/lang/Object; #22 = Methodref #14.#23 // OuterClassAfter11$InnerClass."<init>":(LOuterClassAfter11;)V #23 = NameAndType #5:#24 // "<init>":(LOuterClassAfter11;)V #24 = Utf8 (LOuterClassAfter11;)V #25 = Methodref #14.#26 // OuterClassAfter11$InnerClass.accessOuterPrivateField:()V #26 = NameAndType #27:#6 // accessOuterPrivateField:()V #27 = Utf8 accessOuterPrivateField #28 = Class #29 // OuterClassAfter11$StaticNested #29 = Utf8 OuterClassAfter11$StaticNested #30 = Methodref #28.#3 // OuterClassAfter11$StaticNested."<init>":()V #31 = Methodref #28.#32 // OuterClassAfter11$StaticNested.accessOuterPrivateField:(LOuterClassAfter11;)V #32 = NameAndType #27:#24 // accessOuterPrivateField:(LOuterClassAfter11;)V #33 = Utf8 Code #34 = Utf8 LineNumberTable #35 = Utf8 main #36 = Utf8 ([Ljava/lang/String;)V #37 = Utf8 SourceFile #38 = Utf8 OuterClassAfter11.java #39 = Utf8 NestMembers #40 = Utf8 InnerClasses #41 = Utf8 InnerClass #42 = Utf8 StaticNested { public OuterClassAfter11(); descriptor: ()V flags: (0x0001) ACC_PUBLIC Code: stack=2, locals=1, args_size=1 0: aload_0 1: invokespecial #1 // Method java/lang/Object."<init>":()V 4: aload_0 5: bipush 10 7: putfield #7 // Field privateField:I 10: return LineNumberTable: line 1: 0 line 2: 4 public static void main(java.lang.String[]); descriptor: ([Ljava/lang/String;)V flags: (0x0009) ACC_PUBLIC, ACC_STATIC Code: stack=4, locals=4, args_size=1 0: new #8 // class OuterClassAfter11 3: dup 4: invokespecial #13 // Method "<init>":()V 7: astore_1 8: new #14 // class OuterClassAfter11$InnerClass 11: dup 12: aload_1 13: dup 14: invokestatic #16 // Method java/util/Objects.requireNonNull:(Ljava/lang/Object;)Ljava/lang/Object; 17: pop 18: invokespecial #22 // Method OuterClassAfter11$InnerClass."<init>":(LOuterClassAfter11;)V 21: astore_2 22: aload_2 23: invokevirtual #25 // Method OuterClassAfter11$InnerClass.accessOuterPrivateField:()V 26: new #28 // class OuterClassAfter11$StaticNested 29: dup 30: invokespecial #30 // Method OuterClassAfter11$StaticNested."<init>":()V 33: astore_3 34: aload_3 35: aload_1 36: invokevirtual #31 // Method OuterClassAfter11$StaticNested.accessOuterPrivateField:(LOuterClassAfter11;)V 39: return LineNumberTable: line 17: 0 line 18: 8 line 19: 22 line 20: 26 line 21: 34 line 22: 39 } SourceFile: "OuterClassAfter11.java" NestMembers: OuterClassAfter11$StaticNested OuterClassAfter11$InnerClass InnerClasses: public #41= #14 of #8; // InnerClass=class OuterClassAfter11$InnerClass of class OuterClassAfter11 public static #42= #28 of #8; // StaticNested=class OuterClassAfter11$StaticNested of class OuterClassAfter11

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$ javap -v OuterClassAfter11.class

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Classfile /D:/Ethan/workspace/lab/example/text/java-newfeatures/src/OuterClassAfter11.class

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  Last modified 2025年1月20日; size 730 bytes

4

  SHA-256 checksum fda0dbffcb4d714172e20c1dce54e80a9ebd13ff08e01b48ee3be130f100dbdd

5

  Compiled from "OuterClassAfter11.java"

6

public class OuterClassAfter11

7

  minor version: 0

8

  major version: 61

9

  flags: (0x0021) ACC_PUBLIC, ACC_SUPER

10

  this_class: #8                          // OuterClassAfter11

11

  super_class: #2                         // java/lang/Object

12

  interfaces: 0, fields: 1, methods: 2, attributes: 3

13

Constant pool:

14

   #1 = Methodref          #2.#3          // java/lang/Object."<init>":()V

15

   #2 = Class              #4             // java/lang/Object

16

   #3 = NameAndType        #5:#6          // "<init>":()V

17

   #4 = Utf8               java/lang/Object

18

   #5 = Utf8               <init>

19

   #6 = Utf8               ()V

20

   #7 = Fieldref           #8.#9          // OuterClassAfter11.privateField:I

21

   #8 = Class              #10            // OuterClassAfter11

22

   #9 = NameAndType        #11:#12        // privateField:I

23

  #10 = Utf8               OuterClassAfter11

24

  #11 = Utf8               privateField

25

  #12 = Utf8               I

26

  #13 = Methodref          #8.#3          // OuterClassAfter11."<init>":()V

27

  #14 = Class              #15            // OuterClassAfter11$InnerClass

28

  #15 = Utf8               OuterClassAfter11$InnerClass

29

  #16 = Methodref          #17.#18        // java/util/Objects.requireNonNull:(Ljava/lang/Object;)Ljava/lang/Object;

30

  #17 = Class              #19            // java/util/Objects

31

  #18 = NameAndType        #20:#21        // requireNonNull:(Ljava/lang/Object;)Ljava/lang/Object;

32

  #19 = Utf8               java/util/Objects

33

  #20 = Utf8               requireNonNull

34

  #21 = Utf8               (Ljava/lang/Object;)Ljava/lang/Object;

35

  #22 = Methodref          #14.#23        // OuterClassAfter11$InnerClass."<init>":(LOuterClassAfter11;)V

36

  #23 = NameAndType        #5:#24         // "<init>":(LOuterClassAfter11;)V

37

  #24 = Utf8               (LOuterClassAfter11;)V

38

  #25 = Methodref          #14.#26        // OuterClassAfter11$InnerClass.accessOuterPrivateField:()V

39

  #26 = NameAndType        #27:#6         // accessOuterPrivateField:()V

40

  #27 = Utf8               accessOuterPrivateField

41

  #28 = Class              #29            // OuterClassAfter11$StaticNested

42

  #29 = Utf8               OuterClassAfter11$StaticNested

43

  #30 = Methodref          #28.#3         // OuterClassAfter11$StaticNested."<init>":()V

44

  #31 = Methodref          #28.#32        // OuterClassAfter11$StaticNested.accessOuterPrivateField:(LOuterClassAfter11;)V

45

  #32 = NameAndType        #27:#24        // accessOuterPrivateField:(LOuterClassAfter11;)V

46

  #33 = Utf8               Code

47

  #34 = Utf8               LineNumberTable

48

  #35 = Utf8               main

49

  #36 = Utf8               ([Ljava/lang/String;)V

50

  #37 = Utf8               SourceFile

51

  #38 = Utf8               OuterClassAfter11.java

52

  #39 = Utf8               NestMembers

53

  #40 = Utf8               InnerClasses

54

  #41 = Utf8               InnerClass

55

  #42 = Utf8               StaticNested

56

{

57

  public OuterClassAfter11();

58

    descriptor: ()V

59

    flags: (0x0001) ACC_PUBLIC

60

    Code:

61

      stack=2, locals=1, args_size=1

62

         0: aload_0

63

         1: invokespecial #1                  // Method java/lang/Object."<init>":()V

64

         4: aload_0

65

         5: bipush        10

66

         7: putfield      #7                  // Field privateField:I

67

        10: return

68

      LineNumberTable:

69

        line 1: 0

70

        line 2: 4

71

​

72

  public static void main(java.lang.String[]);

73

    descriptor: ([Ljava/lang/String;)V

74

    flags: (0x0009) ACC_PUBLIC, ACC_STATIC

75

    Code:

76

      stack=4, locals=4, args_size=1

77

         0: new           #8                  // class OuterClassAfter11

78

         3: dup

79

         4: invokespecial #13                 // Method "<init>":()V

80

         7: astore_1

81

         8: new           #14                 // class OuterClassAfter11$InnerClass

82

        11: dup

83

        12: aload_1

84

        13: dup

85

        14: invokestatic  #16                 // Method java/util/Objects.requireNonNull:(Ljava/lang/Object;)Ljava/lang/Object;

86

        17: pop

87

        18: invokespecial #22                 // Method OuterClassAfter11$InnerClass."<init>":(LOuterClassAfter11;)V

88

        21: astore_2

89

        22: aload_2

90

        23: invokevirtual #25                 // Method OuterClassAfter11$InnerClass.accessOuterPrivateField:()V

91

        26: new           #28                 // class OuterClassAfter11$StaticNested

92

        29: dup

93

        30: invokespecial #30                 // Method OuterClassAfter11$StaticNested."<init>":()V

94

        33: astore_3

95

        34: aload_3

96

        35: aload_1

97

        36: invokevirtual #31                 // Method OuterClassAfter11$StaticNested.accessOuterPrivateField:(LOuterClassAfter11;)V

98

        39: return

99

      LineNumberTable:

100

        line 17: 0

101

        line 18: 8

102

        line 19: 22

103

        line 20: 26

104

        line 21: 34

105

        line 22: 39

106

}

107

SourceFile: "OuterClassAfter11.java"

108

NestMembers:

109

  OuterClassAfter11$StaticNested

110

  OuterClassAfter11$InnerClass

111

InnerClasses:

112

  public #41= #14 of #8;                  // InnerClass=class OuterClassAfter11$InnerClass of class OuterClassAfter11

113

  public static #42= #28 of #8;           // StaticNested=class OuterClassAfter11$StaticNested of class OuterClassAfter11

114

​

觀察 OuterClassAfter11$InnerClass.class 檔案：

返回目錄

• NestHost 屬性:
  • 在 javap -v OuterClassAfter11$InnerClass.class 的輸出中，我們看到 NestHost: class OuterClassAfter11。這表示 InnerClass 的 Nest Host 是 OuterClassAfter11，也驗證了它是 OuterClassAfter11 Nest 的成員。

$ javap -v OuterClassAfter11\$InnerClass.class Classfile /D:/Ethan/workspace/lab/example/text/java-newfeatures/src/OuterClassAfter11$InnerClass.class Last modified 2025年1月20日; size 609 bytes SHA-256 checksum 0157ba0bc92487c059993cd4c308a362d69423c47cf6c888c1411d307ff3229f Compiled from "OuterClassAfter11.java" public class OuterClassAfter11$InnerClass minor version: 0 major version: 61 flags: (0x0021) ACC_PUBLIC, ACC_SUPER this_class: #2 // OuterClassAfter11$InnerClass super_class: #8 // java/lang/Object interfaces: 0, fields: 1, methods: 2, attributes: 3 Constant pool: #1 = Fieldref #2.#3 // OuterClassAfter11$InnerClass.this$0:LOuterClassAfter11; #2 = Class #4 // OuterClassAfter11$InnerClass #3 = NameAndType #5:#6 // this$0:LOuterClassAfter11; #4 = Utf8 OuterClassAfter11$InnerClass #5 = Utf8 this$0 #6 = Utf8 LOuterClassAfter11; #7 = Methodref #8.#9 // java/lang/Object."<init>":()V #8 = Class #10 // java/lang/Object #9 = NameAndType #11:#12 // "<init>":()V #10 = Utf8 java/lang/Object #11 = Utf8 <init> #12 = Utf8 ()V #13 = Fieldref #14.#15 // java/lang/System.out:Ljava/io/PrintStream; #14 = Class #16 // java/lang/System #15 = NameAndType #17:#18 // out:Ljava/io/PrintStream; #16 = Utf8 java/lang/System #17 = Utf8 out #18 = Utf8 Ljava/io/PrintStream; #19 = Fieldref #20.#21 // OuterClassAfter11.privateField:I #20 = Class #22 // OuterClassAfter11 #21 = NameAndType #23:#24 // privateField:I #22 = Utf8 OuterClassAfter11 #23 = Utf8 privateField #24 = Utf8 I #25 = Methodref #26.#27 // java/io/PrintStream.println:(I)V #26 = Class #28 // java/io/PrintStream #27 = NameAndType #29:#30 // println:(I)V #28 = Utf8 java/io/PrintStream #29 = Utf8 println #30 = Utf8 (I)V #31 = Utf8 (LOuterClassAfter11;)V #32 = Utf8 Code #33 = Utf8 LineNumberTable #34 = Utf8 accessOuterPrivateField #35 = Utf8 SourceFile #36 = Utf8 OuterClassAfter11.java #37 = Utf8 NestHost #38 = Utf8 InnerClasses #39 = Utf8 InnerClass { final OuterClassAfter11 this$0; descriptor: LOuterClassAfter11; flags: (0x1010) ACC_FINAL, ACC_SYNTHETIC public OuterClassAfter11$InnerClass(OuterClassAfter11); descriptor: (LOuterClassAfter11;)V flags: (0x0001) ACC_PUBLIC Code: stack=2, locals=2, args_size=2 0: aload_0 1: aload_1 2: putfield #1 // Field this$0:LOuterClassAfter11; 5: aload_0 6: invokespecial #7 // Method java/lang/Object."<init>":()V 9: return LineNumberTable: line 4: 0 public void accessOuterPrivateField(); descriptor: ()V flags: (0x0001) ACC_PUBLIC Code: stack=2, locals=1, args_size=1 0: getstatic #13 // Field java/lang/System.out:Ljava/io/PrintStream; 3: aload_0 4: getfield #1 // Field this$0:LOuterClassAfter11; 7: getfield #19 // Field OuterClassAfter11.privateField:I 10: invokevirtual #25 // Method java/io/PrintStream.println:(I)V 13: return LineNumberTable: line 6: 0 line 7: 13 } SourceFile: "OuterClassAfter11.java" NestHost: class OuterClassAfter11 InnerClasses: public #39= #2 of #20; // InnerClass=class OuterClassAfter11$InnerClass of class OuterClassAfter11

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1

$ javap -v OuterClassAfter11\$InnerClass.class

2

Classfile /D:/Ethan/workspace/lab/example/text/java-newfeatures/src/OuterClassAfter11$InnerClass.class

3

  Last modified 2025年1月20日; size 609 bytes

4

  SHA-256 checksum 0157ba0bc92487c059993cd4c308a362d69423c47cf6c888c1411d307ff3229f

5

  Compiled from "OuterClassAfter11.java"

6

public class OuterClassAfter11$InnerClass

7

  minor version: 0

8

  major version: 61

9

  flags: (0x0021) ACC_PUBLIC, ACC_SUPER

10

  this_class: #2                          // OuterClassAfter11$InnerClass

11

  super_class: #8                         // java/lang/Object

12

  interfaces: 0, fields: 1, methods: 2, attributes: 3

13

Constant pool:

14

   #1 = Fieldref           #2.#3          // OuterClassAfter11$InnerClass.this$0:LOuterClassAfter11;

15

   #2 = Class              #4             // OuterClassAfter11$InnerClass

16

   #3 = NameAndType        #5:#6          // this$0:LOuterClassAfter11;

17

   #4 = Utf8               OuterClassAfter11$InnerClass

18

   #5 = Utf8               this$0

19

   #6 = Utf8               LOuterClassAfter11;

20

   #7 = Methodref          #8.#9          // java/lang/Object."<init>":()V

21

   #8 = Class              #10            // java/lang/Object

22

   #9 = NameAndType        #11:#12        // "<init>":()V

23

  #10 = Utf8               java/lang/Object

24

  #11 = Utf8               <init>

25

  #12 = Utf8               ()V

26

  #13 = Fieldref           #14.#15        // java/lang/System.out:Ljava/io/PrintStream;

27

  #14 = Class              #16            // java/lang/System

28

  #15 = NameAndType        #17:#18        // out:Ljava/io/PrintStream;

29

  #16 = Utf8               java/lang/System

30

  #17 = Utf8               out

31

  #18 = Utf8               Ljava/io/PrintStream;

32

  #19 = Fieldref           #20.#21        // OuterClassAfter11.privateField:I

33

  #20 = Class              #22            // OuterClassAfter11

34

  #21 = NameAndType        #23:#24        // privateField:I

35

  #22 = Utf8               OuterClassAfter11

36

  #23 = Utf8               privateField

37

  #24 = Utf8               I

38

  #25 = Methodref          #26.#27        // java/io/PrintStream.println:(I)V

39

  #26 = Class              #28            // java/io/PrintStream

40

  #27 = NameAndType        #29:#30        // println:(I)V

41

  #28 = Utf8               java/io/PrintStream

42

  #29 = Utf8               println

43

  #30 = Utf8               (I)V

44

  #31 = Utf8               (LOuterClassAfter11;)V

45

  #32 = Utf8               Code

46

  #33 = Utf8               LineNumberTable

47

  #34 = Utf8               accessOuterPrivateField

48

  #35 = Utf8               SourceFile

49

  #36 = Utf8               OuterClassAfter11.java

50

  #37 = Utf8               NestHost

51

  #38 = Utf8               InnerClasses

52

  #39 = Utf8               InnerClass

53

{

54

  final OuterClassAfter11 this$0;

55

    descriptor: LOuterClassAfter11;

56

    flags: (0x1010) ACC_FINAL, ACC_SYNTHETIC

57

​

58

  public OuterClassAfter11$InnerClass(OuterClassAfter11);

59

    descriptor: (LOuterClassAfter11;)V

60

    flags: (0x0001) ACC_PUBLIC

61

    Code:

62

      stack=2, locals=2, args_size=2

63

         0: aload_0

64

         1: aload_1

65

         2: putfield      #1                  // Field this$0:LOuterClassAfter11;

66

         5: aload_0

67

         6: invokespecial #7                  // Method java/lang/Object."<init>":()V

68

         9: return

69

      LineNumberTable:

70

        line 4: 0

71

​

72

  public void accessOuterPrivateField();

73

    descriptor: ()V

74

    flags: (0x0001) ACC_PUBLIC

75

    Code:

76

      stack=2, locals=1, args_size=1

77

         0: getstatic     #13                 // Field java/lang/System.out:Ljava/io/PrintStream;

78

         3: aload_0

79

         4: getfield      #1                  // Field this$0:LOuterClassAfter11;

80

         7: getfield      #19                 // Field OuterClassAfter11.privateField:I

81

        10: invokevirtual #25                 // Method java/io/PrintStream.println:(I)V

82

        13: return

83

      LineNumberTable:

84

        line 6: 0

85

        line 7: 13

86

}

87

SourceFile: "OuterClassAfter11.java"

88

NestHost: class OuterClassAfter11

89

InnerClasses:

90

  public #39= #2 of #20;                  // InnerClass=class OuterClassAfter11$InnerClass of class OuterClassAfter11

91

​

無橋接方法分析:

• Constant pool 中直接引用 #19 = Fieldref #20.#21 : InnerClass 的常數池直接包含了對 OuterClassAfter11.privateField 的引用。
• accessOuterPrivateField() 方法中的 7: getfield #19 : 在 accessOuterPrivateField() 方法中，InnerClass 直接使用 getfield 指令來存取外部類別 OuterClassAfter11 的 privateField，而沒有呼叫任何橋接方法。

觀察 OuterClassAfter11$StaticNested.class 檔案：

返回目錄

• NestHost 屬性:
  • 在 javap -v OuterClassAfter11$StaticNested.class 的輸出中，我們看到 NestHost: class OuterClassAfter11。這表示 StaticNested 的 Nest Host 是 OuterClassAfter11，也驗證了它是 OuterClassAfter11 Nest 的成員。

$ javap -v OuterClassAfter11\$StaticNested.class Classfile /D:/Ethan/workspace/lab/example/text/java-newfeatures/src/OuterClassAfter11$StaticNested.class Last modified 2025年1月20日; size 556 bytes SHA-256 checksum f1a06b849b734cef7ba2c2196188fc2ee820022fc1ecf0161b1bdad74e4ff39a Compiled from "OuterClassAfter11.java" public class OuterClassAfter11$StaticNested minor version: 0 major version: 61 flags: (0x0021) ACC_PUBLIC, ACC_SUPER this_class: #25 // OuterClassAfter11$StaticNested super_class: #2 // java/lang/Object interfaces: 0, fields: 0, methods: 2, attributes: 3 Constant pool: #1 = Methodref #2.#3 // java/lang/Object."<init>":()V #2 = Class #4 // java/lang/Object #3 = NameAndType #5:#6 // "<init>":()V #4 = Utf8 java/lang/Object #5 = Utf8 <init> #6 = Utf8 ()V #7 = Fieldref #8.#9 // java/lang/System.out:Ljava/io/PrintStream; #8 = Class #10 // java/lang/System #9 = NameAndType #11:#12 // out:Ljava/io/PrintStream; #10 = Utf8 java/lang/System #11 = Utf8 out #12 = Utf8 Ljava/io/PrintStream; #13 = Fieldref #14.#15 // OuterClassAfter11.privateField:I #14 = Class #16 // OuterClassAfter11 #15 = NameAndType #17:#18 // privateField:I #16 = Utf8 OuterClassAfter11 #17 = Utf8 privateField #18 = Utf8 I #19 = Methodref #20.#21 // java/io/PrintStream.println:(I)V #20 = Class #22 // java/io/PrintStream #21 = NameAndType #23:#24 // println:(I)V #22 = Utf8 java/io/PrintStream #23 = Utf8 println #24 = Utf8 (I)V #25 = Class #26 // OuterClassAfter11$StaticNested #26 = Utf8 OuterClassAfter11$StaticNested #27 = Utf8 Code #28 = Utf8 LineNumberTable #29 = Utf8 accessOuterPrivateField #30 = Utf8 (LOuterClassAfter11;)V #31 = Utf8 SourceFile #32 = Utf8 OuterClassAfter11.java #33 = Utf8 NestHost #34 = Utf8 InnerClasses #35 = Utf8 StaticNested { public OuterClassAfter11$StaticNested(); descriptor: ()V flags: (0x0001) ACC_PUBLIC Code: stack=1, locals=1, args_size=1 0: aload_0 1: invokespecial #1 // Method java/lang/Object."<init>":()V 4: return LineNumberTable: line 10: 0 public void accessOuterPrivateField(OuterClassAfter11); descriptor: (LOuterClassAfter11;)V flags: (0x0001) ACC_PUBLIC Code: stack=2, locals=2, args_size=2 0: getstatic #7 // Field java/lang/System.out:Ljava/io/PrintStream; 3: aload_1 4: getfield #13 // Field OuterClassAfter11.privateField:I 7: invokevirtual #19 // Method java/io/PrintStream.println:(I)V 10: return LineNumberTable: line 12: 0 line 13: 10 } SourceFile: "OuterClassAfter11.java" NestHost: class OuterClassAfter11 InnerClasses: public static #35= #25 of #14; // StaticNested=class OuterClassAfter11$StaticNested of class OuterClassAfter11

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1

$ javap -v OuterClassAfter11\$StaticNested.class

2

Classfile /D:/Ethan/workspace/lab/example/text/java-newfeatures/src/OuterClassAfter11$StaticNested.class

3

  Last modified 2025年1月20日; size 556 bytes

4

  SHA-256 checksum f1a06b849b734cef7ba2c2196188fc2ee820022fc1ecf0161b1bdad74e4ff39a

5

  Compiled from "OuterClassAfter11.java"

6

public class OuterClassAfter11$StaticNested

7

  minor version: 0

8

  major version: 61

9

  flags: (0x0021) ACC_PUBLIC, ACC_SUPER

10

  this_class: #25                         // OuterClassAfter11$StaticNested

11

  super_class: #2                         // java/lang/Object

12

  interfaces: 0, fields: 0, methods: 2, attributes: 3

13

Constant pool:

14

   #1 = Methodref          #2.#3          // java/lang/Object."<init>":()V

15

   #2 = Class              #4             // java/lang/Object

16

   #3 = NameAndType        #5:#6          // "<init>":()V

17

   #4 = Utf8               java/lang/Object

18

   #5 = Utf8               <init>

19

   #6 = Utf8               ()V

20

   #7 = Fieldref           #8.#9          // java/lang/System.out:Ljava/io/PrintStream;

21

   #8 = Class              #10            // java/lang/System

22

   #9 = NameAndType        #11:#12        // out:Ljava/io/PrintStream;

23

  #10 = Utf8               java/lang/System

24

  #11 = Utf8               out

25

  #12 = Utf8               Ljava/io/PrintStream;

26

  #13 = Fieldref           #14.#15        // OuterClassAfter11.privateField:I

27

  #14 = Class              #16            // OuterClassAfter11

28

  #15 = NameAndType        #17:#18        // privateField:I

29

  #16 = Utf8               OuterClassAfter11

30

  #17 = Utf8               privateField

31

  #18 = Utf8               I

32

  #19 = Methodref          #20.#21        // java/io/PrintStream.println:(I)V

33

  #20 = Class              #22            // java/io/PrintStream

34

  #21 = NameAndType        #23:#24        // println:(I)V

35

  #22 = Utf8               java/io/PrintStream

36

  #23 = Utf8               println

37

  #24 = Utf8               (I)V

38

  #25 = Class              #26            // OuterClassAfter11$StaticNested

39

  #26 = Utf8               OuterClassAfter11$StaticNested

40

  #27 = Utf8               Code

41

  #28 = Utf8               LineNumberTable

42

  #29 = Utf8               accessOuterPrivateField

43

  #30 = Utf8               (LOuterClassAfter11;)V

44

  #31 = Utf8               SourceFile

45

  #32 = Utf8               OuterClassAfter11.java

46

  #33 = Utf8               NestHost

47

  #34 = Utf8               InnerClasses

48

  #35 = Utf8               StaticNested

49

{

50

  public OuterClassAfter11$StaticNested();

51

    descriptor: ()V

52

    flags: (0x0001) ACC_PUBLIC

53

    Code:

54

      stack=1, locals=1, args_size=1

55

         0: aload_0

56

         1: invokespecial #1                  // Method java/lang/Object."<init>":()V

57

         4: return

58

      LineNumberTable:

59

        line 10: 0

60

​

61

  public void accessOuterPrivateField(OuterClassAfter11);

62

    descriptor: (LOuterClassAfter11;)V

63

    flags: (0x0001) ACC_PUBLIC

64

    Code:

65

      stack=2, locals=2, args_size=2

66

         0: getstatic     #7                  // Field java/lang/System.out:Ljava/io/PrintStream;

67

         3: aload_1

68

         4: getfield      #13                 // Field OuterClassAfter11.privateField:I

69

         7: invokevirtual #19                 // Method java/io/PrintStream.println:(I)V

70

        10: return

71

      LineNumberTable:

72

        line 12: 0

73

        line 13: 10

74

}

75

SourceFile: "OuterClassAfter11.java"

76

NestHost: class OuterClassAfter11

77

InnerClasses:

78

  public static #35= #25 of #14;          // StaticNested=class OuterClassAfter11$StaticNested of class OuterClassAfter11

79

​

無橋接方法分析:

• Constant pool 中直接引用 #13 = Fieldref #14.#15 : StaticNested 的常數池直接包含了對 OuterClassAfter11.privateField:I 的引用。
• accessOuterPrivateField() 方法中的 4: getfield #13 : 在 accessOuterPrivateField() 方法中， StaticNested 直接使用 getfield 指令來存取外部類別 OuterClassAfter11 的 privateField，而沒有呼叫任何橋接方法。

Nest:

返回目錄

• 靜態巢狀類別 (StaticNested) 的 Nest 成員身份： 即使 StaticNested 是靜態的，它依然屬於 OuterClassAfter11 的 Nest。這與傳統的 Java 內部類別概念稍有不同，因為過去靜態巢狀類別通常不被視為外部類別的「一部分」。
• 存取控制： Nest 關係讓內部類別可以更自然地存取外部類別的私有成員，而無需編譯器生成額外的橋接方法，提升了程式碼的效率和可讀性。
• this$0 欄位： InnerClass 有一個 final OuterClassAfter11 this$0 欄位，這是一個隱式的外部類別實例的引用，用於存取外部類別的成員。在巢狀存取控制中，這個引用仍然存在，但存取私有成員不再需要橋接方法。
• requireNonNull 方法： 在 main 方法中， java/util/Objects.requireNonNull 方法被用來檢測 OuterClassAfter11 的實例是否為空，確保在使用前是有效的物件。

小結

在 Java 11 及以後，由於 Nest 的概念，編譯器不再需要生成橋接方法：

• OuterClassAfter11 和 OuterClassAfter11$InnerClass、OuterClassAfter11$StaticClass 被視為同一個 Nest 的成員。
• InnerClass 和 StaticClass 可以直接存取 OuterClassAfter11 的私有成員 privateField。
• 因此，編譯器不再需要自動生成 access$000 這種橋接方法，程式碼更加精簡，執行效率也更高。

四、Java 12 至 17 的相關演進

返回目錄

Java 11 引入巢狀存取控制之後，後續版本並沒有對此特性進行重大修改，這體現了該機制的成熟和穩定性。Java 12 至 17 延續了這一機制，確保了程式碼的穩定性和高效性。這些版本主要關注在其他方面的語言特性和效能優化，而巢狀存取控制已經相對完善。具體來說：

• 持續穩定性: 雖然 Java 12 至 17 沒有針對巢狀存取控制引入重大變更，但這些版本在 JVM 層面持續優化，確保了 Nest-Based Access Control 的穩定性和效能。
• 與其他特性的互動: Nest-Based Access Control 是一個底層機制，它與 Java 後續版本中引入的其他語言特性 (如 Record、Sealed Class) 相容良好。開發者可以安全地在這些新特性中使用巢狀類別，享受 Nest 帶來的好處。
• 工具改進: javap 工具在 Java 12 以後的版本中，對 Nest 相關資訊的輸出更加完整和易讀，方便開發者分析類別檔案。
• 效能提升: 後續版本對 JVM 的不斷優化，間接提升了使用巢狀存取控制的程式碼的執行效能。例如，JIT 編譯器可以更好地優化使用 Nest 的程式碼。
• 版本兼容性: 确保 Java 11 及之后編譯的代碼，在不同版本的 JVM 上運行都具有良好的兼容性，無需額外處理。
• 沒有新屬性: Java 12 至 17 没有引入新的類別文件屬性來擴展巢狀存取控制，說明 Java 11 引入的機制已经足够完整且满足需求。

總結來說，Java 12 至 17 的演進，主要是對 Java 11 引入的巢狀存取控制機制的穩定性和效能做進一步的保證，並將其融入到後續版本的新特性中。

五、總結

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Java 11 引入的巢狀存取控制 (Nest-Based Access Control) 是一項重要的改進，解決了舊有巢狀類別存取機制中的問題，帶來了以下優點：

• 消除橋接方法: 編譯器不再需要生成橋接方法，程式碼更加清晰易懂，類別檔案也更加簡潔。這不僅減少了類別檔案的大小，也減少了開發者在除錯時的困擾。
• 提升存取效能: 巢狀類別可以直接存取外部類別的私有成員，無需經過中轉，提高了運行效率。在效能敏感的程式碼中，這種優化尤為重要。
• 簡化虛擬機行為: 虛擬機的載入和執行流程得到簡化，提高了效率和可維護性。
• 更自然的存取模型: 讓內部類別和外部類別的關係更加緊密，符合程式開發者的直覺。
• 促進設計模式: 巢狀類別常常用於實現特定設計模式，Nest-Based Access Control 使得這些模式的實現更加有效率。
• 對微服務架構的影響: 雖然 Nest 主要是語言層面的機制，但其在微服務架構中也有應用，例如，服務內的私有類別之間的存取更加高效。
• 鼓勵程式碼模組化: 鼓勵開發者合理使用巢狀類別，提高程式碼的模組化和可維護性。
• 現代Java開發的必要知識: 理解 Nest-Based Access Control 是現代 Java 開發者的基本要求，有助於編寫更高效、更可靠的程式碼。
• 除錯優化: javap 工具不僅是分析工具，更是除錯與效能調優的利器，透過觀察 bytecode 可以更深入理解程式碼的行為。

從 Java 11 開始，巢狀存取控制機制在 Java 語言中成為標準，並且在 Java 12 至 17 中持續得到應用，這證明了這項改進的價值和重要性。開發者應當理解巢狀存取控制的原理，以撰寫更高效、更易於維護的 Java 程式碼。

此外，使用 javap 工具檢視 bytecode 文件，能夠幫助我們更深入地理解底層實現，對於除錯或優化程式碼都有很大的幫助。

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