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Kotlin字节码层探究构造函数与成员变量和init代码块执行顺序
来源：jb51　　时间：2022/11/17 8:48:32　　对本文有异议

之前写了一篇文章，从Java语法的角度分析了Kotlin构造函数、成员变量初始化、init代码块三者的执行顺序：

Kotlin构造函数与成员变量和init代码块执行顺序详细讲解

这次再从字节码的角度分析它们的执行顺序。

还是用之前那个例子：

  1. class InitOrderDemo(name: String) {
  2.     val firstProperty = "First property: $name".also(::println)
  3.     init {
  4.         println("First initializer block that prints ${name}")
  5.     }
  6.     val secondProperty = "Second property: ${name.length}".also(::println)
  7.     init {
  8.         println("Second initializer block that prints ${name.length}")
  9.     }
  10. }

调用InitOrderDemo(“hello”)打印的结果如下：

First property: hello
First initializer block that prints hello
Second property: 5
Second initializer block that prints 5

可以看到执行顺序，是按照它们声明的顺序执行。

将上面Koltin代码转成字节码之后，显示内容如下：

  1. // ================com/devnn/javalib/InitOrderDemo.class =================
  2. // class version 52.0 (52)
  3. // access flags 0x31
  4. public final class com/devnn/javalib/InitOrderDemo {
  5.   // access flags 0x12
  6.   private final Ljava/lang/String; firstProperty
  7.   @Lorg/jetbrains/annotations/NotNull;() // invisible
  8.   // access flags 0x11
  9.   public final getFirstProperty()Ljava/lang/String;
  10.   @Lorg/jetbrains/annotations/NotNull;() // invisible
  11.    L0
  12.     LINENUMBER 4 L0
  13.     ALOAD 0
  14.     GETFIELD com/devnn/javalib/InitOrderDemo.firstProperty : Ljava/lang/String;
  15.     ARETURN
  16.    L1
  17.     LOCALVARIABLE this Lcom/devnn/javalib/InitOrderDemo; L0 L1 0
  18.     MAXSTACK = 1
  19.     MAXLOCALS = 1
  20.   // access flags 0x12
  21.   private final Ljava/lang/String; secondProperty
  22.   @Lorg/jetbrains/annotations/NotNull;() // invisible
  23.   // access flags 0x11
  24.   public final getSecondProperty()Ljava/lang/String;
  25.   @Lorg/jetbrains/annotations/NotNull;() // invisible
  26.    L0
  27.     LINENUMBER 10 L0
  28.     ALOAD 0
  29.     GETFIELD com/devnn/javalib/InitOrderDemo.secondProperty : Ljava/lang/String;
  30.     ARETURN
  31.    L1
  32.     LOCALVARIABLE this Lcom/devnn/javalib/InitOrderDemo; L0 L1 0
  33.     MAXSTACK = 1
  34.     MAXLOCALS = 1
  35.   // access flags 0x1
  36.   public <init>(Ljava/lang/String;)V
  37.     // annotable parameter count: 1 (visible)
  38.     // annotable parameter count: 1 (invisible)
  39.     @Lorg/jetbrains/annotations/NotNull;() // invisible, parameter 0
  40.    L0
  41.     ALOAD 1
  42.     LDC "name"
  43.     INVOKESTATIC kotlin/jvm/internal/Intrinsics.checkNotNullParameter (Ljava/lang/Object;Ljava/lang/String;)V
  44.    L1
  45.     LINENUMBER 3 L1
  46.     ALOAD 0
  47.     INVOKESPECIAL java/lang/Object.<init> ()V
  48.    L2
  49.     LINENUMBER 4 L2
  50.     ALOAD 0
  51.     NEW java/lang/StringBuilder
  52.     DUP
  53.     INVOKESPECIAL java/lang/StringBuilder.<init> ()V
  54.     LDC "First property: "
  55.     INVOKEVIRTUAL java/lang/StringBuilder.append (Ljava/lang/String;)Ljava/lang/StringBuilder;
  56.     ALOAD 1
  57.     INVOKEVIRTUAL java/lang/StringBuilder.append (Ljava/lang/String;)Ljava/lang/StringBuilder;
  58.     INVOKEVIRTUAL java/lang/StringBuilder.toString ()Ljava/lang/String;
  59.     ASTORE 2
  60.    L3
  61.     ALOAD 2
  62.     ASTORE 3
  63.    L4
  64.     LINENUMBER 17 L4
  65.     ASTORE 5
  66.    L5
  67.     ICONST_0
  68.     ISTORE 4
  69.    L6
  70.     LINENUMBER 4 L6
  71.    L7
  72.     GETSTATIC java/lang/System.out : Ljava/io/PrintStream;
  73.     ALOAD 3
  74.     INVOKEVIRTUAL java/io/PrintStream.println (Ljava/lang/Object;)V
  75.    L8
  76.    L9
  77.    L10
  78.     GETSTATIC kotlin/Unit.INSTANCE : Lkotlin/Unit;
  79.     ASTORE 6
  80.     ALOAD 5
  81.    L11
  82.     LINENUMBER 4 L11
  83.    L12
  84.     ALOAD 2
  85.    L13
  86.     PUTFIELD com/devnn/javalib/InitOrderDemo.firstProperty : Ljava/lang/String;
  87.    L14
  88.     LINENUMBER 6 L14
  89.     NOP
  90.    L15
  91.     LINENUMBER 7 L15
  92.     NEW java/lang/StringBuilder
  93.     DUP
  94.     INVOKESPECIAL java/lang/StringBuilder.<init> ()V
  95.     LDC "First initializer block that prints "
  96.     INVOKEVIRTUAL java/lang/StringBuilder.append (Ljava/lang/String;)Ljava/lang/StringBuilder;
  97.     ALOAD 1
  98.     INVOKEVIRTUAL java/lang/StringBuilder.append (Ljava/lang/String;)Ljava/lang/StringBuilder;
  99.     INVOKEVIRTUAL java/lang/StringBuilder.toString ()Ljava/lang/String;
  100.     ASTORE 2
  101.    L16
  102.     GETSTATIC java/lang/System.out : Ljava/io/PrintStream;
  103.     ALOAD 2
  104.     INVOKEVIRTUAL java/io/PrintStream.println (Ljava/lang/Object;)V
  105.    L17
  106.    L18
  107.     LINENUMBER 8 L18
  108.     NOP
  109.    L19
  110.     LINENUMBER 10 L19
  111.     ALOAD 0
  112.     NEW java/lang/StringBuilder
  113.     DUP
  114.     INVOKESPECIAL java/lang/StringBuilder.<init> ()V
  115.     LDC "Second property: "
  116.     INVOKEVIRTUAL java/lang/StringBuilder.append (Ljava/lang/String;)Ljava/lang/StringBuilder;
  117.     ALOAD 1
  118.     INVOKEVIRTUAL java/lang/String.length ()I
  119.     INVOKEVIRTUAL java/lang/StringBuilder.append (I)Ljava/lang/StringBuilder;
  120.     INVOKEVIRTUAL java/lang/StringBuilder.toString ()Ljava/lang/String;
  121.     ASTORE 2
  122.    L20
  123.     ALOAD 2
  124.     ASTORE 3
  125.    L21
  126.     LINENUMBER 17 L21
  127.     ASTORE 5
  128.    L22
  129.     ICONST_0
  130.     ISTORE 4
  131.    L23
  132.     LINENUMBER 10 L23
  133.    L24
  134.     GETSTATIC java/lang/System.out : Ljava/io/PrintStream;
  135.     ALOAD 3
  136.     INVOKEVIRTUAL java/io/PrintStream.println (Ljava/lang/Object;)V
  137.    L25
  138.    L26
  139.    L27
  140.     GETSTATIC kotlin/Unit.INSTANCE : Lkotlin/Unit;
  141.     ASTORE 6
  142.     ALOAD 5
  143.    L28
  144.     LINENUMBER 10 L28
  145.    L29
  146.     ALOAD 2
  147.    L30
  148.     PUTFIELD com/devnn/javalib/InitOrderDemo.secondProperty : Ljava/lang/String;
  149.    L31
  150.     LINENUMBER 12 L31
  151.     NOP
  152.    L32
  153.     LINENUMBER 13 L32
  154.     NEW java/lang/StringBuilder
  155.     DUP
  156.     INVOKESPECIAL java/lang/StringBuilder.<init> ()V
  157.     LDC "Second initializer block that prints "
  158.     INVOKEVIRTUAL java/lang/StringBuilder.append (Ljava/lang/String;)Ljava/lang/StringBuilder;
  159.     ALOAD 1
  160.     INVOKEVIRTUAL java/lang/String.length ()I
  161.     INVOKEVIRTUAL java/lang/StringBuilder.append (I)Ljava/lang/StringBuilder;
  162.     INVOKEVIRTUAL java/lang/StringBuilder.toString ()Ljava/lang/String;
  163.     ASTORE 2
  164.    L33
  165.     GETSTATIC java/lang/System.out : Ljava/io/PrintStream;
  166.     ALOAD 2
  167.     INVOKEVIRTUAL java/io/PrintStream.println (Ljava/lang/Object;)V
  168.    L34
  169.    L35
  170.     LINENUMBER 14 L35
  171.     RETURN
  172.    L36
  173.     LOCALVARIABLE p1 Ljava/lang/Object; L5 L10 3
  174.     LOCALVARIABLE $i$a$-unknown-InitOrderDemo$firstProperty$1 I L6 L10 4
  175.     LOCALVARIABLE p1 Ljava/lang/Object; L22 L27 3
  176.     LOCALVARIABLE $i$a$-unknown-InitOrderDemo$secondProperty$1 I L23 L27 4
  177.     LOCALVARIABLE this Lcom/devnn/javalib/InitOrderDemo; L0 L36 0
  178.     LOCALVARIABLE name Ljava/lang/String; L0 L36 1
  179.     MAXSTACK = 3
  180.     MAXLOCALS = 7
  181. }

可以看到上面的构造函数、成员变量初始化和init代码块，按照声明都被放到了字节码的init代码块中了。

字节码的init初始化器其实就是类的构造函数。将Java代码转成字节码也是存在init构造函数。

下面看一个Java示例，加深对字节码的init初始化块的认识。

  1. package com.devnn.javalib;
  2. public class JavaInit {
  3.     String firstName = "Steven";
  4.     {
  5.         System.out.println("This is init block");
  6.     }
  7.     JavaInit(String secondName) {
  8.         System.out.println("firstName=" + firstName);
  9.         System.out.println("secondName=" + secondName);
  10.     }
  11.     public static void main(String[] args) {
  12.         new JavaInit("Jobs");
  13.     }
  14. }

运行main函数打印结果如下：

This is init block
firstName=Steven
secondName=Jobs

将上面的JavaInit类转成字节码之后的内容如下：

  1. // class version 51.0 (51)
  2. // access flags 0x21
  3. public class com/devnn/javalib/JavaInit {
  4.   // compiled from: JavaInit.java
  5.   // access flags 0x0
  6.   Ljava/lang/String; firstName
  7.   // access flags 0x0
  8.   <init>(Ljava/lang/String;)V
  9.    L0
  10.     LINENUMBER 10 L0
  11.     ALOAD 0
  12.     INVOKESPECIAL java/lang/Object.<init> ()V
  13.    L1
  14.     LINENUMBER 4 L1
  15.     ALOAD 0
  16.     LDC "Steven"
  17.     PUTFIELD com/devnn/javalib/JavaInit.firstName : Ljava/lang/String;
  18.    L2
  19.     LINENUMBER 7 L2
  20.     GETSTATIC java/lang/System.out : Ljava/io/PrintStream;
  21.     LDC "This is init block"
  22.     INVOKEVIRTUAL java/io/PrintStream.println (Ljava/lang/String;)V
  23.    L3
  24.     LINENUMBER 11 L3
  25.     GETSTATIC java/lang/System.out : Ljava/io/PrintStream;
  26.     NEW java/lang/StringBuilder
  27.     DUP
  28.     INVOKESPECIAL java/lang/StringBuilder.<init> ()V
  29.     LDC "firstName="
  30.     INVOKEVIRTUAL java/lang/StringBuilder.append (Ljava/lang/String;)Ljava/lang/StringBuilder;
  31.     ALOAD 0
  32.     GETFIELD com/devnn/javalib/JavaInit.firstName : Ljava/lang/String;
  33.     INVOKEVIRTUAL java/lang/StringBuilder.append (Ljava/lang/String;)Ljava/lang/StringBuilder;
  34.     INVOKEVIRTUAL java/lang/StringBuilder.toString ()Ljava/lang/String;
  35.     INVOKEVIRTUAL java/io/PrintStream.println (Ljava/lang/String;)V
  36.    L4
  37.     LINENUMBER 12 L4
  38.     GETSTATIC java/lang/System.out : Ljava/io/PrintStream;
  39.     NEW java/lang/StringBuilder
  40.     DUP
  41.     INVOKESPECIAL java/lang/StringBuilder.<init> ()V
  42.     LDC "secondName="
  43.     INVOKEVIRTUAL java/lang/StringBuilder.append (Ljava/lang/String;)Ljava/lang/StringBuilder;
  44.     ALOAD 1
  45.     INVOKEVIRTUAL java/lang/StringBuilder.append (Ljava/lang/String;)Ljava/lang/StringBuilder;
  46.     INVOKEVIRTUAL java/lang/StringBuilder.toString ()Ljava/lang/String;
  47.     INVOKEVIRTUAL java/io/PrintStream.println (Ljava/lang/String;)V
  48.    L5
  49.     LINENUMBER 13 L5
  50.     RETURN
  51.    L6
  52.     LOCALVARIABLE this Lcom/devnn/javalib/JavaInit; L0 L6 0
  53.     LOCALVARIABLE secondName Ljava/lang/String; L0 L6 1
  54.     MAXSTACK = 3
  55.     MAXLOCALS = 2
  56.   // access flags 0x9
  57.   public static main([Ljava/lang/String;)V
  58.    L0
  59.     LINENUMBER 16 L0
  60.     NEW com/devnn/javalib/JavaInit
  61.     DUP
  62.     LDC "Jobs"
  63.     INVOKESPECIAL com/devnn/javalib/JavaInit.<init> (Ljava/lang/String;)V
  64.     POP
  65.    L1
  66.     LINENUMBER 17 L1
  67.     RETURN
  68.    L2
  69.     LOCALVARIABLE args [Ljava/lang/String; L0 L2 0
  70.     MAXSTACK = 3
  71.     MAXLOCALS = 1
  72. }

可见，Java类的成员变量初始化、构造函数、构造块同样都被拷贝进了init代码块中。那么它们是否存在顺序问题呢？

将上面JavaInit类的firname成员变量放到初始化块下面试试：

  1. package com.devnn.javalib;
  2. public class JavaInit {
  3.     {
  4.         System.out.println("This is init block");
  5.     }
  6.     JavaInit(String secondName) {
  7.         System.out.println("firstName=" + firstName);
  8.         System.out.println("secondName=" + secondName);
  9.     }
  10.     String firstName = "Steven";
  11.     public static void main(String[] args) {
  12.         new JavaInit("Jobs");
  13.     }
  14. }

查看字节码：

  1. // class version 51.0 (51)
  2. // access flags 0x21
  3. public class com/devnn/javalib/JavaInit {
  4.   // compiled from: JavaInit.java
  5.   // access flags 0x0
  6.   Ljava/lang/String; firstName
  7.   // access flags 0x0
  8.   <init>(Ljava/lang/String;)V
  9.    L0
  10.     LINENUMBER 8 L0
  11.     ALOAD 0
  12.     INVOKESPECIAL java/lang/Object.<init> ()V
  13.    L1
  14.     LINENUMBER 5 L1
  15.     GETSTATIC java/lang/System.out : Ljava/io/PrintStream;
  16.     LDC "This is init block"
  17.     INVOKEVIRTUAL java/io/PrintStream.println (Ljava/lang/String;)V
  18.    L2
  19.     LINENUMBER 13 L2
  20.     ALOAD 0
  21.     LDC "Steven"
  22.     PUTFIELD com/devnn/javalib/JavaInit.firstName : Ljava/lang/String;
  23.    L3
  24.     LINENUMBER 9 L3
  25.     GETSTATIC java/lang/System.out : Ljava/io/PrintStream;
  26.     NEW java/lang/StringBuilder
  27.     DUP
  28.     INVOKESPECIAL java/lang/StringBuilder.<init> ()V
  29.     LDC "firstName="
  30.     INVOKEVIRTUAL java/lang/StringBuilder.append (Ljava/lang/String;)Ljava/lang/StringBuilder;
  31.     ALOAD 0
  32.     GETFIELD com/devnn/javalib/JavaInit.firstName : Ljava/lang/String;
  33.     INVOKEVIRTUAL java/lang/StringBuilder.append (Ljava/lang/String;)Ljava/lang/StringBuilder;
  34.     INVOKEVIRTUAL java/lang/StringBuilder.toString ()Ljava/lang/String;
  35.     INVOKEVIRTUAL java/io/PrintStream.println (Ljava/lang/String;)V
  36.    L4
  37.     LINENUMBER 10 L4
  38.     GETSTATIC java/lang/System.out : Ljava/io/PrintStream;
  39.     NEW java/lang/StringBuilder
  40.     DUP
  41.     INVOKESPECIAL java/lang/StringBuilder.<init> ()V
  42.     LDC "secondName="
  43.     INVOKEVIRTUAL java/lang/StringBuilder.append (Ljava/lang/String;)Ljava/lang/StringBuilder;
  44.     ALOAD 1
  45.     INVOKEVIRTUAL java/lang/StringBuilder.append (Ljava/lang/String;)Ljava/lang/StringBuilder;
  46.     INVOKEVIRTUAL java/lang/StringBuilder.toString ()Ljava/lang/String;
  47.     INVOKEVIRTUAL java/io/PrintStream.println (Ljava/lang/String;)V
  48.    L5
  49.     LINENUMBER 11 L5
  50.     RETURN
  51.    L6
  52.     LOCALVARIABLE this Lcom/devnn/javalib/JavaInit; L0 L6 0
  53.     LOCALVARIABLE secondName Ljava/lang/String; L0 L6 1
  54.     MAXSTACK = 3
  55.     MAXLOCALS = 2
  56.   // access flags 0x9
  57.   public static main([Ljava/lang/String;)V
  58.    L0
  59.     LINENUMBER 17 L0
  60.     NEW com/devnn/javalib/JavaInit
  61.     DUP
  62.     LDC "Jobs"
  63.     INVOKESPECIAL com/devnn/javalib/JavaInit.<init> (Ljava/lang/String;)V
  64.     POP
  65.    L1
  66.     LINENUMBER 18 L1
  67.     RETURN
  68.    L2
  69.     LOCALVARIABLE args [Ljava/lang/String; L0 L2 0
  70.     MAXSTACK = 3
  71.     MAXLOCALS = 1
  72. }

可见，Java类的成员变量初始化、构造函数、构造块同样都被拷贝进了字节码init代码块中。Java的成员变量初始化和构造块也是按声明顺序执行。不同的是，Java的构造函数代码始终放在了字节码init代码块的后面。

字节码的init初始化块，其实就是类的真正的构造函数。Kotlin多个init代码块都是按照顺序拷贝进了字节码的init初始化块中，可以理解为它们是构造函数的组成部分。

Java和kotlin成员变量的初始化都是放到字节码的init代码块中，也就是在构造函数中执行的。

到此这篇关于Kotlin字节码层探究构造函数与成员变量和init代码块执行顺序的文章就介绍到这了,更多相关Kotlin构造函数内容请搜索w3xue以前的文章或继续浏览下面的相关文章希望大家以后多多支持w3xue！

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