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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

作者：devnn

这篇文章主要介绍了字节码层Kotlin构造函数与成员变量和init代码块执行顺序，kotlin里面的构造函数分为主构造函数和次构造函数。主构造函数只能有一个，次构造函数个数不限制，可以有一个或者多个

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：

class InitOrderDemo(name: String) {
    val firstProperty = "First property: $name".also(::println)
    init {
        println("First initializer block that prints ${name}")
    }
    val secondProperty = "Second property: ${name.length}".also(::println)
    init {
        println("Second initializer block that prints ${name.length}")
    }
}

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：

// ================com/devnn/javalib/InitOrderDemo.class =================
// class version 52.0 (52)
// access flags 0x31
public final class com/devnn/javalib/InitOrderDemo {
  // access flags 0x12
  private final Ljava/lang/String; firstProperty
  @Lorg/jetbrains/annotations/NotNull;() // invisible
  // access flags 0x11
  public final getFirstProperty()Ljava/lang/String;
  @Lorg/jetbrains/annotations/NotNull;() // invisible
   L0
    LINENUMBER 4 L0
    ALOAD 0
    GETFIELD com/devnn/javalib/InitOrderDemo.firstProperty : Ljava/lang/String;
    ARETURN
   L1
    LOCALVARIABLE this Lcom/devnn/javalib/InitOrderDemo; L0 L1 0
    MAXSTACK = 1
    MAXLOCALS = 1
  // access flags 0x12
  private final Ljava/lang/String; secondProperty
  @Lorg/jetbrains/annotations/NotNull;() // invisible
  // access flags 0x11
  public final getSecondProperty()Ljava/lang/String;
  @Lorg/jetbrains/annotations/NotNull;() // invisible
   L0
    LINENUMBER 10 L0
    ALOAD 0
    GETFIELD com/devnn/javalib/InitOrderDemo.secondProperty : Ljava/lang/String;
    ARETURN
   L1
    LOCALVARIABLE this Lcom/devnn/javalib/InitOrderDemo; L0 L1 0
    MAXSTACK = 1
    MAXLOCALS = 1
  // access flags 0x1
  public <init>(Ljava/lang/String;)V
    // annotable parameter count: 1 (visible)
    // annotable parameter count: 1 (invisible)
    @Lorg/jetbrains/annotations/NotNull;() // invisible, parameter 0
   L0
    ALOAD 1
    LDC "name"
    INVOKESTATIC kotlin/jvm/internal/Intrinsics.checkNotNullParameter (Ljava/lang/Object;Ljava/lang/String;)V
   L1
    LINENUMBER 3 L1
    ALOAD 0
    INVOKESPECIAL java/lang/Object.<init> ()V
   L2
    LINENUMBER 4 L2
    ALOAD 0
    NEW java/lang/StringBuilder
    DUP
    INVOKESPECIAL java/lang/StringBuilder.<init> ()V
    LDC "First property: "
    INVOKEVIRTUAL java/lang/StringBuilder.append (Ljava/lang/String;)Ljava/lang/StringBuilder;
    ALOAD 1
    INVOKEVIRTUAL java/lang/StringBuilder.append (Ljava/lang/String;)Ljava/lang/StringBuilder;
    INVOKEVIRTUAL java/lang/StringBuilder.toString ()Ljava/lang/String;
    ASTORE 2
   L3
    ALOAD 2
    ASTORE 3
   L4
    LINENUMBER 17 L4
    ASTORE 5
   L5
    ICONST_0
    ISTORE 4
   L6
    LINENUMBER 4 L6
   L7
    GETSTATIC java/lang/System.out : Ljava/io/PrintStream;
    ALOAD 3
    INVOKEVIRTUAL java/io/PrintStream.println (Ljava/lang/Object;)V
   L8
   L9
   L10
    GETSTATIC kotlin/Unit.INSTANCE : Lkotlin/Unit;
    ASTORE 6
    ALOAD 5
   L11
    LINENUMBER 4 L11
   L12
    ALOAD 2
   L13
    PUTFIELD com/devnn/javalib/InitOrderDemo.firstProperty : Ljava/lang/String;
   L14
    LINENUMBER 6 L14
    NOP
   L15
    LINENUMBER 7 L15
    NEW java/lang/StringBuilder
    DUP
    INVOKESPECIAL java/lang/StringBuilder.<init> ()V
    LDC "First initializer block that prints "
    INVOKEVIRTUAL java/lang/StringBuilder.append (Ljava/lang/String;)Ljava/lang/StringBuilder;
    ALOAD 1
    INVOKEVIRTUAL java/lang/StringBuilder.append (Ljava/lang/String;)Ljava/lang/StringBuilder;
    INVOKEVIRTUAL java/lang/StringBuilder.toString ()Ljava/lang/String;
    ASTORE 2
   L16
    GETSTATIC java/lang/System.out : Ljava/io/PrintStream;
    ALOAD 2
    INVOKEVIRTUAL java/io/PrintStream.println (Ljava/lang/Object;)V
   L17
   L18
    LINENUMBER 8 L18
    NOP
   L19
    LINENUMBER 10 L19
    ALOAD 0
    NEW java/lang/StringBuilder
    DUP
    INVOKESPECIAL java/lang/StringBuilder.<init> ()V
    LDC "Second property: "
    INVOKEVIRTUAL java/lang/StringBuilder.append (Ljava/lang/String;)Ljava/lang/StringBuilder;
    ALOAD 1
    INVOKEVIRTUAL java/lang/String.length ()I
    INVOKEVIRTUAL java/lang/StringBuilder.append (I)Ljava/lang/StringBuilder;
    INVOKEVIRTUAL java/lang/StringBuilder.toString ()Ljava/lang/String;
    ASTORE 2
   L20
    ALOAD 2
    ASTORE 3
   L21
    LINENUMBER 17 L21
    ASTORE 5
   L22
    ICONST_0
    ISTORE 4
   L23
    LINENUMBER 10 L23
   L24
    GETSTATIC java/lang/System.out : Ljava/io/PrintStream;
    ALOAD 3
    INVOKEVIRTUAL java/io/PrintStream.println (Ljava/lang/Object;)V
   L25
   L26
   L27
    GETSTATIC kotlin/Unit.INSTANCE : Lkotlin/Unit;
    ASTORE 6
    ALOAD 5
   L28
    LINENUMBER 10 L28
   L29
    ALOAD 2
   L30
    PUTFIELD com/devnn/javalib/InitOrderDemo.secondProperty : Ljava/lang/String;
   L31
    LINENUMBER 12 L31
    NOP
   L32
    LINENUMBER 13 L32
    NEW java/lang/StringBuilder
    DUP
    INVOKESPECIAL java/lang/StringBuilder.<init> ()V
    LDC "Second initializer block that prints "
    INVOKEVIRTUAL java/lang/StringBuilder.append (Ljava/lang/String;)Ljava/lang/StringBuilder;
    ALOAD 1
    INVOKEVIRTUAL java/lang/String.length ()I
    INVOKEVIRTUAL java/lang/StringBuilder.append (I)Ljava/lang/StringBuilder;
    INVOKEVIRTUAL java/lang/StringBuilder.toString ()Ljava/lang/String;
    ASTORE 2
   L33
    GETSTATIC java/lang/System.out : Ljava/io/PrintStream;
    ALOAD 2
    INVOKEVIRTUAL java/io/PrintStream.println (Ljava/lang/Object;)V
   L34
   L35
    LINENUMBER 14 L35
    RETURN
   L36
    LOCALVARIABLE p1 Ljava/lang/Object; L5 L10 3
    LOCALVARIABLE $i$a$-unknown-InitOrderDemo$firstProperty$1 I L6 L10 4
    LOCALVARIABLE p1 Ljava/lang/Object; L22 L27 3
    LOCALVARIABLE $i$a$-unknown-InitOrderDemo$secondProperty$1 I L23 L27 4
    LOCALVARIABLE this Lcom/devnn/javalib/InitOrderDemo; L0 L36 0
    LOCALVARIABLE name Ljava/lang/String; L0 L36 1
    MAXSTACK = 3
    MAXLOCALS = 7
}

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。

package com.devnn.javalib;
public class JavaInit {
    String firstName = "Steven";
    {
        System.out.println("This is init block");
    }
    JavaInit(String secondName) {
        System.out.println("firstName=" + firstName);
        System.out.println("secondName=" + secondName);
    }
    public static void main(String[] args) {
        new JavaInit("Jobs");
    }
}

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：

// class version 51.0 (51)
// access flags 0x21
public class com/devnn/javalib/JavaInit {
  // compiled from: JavaInit.java
  // access flags 0x0
  Ljava/lang/String; firstName
  // access flags 0x0
  <init>(Ljava/lang/String;)V
   L0
    LINENUMBER 10 L0
    ALOAD 0
    INVOKESPECIAL java/lang/Object.<init> ()V
   L1
    LINENUMBER 4 L1
    ALOAD 0
    LDC "Steven"
    PUTFIELD com/devnn/javalib/JavaInit.firstName : Ljava/lang/String;
   L2
    LINENUMBER 7 L2
    GETSTATIC java/lang/System.out : Ljava/io/PrintStream;
    LDC "This is init block"
    INVOKEVIRTUAL java/io/PrintStream.println (Ljava/lang/String;)V
   L3
    LINENUMBER 11 L3
    GETSTATIC java/lang/System.out : Ljava/io/PrintStream;
    NEW java/lang/StringBuilder
    DUP
    INVOKESPECIAL java/lang/StringBuilder.<init> ()V
    LDC "firstName="
    INVOKEVIRTUAL java/lang/StringBuilder.append (Ljava/lang/String;)Ljava/lang/StringBuilder;
    ALOAD 0
    GETFIELD com/devnn/javalib/JavaInit.firstName : Ljava/lang/String;
    INVOKEVIRTUAL java/lang/StringBuilder.append (Ljava/lang/String;)Ljava/lang/StringBuilder;
    INVOKEVIRTUAL java/lang/StringBuilder.toString ()Ljava/lang/String;
    INVOKEVIRTUAL java/io/PrintStream.println (Ljava/lang/String;)V
   L4
    LINENUMBER 12 L4
    GETSTATIC java/lang/System.out : Ljava/io/PrintStream;
    NEW java/lang/StringBuilder
    DUP
    INVOKESPECIAL java/lang/StringBuilder.<init> ()V
    LDC "secondName="
    INVOKEVIRTUAL java/lang/StringBuilder.append (Ljava/lang/String;)Ljava/lang/StringBuilder;
    ALOAD 1
    INVOKEVIRTUAL java/lang/StringBuilder.append (Ljava/lang/String;)Ljava/lang/StringBuilder;
    INVOKEVIRTUAL java/lang/StringBuilder.toString ()Ljava/lang/String;
    INVOKEVIRTUAL java/io/PrintStream.println (Ljava/lang/String;)V
   L5
    LINENUMBER 13 L5
    RETURN
   L6
    LOCALVARIABLE this Lcom/devnn/javalib/JavaInit; L0 L6 0
    LOCALVARIABLE secondName Ljava/lang/String; L0 L6 1
    MAXSTACK = 3
    MAXLOCALS = 2
  // access flags 0x9
  public static main([Ljava/lang/String;)V
   L0
    LINENUMBER 16 L0
    NEW com/devnn/javalib/JavaInit
    DUP
    LDC "Jobs"
    INVOKESPECIAL com/devnn/javalib/JavaInit.<init> (Ljava/lang/String;)V
    POP
   L1
    LINENUMBER 17 L1
    RETURN
   L2
    LOCALVARIABLE args [Ljava/lang/String; L0 L2 0
    MAXSTACK = 3
    MAXLOCALS = 1
}

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：

package com.devnn.javalib;
public class JavaInit {
    {
        System.out.println("This is init block");
    }
    JavaInit(String secondName) {
        System.out.println("firstName=" + firstName);
        System.out.println("secondName=" + secondName);
    }
    String firstName = "Steven";
    public static void main(String[] args) {
        new JavaInit("Jobs");
    }
}

查看字节码：

// class version 51.0 (51)
// access flags 0x21
public class com/devnn/javalib/JavaInit {
  // compiled from: JavaInit.java
  // access flags 0x0
  Ljava/lang/String; firstName
  // access flags 0x0
  <init>(Ljava/lang/String;)V
   L0
    LINENUMBER 8 L0
    ALOAD 0
    INVOKESPECIAL java/lang/Object.<init> ()V
   L1
    LINENUMBER 5 L1
    GETSTATIC java/lang/System.out : Ljava/io/PrintStream;
    LDC "This is init block"
    INVOKEVIRTUAL java/io/PrintStream.println (Ljava/lang/String;)V
   L2
    LINENUMBER 13 L2
    ALOAD 0
    LDC "Steven"
    PUTFIELD com/devnn/javalib/JavaInit.firstName : Ljava/lang/String;
   L3
    LINENUMBER 9 L3
    GETSTATIC java/lang/System.out : Ljava/io/PrintStream;
    NEW java/lang/StringBuilder
    DUP
    INVOKESPECIAL java/lang/StringBuilder.<init> ()V
    LDC "firstName="
    INVOKEVIRTUAL java/lang/StringBuilder.append (Ljava/lang/String;)Ljava/lang/StringBuilder;
    ALOAD 0
    GETFIELD com/devnn/javalib/JavaInit.firstName : Ljava/lang/String;
    INVOKEVIRTUAL java/lang/StringBuilder.append (Ljava/lang/String;)Ljava/lang/StringBuilder;
    INVOKEVIRTUAL java/lang/StringBuilder.toString ()Ljava/lang/String;
    INVOKEVIRTUAL java/io/PrintStream.println (Ljava/lang/String;)V
   L4
    LINENUMBER 10 L4
    GETSTATIC java/lang/System.out : Ljava/io/PrintStream;
    NEW java/lang/StringBuilder
    DUP
    INVOKESPECIAL java/lang/StringBuilder.<init> ()V
    LDC "secondName="
    INVOKEVIRTUAL java/lang/StringBuilder.append (Ljava/lang/String;)Ljava/lang/StringBuilder;
    ALOAD 1
    INVOKEVIRTUAL java/lang/StringBuilder.append (Ljava/lang/String;)Ljava/lang/StringBuilder;
    INVOKEVIRTUAL java/lang/StringBuilder.toString ()Ljava/lang/String;
    INVOKEVIRTUAL java/io/PrintStream.println (Ljava/lang/String;)V
   L5
    LINENUMBER 11 L5
    RETURN
   L6
    LOCALVARIABLE this Lcom/devnn/javalib/JavaInit; L0 L6 0
    LOCALVARIABLE secondName Ljava/lang/String; L0 L6 1
    MAXSTACK = 3
    MAXLOCALS = 2
  // access flags 0x9
  public static main([Ljava/lang/String;)V
   L0
    LINENUMBER 17 L0
    NEW com/devnn/javalib/JavaInit
    DUP
    LDC "Jobs"
    INVOKESPECIAL com/devnn/javalib/JavaInit.<init> (Ljava/lang/String;)V
    POP
   L1
    LINENUMBER 18 L1
    RETURN
   L2
    LOCALVARIABLE args [Ljava/lang/String; L0 L2 0
    MAXSTACK = 3
    MAXLOCALS = 1
}

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脚本之家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脚本之家！

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