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

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：

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 (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. ()V L2 LINENUMBER 4 L2 ALOAD 0 NEW java/lang/StringBuilder DUP INVOKESPECIAL java/lang/StringBuilder. ()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. ()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. ()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. ()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 (Ljava/lang/String;)V L0 LINENUMBER 10 L0 ALOAD 0 INVOKESPECIAL java/lang/Object. ()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. ()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. ()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. (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 (Ljava/lang/String;)V L0 LINENUMBER 8 L0 ALOAD 0 INVOKESPECIAL java/lang/Object. ()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. ()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. ()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. (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。

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