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
devnn 人气:0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。
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