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Kotlin字節(jié)碼層探究構造函數(shù)與成員變量和init代碼塊執(zhí)行順序_Android

作者：devnn ? 更新時間： 2022-12-12 編程語言

之前寫了一篇文章，從Java語法的角度分析了Kotlin構造函數(shù)、成員變量初始化、init代碼塊三者的執(zhí)行順序：

Kotlin構造函數(shù)與成員變量和init代碼塊執(zhí)行順序詳細講解

這次再從字節(jié)碼的角度分析它們的執(zhí)行順序。

還是用之前那個例子：

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}")
    }
}

調用InitOrderDemo(“hello”)打印的結果如下：

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

可以看到執(zhí)行順序，是按照它們聲明的順序執(zhí)行。

將上面Koltin代碼轉成字節(jié)碼之后，顯示內(nèi)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
}

可以看到上面的構造函數(shù)、成員變量初始化和init代碼塊，按照聲明都被放到了字節(jié)碼的init代碼塊中了。

字節(jié)碼的init初始化器其實就是類的構造函數(shù)。將Java代碼轉成字節(jié)碼也是存在init構造函數(shù)。

下面看一個Java示例，加深對字節(jié)碼的init初始化塊的認識。

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");
    }
}

運行main函數(shù)打印結果如下：

This is init block
firstName=Steven
secondName=Jobs

將上面的JavaInit類轉成字節(jié)碼之后的內(nèi)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
}

可見，Java類的成員變量初始化、構造函數(shù)、構造塊同樣都被拷貝進了init代碼塊中。那么它們是否存在順序問題呢？

將上面JavaInit類的firname成員變量放到初始化塊下面試試：

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");
    }
}

查看字節(jié)碼：

// 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
}

可見，Java類的成員變量初始化、構造函數(shù)、構造塊同樣都被拷貝進了字節(jié)碼init代碼塊中。Java的成員變量初始化和構造塊也是按聲明順序執(zhí)行。不同的是，Java的構造函數(shù)代碼始終放在了字節(jié)碼init代碼塊的后面。

字節(jié)碼的init初始化塊，其實就是類的真正的構造函數(shù)。Kotlin多個init代碼塊都是按照順序拷貝進了字節(jié)碼的init初始化塊中，可以理解為它們是構造函數(shù)的組成部分。

Java和kotlin成員變量的初始化都是放到字節(jié)碼的init代碼塊中，也就是在構造函數(shù)中執(zhí)行的。

原文鏈接：https://blog.csdn.net/devnn/article/details/127620245

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Kotlin字節(jié)碼層探究構造函數(shù)與成員變量和init代碼塊執(zhí)行順序_Android

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