Kotlin构造函数与成员变量和init代码块执行顺序详细讲解

来源：jb51　　时间：2022/11/17 8:48:40　　对本文有异议

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}")
    }
}

官方示例仅仅是展示了打印结果，没有进行深入剖析。

下面围绕下面几个问题进行剖析。

为什么成员变量可以直接使用构造函数的参数？

成员变量和init初始化器执行先后顺序是什么？

构造函数与init初始化器执行先后顺序是什么？

以上代码，调用InitOrderDemo(“hello”)，打印结果如下：

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。

那么为什么会这样呢？将这段代码转化成Java代码就会一目了然了。

将kotlin转成Java代码的操作步骤：

打开某个Kotlin类 -> 点击android studio菜单栏上的tools- > kotlin -> Decompile kotlin to java

转化成Java代码如下：

import kotlin.Metadata;
import kotlin.Unit;
import kotlin.jvm.internal.Intrinsics;
import org.jetbrains.annotations.NotNull;
public final class InitOrderDemo {
   @NotNull
   private final String firstProperty;
   @NotNull
   private final String secondProperty;
   @NotNull
   public final String getFirstProperty() {
      return this.firstProperty;
   }
   @NotNull
   public final String getSecondProperty() {
      return this.secondProperty;
   }
   public InitOrderDemo(@NotNull String name) {
      Intrinsics.checkNotNullParameter(name, "name");
      super();
      String var2 = "First property: " + name;
      boolean var3 = false;
      boolean var4 = false;
      int var6 = false;
      boolean var7 = false;
      System.out.println(var2);
      Unit var9 = Unit.INSTANCE;
      this.firstProperty = var2;
      var2 = "First initializer block that prints " + name;
      var3 = false;
      System.out.println(var2);
      var2 = "Second property: " + name.length();
      var3 = false;
      var4 = false;
      var6 = false;
      var7 = false;
      System.out.println(var2);
      var9 = Unit.INSTANCE;
      this.secondProperty = var2;
      var2 = "Second initializer block that prints " + name.length();
      var3 = false;
      System.out.println(var2);
   }
}

可以看到，Kotlin的成员变量初始化是放在构造函数当中的，init代码块也是"拷贝"到了构造函数当中，并且是按照声明顺序"拷贝"的，所以它们都是构造函数的一部分。所以kotlin跟Java还是不一样的，Java成员变量是优先于构造函数初始化，Kotlin按照声明顺序初始化。

而且可以看到Kotlin用val声明的只读变量在Java里只是加一个final关键字，生成了get方法没有set方法。

下面将以上kotlin代码略微改一下，给主构造函数参数加上默认值看看会怎么样，将val改成var变量看看有啥不同，并且添加一个次构造函数看看代码执行顺序：

class InitOrderDemo(name:String="test") {
    val firstProperty = "First property: $name".also(::println)
    init {
        println("First initializer block that prints ${name}")
    }
    var secondProperty = "Second property: ${name.length}".also(::println)
    init {
        println("Second initializer block that prints ${name.length}")
    }
    constructor(name:String,age:Int):this(name){
        println("Sub constructor block that prints age= ${age}")
    }
}

转化成Java代码如下：

public final class InitOrderDemo {
   @NotNull
   private final String firstProperty;
   @NotNull
   private String secondProperty;
   @NotNull
   public final String getFirstProperty() {
      return this.firstProperty;
   }
   @NotNull
   public final String getSecondProperty() {
      return this.secondProperty;
   }
   public final void setSecondProperty(@NotNull String var1) {
      Intrinsics.checkNotNullParameter(var1, "<set-?>");
      this.secondProperty = var1;
   }
   public InitOrderDemo(@NotNull String name) {
      Intrinsics.checkNotNullParameter(name, "name");
      super();
      String var2 = "First property: " + name;
      boolean var3 = false;
      boolean var4 = false;
      int var6 = false;
      boolean var7 = false;
      System.out.println(var2);
      Unit var9 = Unit.INSTANCE;
      this.firstProperty = var2;
      var2 = "First initializer block that prints " + name;
      var3 = false;
      System.out.println(var2);
      var2 = "Second property: " + name.length();
      var3 = false;
      var4 = false;
      var6 = false;
      var7 = false;
      System.out.println(var2);
      var9 = Unit.INSTANCE;
      this.secondProperty = var2;
      var2 = "Second initializer block that prints " + name.length();
      var3 = false;
      System.out.println(var2);
   }
   // $FF: synthetic method
   public InitOrderDemo(String var1, int var2, DefaultConstructorMarker var3) {
      if ((var2 & 1) != 0) {
         var1 = "test";
      }
 
      this(var1);
   }
   public InitOrderDemo() {
      this((String)null, 1, (DefaultConstructorMarker)null);
   }
   public InitOrderDemo(@NotNull String name, int age) {
      Intrinsics.checkNotNullParameter(name, "name");
      this(name);
      String var3 = "Sub constructor block that prints age= " + age;
      boolean var4 = false;
      System.out.println(var3);
   }
}