为什么实例初始化块在构造函数之前执行 [英] Why are instance initialization blocks executed before constructors

问题描述

我知道静态块是在加载类时初始化的，并且由于该类在程序中仅加载一次，因此它们仅被初始化一次。

I know that static blocks are initialized at the time the class is loaded and since the class is loaded only once in a program,they are initialized only once.

每次创建该类的实例时都会初始化IIB（实例初始化块），构造函数也是如此：它们在对象创建期间执行。

IIB (Instance initialization blocks) are initialized every time an instance of the class is made, and the same for constructors: they are executed during object creation.

我不了解为什么在下面的程序IIB中优先于构造函数执行。
代码-

I don't understand why in the below program IIB is executed in prior to Constructors. Code-

public class Hello {

    public static void main(String args[]) {
        C obj = new C();
    }
}

class A {

    static {
        System.out.println("Inside static block of A");
    }

    {
        System.out.println("Inside IIB of A");
    }

    A() {
        System.out.println("Inside NO-ARGS constructor of A");
    }
}

class B extends A {

    static {
        System.out.println("Inside static block of B");
    }

    {
        System.out.println("Inside IIB of B");
    }

    B() {
        System.out.println("Inside NO-ARGS constructor of B");
    }
}

class C extends B {

    static {
        System.out.println("Inside static block of C");
    }

    {
        System.out.println("Inside IIB of C");
    }

    C() {
        System.out.println("Inside NO-ARGS constructor of C");
    }
}

为什么与构造函数相比，首先执行IIB？ p>

Why IIB is executed first compared to constructors?

推荐答案

Java编译器注入初始化程序块在您的构造函数的开头（在调用超级构造函数之后）。为了使您更好地理解，我编译了以下类

Java compiler injects initializer blocks at the beginning of your constructors (after calling a superconstructor). To give you a better understanding I've compiled the following class

public class Foo extends SuperFoo {
    private String foo1 = "hello";
    private String foo2;
    private String foo3;
    {
        foo2 = "world";
    }
    public Foo() {
        foo3 = "!!!";
    }
}

并通过 javap 反编译器：

and run it through a javap decompiler:

Compiled from "Foo.java"
public class Foo extends SuperFoo {
  private java.lang.String foo1;
  private java.lang.String foo2;
  private java.lang.String foo3;
  public Foo();
    Code:
       0: aload_0
       1: invokespecial #12                 // Method SuperFoo."<init>":()V
       4: aload_0
       5: ldc           #14                 // String hello
       7: putfield      #16                 // Field foo1:Ljava/lang/String;
      10: aload_0
      11: ldc           #18                 // String world
      13: putfield      #20                 // Field foo2:Ljava/lang/String;
      16: aload_0
      17: ldc           #22                 // String !!!
      19: putfield      #24                 // Field foo3:Ljava/lang/String;
      22: return
}

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