인스턴스 변수의 초기화 순서

kha212 commented 1 year ago

인스턴스 초기화 블록 후 인스턴스 변수 명시적 초기화시 값 변경됨 해당 소스가 오류없이 되는 이유 확인 필요

class Test { 
 {
    tmp = 2;  
    System.out.println("초기화 블록");
    System.out.println(this.tmp);
  }
  float tmp = 1;
}

class Main {
  public static void main(String[] args) {
    Test t = new Test();
    System.out.println("[Main] t.tmp : " + t.tmp);
    t.tmp = 100;
    System.out.println("[Main] t.tmp : " + t.tmp);
  }
}
[결과]
초기화 블록
2.0
[Main] t.tmp : 1.0
[Main] t.tmp : 100.0

pej4303 commented 1 year ago

링크 참조

URL : https://stackoverflow.com/questions/74580273/initialization-order-of-instance-variables-in-java

pej4303 commented 1 year ago

java 바이너리 코드를 확인해본 결과 순차적으로 코드 실행을 하는 것으로 판단됨.

public class InitializationBlock {
    int tmp = 1;
    {
        tmp = 3;  
        System.out.println("초기화 블록");
        System.out.println(this.tmp);
    }

    public static void main(String[] args) {
        InitializationBlock t = new InitializationBlock();
       System.out.println("[Main] t.tmp : " + t.tmp);
    }
}
[결과]
초기화 블록
3
[Main] t.tmp : 3

iconst_1 -> iconst_3 : 1로 값을 셋팅하고 다시 3으로 값을 셋팅

public InitializationBlock();
Code:
   0: aload_0
   1: invokespecial #1                  // Method java/lang/Object."<init>":()V
   4: aload_0
   5: iconst_1 
   6: putfield      #2                  // Field tmp:I
   9: aload_0
  10: iconst_3
  11: putfield      #2                  // Field tmp:I
  14: getstatic     #3                  // Field java/lang/System.out:Ljava/io/PrintStream;
  17: ldc           #4                  // String 초기화 블록
  19: invokevirtual #5                  // Method java/io/PrintStream.println:(Ljava/lang/String;)V
  22: getstatic     #3                  // Field java/lang/System.out:Ljava/io/PrintStream;
  25: aload_0
  26: getfield      #2                  // Field tmp:I
  29: invokevirtual #6                  // Method java/io/PrintStream.println:(I)V
  32: return

public static void main(java.lang.String[]);
Code:
   0: new           #7                  // class InitializationBlock
   3: dup
   4: invokespecial #8                  // Method "<init>":()V
   7: astore_1
   8: getstatic     #3                  // Field java/lang/System.out:Ljava/io/PrintStream;
  11: new           #9                  // class java/lang/StringBuilder
  14: dup
  15: invokespecial #10                 // Method java/lang/StringBuilder."<init>":()V
  18: ldc           #11                 // String [Main] t.tmp :
  20: invokevirtual #12                 // Method java/lang/StringBuilder.append:(Ljava/lang/String;)Ljava/lang/StringBuilder;
  23: aload_1
  24: getfield      #2                  // Field tmp:I
  27: invokevirtual #13                 // Method java/lang/StringBuilder.append:(I)Ljava/lang/StringBuilder;
  30: invokevirtual #14                 // Method java/lang/StringBuilder.toString:()Ljava/lang/String;
  33: invokevirtual #5                  // Method java/io/PrintStream.println:(Ljava/lang/String;)V
  36: return

public class InitializationBlock {
{
    tmp = 3;  
    System.out.println("초기화 블록");
    System.out.println(this.tmp);
}
int tmp = 1;

public static void main(String[] args) {
    InitializationBlock t = new InitializationBlock();
    System.out.println("[Main] t.tmp : " + t.tmp);
}
}
[결과]
초기화 블록
3
[Main] t.tmp : 1

iconst_3 -> iconst_1 : 3으로 값을 셋팅하고 다시 1로 값을 셋팅

public InitializationBlock();
Code:
   0: aload_0
   1: invokespecial #1                  // Method java/lang/Object."<init>":()V
   4: aload_0
   5: iconst_3
   6: putfield      #2                  // Field tmp:I
   9: getstatic     #3                  // Field java/lang/System.out:Ljava/io/PrintStream;
  12: ldc           #4                  // String 초기화 블록
  14: invokevirtual #5                  // Method java/io/PrintStream.println:(Ljava/lang/String;)V
  17: getstatic     #3                  // Field java/lang/System.out:Ljava/io/PrintStream;
  20: aload_0
  21: getfield      #2                  // Field tmp:I
  24: invokevirtual #6                  // Method java/io/PrintStream.println:(I)V
  27: aload_0
  28: iconst_1
  29: putfield      #2                  // Field tmp:I
  32: return

public static void main(java.lang.String[]);
Code:
   0: new           #7                  // class InitializationBlock
   3: dup
   4: invokespecial #8                  // Method "<init>":()V
   7: astore_1
   8: getstatic     #3                  // Field java/lang/System.out:Ljava/io/PrintStream;
  11: new           #9                  // class java/lang/StringBuilder
  14: dup
  15: invokespecial #10                 // Method java/lang/StringBuilder."<init>":()V
  18: ldc           #11                 // String [Main] t.tmp :
  20: invokevirtual #12                 // Method java/lang/StringBuilder.append:(Ljava/lang/String;)Ljava/lang/StringBuilder;
  23: aload_1
  24: getfield      #2                  // Field tmp:I
  27: invokevirtual #13                 // Method java/lang/StringBuilder.append:(I)Ljava/lang/StringBuilder;
  30: invokevirtual #14                 // Method java/lang/StringBuilder.toString:()Ljava/lang/String;
  33: invokevirtual #5                  // Method java/io/PrintStream.println:(Ljava/lang/String;)V
  36: return

pej4303 / java-study

인스턴스 변수의 초기화 순서 #3