Java boxing 및 unboxing에 대한 자세한 설명
머리말:
boxing 및 unboxing의 개념을 이해하려면 Java 데이터 유형을 이해해야 합니다.
Boxing: 기본 유형은 해당 참조 유형으로 래핑되어 객체의 속성을 제공합니다. Int는 Integer로 패키징되고, float는 Float로 패키징됩니다
Unboxing: boxing과 달리 참조 유형 객체는 값 유형 데이터로 단순화됩니다
Integer a = 100; 这是自动装箱 (编译器调用的是static Integer valueOf(int i)) int b = new Integer(100); 这是自动拆箱
다음 코드를 보세요
m1
public class DataType { public static void main(String args[]) { DataType dt = new DataType(); dt.m11(); dt.m12(); } public void m11() { Integer a = new Integer(100); Integer b = 100; System.out.println("m11 result " + (a == b)); } public void m12() { Integer a = new Integer(128); Integer b = 128; System.out.println("m12 result " + (a == b)); } }
인쇄 결과는 무엇인가요?
m11 result false m12 result false
"==" 주소를 비교한 결과 두 객체 a와 b의 주소가 다르다는 것, 즉 두 객체이므로 둘 다 거짓
javap을 통해 바이트코드를 구문 분석하면 내용은 다음과 같습니다
public void m11(); Code: 0: new #44; //class java/lang/Integer 3: dup 4: bipush 100 6: invokespecial #46; //Method java/lang/Integer."<init>":(I)V 9: astore_1 10: bipush 100 12: invokestatic #49; //Method java/lang/Integer.valueOf:(I)Ljava/lang/In teger; 15: astore_2 16: getstatic #53; //Field java/lang/System.out:Ljava/io/PrintStream; 19: new #59; //class java/lang/StringBuilder 22: dup 23: ldc #61; //String m11 result 25: invokespecial #63; //Method java/lang/StringBuilder."<init>":(Ljava/la ng/String;)V 28: aload_1 29: aload_2 30: if_acmpne 37 33: iconst_1 34: goto 38 37: iconst_0 38: invokevirtual #66; //Method java/lang/StringBuilder.append:(Z)Ljava/la ng/StringBuilder; 41: invokevirtual #70; //Method java/lang/StringBuilder.toString:()Ljava/l ang/String; 44: invokevirtual #74; //Method java/io/PrintStream.println:(Ljava/lang/St ring;)V 47: return public void m12(); Code: 0: new #44; //class java/lang/Integer 3: dup 4: sipush 128 7: invokespecial #46; //Method java/lang/Integer."<init>":(I)V 10: astore_1 11: sipush 128 14: invokestatic #49; //Method java/lang/Integer.valueOf:(I)Ljava/lang/In teger; 17: astore_2 18: getstatic #53; //Field java/lang/System.out:Ljava/io/PrintStream; 21: new #59; //class java/lang/StringBuilder 24: dup 25: ldc #82; //String m12 result 27: invokespecial #63; //Method java/lang/StringBuilder."<init>":(Ljava/la ng/String;)V 30: aload_1 31: aload_2 32: if_acmpne 39 35: iconst_1 36: goto 40 39: iconst_0 40: invokevirtual #66; //Method java/lang/StringBuilder.append:(Z)Ljava/la ng/StringBuilder; 43: invokevirtual #70; //Method java/lang/StringBuilder.toString:()Ljava/l ang/String; 46: invokevirtual #74; //Method java/io/PrintStream.println:(Ljava/lang/St ring;)V 49: return </init></init></init></init>
m2
public class DataType { public static void main(String args[]) { DataType dt = new DataType(); dt.m21(); dt.m22(); } public void m21() { Integer a = new Integer(100); Integer b = new Integer(100); System.out.println("m21 result " + (a == b)); } public void m22() { Integer a = new Integer(128); Integer b = new Integer(128); System.out.println("m22 result " + (a == b)); } }
인쇄된 결과는
m21 result false m22 result false
a와 b는 여전히 두 객체입니다
javap 구문 분석된 콘텐츠
public void m21(); Code: 0: new #44; //class java/lang/Integer 3: dup 4: bipush 100 6: invokespecial #46; //Method java/lang/Integer."<init>":(I)V 9: astore_1 10: new #44; //class java/lang/Integer 13: dup 14: bipush 100 16: invokespecial #46; //Method java/lang/Integer."<init>":(I)V 19: astore_2 20: getstatic #53; //Field java/lang/System.out:Ljava/io/PrintStream; 23: new #59; //class java/lang/StringBuilder 26: dup 27: ldc #84; //String m21 result 29: invokespecial #63; //Method java/lang/StringBuilder."<init>":(Ljava/la ng/String;)V 32: aload_1 33: aload_2 34: if_acmpne 41 37: iconst_1 38: goto 42 41: iconst_0 42: invokevirtual #66; //Method java/lang/StringBuilder.append:(Z)Ljava/la ng/StringBuilder; 45: invokevirtual #70; //Method java/lang/StringBuilder.toString:()Ljava/l ang/String; 48: invokevirtual #74; //Method java/io/PrintStream.println:(Ljava/lang/St ring;)V 51: return public void m22(); Code: 0: new #44; //class java/lang/Integer 3: dup 4: sipush 128 7: invokespecial #46; //Method java/lang/Integer."<init>":(I)V 10: astore_1 11: new #44; //class java/lang/Integer 14: dup 15: sipush 128 18: invokespecial #46; //Method java/lang/Integer."<init>":(I)V 21: astore_2 22: getstatic #53; //Field java/lang/System.out:Ljava/io/PrintStream; 25: new #59; //class java/lang/StringBuilder 28: dup 29: ldc #86; //String m22 result 31: invokespecial #63; //Method java/lang/StringBuilder."<init>":(Ljava/la ng/String;)V 34: aload_1 35: aload_2 36: if_acmpne 43 39: iconst_1 40: goto 44 43: iconst_0 44: invokevirtual #66; //Method java/lang/StringBuilder.append:(Z)Ljava/la ng/StringBuilder; 47: invokevirtual #70; //Method java/lang/StringBuilder.toString:()Ljava/l ang/String; 50: invokevirtual #74; //Method java/io/PrintStream.println:(Ljava/lang/St ring;)V 53: return
m3
public class DataType { public static void main(String args[]) { DataType dt = new DataType(); dt.m31(); dt.m32(); } public void m31() { Integer a = 100; Integer b = 100; System.out.println("m31 result " + (a == b)); } public void m32() { Integer a = 128; Integer b = 128; System.out.println("m32 result " + (a == b)); } }
인쇄 결과
m31 result true m32 result false
왜 첫 번째는 참이고 두 번째는 거짓인가요? javap
javap parsed content
public void m31(); Code: 0: bipush 100 2: invokestatic #49; //Method java/lang/Integer.valueOf:(I)Ljava/lang/In teger; 5: astore_1 6: bipush 100 8: invokestatic #49; //Method java/lang/Integer.valueOf:(I)Ljava/lang/In teger; 11: astore_2 12: getstatic #53; //Field java/lang/System.out:Ljava/io/PrintStream; 15: new #59; //class java/lang/StringBuilder 18: dup 19: ldc #88; //String m31 result 21: invokespecial #63; //Method java/lang/StringBuilder."<init>":(Ljava/la ng/String;)V 24: aload_1 25: aload_2 26: if_acmpne 33 29: iconst_1 30: goto 34 33: iconst_0 34: invokevirtual #66; //Method java/lang/StringBuilder.append:(Z)Ljava/la ng/StringBuilder; 37: invokevirtual #70; //Method java/lang/StringBuilder.toString:()Ljava/l ang/String; 40: invokevirtual #74; //Method java/io/PrintStream.println:(Ljava/lang/St ring;)V 43: return public void m32(); Code: 0: sipush 128 3: invokestatic #49; //Method java/lang/Integer.valueOf:(I)Ljava/lang/In teger; 6: astore_1 7: sipush 128 10: invokestatic #49; //Method java/lang/Integer.valueOf:(I)Ljava/lang/In teger; 13: astore_2 14: getstatic #53; //Field java/lang/System.out:Ljava/io/PrintStream; 17: new #59; //class java/lang/StringBuilder 20: dup 21: ldc #90; //String m32 result 23: invokespecial #63; //Method java/lang/StringBuilder."<init>":(Ljava/la ng/String;)V 26: aload_1 27: aload_2 28: if_acmpne 35 31: iconst_1 32: goto 36 35: iconst_0 36: invokevirtual #66; //Method java/lang/StringBuilder.append:(Z)Ljava/la ng/StringBuilder; 39: invokevirtual #70; //Method java/lang/StringBuilder.toString:()Ljava/l ang/String; 42: invokevirtual #74; //Method java/io/PrintStream.println:(Ljava/lang/St ring;)V 45: return
m4
public class DataType { public static void main(String args[]) { DataType dt = new DataType(); dt.m41(); dt.m42(); } public void m41() { Integer a = Integer.valueOf(100); Integer b = 100; System.out.println("m41 result " + (a == b)); } public void m42() { Integer a = Integer.valueOf(128); Integer b = 128; System.out.println("m42 result " + (a == b)); } }
결과 인쇄
로 파싱된 데이터 관찰
m41 result true m42 result falsejavap 분석 콘텐츠
public void m41(); Code: 0: bipush 100 2: invokestatic #49; //Method java/lang/Integer.valueOf:(I)Ljava/lang/In teger; 5: astore_1 6: bipush 100 8: invokestatic #49; //Method java/lang/Integer.valueOf:(I)Ljava/lang/In teger; 11: astore_2 12: getstatic #53; //Field java/lang/System.out:Ljava/io/PrintStream; 15: new #59; //class java/lang/StringBuilder 18: dup 19: ldc #92; //String m41 result 21: invokespecial #63; //Method java/lang/StringBuilder."<init>":(Ljava/la ng/String;)V 24: aload_1 25: aload_2 26: if_acmpne 33 29: iconst_1 30: goto 34 33: iconst_0 34: invokevirtual #66; //Method java/lang/StringBuilder.append:(Z)Ljava/la ng/StringBuilder; 37: invokevirtual #70; //Method java/lang/StringBuilder.toString:()Ljava/l ang/String; 40: invokevirtual #74; //Method java/io/PrintStream.println:(Ljava/lang/St ring;)V 43: return public void m42(); Code: 0: sipush 128 3: invokestatic #49; //Method java/lang/Integer.valueOf:(I)Ljava/lang/In teger; 6: astore_1 7: sipush 128 10: invokestatic #49; //Method java/lang/Integer.valueOf:(I)Ljava/lang/In teger; 13: astore_2 14: getstatic #53; //Field java/lang/System.out:Ljava/io/PrintStream; 17: new #59; //class java/lang/StringBuilder 20: dup 21: ldc #94; //String m42 result 23: invokespecial #63; //Method java/lang/StringBuilder."<init>":(Ljava/la ng/String;)V 26: aload_1 27: aload_2 28: if_acmpne 35 31: iconst_1 32: goto 36 35: iconst_0 36: invokevirtual #66; //Method java/lang/StringBuilder.append:(Z)Ljava/la ng/StringBuilder; 39: invokevirtual #70; //Method java/lang/StringBuilder.toString:()Ljava/l ang/String; 42: invokevirtual #74; //Method java/io/PrintStream.println:(Ljava/lang/St ring;)V 45: return }분석
javap은 Java와 함께 제공됩니다. Java 컴파일러에서 생성된 바이트코드를 볼 수 있습니다(위 코드는 javap -c DataType만 사용함). 구체적인 사용 방법은 Google에서 확인하세요. 먼저 m4를 보면, 실행 결과에 "true"가 나타나는 이유는 무엇입니까? True는 a와 b가 동일한 개체라는 것을 의미합니다. 그런데 객체 a는 Integer.valueOf()를 호출하여 생성되고, 객체 b는 자동 박싱에 의해 생성되는 이유는 무엇입니까? 바이트코드를 다시 살펴보겠습니다. 결국 Java 프로그램은 바이트코드를 실행하기 위해 가상 머신에 의존하여 구현됩니다. m41 이 메서드는 valueOf()를 한 번만 적용하지만 바이트코드에 두 번 나타나며 이는 autoboxing 중에 valueOf()도 호출된다는 것을 나타냅니다. 다음은 valueOf()의 구체적인 구현입니다.
/** * Returns a <tt>Integer</tt> instance representing the specified * <tt>int</tt> value. * If a new <tt>Integer</tt> instance is not required, this method * should generally be used in preference to the constructor * {@link #Integer(int)}, as this method is likely to yield * significantly better space and time performance by caching * frequently requested values. * * @param i an <code>int</code> value. * @return a <tt>Integer</tt> instance representing <tt>i</tt>. * @since 1.5 */ public static Integer valueOf(int i) { final int offset = 128; if (i >= -128 && i <= 127) { // must cache return IntegerCache.cache[i + offset]; } return new Integer(i); }[-128, 127] 사이의 숫자, valueOf는 캐시 객체를 반환합니다. 이므로 두 호출은 동일한 객체를 반환합니다. 읽어주셔서 감사합니다. 도움이 되기를 바랍니다. 이 사이트를 지원해 주셔서 감사합니다! 더 많은 Java boxing 및 unboxing 자세한 설명과 예제 코드 관련 기사를 보려면 PHP 중국어 웹사이트를 주목하세요!