Java auto boxing의 컴파일 방식과 Integer 캐시

public class Boxing {
    public static void main(String[] args) {
        int a = 1;
        Integer b = 1;
        int c = b;
        System.out.println(a == b);
        System.out.println(a == c);
    }
}

  public static void main(java.lang.String[]);
    Code:
         0: iconst_1
         1: istore_1
         2: iconst_1
         3: invokestatic  #7    // Method java/lang/Integer.valueOf:(I)Ljava/lang/Integer;
         6: astore_2
         7: aload_2
         8: invokevirtual #13   // Method java/lang/Integer.intValue:()I
        11: istore_3
        12: getstatic     #17   // Field java/lang/System.out:Ljava/io/PrintStream;
        15: iload_1
        16: aload_2
        17: invokevirtual #13   // Method java/lang/Integer.intValue:()I
        20: if_icmpne     27
        ...

public class Boxing {
    public static void main(String[] stringArray) {
        int n = 1;
        Integer n2 = Integer.valueOf((int)1);
        int n3 = n2.intValue();
        System.out.println((n == n2.intValue() ? 1 : 0) != 0);
        System.out.println((n == n3 ? 1 : 0) != 0);
    }
}

public class Boxing {
    public static void main(String[] stringArray) {
        int n = 1;
        Integer n2 = 1;
        int n3 = n2;
        System.out.println(n == n2);
        System.out.println(n == n3);
    }
}

    private static final class IntegerCache {
        static final int low = -128;
        static final int high;

        @Stable
        static final Integer[] cache;
        static Integer[] archivedCache;

        static {
            // high value may be configured by property
            int h = 127;
            String integerCacheHighPropValue =
                VM.getSavedProperty("java.lang.Integer.IntegerCache.high");
            ...
        }
    }

    public static Integer valueOf(int i) {
        if (i >= IntegerCache.low && i <= IntegerCache.high)
            return IntegerCache.cache[i + (-IntegerCache.low)];
        return new Integer(i);
    }

public class Identity {
    public static void main(String[] args) {
        Integer a = 127;
        Integer b = 127;
        Integer c = 128;
        Integer d = 128;
        System.out.println("a == b : " + (a == b));
        System.out.println("c == d : " + (c == d));
        System.out.println("c.equals(d) : " + c.equals(d));
    }
}

a == b : true
c == d : false
c.equals(d) : true

M P Bx: Method study.Test.testInteger() invokes inefficient new Integer(int) constructor;
        use Integer.valueOf(int) instead

$ javac -Xlint:deprecation OldStyle.java
OldStyle.java:3: warning: [deprecation] Integer(int) in Integer has been deprecated
        Integer c = new Integer(1);
                    ^
OldStyle.java:4: warning: [deprecation] Double(double) in Double has been deprecated
        Double d = new Double(1.0);
                   ^
2 warnings

    /**
     * @deprecated
     * It is rarely appropriate to use this constructor. The static factory
     * {@link #valueOf(int)} is generally a better choice, as it is
     * likely to yield significantly better space and time performance.
     */
    @Deprecated(since="9")
    public Integer(int value) {

static long boxed() {
    Long sum = 0L;
    for (long i = 0; i < Integer.MAX_VALUE; i++) {
        sum += i;
    }
    return sum;
}

    static long boxed() {
        Long l = Long.valueOf((long)0L);
        for (long i = 0L; i < Integer.MAX_VALUE; ++i) {
            l = Long.valueOf((long)(l.longValue() + i));
        }
        return l.longValue();
    }

  static long boxed();
    Code:
         0: lconst_0
         1: invokestatic  #7    // Method java/lang/Long.valueOf:(J)Ljava/lang/Long;
         4: astore_0
         5: lconst_0
         6: lstore_1
         7: lload_1
         8: ldc2_w        #15   // long 2147483647l
        11: lcmp
        12: ifge          32
        15: aload_0
        16: invokevirtual #17   // Method java/lang/Long.longValue:()J
        19: lload_1
        20: ladd
        21: invokestatic  #7    // Method java/lang/Long.valueOf:(J)Ljava/lang/Long;
        24: astore_0
        25: lload_1
        26: lconst_1
        27: ladd
        28: lstore_1
        29: goto          7

Long: 5122 ms, long: 507 ms
Long: 4987 ms, long: 516 ms
Long: 4919 ms, long: 513 ms

    public static Double valueOf(double d) {
        return new Double(d);
    }

$ ~/.sdkman/candidates/java/21.0.8-tem/bin/javac OldStyle.java
OldStyle.java:3: warning: [removal] Integer(int) in Integer has been deprecated and marked for removal
        Integer c = new Integer(1);
                    ^
OldStyle.java:4: warning: [removal] Double(double) in Double has been deprecated and marked for removal
        Double d = new Double(1.0);
                   ^
2 warnings

$ ~/.sdkman/candidates/java/25-tem/bin/javac OldStyle.java
Note: OldStyle.java uses or overrides a deprecated API.
Note: Recompile with -Xlint:deprecation for details.