深入理解Java自动装箱和自动拆箱（反编译字节码理解每条指令）

// 自动装箱 相当于 Integer a = Integer.valueOf(66);
Integer a = 66;

Integer i = new Integer(100);
// 自动拆箱 相当于 int n = i.intValue();
int n = i;

/**
 * 自动装箱和自去拆箱 <br/>
 *
 * @author yubaba
 * @date 2023/7/15 08:53:47
 */
public class AutoPack {
    public static void main(String[] args) {
        Integer i = 66;
        int n = i;
    }
}

Classfile /Users/changx/Documents/ws/test/AutoPack.class
  Last modified 2023-7-15; size 375 bytes
  MD5 checksum 2d1985a02dfdf427efd0659a02bfb263
  Compiled from "AutoPack.java"
public class AutoPack
  minor version: 0
  major version: 52
  flags: ACC_PUBLIC, ACC_SUPER
Constant pool:
   #1 = Methodref          #5.#14         // java/lang/Object."<init>":()V
   #2 = Methodref          #15.#16        // java/lang/Integer.valueOf:(I)Ljava/lang/Integer;
   #3 = Methodref          #15.#17        // java/lang/Integer.intValue:()I
   #4 = Class              #18            // AutoPack
   #5 = Class              #19            // java/lang/Object
   #6 = Utf8               <init>
   #7 = Utf8               ()V
   #8 = Utf8               Code
   #9 = Utf8               LineNumberTable
  #10 = Utf8               main
  #11 = Utf8               ([Ljava/lang/String;)V
  #12 = Utf8               SourceFile
  #13 = Utf8               AutoPack.java
  #14 = NameAndType        #6:#7          // "<init>":()V
  #15 = Class              #20            // java/lang/Integer
  #16 = NameAndType        #21:#22        // valueOf:(I)Ljava/lang/Integer;
  #17 = NameAndType        #23:#24        // intValue:()I
  #18 = Utf8               AutoPack
  #19 = Utf8               java/lang/Object
  #20 = Utf8               java/lang/Integer
  #21 = Utf8               valueOf
  #22 = Utf8               (I)Ljava/lang/Integer;
  #23 = Utf8               intValue
  #24 = Utf8               ()I
{
  public AutoPack();
    descriptor: ()V
    flags: ACC_PUBLIC
    Code:
      stack=1, locals=1, args_size=1
         0: aload_0
         1: invokespecial #1                  // Method java/lang/Object."<init>":()V
         4: return
      LineNumberTable:
        line 7: 0
  public static void main(java.lang.String[]);
    descriptor: ([Ljava/lang/String;)V
    flags: ACC_PUBLIC, ACC_STATIC
    Code:
      stack=1, locals=3, args_size=1
         0: bipush        66
         2: invokestatic  #2                  // Method java/lang/Integer.valueOf:(I)Ljava/lang/Integer;
         5: astore_1
         6: aload_1
         7: invokevirtual #3                  // Method java/lang/Integer.intValue:()I
        10: istore_2
        11: return
      LineNumberTable:
        line 10: 0
        line 11: 6
        line 12: 11
}
SourceFile: "AutoPack.java"

public class Main {
    public static void main(String[] args) {
        Integer i1 = 66;
        Integer i2 = 66;
        Integer i3 = 166;
        Integer i4 = 166;
        System.out.println(i1==i2);
        System.out.println(i3==i4);
    }
}

true
false

// jdk 1.8
public final class Integer extends Number implements Comparable<Integer> {
    ...
    private static class IntegerCache {
        static final int low = -128;
        static final int high;
        static final Integer cache[];
        static {
            int h = 127;
            String integerCacheHighPropValue = sun.misc.VM.getSavedProperty("java.lang.Integer.IntegerCache.high");
            if (integerCacheHighPropValue != null) {
                try {
                    int i = parseInt(integerCacheHighPropValue);
                    i = Math.max(i, 127);
                    h = Math.min(i, Integer.MAX_VALUE - (-low) -1);
                } catch( NumberFormatException nfe) {
                    // If the property cannot be parsed into an int, ignore it.
                }
            } 
            high = h;
            cache = new Integer[(high - low) + 1];
            int j = low;
            for(int k = 0; k < cache.length; k++)
                cache[k] = new Integer(j++);
            // range [-128, 127] must be interned (JLS7 5.1.7)
            assert IntegerCache.high >= 127;
        }
        private IntegerCache() {}
    }
    ...
    public static Integer valueOf(int i) {
        if (i >= Integer.IntegerCache.low && i <= Integer.IntegerCache.high)
        return Integer.IntegerCache.cache[i + (-Integer.IntegerCache.low)];
        return new Integer(i);
    }
    ...
}

public class Main {
    public static void main(String[] args) {    
        Double d1 = 66.0;
        Double d2 = 66.0;
        Double d3 = 166.0;
        Double d4 = 166.0; 
        System.out.println(i1==i2);
        System.out.println(i3==i4);
    }
}

public class Main {
    public static void main(String[] args) { 
        Integer a = 2;
        Integer b = 4;
        Integer c = 6;
        Integer d = 6;
        Integer e = 166;
        Integer f = 166;
        Long g = 6L;
        Long h = 4L;
        System.out.println(c==d);         //1
        System.out.println(e==f);         //2
        System.out.println(c==(a+b));     //3
        System.out.println(c.equals(a+b));//4
        System.out.println(g==(a+b));     //5
        System.out.println(g.equals(a+b));//6
        System.out.println(g.equals(a+h));//7
    }
}