java中Integer常量池

    我们先看一个关于Integer的例子

public static void main(String[] args) {
		// TeODO Auto-generated method stu
        Integer i1 = 40;
        Integer i2 = 40;
        Integer i3 = 0;
        Integer i4 = new Integer(40);
        Integer i5 = new Integer(40);
        Integer i6 = new Integer(0);
        
        System.out.println("i1=i2	" + (i1 == i2));
        System.out.println("i1=i2+i3	" + (i1 == i2 + i3));
        System.out.println("i4=i5	" + (i4 == i5));
        System.out.println("i4=i5+i6	" + (i4 == i5 + i6));    
        
        System.out.println();     
	}
}

 结果：

i1=i2     true
i1=i2+i3  true
i4=i5     false
i4=i5+i6  true

 再看一个例子

public static void main(String[] args) {
	// TeODO Auto-generated method stu
        Integer i1 = 400;
        Integer i2 = 400;
        Integer i3 = 0;
        Integer i4 = new Integer(400);
        Integer i5 = new Integer(400);
        Integer i6 = new Integer(0);
        
        System.out.println("i1=i2	" + (i1 == i2));
        System.out.println("i1=i2+i3	" + (i1 == i2 + i3));
        System.out.println("i4=i5	" + (i4 == i5));
        System.out.println("i4=i5+i6	" + (i4 == i5 + i6));    
        
        System.out.println();     
	}
}

 结果

i1=i2     false
i1=i2+i3  true
i4=i5     false
i4=i5+i6  true

 比较上面两个例子，我们发现：两个例子代码几乎一样，就是值不一样，40和400的区别怎么会造成如此大的结果差别呢？？

我们用上篇博客的方法，用javap看他们的反编译结果

public class com.study.main.Test extends java.lang.Object{
public com.study.main.Test();
  Code:
   0:   aload_0
   1:   invokespecial   #1; //Method java/lang/Object."<init>":()V
   4:   return

public static void main(java.lang.String[]);
  Code:
   0:   sipush  400 //40时我们用cipush(下同）  cipush(char 类型)的值范围是-128——127,sipush（short 类型）的值范围是2^15——2^15-1,
   3:   invokestatic    #2; //Method java/lang/Integer.valueOf:(I)Ljava/lang/Int
eger;
   6:   astore_1
   7:   sipush  400
   10:  invokestatic    #2; //Method java/lang/Integer.valueOf:(I)Ljava/lang/Int
eger;
   13:  astore_2
   14:  iconst_0
   15:  invokestatic    #2; //Method java/lang/Integer.valueOf:(I)Ljava/lang/Int
eger;
   18:  astore_3
   19:  new     #3; //class java/lang/Integer
   22:  dup
   23:  sipush  400
   26:  invokespecial   #4; //Method java/lang/Integer."<init>":(I)V
   29:  astore  4
   31:  new     #3; //class java/lang/Integer
   34:  dup
   35:  sipush  400
   38:  invokespecial   #4; //Method java/lang/Integer."<init>":(I)V
   41:  astore  5
   43:  new     #3; //class java/lang/Integer
   46:  dup
   47:  iconst_0
   48:  invokespecial   #4; //Method java/lang/Integer."<init>":(I)V
   51:  astore  6
   53:  getstatic       #5; //Field java/lang/System.out:Ljava/io/PrintStream;
   56:  new     #6; //class java/lang/StringBuilder
   59:  dup
   60:  invokespecial   #7; //Method java/lang/StringBuilder."<init>":()V
   63:  ldc     #8; //String i1=i2	
   65:  invokevirtual   #9; //Method java/lang/StringBuilder.append:(Ljava/lang/
String;)Ljava/lang/StringBuilder;
   68:  aload_1
   69:  aload_2
   70:  if_acmpne       77
   73:  iconst_1
   74:  goto    78
   77:  iconst_0
   78:  invokevirtual   #10; //Method java/lang/StringBuilder.append:(Z)Ljava/la
ng/StringBuilder;
   81:  invokevirtual   #11; //Method java/lang/StringBuilder.toString:()Ljava/l
ang/String;
   84:  invokevirtual   #12; //Method java/io/PrintStream.println:(Ljava/lang/St
ring;)V
   87:  getstatic       #5; //Field java/lang/System.out:Ljava/io/PrintStream;
   90:  new     #6; //class java/lang/StringBuilder
   93:  dup
   94:  invokespecial   #7; //Method java/lang/StringBuilder."<init>":()V
   97:  ldc     #13; //String i1=i2+i3	
   99:  invokevirtual   #9; //Method java/lang/StringBuilder.append:(Ljava/lang/
String;)Ljava/lang/StringBuilder;
   102: aload_1
   103: invokevirtual   #14; //Method java/lang/Integer.intValue:()I
   106: aload_2
   107: invokevirtual   #14; //Method java/lang/Integer.intValue:()I
   110: aload_3
   111: invokevirtual   #14; //Method java/lang/Integer.intValue:()I
   114: iadd
   115: if_icmpne       122
   118: iconst_1
   119: goto    123
   122: iconst_0
   123: invokevirtual   #10; //Method java/lang/StringBuilder.append:(Z)Ljava/la
ng/StringBuilder;
   126: invokevirtual   #11; //Method java/lang/StringBuilder.toString:()Ljava/l
ang/String;
   129: invokevirtual   #12; //Method java/io/PrintStream.println:(Ljava/lang/St
ring;)V
   132: getstatic       #5; //Field java/lang/System.out:Ljava/io/PrintStream;
   135: new     #6; //class java/lang/StringBuilder
   138: dup
   139: invokespecial   #7; //Method java/lang/StringBuilder."<init>":()V
   142: ldc     #15; //String i4=i5	
   144: invokevirtual   #9; //Method java/lang/StringBuilder.append:(Ljava/lang/
String;)Ljava/lang/StringBuilder;
   147: aload   4
   149: aload   5
   151: if_acmpne       158
   154: iconst_1
   155: goto    159
   158: iconst_0
   159: invokevirtual   #10; //Method java/lang/StringBuilder.append:(Z)Ljava/la
ng/StringBuilder;
   162: invokevirtual   #11; //Method java/lang/StringBuilder.toString:()Ljava/l
ang/String;
   165: invokevirtual   #12; //Method java/io/PrintStream.println:(Ljava/lang/St
ring;)V
   168: getstatic       #5; //Field java/lang/System.out:Ljava/io/PrintStream;
   171: new     #6; //class java/lang/StringBuilder
   174: dup
   175: invokespecial   #7; //Method java/lang/StringBuilder."<init>":()V
   178: ldc     #16; //String i4=i5+i6	
   180: invokevirtual   #9; //Method java/lang/StringBuilder.append:(Ljava/lang/
String;)Ljava/lang/StringBuilder;
   183: aload   4
   185: invokevirtual   #14; //Method java/lang/Integer.intValue:()I
   188: aload   5
   190: invokevirtual   #14; //Method java/lang/Integer.intValue:()I
   193: aload   6
   195: invokevirtual   #14; //Method java/lang/Integer.intValue:()I
   198: iadd
   199: if_icmpne       206
   202: iconst_1
   203: goto    207
   206: iconst_0
   207: invokevirtual   #10; //Method java/lang/StringBuilder.append:(Z)Ljava/la
ng/StringBuilder;
   210: invokevirtual   #11; //Method java/lang/StringBuilder.toString:()Ljava/l
ang/String;
   213: invokevirtual   #12; //Method java/io/PrintStream.println:(Ljava/lang/St
ring;)V
   216: getstatic       #5; //Field java/lang/System.out:Ljava/io/PrintStream;
   219: invokevirtual   #17; //Method java/io/PrintStream.println:()V
   222: return

}

 比较后我们惊人的发现，两者其实没有什么区别

只有这里：

 0:   sipush  400 //40时我们用cipush(下同）  cipush(char 类型)的值范围是-128——127,sipush（short 类型）的值范围是2^15——2^15-1

我们看到第12行： 3:   invokestatic    #2; //Method java/lang/Integer.valueOf:(I)Ljava/lang/Int

在Integer中是调用Integer.valueOf初始化对象的

Integer.valueOf函数

 public static Integer valueOf(int i) {
        if(i >= -128 && i <= IntegerCache.high)
            return IntegerCache.cache[i + 128];
        else
            return new Integer(i);
    }

IntegerCache

  private static class IntegerCache {
        static final int high;
        static final Integer cache[];

        static {
            final int low = -128;

            // high value may be configured by property
            int h = 127;
            if (integerCacheHighPropValue != null) {
                // Use Long.decode here to avoid invoking methods that
                // require Integer's autoboxing cache to be initialized
                int i = Long.decode(integerCacheHighPropValue).intValue();
                i = Math.max(i, 127);
                // Maximum array size is Integer.MAX_VALUE
                h = Math.min(i, Integer.MAX_VALUE - -low);
            }
            high = h;

            cache = new Integer[(high - low) + 1];
            int j = low;
            for(int k = 0; k < cache.length; k++)
                cache[k] = new Integer(j++);
        }

        private IntegerCache() {}
    }

通过研究发现：Integer.valueOf()中有个内部类IntegerCache（类似于一个常量数组，也叫对象池），它维护了一个Integer数组cache，长度为（128+127+1）=256；Integer类中还有一个Static Block(静态块)，如上所示。

从这个静态块可以看出，Integer已经默认创建了数值【-128-127】的Integer缓存数据。所以使用Integer i1=40时，JVM会直接在该在对象池找到该值的引用。

也就是说这种方式声明一个Integer对象时，JVM首先会在Integer对象的缓存池中查找有木有值为40的对象，如果有直接返回该对象的引用；如果没有，则使用New keyword创建一个对象，并返回该对象的引用地址。因为Java中【==】比较的是两个对象是否是同一个引用（即比较内存地址），i2和i2都是引用的同一个对象，So i1==i2结果为”true“；而使用new方式创建的i4=new Integer(40)、i5=new Integer(40)，虽然他们的值相等，但是每次都会重新Create新的Integer对象，不会被放入到对象池中，所以他们不是同一个引用，输出false。

　　对于i1==i2+i3、i4==i5+i6结果为True，是因为，Java的数学计算是在内存栈里操作的，Java会对i5、i6进行拆箱操作，其实比较的是基本类型（40=40+0），他们的值相同，因此结果为True。

但是但为400时，这个时候已经超过了cache的范围，所以都得重新初始化对象