Java Integer装箱拆箱与缓存机制解析-优快云博客

1. 示例代码

接下来，以一段代码来解释Integer的装箱与拆箱

public class IntegerTest {
    public static void main(String[] args) {
		//装箱
        Integer obj=1;
		//拆箱
        int result=obj*2;

		//不在-128~127之间，结果为false
        Integer a=200;
        Integer b=200;
        System.out.println(a==b);
        //在-128~127之间，结果为true
        Integer c=100;
        Integer d=100;
        System.out.println(c==d);

    }
}

然后对这段代码进行编译和反编译:

javac IntegerTest.java
javap -c IntegerTest

反编译结果为:

Compiled from "IntegerTest.java"
public class grammer.integer_test.IntegerTest {
  public grammer.integer_test.IntegerTest();
    Code:
       0: aload_0
       1: invokespecial #1                  // Method java/lang/Object."<init>":()V
       4: return

  public static void main(java.lang.String[]);
    Code:
       0: iconst_1
       1: invokestatic  #7                  // Method java/lang/Integer.valueOf:(I)Ljava/lang/Integer;
       4: astore_1
       5: aload_1
       6: invokevirtual #13                 // Method java/lang/Integer.intValue:()I
       9: iconst_2
      10: imul
      11: istore_2
      12: sipush        200
      15: invokestatic  #7                  // Method java/lang/Integer.valueOf:(I)Ljava/lang/Integer;
      18: astore_3
      19: sipush        200
      22: invokestatic  #7                  // Method java/lang/Integer.valueOf:(I)Ljava/lang/Integer;
      25: astore        4
      27: getstatic     #17                 // Field java/lang/System.out:Ljava/io/PrintStream;
      30: aload_3
      31: aload         4
      33: if_acmpne     40
      36: iconst_1
      37: goto          41
      40: iconst_0
      41: invokevirtual #23                 // Method java/io/PrintStream.println:(Z)V
      44: bipush        100
      46: invokestatic  #7                  // Method java/lang/Integer.valueOf:(I)Ljava/lang/Integer;
      49: astore        5
      51: bipush        100
      53: invokestatic  #7                  // Method java/lang/Integer.valueOf:(I)Ljava/lang/Integer;
      56: astore        6
      58: getstatic     #17                 // Field java/lang/System.out:Ljava/io/PrintStream;
      61: aload         5
      63: aload         6
      65: if_acmpne     72
      68: iconst_1
      69: goto          73
      72: iconst_0
      73: invokevirtual #23                 // Method java/io/PrintStream.println:(Z)V
      76: return
}

2. 结果分析

2.1 装箱与拆箱的底层实现

由反编译的结果可以看出来:

装箱：底层调用的是Integer.valueOf()方法

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

拆箱: 底层调用的是Integer.intValue()方法

public int intValue() {
    return value;
}

2.2 使用==比较Integer结果为什么会有差异?

从示例代码可以看出：a和b都是200的时候，使用双等号比较结果是false；而c和d都是1的时候，使用双等号比较结果是true。
那到底是为什么呢？原因很简单，Integer内部缓存了-128~127之间的Integer对象，当处于这个范围时会直接返回缓存对象；而超过这个范围，是重新创建对象.众所周知，==比较的是内存地址.缓存对象都是同一个，那内存地址自然也是一致的.

以上是结论，那么依据是什么呢？依据就是装箱的代码:

    public static Integer valueOf(int i) {
    //如下IntegerCache定义：IntegerCache.low=-128,而IntegerCache.high=127
        if (i >= IntegerCache.low && i <= IntegerCache.high)
            return IntegerCache.cache[i + (-IntegerCache.low)];
        return new Integer(i);
    }
    
  private static class IntegerCache {
        static final int low = -128;
        static final int high;
        static final Integer cache[];

        static {
            // high value may be configured by property
            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);
                    // Maximum array size is Integer.MAX_VALUE
                    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() {}
    }