java拆箱装箱理解以Long long为例

学过Java的同窗或多或少都听过自动装箱拆箱,下边经过代码和字节码文件加深下对自动拆箱装箱的理解.

1为何要有包装类型?

做为和基本数据类型对应的类类型存在，方便涉及到对象的操做,好比泛型必需要求咱们是对象数据类型.

2 自动装箱拆箱发生在何时?

自动拆箱装箱发生在代码编译期间.

经过例子来看下自动拆箱装箱是怎么作的:

public static void main(String[] args) { 
        Long a = 100L;
        Long b = 100L;
        long c = 100L;
        Long d = new Long(100);

        Long e = 1000L;
        Long f = 1000L;
        
        System.out.println(a == b);
        System.out.println(a == c);
        System.out.println(a == d);
        System.out.println(c == d);
        System.out.println(e == f);
    }
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下面先公布答案: true true false true false

蒙 a==b为true; 为何 e==f就是false? a和c是怎么比较的? a和d又是什么状况?

接下来咱们经过字节码文件看看到底有什么奥秘:

public static void main(java.lang.String[]);
    Code:
       0: ldc2_w        #2 // long 100l
       3: invokestatic  #4 // Method java/lang/Long.valueOf:(J)Ljava/lang/Long;
       6: astore_1
       7: ldc2_w        #2 // long 100l
      10: invokestatic  #4 // Method java/lang/Long.valueOf:(J)Ljava/lang/Long;
      13: astore_2
      14: ldc2_w        #2 // long 100l
      17: lstore_3
      18: new           #5 // class java/lang/Long
      21: dup
      22: ldc2_w        #2 // long 100l
      25: invokespecial #6 // Method java/lang/Long."<init>":(J)V
      28: astore        5
      30: ldc2_w        #7 // long 1000l
      33: invokestatic  #4 // Method java/lang/Long.valueOf:(J)Ljava/lang/Long;
      36: astore        6
      38: ldc2_w        #7 // long 1000l
      41: invokestatic  #4 // Method java/lang/Long.valueOf:(J)Ljava/lang/Long;
      44: astore        7
      46: getstatic     #9 // Field java/lang/System.out:Ljava/io/PrintStream;
      49: aload_1
      50: aload_2
      51: if_acmpne     58
      54: iconst_1
      55: goto          59
      58: iconst_0
      59: invokevirtual #10 // Method java/io/PrintStream.println:(Z)V
      62: getstatic     #9 // Field java/lang/System.out:Ljava/io/PrintStream;
      65: aload_1
      66: invokevirtual #11 // Method java/lang/Long.longValue:()J
      69: lload_3
      70: lcmp
      71: ifne          78
      74: iconst_1
      75: goto          79
      78: iconst_0
      79: invokevirtual #10 // Method java/io/PrintStream.println:(Z)V
      82: getstatic     #9 // Field java/lang/System.out:Ljava/io/PrintStream;
      85: aload_1
      86: aload         5
      88: if_acmpne     95
      91: iconst_1
      92: goto          96
      95: iconst_0
      96: invokevirtual #10 // Method java/io/PrintStream.println:(Z)V
      99: getstatic     #9 // Field java/lang/System.out:Ljava/io/PrintStream;
     102: lload_3
     103: aload         5
     105: invokevirtual #11 // Method java/lang/Long.longValue:()J
     108: lcmp
     109: ifne          116
     112: iconst_1
     113: goto          117
     116: iconst_0
     117: invokevirtual #10 // Method java/io/PrintStream.println:(Z)V
     120: getstatic     #9 // Field java/lang/System.out:Ljava/io/PrintStream;
     123: aload         6
     125: aload         7
     127: if_acmpne     134
     130: iconst_1
     131: goto          135
     134: iconst_0
     135: invokevirtual #10 // Method java/io/PrintStream.println:(Z)V
     138: return
}
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鬼画符?咱们看下这些鬼到底什么意思.

3: invokestatic #4 // Method java/lang/Long.valueOf:(J)Ljava/lang/Long; 意思是执行 Long 的valueof()方法 参数为基本类型 返回值为Long类型 看看Long的valueof方法

public static Long valueOf(long l) {
        final int offset = 128;
        if (l >= -128 && l <= 127) { // will cache
            return LongCache.cache[(int)l + offset];
        }
        return new Long(l);
    }
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若是传入的基本类型在-128-127以内就,就从LongCache中取数据返回给咱们.看下LongCache干了啥

private static class LongCache {
        private LongCache(){}
        static final Long cache[] = new Long[-(-128) + 127 + 1];
        static {
            for(int i = 0; i < cache.length; i++)
                cache[i] = new Long(i - 128);
        }
    }
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原来如此,***-128-127就直接返回给咱们chche中的数据,超出这个范围直接new一个给咱们***.a==b为true,e==f为false就说得通了.

65: aload_1
  66: invokevirtual #11                 // Method java/lang/Long.longValue:()J
  69: lload_3
  70: lcmp
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拿出变量a,执行Long.longValue()返回一个基本数据类型,在和c比较.看下longValue方法

/**
     * Returns the value of this {@code Long} as a
     * {@code long} value.
     */
    public long longValue() {
        return value;
    }
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这部就是两个基本数据类型比较吗.

85: aload_1
      86: aload         5
      88: if_acmpne     95
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取出a和d直接比较内存地址是否同样.铁定不同呀.

剩下的你们能够本身看下,有不明白的能够评论问.

3 总结

自动拆箱装箱没有什么神秘的.字节码能够告诉咱们很东西.若是某些概念理解不了,试着看看字节码文件.说不定会豁然开朗.

若是发现文章中有不妥之处,但愿你们指出,共同进步.