java拆箱裝箱理解以Long long爲例

學過Java的同窗或多或少都聽過自動裝箱拆箱,下邊經過代碼和字節碼文件加深下對自動拆箱裝箱的理解.

1爲何要有包裝類型?

做爲和基本數據類型對應的類類型存在，方便涉及到對象的操做,好比泛型必需要求咱們是對象數據類型.

2 自動裝箱拆箱發生在何時?

自動拆箱裝箱發生在代碼編譯期間.

經過例子來看下自動拆箱裝箱是怎麼作的:

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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下面先公佈答案: true true false true false

蒙 a==b爲true; 爲何 e==f就是false? a和c是怎麼比較的? a和d又是什麼狀況?

接下來咱們經過字節碼文件看看到底有什麼奧祕:

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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鬼畫符?咱們看下這些鬼到底什麼意思.

3: invokestatic #4 // Method java/lang/Long.valueOf:(J)Ljava/lang/Long; 意思是執行 Long 的valueof()方法 參數爲基本類型 返回值爲Long類型 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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若是傳入的基本類型在-128-127以內就,就從LongCache中取數據返回給咱們.看下LongCache幹了啥

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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原來如此,***-128-127就直接返回給咱們chche中的數據,超出這個範圍直接new一個給咱們***.a==b爲true,e==f爲false就說得通了.

65: aload_1
  66: invokevirtual #11                 // Method java/lang/Long.longValue:()J
  69: lload_3
  70: lcmp
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拿出變量a,執行Long.longValue()返回一個基本數據類型,在和c比較.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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這部就是兩個基本數據類型比較嗎.

85: aload_1
      86: aload         5
      88: if_acmpne     95
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取出a和d直接比較內存地址是否同樣.鐵定不同呀.

剩下的你們能夠本身看下,有不明白的能夠評論問.

3 總結

自動拆箱裝箱沒有什麼神祕的.字節碼能夠告訴咱們很東西.若是某些概念理解不了,試着看看字節碼文件.說不定會豁然開朗.

若是發現文章中有不妥之處,但願你們指出,共同進步.