深入理解JAVA虛擬機-自動裝箱、拆箱

 

 

下面爲第14行到26行的字節碼編譯文件，可以看到虛擬機是如何自動裝箱和拆箱的，末尾會給出輸出答案

   L1
    LINENUMBER 14 L1  
    ICONST_1
    INVOKESTATIC java/lang/Integer.valueOf (I)Ljava/lang/Integer;
    ASTORE 2        //存入ICONST_1+Integer裝箱
   L2
    LINENUMBER 15 L2
    ICONST_2
    INVOKESTATIC java/lang/Integer.valueOf (I)Ljava/lang/Integer;
    ASTORE 3        //存入ICONST_2+Integer裝箱
   L3
    LINENUMBER 16 L3
    ICONST_3
    INVOKESTATIC java/lang/Integer.valueOf (I)Ljava/lang/Integer;
    ASTORE 4       //存入ICONST_3+Integer裝箱
   L4
    LINENUMBER 17 L4
    ICONST_3 
    INVOKESTATIC java/lang/Integer.valueOf (I)Ljava/lang/Integer;
    ASTORE 5       //存入ICONST_3+Integer裝箱
   L5
    LINENUMBER 18 L5
    SIPUSH 321
    INVOKESTATIC java/lang/Integer.valueOf (I)Ljava/lang/Integer;
    ASTORE 6          //存入321+Integer裝箱
   L6
    LINENUMBER 19 L6
    SIPUSH 321    
    INVOKESTATIC java/lang/Integer.valueOf (I)Ljava/lang/Integer;
    ASTORE 7          //存入321+Integer裝箱
   L7
    LINENUMBER 20 L7
    LDC 3
    INVOKESTATIC java/lang/Long.valueOf (J)Ljava/lang/Long;
    ASTORE 8          //存入3+Long裝箱
   L8
    LINENUMBER 21 L8
    GETSTATIC java/lang/System.out : Ljava/io/PrintStream;
    ALOAD 4
    ALOAD 5               //這裏的4和5都是 
    IF_ACMPNE L9
    ICONST_1
    GOTO L10
   L9
   FRAME FULL [[Ljava/lang/String; java/util/List java/lang/Integer java/lang/Integer java/lang/Integer java/lang/Integer java/lang/Integer java/lang/Integer java/lang/Long] [java/io/PrintStream]
    ICONST_0
   L10
   FRAME FULL [[Ljava/lang/String; java/util/List java/lang/Integer java/lang/Integer java/lang/Integer java/lang/Integer java/lang/Integer java/lang/Integer java/lang/Long] [java/io/PrintStream I]
    INVOKEVIRTUAL java/io/PrintStream.println (Z)V
   L11
    LINENUMBER 22 L11
    GETSTATIC java/lang/System.out : Ljava/io/PrintStream;
    ALOAD 6                            
    ALOAD 7
    IF_ACMPNE L12                                //不涉及算術運算比較對象
    ICONST_1
    GOTO L13
   L12
   FRAME SAME1 java/io/PrintStream
    ICONST_0
   L13
   FRAME FULL [[Ljava/lang/String; java/util/List java/lang/Integer java/lang/Integer java/lang/Integer java/lang/Integer java/lang/Integer java/lang/Integer java/lang/Long] [java/io/PrintStream I]
    INVOKEVIRTUAL java/io/PrintStream.println (Z)V
   L14
    LINENUMBER 23 L14                            //涉及算數運算IADD的時候回自動拆箱比較數值
    GETSTATIC java/lang/System.out : Ljava/io/PrintStream;
    ALOAD 4
    INVOKEVIRTUAL java/lang/Integer.intValue ()I  //拆箱
    ALOAD 2
    INVOKEVIRTUAL java/lang/Integer.intValue ()I  //拆箱
    ALOAD 3
    INVOKEVIRTUAL java/lang/Integer.intValue ()I  //拆箱
    IADD 
    IF_ICMPNE L15                                 //比較int
    ICONST_1
    GOTO L16
   L15
   FRAME SAME1 java/io/PrintStream
    ICONST_0
   L16
   FRAME FULL [[Ljava/lang/String; java/util/List java/lang/Integer java/lang/Integer java/lang/Integer java/lang/Integer java/lang/Integer java/lang/Integer java/lang/Long] [java/io/PrintStream I]
    INVOKEVIRTUAL java/io/PrintStream.println (Z)V
   L17
    LINENUMBER 24 L17
    GETSTATIC java/lang/System.out : Ljava/io/PrintStream;
    ALOAD 4
    ALOAD 2
    INVOKEVIRTUAL java/lang/Integer.intValue ()I  //拆箱
    ALOAD 3
    INVOKEVIRTUAL java/lang/Integer.intValue ()I  //拆箱
    IADD                                          //算術IADD
    INVOKESTATIC java/lang/Integer.valueOf (I)Ljava/lang/Integer;//拆完了裝箱
    INVOKEVIRTUAL java/lang/Integer.equals (Ljava/lang/Object;)Z//比較對象
    INVOKEVIRTUAL java/io/PrintStream.println (Z)V
   L18
    LINENUMBER 25 L18                        //涉及算數運算IADD的時候回自動拆箱比較數值
    GETSTATIC java/lang/System.out : Ljava/io/PrintStream;
    ALOAD 8
    INVOKEVIRTUAL java/lang/Long.longValue ()J
    ALOAD 2
    INVOKEVIRTUAL java/lang/Integer.intValue ()I
    ALOAD 3
    INVOKEVIRTUAL java/lang/Integer.intValue ()I
    IADD
    I2L
    LCMP
    IFNE L19
    ICONST_1
    GOTO L20
   L19
   FRAME SAME1 java/io/PrintStream
    ICONST_0
   L20
   FRAME FULL [[Ljava/lang/String; java/util/List java/lang/Integer java/lang/Integer java/lang/Integer java/lang/Integer java/lang/Integer java/lang/Integer java/lang/Long] [java/io/PrintStream I]
    INVOKEVIRTUAL java/io/PrintStream.println (Z)V
   L21            
    LINENUMBER 26 L21                        //Long.equals (Object) instanceof檢查不通過
    GETSTATIC java/lang/System.out : Ljava/io/PrintStream;
    ALOAD 8
    ALOAD 2
    INVOKEVIRTUAL java/lang/Integer.intValue ()I
    ALOAD 3
    INVOKEVIRTUAL java/lang/Integer.intValue ()I
    IADD
    INVOKESTATIC java/lang/Integer.valueOf (I)Ljava/lang/Integer;
    INVOKEVIRTUAL java/lang/Long.equals (Ljava/lang/Object;)Z
    INVOKEVIRTUAL java/io/PrintStream.println (Z)V
   L22
    LINENUMBER 27 L22
    RETURN
   L23
    LOCALVARIABLE args [Ljava/lang/String; L0 L23 0
    LOCALVARIABLE list Ljava/util/List; L1 L23 1
    // signature Ljava/util/List<Ljava/lang/Integer;>;
    // declaration: list extends java.util.List<java.lang.Integer>
    LOCALVARIABLE a Ljava/lang/Integer; L2 L23 2
    LOCALVARIABLE b Ljava/lang/Integer; L3 L23 3
    LOCALVARIABLE c Ljava/lang/Integer; L4 L23 4
    LOCALVARIABLE d Ljava/lang/Integer; L5 L23 5
    LOCALVARIABLE e Ljava/lang/Integer; L6 L23 6
    LOCALVARIABLE f Ljava/lang/Integer; L7 L23 7
    LOCALVARIABLE g Ljava/lang/Long; L8 L23 8
    MAXSTACK = 5
    MAXLOCALS = 9
}

輸出： 

true
false
true
true
true
false