对象的生成过程

好长时间没看java了，有些东西得复习一下。

现在存在两个类Father和Child，如果我们new  Child();或者new Child("李明");这个过程发生了什么？

先看Father:

 1 public class Father{
 2         static{
 3             System.out.println("Father_jingtaichushihuakuai");
 4         }
 5         public static String static_str="Father_jingtaiziduan";
 6         {
 7             System.out.println("Father_chushihuakuai");
 8         }
 9         public String str="Father_ziduan";
10         public Father(){
11             System.out.println("Father_wucangouzaohanshu");
12         }
13         public Father(String str){
14             System.out.println("Father_youcangouzaohanshu");
15         }
16     }

再看Child:

 1 public class Child extends Father{
 2         public static String static_str="Child_jingtaiziduan";  
 3         static{
 4             System.out.println("Child_jingtaichushihuakuai");  
 5         }  
 6         public String str="Child_ziduan";  
 7         {  
 8             System.out.println("Child_chushihuakuai");  
 9         }  
10         public Child(){  
11             System.out.println("Child_wucangouzaohanshu");  
12         }  
13         public Child(String str){  
14             System.out.println("Child_youcangouzaohanshu");  
15         }  
16         public static void main(String[] args) {  
17             new Child();
18             System.out.println("×××××××分割线×××××××");
19             new Child("abc");
20             System.exit(0);
21       
22         }  
23 }

再看结果：

 1 Father_jingtaichushihuakuai
 2 Child_jingtaichushihuakuai
 3 Father_chushihuakuai
 4 Father_wucangouzaohanshu
 5 Child_chushihuakuai
 6 Child_wucangouzaohanshu
 7 ×××××××分割线×××××××
 8 Father_chushihuakuai
 9 Father_wucangouzaohanshu
10 Child_chushihuakuai
11 Child_youcangouzaohanshu


分析原因之前，先说明一点，Father类默认继承Object.

附：

Father.class文件 
 1 // class version 50.0 (50)
 2 // access flags 33
 3 public class com/ibeen/test/Father {
 4 
 5   // compiled from: Father.java
 6 
 7 // access flags 9
 8   public static Ljava/lang/String; static_str
 9 
10   // access flags 1
11   public Ljava/lang/String; str
12 
13   // access flags 8
14   static <clinit>()V
15    L0 (0)
16     LINENUMBER 5 L0
17     GETSTATIC java/lang/System.out : Ljava/io/PrintStream;
18     LDC "Father_jingtaichushihuakuai"
19     INVOKEVIRTUAL java/io/PrintStream.println(Ljava/lang/String;)V
20    L1 (4)
21     LINENUMBER 7 L1
22     LDC "Father_jingtaiziduan"
23     PUTSTATIC com/ibeen/test/Father.static_str : Ljava/lang/String;
24    L2 (7)
25     LINENUMBER 3 L2
26     RETURN
27     MAXSTACK = 2
28     MAXLOCALS = 0
29 
30   // access flags 1
31   public <init>()V
32    L0 (0)
33     LINENUMBER 12 L0
34     ALOAD 0
35     INVOKESPECIAL java/lang/Object.<init>()V
36    L1 (3)
37     LINENUMBER 9 L1
38     GETSTATIC java/lang/System.out : Ljava/io/PrintStream;
39     LDC "Father_chushihuakuai"
40     INVOKEVIRTUAL java/io/PrintStream.println(Ljava/lang/String;)V
41    L2 (7)
42     LINENUMBER 11 L2
43     ALOAD 0
44     LDC "Father_ziduan"
45     PUTFIELD com/ibeen/test/Father.str : Ljava/lang/String;
46    L3 (11)
47     LINENUMBER 13 L3
48     GETSTATIC java/lang/System.out : Ljava/io/PrintStream;
49     LDC "Father_wucangouzaohanshu"
50     INVOKEVIRTUAL java/io/PrintStream.println(Ljava/lang/String;)V
51    L4 (15)
52     LINENUMBER 14 L4
53     RETURN
54    L5 (17)
55     LOCALVARIABLE this Lcom/ibeen/test/Father; L0 L5 0
56     MAXSTACK = 2
57     MAXLOCALS = 1
58 
59   // access flags 1
60   public <init>(Ljava/lang/String;)V
61    L0 (0)
62     LINENUMBER 15 L0
63     ALOAD 0
64     INVOKESPECIAL java/lang/Object.<init>()V
65    L1 (3)
66     LINENUMBER 9 L1
67     GETSTATIC java/lang/System.out : Ljava/io/PrintStream;
68     LDC "Father_chushihuakuai"
69     INVOKEVIRTUAL java/io/PrintStream.println(Ljava/lang/String;)V
70    L2 (7)
71     LINENUMBER 11 L2
72     ALOAD 0
73     LDC "Father_ziduan"
74     PUTFIELD com/ibeen/test/Father.str : Ljava/lang/String;
75    L3 (11)
76     LINENUMBER 16 L3
77     GETSTATIC java/lang/System.out : Ljava/io/PrintStream;
78     LDC "Father_youcangouzaohanshu"
79     INVOKEVIRTUAL java/io/PrintStream.println(Ljava/lang/String;)V
80    L4 (15)
81     LINENUMBER 17 L4
82     RETURN
83    L5 (17)
84     LOCALVARIABLE this Lcom/ibeen/test/Father; L0 L5 0
85     LOCALVARIABLE str Ljava/lang/String; L0 L5 1
86     MAXSTACK = 2
87     MAXLOCALS = 2
88 }
Child.class文件 
  1 // class version 50.0 (50)
  2 // access flags 33
  3 public class com/ibeen/test/Child extends com/ibeen/test/Father  {
  4 
  5   // compiled from: Child.java
  6 
  7 // access flags 9
  8   public static Ljava/lang/String; static_str
  9 
 10   // access flags 1
 11   public Ljava/lang/String; str
 12 
 13   // access flags 8
 14   static <clinit>()V
 15    L0 (0)
 16     LINENUMBER 4 L0
 17     LDC "Child_jingtaiziduan"
 18     PUTSTATIC com/ibeen/test/Child.static_str : Ljava/lang/String;
 19    L1 (3)
 20     LINENUMBER 6 L1
 21     GETSTATIC java/lang/System.out : Ljava/io/PrintStream;
 22     LDC "Child_jingtaichushihuakuai"
 23     INVOKEVIRTUAL java/io/PrintStream.println(Ljava/lang/String;)V
 24    L2 (7)
 25     LINENUMBER 3 L2
 26     RETURN
 27     MAXSTACK = 2
 28     MAXLOCALS = 0
 29 
 30   // access flags 1
 31   public <init>()V
 32    L0 (0)
 33     LINENUMBER 12 L0
 34     ALOAD 0
 35     INVOKESPECIAL com/ibeen/test/Father.<init>()V
 36    L1 (3)
 37     LINENUMBER 8 L1
 38     ALOAD 0
 39     LDC "Child_ziduan"
 40     PUTFIELD com/ibeen/test/Child.str : Ljava/lang/String;
 41    L2 (7)
 42     LINENUMBER 10 L2
 43     GETSTATIC java/lang/System.out : Ljava/io/PrintStream;
 44     LDC "Child_chushihuakuai"
 45     INVOKEVIRTUAL java/io/PrintStream.println(Ljava/lang/String;)V
 46    L3 (11)
 47     LINENUMBER 13 L3
 48     GETSTATIC java/lang/System.out : Ljava/io/PrintStream;
 49     LDC "Child_wucangouzaohanshu"
 50     INVOKEVIRTUAL java/io/PrintStream.println(Ljava/lang/String;)V
 51    L4 (15)
 52     LINENUMBER 14 L4
 53     RETURN
 54    L5 (17)
 55     LOCALVARIABLE this Lcom/ibeen/test/Child; L0 L5 0
 56     MAXSTACK = 2
 57     MAXLOCALS = 1
 58 
 59   // access flags 1
 60   public <init>(Ljava/lang/String;)V
 61    L0 (0)
 62     LINENUMBER 15 L0
 63     ALOAD 0
 64     INVOKESPECIAL com/ibeen/test/Father.<init>()V
 65    L1 (3)
 66     LINENUMBER 8 L1
 67     ALOAD 0
 68     LDC "Child_ziduan"
 69     PUTFIELD com/ibeen/test/Child.str : Ljava/lang/String;
 70    L2 (7)
 71     LINENUMBER 10 L2
 72     GETSTATIC java/lang/System.out : Ljava/io/PrintStream;
 73     LDC "Child_chushihuakuai"
 74     INVOKEVIRTUAL java/io/PrintStream.println(Ljava/lang/String;)V
 75    L3 (11)
 76     LINENUMBER 16 L3
 77     GETSTATIC java/lang/System.out : Ljava/io/PrintStream;
 78     LDC "Child_youcangouzaohanshu"
 79     INVOKEVIRTUAL java/io/PrintStream.println(Ljava/lang/String;)V
 80    L4 (15)
 81     LINENUMBER 17 L4
 82     RETURN
 83    L5 (17)
 84     LOCALVARIABLE this Lcom/ibeen/test/Child; L0 L5 0
 85     LOCALVARIABLE str Ljava/lang/String; L0 L5 1
 86     MAXSTACK = 2
 87     MAXLOCALS = 2
 88 
 89   // access flags 9
 90   public static main([Ljava/lang/String;)V
 91    L0 (0)
 92     LINENUMBER 19 L0
 93     NEW com/ibeen/test/Child
 94     INVOKESPECIAL com/ibeen/test/Child.<init>()V
 95    L1 (3)
 96     LINENUMBER 20 L1
 97     GETSTATIC java/lang/System.out : Ljava/io/PrintStream;
 98     LDC "\u00d7\u00d7\u00d7\u00d7\u00d7\u00d7\u00d7\u5206\u5272\u7ebf\u00d7\u00d7\u00d7\u00d7\u00d7\u00d7\u00d7"
 99     INVOKEVIRTUAL java/io/PrintStream.println(Ljava/lang/String;)V
100    L2 (7)
101     LINENUMBER 21 L2
102     NEW com/ibeen/test/Child
103     LDC "abc"
104     INVOKESPECIAL com/ibeen/test/Child.<init>(Ljava/lang/String;)V
105    L3 (11)
106     LINENUMBER 22 L3
107     ICONST_0
108     INVOKESTATIC java/lang/System.exit(I)V
109    L4 (14)
110     LINENUMBER 76 L4
111     RETURN
112    L5 (16)
113     LOCALVARIABLE args [Ljava/lang/String; L0 L5 0
114     MAXSTACK = 2
115     MAXLOCALS = 1
116 }

排除Object，来看打印结果

得出一个小小的结论：

 

　　　　new Child()这个过程：

  1. 父类的静态块
  2. 子类的静态块
  3. 父类的非静态块
  4. 父类的无参构造方法
  5. 子类的非静态块
  6. 子类的无参构造方法　　　　　　

　　　　new Child("abc")这个过程:

  1. 父类的非静态块
  2. 父类的无参构造方法
  3. 子类的非静态块
  4. 子类的无参构造方法

这两个过程的差别在于第一个过程的中的前2个步骤，事实上这两个步骤和这两个过程没有任何关系。

静态变量和静态块在声明的时候仅被初始化一次。和创建对象与否无关。

可能会有人问，那么静态变量什么时候初始化呢？

我们来看Child.class文件中

  static <clinit>()V
   L0 (0)
    LINENUMBER 4 L0
    LDC "Child_jingtaiziduan"
    PUTSTATIC com/ibeen/test/Child.static_str : Ljava/lang/String;
   L1 (3)
    LINENUMBER 6 L1
    GETSTATIC java/lang/System.out : Ljava/io/PrintStream;
    LDC "Child_jingtaichushihuakuai"
    INVOKEVIRTUAL java/io/PrintStream.println(Ljava/lang/String;)V
   L2 (7)
    LINENUMBER 3 L2
    RETURN
    MAXSTACK = 2
    MAXLOCALS = 0

这一区域只执行一次，顺序很明显了：先初始化静态变量， 然后再执行静态块。

事实上是在定义类的时候，静态变量和静态块哪个在前面哪个先执行。

同样的方法看非静态变量和非静态块同样如此。

关于静态部分，可以得出：

  1. 父类的静态变量的初始化和静态块的执行先于子类
  2. 同一个类中，静态变量和静态块哪个先定义哪个先初始化（执行）

ps:静态变量和静态块的初始化在什么时候执行，我也不知道，我猜，在jvm加载的时候，你猜呢？

关于对象的生成，可以得出：

  1. 父类的非静态块或者非静态变量，哪个先定义哪个先初始化（执行）；
  2. 父类的构造函数；
  3. 子类的非静态块或者非静态变量，哪个先定义哪个先初始化（执行）；
  4. 子类的构造函数；

ps:如果子类被调用的构造方法中没有显示调用父类的有参构造方法，将默认调用父类的无参构造方法

posted on 2011-11-19 18:28  ibeen  阅读(383)  评论(0)    收藏  举报

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