class loading: an example

Yao Yao on March 25, 2009

class loading 是一个比较复杂的过程。一般说来,类是在其 static member 被访问时被加载的。在加载时会做的一件事是:初始化 static member 和 static 代码段 (static block, i.e. static {......}),当然,static 是只会被执行 only once 的。

以下是一个例子 (adapted from Chapter 7, Thinking in Java, Fourth Edition):

import static java.lang.System.out;  
  
class Insect {  
    private int i = 1;  
    protected int j;  
    private static int x1 = printInit("static Insect.x1 initialized");  
      
    Insect() {  
        out.println("Insect constructor");  
        out.println("i = " + i + ", j = " + j + ", x1 = " + x1);  
        this.j = 2;  
    }  
      
    static int printInit(String s) {  
        out.println(s);  
  
        return 3;  
    }  
}  
  
public class Beetle extends Insect {  
    private int k = printInit("Beetle.k initialized");  
    private static int x2 = printInit("static Beetle.x2 initialized");  
      
    public Beetle() {  
        out.println("Beetle constructor");  
        out.println("j = " + j + ", k = " + k + ", x2 = " + x2);  
    }  
   
    public static void main(String[] args) {  
        Beetle b = new Beetle();  
    }  
}  
//output:  
/* 
    static Insect.x1 initialized 
    static Beetle.x2 initialized 
    Insect constructor 
    i = 1, j = 0, x1 = 3 
    Beetle.k initialized 
    Beetle constructor 
    j = 2, k = 3, x2 = 3 
*/ 
  1. 首先,访问 Beetle.main(),是个 static,好,加载 Beetle.class;
    • 发现 Beetle extends Insect,好,加载 Insect.class (如果 Insect 还有 base class,则持续这一过程,直到找到 root base class);
      • 初始化 private static int x1 (=3),打印 “static Insect.x1 initialized”;
      • Insect.class 加载完毕
    • 继续加载 Beetle.class;
      • 初始化 private static int x2 (=3),打印 “static Beetle.x2 initialized”;
      • Beetle.class 加载完毕,
  2. 开始执行 Beetle.main()
    • 要 new 一个 Beetle,须先 new 一个 Insect
      • 在执行 Insect constructor 前,先要初始化 member,由于 private static int x1 是 static 且已经初始化了,所以这次只初始化 private int i (=1) 和 protected int j (=0 by default);
      • 执行 Insect constructor,打印 “Insect constructor” 和 “i = 1, j = 0, x1 = 3”,然后 j = 2
      • Insect constructor 执行完毕,即 new Insect 过程完毕
    • 继续 new Beetle()
      • 同理,在执行 Beetle constructor 之前,要先初始化 member,这里是初始化 private int k (=3),打印 “Beetle.k initialized”;
      • 执行 Beetle constructor,打印 “Beetle constructor” 和 “j = 2, k = 3, x2 = 3”;
      • Beetle constructor 执行完毕,即 new Beetle() 过程完毕
  3. Beetle.main() 执行完毕。


2009年03月27日补充:more details see warning: 在构造器中请谨慎使用被覆写方法


2009年09月04日归纳class loading: further discussion involving steps of instance creation



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