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Java static 初始化顺序

前言

在之前用 java 的时候，总是对 static 的初始化顺序存有疑惑，今天在重撸 Thinking in Java ,一下子豁然开朗，特此做下笔记。

初始化顺序

类内，总是先初始化字段，字段定义的先后顺序决定了初始化的顺序，然后再初始化构造器。


// When the constructor is called to create a
// Window object, you'll see a message:
class Window {
  Window(int marker) { print("Window(" + marker + ")"); }
}

class House {
  Window w1 = new Window(1); // Before constructor
  House() {
    // Show that we're in the constructor:
    print("House()");
    w3 = new Window(33); // Reinitialize w3
  }
  Window w2 = new Window(2); // After constructor
  void f() { print("f()"); }
  Window w3 = new Window(3); // At end
}

public class OrderOfInitialization {
  public static void main(String[] args) {
    House h = new House();
    h.f(); // Shows that construction is done
  }
} 
/* Output:
Window(1)
Window(2)
Window(3)
House()
Window(33)
f()
*///:~

`static` 数据的初始化

加上 static 限定的字段，是所谓的类字段，也就是说这个字段的拥有者不是对象而是类。无论创建多少对象， static 数据都只有一份。

类内总是先初始化 static 字段，再初始化一般字段。接着初始化构造器。但是如果不创建这个类的对象，那这个对象是不会进行初始化的，并且只执行一次。

如下面的代码，在 StaticInitialization 类中，先初始化 static Table table = new Table(); ，然后才去初始化 Table 对象，不然是不会被初始化的。

class Bowl {
  Bowl(int marker) {
    print("Bowl(" + marker + ")");
  }
  void f1(int marker) {
    print("f1(" + marker + ")");
  }
}

class Table {
  static Bowl bowl1 = new Bowl(1);
  Table() {
    print("Table()");
    bowl2.f1(1);
  }
  void f2(int marker) {
    print("f2(" + marker + ")");
  }
  static Bowl bowl2 = new Bowl(2);
}

class Cupboard {
  Bowl bowl3 = new Bowl(3);
  static Bowl bowl4 = new Bowl(4);
  Cupboard() {
    print("Cupboard()");
    bowl4.f1(2);
  }
  void f3(int marker) {
    print("f3(" + marker + ")");
  }
  static Bowl bowl5 = new Bowl(5);
}

public class StaticInitialization {
  public static void main(String[] args) {
    print("Creating new Cupboard() in main");
    new Cupboard();
    print("Creating new Cupboard() in main");
    new Cupboard();
    table.f2(1);
    cupboard.f3(1);
  }
  static Table table = new Table();
  static Cupboard cupboard = new Cupboard();
} /* Output:
Bowl(1)
Bowl(2)
Table()
f1(1)
Bowl(4)
Bowl(5)
Bowl(3)
Cupboard()
f1(2)
Creating new Cupboard() in main
Bowl(3)
Cupboard()
f1(2)
Creating new Cupboard() in main
Bowl(3)
Cupboard()
f1(2)
f2(1)
f3(1)
*///:~

显示的静态初始化（也就是静态块）

把多个初始化语句包在一个 static 花括号里，叫做 静态块 ，其实就是把多个static合在一起写了，本质是一样的。只有首次创建对象或者首次访问类的字段时才会执行，而且仅仅一次。

class Cup {
  Cup(int marker) {
    print("Cup(" + marker + ")");
  }
  void f(int marker) {
    print("f(" + marker + ")");
  }
}

class Cups {
  static Cup cup1;
  static Cup cup2;
  static {
    cup1 = new Cup(1);
    cup2 = new Cup(2);
  }
  Cups() {
    print("Cups()");
  }
}

public class ExplicitStatic {
  public static void main(String[] args) {
    print("Inside main()");
    Cups.cup1.f(99);  // (1)
  }
  // static Cups cups1 = new Cups();  // (2)
  // static Cups cups2 = new Cups();  // (2)
} /* Output:
Inside main()
Cup(1)
Cup(2)
f(99)
*///:~

非静态实例初始化

这个没什么好讲的，就是普通初始化，按顺序执行，可以多次执行。

class Mug {
  Mug(int marker) {
    print("Mug(" + marker + ")");
  }
  void f(int marker) {
    print("f(" + marker + ")");
  }
}

public class Mugs {
  Mug mug1;
  Mug mug2;
  {
    mug1 = new Mug(1);
    mug2 = new Mug(2);
    print("mug1 & mug2 initialized");
  }
  Mugs() {
    print("Mugs()");
  }
  Mugs(int i) {
    print("Mugs(int)");
  }
  public static void main(String[] args) {
    print("Inside main()");
    new Mugs();
    print("new Mugs() completed");
    new Mugs(1);
    print("new Mugs(1) completed");
  }
} /* Output:
Inside main()
Mug(1)
Mug(2)
mug1 & mug2 initialized
Mugs()
new Mugs() completed
Mug(1)
Mug(2)
mug1 & mug2 initialized
Mugs(int)
new Mugs(1) completed
*///:~