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。

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// 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。

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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。

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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。

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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
*///:~

Reference

实例代码均来自《Thinking in Java》