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Java中非静态成员变量、静态成员变量的初始化时机。

程序员文章站 2022-05-21 22:10:48
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Java中非静态成员变量、静态成员变量的初始化时机。

非静态变量

我们在这里分析三种结构,着重分析这三种结构的初始化顺序:

  1. 成员变量初始化语句;
  2. 成员变量初始化块;
  3. 构造函数;

示例一:

public class MyTest {

    private String name = "wei.hu";

    public MyTest(String name) {
        System.out.println("This is constructor. Will assign the variable name to: " + name + ".");

        System.out.println("Before the name was modified: " + this.name);
        this.name = name;
        System.out.println("After the name was modified: " + this.name);
    }

    {
        System.out.println("This is initialize block. Will assign the variable name to: chouchou");

        System.out.println("Before the name was modified: " + this.name);
        this.name = "chouchou";
        System.out.println("After the name was modified: " + this.name);
    }

    public String getName() {
        return name;
    }

    public static void main(String[] args) {
        MyTest myTest = new MyTest("mengna");
        System.out.println(myTest.getName());
    }
}


#输出
This is initialize block. Will assign the variable name to: chouchou
Before the name was modified: wei.hu
After the name was modified: chouchou
This is constructor. Will assign the variable name to: mengna.
Before the name was modified: chouchou
After the name was modified: mengna
mengna

分析:
1. 先执行了初始化语句;
2. 再执行了初始化块;
3. 最后执行了构造函数;

示例二:

public class MyTest {

    public MyTest(String name) {
        System.out.println("This is constructor. Will assign the variable name to: " + name + ".");

        System.out.println("Before the name was modified: " + this.name);
        this.name = name;
        System.out.println("After the name was modified: " + this.name);
    }

    private String name = "wei.hu";

    {
        System.out.println("This is initialize block. Will assign the variable name to: chouchou");

        System.out.println("Before the name was modified: " + this.name);
        this.name = "chouchou";
        System.out.println("After the name was modified: " + this.name);
    }

    public String getName() {
        return name;
    }

    public static void main(String[] args) {
        MyTest myTest = new MyTest("mengna");
        System.out.println(myTest.getName());
    }
}

#结果(与示例一相同)
This is initialize block. Will assign the variable name to: chouchou
Before the name was modified: wei.hu
After the name was modified: chouchou
This is constructor. Will assign the variable name to: mengna.
Before the name was modified: chouchou
After the name was modified: mengna
mengna

分析:
尽管我们在此示例中,将初始化语句放到到构造函数后面,但是并不影响 初始化语句 先执行的事实。依然是:
1、先执行初始化语句;
2、在执行初始化块;
3、最后执构造函数;

示例三:

public class MyTest {

    public MyTest(String name) {
        System.out.println("This is constructor. Will assign the variable name to: " + name + ".");

        System.out.println("Before the name was modified: " + this.name);
        this.name = name;
        System.out.println("After the name was modified: " + this.name);
    }

    {
        System.out.println("This is initialize block. Will assign the variable name to: chouchou");

        System.out.println("Before the name was modified: " + this.name);
        this.name = "chouchou";
        System.out.println("After the name was modified: " + this.name);
    }

    private String name = "wei.hu";

    public String getName() {
        return name;
    }

    public static void main(String[] args) {
        MyTest myTest = new MyTest("mengna");
        System.out.println(myTest.getName());
    }
}


#结果
This is initialize block. Will assign the variable name to: chouchou
Before the name was modified: null
After the name was modified: chouchou
This is constructor. Will assign the variable name to: mengna.
Before the name was modified: wei.hu
After the name was modified: mengna
mengna

分析:
注意本示例的结果与上面两个示例的结果不同。
1、当我们想将成员变量name赋值为chouchou之前,发现this.name为null。也就是说初始化语句没有先执行,而是先执行了初始化块;
2、当在执行构造函数时,我们想将成员变量name赋值为mengna,发现赋值之前,this.name不再是chouchou,而是wei.hu,这说明了什么?
    因为初始化块先执行,如果紧接着执行构造函数的话,那么在构造函数赋值语句执行之前,this.name应该是chouchou才对。但是在构造函数赋值语句执行之前,this.name的值变成了wei.hu,那么足以证明:
    1)初始化块先执行;
    2)下来执行了初始化语句;
    3)最后执行了构造函数;

结论:
通过上面三个示例,我们可以发现,对于非静态的成员变量:

  1. 初始化语句、初始化块,总是先于构造函数执行;
  2. 初始化语句、初始化块的和执行顺序,取决于 初始化语句、初始化块在代码中的书写顺序。写在上面的先执行。

静态变量

我们在这里也分析三种结构:

  1. 静态初始化语句;
  2. 静态初始化块;
  3. 构造函数;

示例一:

public class MyTest {

    public static String name = "wei.hu";

    public MyTest() {
        System.out.println("This is constructor. Will assign the variable name to: chouchou");

        System.out.println("Before the name was modified: " + name);
        name = "chouchou";
        System.out.println("After the name was modified: " + name);
    }

    static {
        System.out.println("This is static initialize block. Will assign the variable name to: mengna");

        System.out.println("Before the name was modified: " + name);
        name = "mengna";
        System.out.println("After the name was modified: " + name);
    }

    public static void main(String[] args) {
        System.out.println(MyTest.name);
    }
}


#结果
This is static initialize block. Will assign the variable name to: mengna
Before the name was modified: wei.hu
After the name was modified: mengna
mengna

分析:
通过打印输出,我们发现在执行静态初始快之前,静态变量name已经初始化为wei.hu了。也就是说:
1、静态初始化语句先执行;
2、下来执行静态初始化块;
3、构造函数未执行;

示例二:

public class MyTest {

    public MyTest() {
        System.out.println("This is constructor. Will assign the variable name to: chouchou");

        System.out.println("Before the name was modified: " + MyTest.name);
        name = "chouchou";
        System.out.println("After the name was modified: " + MyTest.name);
    }

    static {
        System.out.println("This is static initialize block. Will assign the variable name to: mengna");

        System.out.println("Before the name was modified: " + MyTest.name);
        name = "mengna";
        System.out.println("After the name was modified: " + MyTest.name);
    }

    public static String name = "wei.hu";

    public static void main(String[] args) {
        System.out.println(MyTest.name);
    }
}


#结果
This is static initialize block. Will assign the variable name to: mengna
Before the name was modified: null
After the name was modified: mengna
wei.hu

分析:
初始化块在对静态变量赋值之前,发现MyTest.name的值为空。 在最后打印出MyTest.name时,发现输出的值是wei.hu,而不是mengna。也就是说,在初始化块执行之后,执行了静态初始化语句。
1、先执行静态初始化块;
2、再执行静态初始化语句;
3、构造函数未执行;

结论:
对于静态字段,初始化有如下规则:
1. 若静态初始化语句在前,静态代码块在后,则先执行静态初始化语句;
2. 若静态代码块在前,静态初始化语句在后,则先执行静态代码块;