Java中非静态成员变量、静态成员变量的初始化时机。

2018年05月22日 11:48:11 SilenceCarrot 阅读数 421

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：

成员变量初始化语句；
成员变量初始化块；
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

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

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：

初始化语句、初始化块，总是先于构造函数执行；
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：

静态初始化语句；
静态初始化块；
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；