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Java中非靜態成員變量、靜態成員變量的初始化時機。

2018年05月22日 11:48:11 SilenceCarrot 閱讀數 421

 

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Java中非靜態成員變量、靜態成員變量的初始化時機。

非靜態變量

我們在這里分析三種結構，着重分析這三種結構的初始化順序：

1. 成員變量初始化語句；
2. 成員變量初始化塊；
3. 構造函數；

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());
    }
}


#輸出
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());
    }
}

#結果（與示例一相同）
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());
    }
}


#結果
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：
注意本示例的結果與上面兩個示例的結果不同。
1、當我們想將成員變量name賦值為chouchou之前，發現this.name為null。也就是說初始化語句沒有先執行，而是先執行了初始化塊；
2、當在執行構造函數時，我們想將成員變量name賦值為mengna，發現賦值之前，this.name不再是chouchou，而是wei.hu，這說明了什么？
    因為初始化塊先執行，如果緊接着執行構造函數的話，那么在構造函數賦值語句執行之前，this.name應該是chouchou才對。但是在構造函數賦值語句執行之前，this.name的值變成了wei.hu，那么足以證明：
    1）初始化塊先執行；
    2）下來執行了初始化語句；
    3）最后執行了構造函數；

結論：
通過上面三個示例，我們可以發現，對於非靜態的成員變量：

1. 初始化語句、初始化塊，總是先於構造函數執行；
2. 初始化語句、初始化塊的和執行順序，取決於 初始化語句、初始化塊在代碼中的書寫順序。寫在上面的先執行。

---

靜態變量

我們在這里也分析三種結構：

1. 靜態初始化語句；
2. 靜態初始化塊；
3. 構造函數；

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);
    }
}


#結果
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：
通過打印輸出，我們發現在執行靜態初始快之前，靜態變量name已經初始化為wei.hu了。也就是說：
1、靜態初始化語句先執行；
2、下來執行靜態初始化塊；
3、構造函數未執行；
--------------------- 

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);
    }
}


#結果
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：
初始化塊在對靜態變量賦值之前，發現MyTest.name的值為空。 在最后打印出MyTest.name時，發現輸出的值是wei.hu，而不是mengna。也就是說，在初始化塊執行之后，執行了靜態初始化語句。
1、先執行靜態初始化塊；
2、再執行靜態初始化語句；
3、構造函數未執行；
--------------------- 

結論：
對於靜態字段，初始化有如下規則：
1. 若靜態初始化語句在前，靜態代碼塊在后，則先執行靜態初始化語句；
2. 若靜態代碼塊在前，靜態初始化語句在后，則先執行靜態代碼塊；