這篇文章主要介紹了Java靜態和非靜態成員變數初始化過程解析,文中通過示例程式碼介紹的非常詳細，對大家的學習或者工作具有一定的參考學習價值,需要的朋友可以參考下

Java中非靜態成員變數、靜態成員變數的初始化時機。

非靜態變數

我們在這裡分析三種結構，著重分析這三種結構的初始化順序：

• 成員變數初始化語句；
• 成員變數初始化塊；
• 建構函式；

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、當我們想將成員變數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、當在執行建構函式時，我們想將成員變數name賦值為mengna，發現賦值之前，this.name不再是chouchou，而是wei.hu，這說明了什麼？
  因為初始化塊先執行，如果緊接著執行建構函式的話，那麼在建構函式賦值語句執行之前，this.name應該是chouchou才對。但是在建構函式賦值語句執行之前，this.name的值變成了wei.hu，那麼足以證明：
  1）初始化塊先執行；
  2）下來執行了初始化語句；
  3）最後執行了建構函式；

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數：

初始化語句、初始化塊，總是先於建構函式執行；

初始化語句、初始化塊的和執行順序，取決於 初始化語句、初始化塊在程式碼中的書寫順序。寫在上面的先執行。

靜態變數

我們在這裡也分析三種結構：

• 靜態初始化語句；
• 靜態初始化塊；
• 建構函式；

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：
通過列印輸出，我們發現在執行靜態初始快之前，靜態變數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、建構函式未執行；
---------------------

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數賦值之前，發現MyTest.name的值為空。 在最後打印出MyTest.name時，發現輸出的值是wei.hu，而不是mengna。也就是說，在初始化塊執行之後，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、建構函式未執行；
---------------------

結論：

對於靜態欄位，初始化有如下規則：

1. 若靜態初始化語句在前，靜態程式碼塊在後，則先執行靜態初始化語句；

2. 若靜態程式碼塊在前，靜態初始化語句在後，則先執行靜態程式碼塊；

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