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Java靜態(tài)和非靜態(tài)成員變量初始化過(guò)程解析

更新時(shí)間：2020年01月09日 15:14:36 作者：戈博折刀

這篇文章主要介紹了Java靜態(tài)和非靜態(tài)成員變量初始化過(guò)程解析,文中通過(guò)示例代碼介紹的非常詳細(xì)，對(duì)大家的學(xué)習(xí)或者工作具有一定的參考學(xué)習(xí)價(jià)值,需要的朋友可以參考下

Java中非靜態(tài)成員變量、靜態(tài)成員變量的初始化時(shí)機(jī)。

非靜態(tài)變量

我們?cè)谶@里分析三種結(jié)構(gòu)，著重分析這三種結(jié)構(gòu)的初始化順序：

成員變量初始化語(yǔ)句；
成員變量初始化塊；
構(gòu)造函數(shù)；

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(jié)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(jié)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(jié)果與上面兩個(gè)示例的結(jié)果不同。
1、當(dāng)我們想將成員變量name賦值為chouchou之前，發(fā)現(xiàn)this.name為null。也就是說(shuō)初始化語(yǔ)句沒(méi)有先執(zhí)行，而是先執(zhí)行了初始化塊；
2、當(dāng)在執(zhí)行構(gòu)造函數(shù)時(shí)，我們想將成員變量name賦值為mengna，發(fā)現(xiàn)賦值之前，this.name不再是chouchou，而是wei.hu，這說(shuō)明了什么？
  因?yàn)槌跏蓟瘔K先執(zhí)行，如果緊接著執(zhí)行構(gòu)造函數(shù)的話，那么在構(gòu)造函數(shù)賦值語(yǔ)句執(zhí)行之前，this.name應(yīng)該是chouchou才對(duì)。但是在構(gòu)造函數(shù)賦值語(yǔ)句執(zhí)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(zhí)行；
  2）下來(lái)執(zhí)行了初始化語(yǔ)句；
  3）最后執(zhí)行了構(gòu)造函數(shù)；

結(jié)論：

通過(guò)上面三個(gè)示例，我們可以發(fā)現(xiàn)，對(duì)于非靜態(tài)的成員變量：

初始化語(yǔ)句、初始化塊，總是先于構(gòu)造函數(shù)執(zhí)行；

初始化語(yǔ)句、初始化塊的和執(zhí)行順序，取決于初始化語(yǔ)句、初始化塊在代碼中的書寫順序。寫在上面的先執(zhí)行。

靜態(tài)變量

我們?cè)谶@里也分析三種結(jié)構(gòu)：

靜態(tài)初始化語(yǔ)句；
靜態(tài)初始化塊；
構(gòu)造函數(shù)；

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(jié)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(guò)打印輸出，我們發(fā)現(xiàn)在執(zhí)行靜態(tài)初始快之前，靜態(tài)變量name已經(jīng)初始化為wei.hu了。也就是說(shuō)：
1、靜態(tài)初始化語(yǔ)句先執(zhí)行；
2、下來(lái)執(zhí)行靜態(tài)初始化塊；
3、構(gòu)造函數(shù)未執(zhí)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(jié)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(duì)靜態(tài)變量賦值之前，發(fā)現(xiàn)MyTest.name的值為空。 在最后打印出MyTest.name時(shí)，發(fā)現(xiàn)輸出的值是wei.hu，而不是mengna。也就是說(shuō)，在初始化塊執(zhí)行之后，執(zhí)行了靜態(tài)初始化語(yǔ)句。
1、先執(zhí)行靜態(tài)初始化塊；
2、再執(zhí)行靜態(tài)初始化語(yǔ)句；
3、構(gòu)造函數(shù)未執(zhí)行；
---------------------

結(jié)論：

對(duì)于靜態(tài)字段，初始化有如下規(guī)則：

1. 若靜態(tài)初始化語(yǔ)句在前，靜態(tài)代碼塊在后，則先執(zhí)行靜態(tài)初始化語(yǔ)句；

2. 若靜態(tài)代碼塊在前，靜態(tài)初始化語(yǔ)句在后，則先執(zhí)行靜態(tài)代碼塊；

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