Java变量初始化详解-优快云博客

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Java 变量类型

实例变量；
类变量；

Java初始化途径

实例变量

声明时，初始化；
非静态初始化块内，初始化；
构造函数内，初始化；

实例1:

public class bean1 {
	int d;
	int e;
	int c;
	int b;
	int f;
	{
		c = 3;
	}
	int a = 1;
	{
		b = 2;
	}
	public bean1() {
		d = 4;
		f = 5;
		e = 6;
	}
	public int geta() {
			return a;
	}
}

使用javap -c bean1.class查看反汇编代码如下：

Compiled from "bean1.java"
public class initialization.bean1 {
  int d;
  int e;
  int c;
  int b;
  int f;
  int a;
  public initialization.bean1();
    Code:
       0: aload_0
       1: invokespecial #1                  // Method java/lang/Object."<init>":()V
       4: aload_0
       5: iconst_3　　//初始化c
       6: putfield      #2                  // Field c:I
       9: aload_0
      10: iconst_1　　//初始化a　　
      11: putfield      #3                  // Field a:I
      14: aload_0
      15: iconst_2　　//初始化b
      16: putfield      #4                  // Field b:I
      19: aload_0
      20: iconst_4　　//初始化d
      21: putfield      #5                  // Field d:I
      24: aload_0
      25: iconst_5
      26: putfield      #6                  // Field f:I
      29: aload_0
      30: bipush        6
      32: putfield      #7                  // Field e:I
      35: return

  public int geta();
    Code:
       0: aload_0
       1: getfield      #3                  // Field a:I
       4: ireturn
}

结论:实例变量的初始化均在public initialization.bean1()当中完成，无论是在声明时直接初始化，还是在代码块当中初始化，本质均在构造函数里面进行初始化，在构造函数当中的初始化顺序取决于在定义直接初始化和代码块初始化中的顺序。

类变量

声明时，初始化；
静态初始化块内，初始化；

实例2:

package initialization;
public class bean2 {
	static int d;
	static int e = 5;
	static int c;
	int b;
	int f;
	int a = 1;
	static {
		c = 3;
		System.out.println("hello,static");
	}
	{
		b = 2;
		System.out.println("hello");
	}
	static {
		d = 4;
	}
	
	public bean2() {
		f = 6;
	}
	
	public int geta() {
			return a;
	}
}

bean2输出结果为：

hello,static
hello

javap -c bean2.class查看反汇编代码如下:

Compiled from "bean2.java"
public class initialization.bean2 {
  static int d;

  static int e;

  static int c;

  int b;

  int f;

  int a;

  public initialization.bean2();
    Code:
       0: aload_0
       1: invokespecial #1                  // Method java/lang/Object."<init>":()V
       4: aload_0
       5: iconst_1	//初始化a
       6: putfield      #2                  // Field a:I
       9: aload_0
      10: iconst_2	//初始化b
      11: putfield      #3                  // Field b:I
      14: aload_0
      15: bipush        6
      17: putfield      #4                  // Field f:I
      20: return

  public int geta();
    Code:
       0: aload_0
       1: getfield      #2                  // Field a:I
       4: ireturn

  static {};
    Code:
       0: iconst_5	//初始化e
       1: putstatic     #5                  // Field e:I
       4: iconst_3	//初始化c
       5: putstatic     #6                  // Field c:I
       8: iconst_4	//初始化d
       9: putstatic     #7                  // Field d:I
      12: return
}

结论:实例变量的初始化均在public initialization.bean2()当中完成，无论是在声明时直接初始化，还是在代码块当中初始化，本质均在构造函数里面进行初始化，在构造函数当中的初始化顺序取决于在定义直接初始化和代码块初始化中的顺序；类变量在static {}中完成，且初始顺序以代码中顺序为主，类变量在实例变量前执行；

关于java final可以查看：java final ;

转载于:https://my.oschina.net/u/3052784/blog/789989