Java成员变量初始化顺序

红太狼 2022-04-14 05:09 308阅读 0赞

Java中成员变量分为两类：1、类变量 2、实例变量

## 1、类变量初始化 ##

类变量简单来说就是被`static`修饰的变量，类变量属于类本身

### 1.1 初始化时机 ###

*  定义类变量时指定初始化
 *  静态代码块中对类变量指定初始化

### 1.2 初始化顺序 ###

上述两种方式在程序中的排列顺序即为执行顺序

class ClassVarInit{
    	static int value=10;
    	static{
    		value=100;
    		name="静态代码块中对类变量指定初始化"；
    	}
    	static String name="定义类变量时指定初始化"；
    	public static void main(String[] args){
    		System.out.println(value);//输出:100
    		System.out.println(name);//输出:"定义类变量时指定初始化"
    	}
    }

利用`javap`工具可以明显的看到类变量的赋值顺序

javap -c ClassVarInit.class

Compiled from "ClassVarInit.java"
    public class demo.ClassVarInit {
      static int value;
    
      static java.lang.String name;
    
      public demo.ClassVarInit();
        Code:
           0: aload_0
           1: invokespecial #1                  // Method java/lang
    /Object."<init>":()V
           4: return
    
      public static void main(java.lang.String[]);
        Code:
           0: getstatic     #2                  // Field java/lang/
    System.out:Ljava/io/PrintStream;
           3: getstatic     #3                  // Field value:I
           6: invokevirtual #4                  // Method java/io/P
    rintStream.println:(I)V
           9: getstatic     #2                  // Field java/lang/
    System.out:Ljava/io/PrintStream;
          12: getstatic     #5                  // Field name:Ljava
    /lang/String;
          15: invokevirtual #6                  // Method java/io/P
    rintStream.println:(Ljava/lang/String;)V
          18: return
    
      static {};
        Code:
           0: bipush        10
           2: putstatic     #3                  // Field value:I
           5: bipush        100               
           7: putstatic     #3                  // Field value:I
          10: ldc           #7                  // String 静态代码
    块中对类变量指定初始化
          12: putstatic     #5                  // Field name:Ljava
    /lang/String;
          15: ldc           #8                  // String 定义类变
    量时指定初始化
          17: putstatic     #5                  // Field name:Ljava
    /lang/String;
          20: return
    }

## 2、实例变量初始化 ##

实例变量属于实例对象

### 2.1 初始化时机 ###

*  定义实例变量时指定初始化
 *  非静态代码块中对实例变量指定初始化
 *  构造器中对实例变量指定初始化

### 2.2 初始化顺序 ###

前两种方式在程序中的排列顺序即为执行顺序，且都优先于构造器初始化

class InstanceVarInit{
    	public InstanceVarInit(int value,String name){
    		this.value=value;	
    		this.name=name;
    	}
    	int value=10;
    	{
    		value=100;
    		name="非静态代码块中对实例变量指定初始化"；
    	}
    	String name="定义实例变量时指定初始化"；
    	public static void main(String[] args){
    		InstanceVarInit obj=new InstanceVarInit(1000,"构造器中对实例变量指定初始化");	
    		System.out.println(obj.value);//输出:1000
    		System.out.println(obj.name);//输出:"构造器中对实例变量指定初始化"
    	}
    }

javap -c InstanceVarInit.class

Compiled from "InstanceVarInit.java"
    public class demo1.InstanceVarInit {
      int value;
    
      java.lang.String name;
    
      public demo1.InstanceVarInit(int, java.lang.String);
        Code:
           0: aload_0
           1: invokespecial #1                  // Method java/lang
    /Object."<init>":()V
           4: aload_0
           5: bipush        10
           7: putfield      #2                  // Field value:I
          10: aload_0
          11: bipush        100
          13: putfield      #2                  // Field value:I
          16: aload_0
          17: ldc           #3                  // String 非静态代
    码块中对实例变量指定初始化
          19: putfield      #4                  // Field name:Ljava
    /lang/String;
          22: aload_0
          23: ldc           #5                  // String 定义实例
    变量时指定初始化
          25: putfield      #4                  // Field name:Ljava
    /lang/String;
          28: aload_0
          29: iload_1
          30: putfield      #2                  // Field value:I
          33: aload_0
          34: aload_2
          35: putfield      #4                  // Field name:Ljava
    /lang/String;
          38: return
    
      public static void main(java.lang.String[]);
        Code:
           0: new           #6                  // class demo1/Inst
    anceVarInit
           3: dup
           4: sipush        1000
           7: ldc           #7                  // String 构造器中
    对实例变量指定初始化
           9: invokespecial #8                  // Method "<init>":
    (ILjava/lang/String;)V
          12: astore_1
          13: getstatic     #9                  // Field java/lang/
    System.out:Ljava/io/PrintStream;
          16: aload_1
          17: getfield      #2                  // Field value:I
          20: invokevirtual #10                 // Method java/io/P
    rintStream.println:(I)V
          23: getstatic     #9                  // Field java/lang/
    System.out:Ljava/io/PrintStream;
          26: aload_1
          27: getfield      #4                  // Field name:Ljava
    /lang/String;
          30: invokevirtual #11                 // Method java/io/P
    rintStream.println:(Ljava/lang/String;)V
          33: return
    }