类各成员加载顺序

我们从两个例子看起：

例子一

package classLoader;

class Singleton{
    
    private static Singleton singleton = new Singleton();
    public static int counter1;
    public static int counter2=4;

    private Singleton(){
        Singleton.counter1++;
        Singleton.counter2++;
    }

    public static Singleton getInstance(){
        return singleton;
    }
}

public class SingletonTest{
    public static void main(String[] args){
        Singleton singleton = Singleton.getInstance();
        System.out.println("counter1:"+singleton.counter1);
        System.out.println("counter2:"+singleton.counter2);
    }
}

输出结果：

例子二：

package classLoader;

class Singleton{
    
    public static int counter1;
    public static int counter2=4;
    private static Singleton singleton = new Singleton();

    private Singleton(){
        Singleton.counter1++;
        Singleton.counter2++;
    }

    public static Singleton getInstance(){
        return singleton;
    }
}

public class SingletonTest{
    public static void main(String[] args){
        Singleton singleton = Singleton.getInstance();
        System.out.println("counter1:"+singleton.counter1);
        System.out.println("counter2:"+singleton.counter2);
    }
}

这两个例子只有一处不同，只是调整了 public static int counter1;  public static int counter2=4;  private static Singleton singleton = new Singleton();这三个静态变量的顺序，我们再看看结果：

这是为什么呢？原因就在于加载类时各个成员的加载顺序不同。

我们看一下类的各个成员加载顺序，直接看例子：

package classLoader;

public class Demo {  
      
        public static void main(String[] args) {  
            subs1 = new Sub();  
        }  
    }  
      
    class Super {  
      
        static int a = getA();  
      
        static {  
            System.out.println("加载Super的静态块");  
        }  
      
        int b = getB();  
      
        {  
            System.out.println("加载Super的普通块");  
        }  
      
        Super() {  
            System.out.println("加载Super的构造器");  
        }  
      
        static int getA() {  
            System.out.println("加载Super的静态变量");  
            return 1;  
        }  
      
        static int getB() {  
            System.out.println("加载Super的实例变量");  
            return 2;  
        }  
      
    }  
      
    class Sub extends Super {  
      
        static int c = getC();  
      
        static {  
            System.out.println("加载Sub的静态块");  
        }  
      
        int d = getD();  
      
        {  
            System.out.println("加载Sub的普通块");  
        }  
      
        Sub() {  
            System.out.println("加载Sub的构造器");  
        }  
      
        static int getC() {  
            System.out.println("加载Sub的静态变量");  
            return 3;  
        }  
      
        static int getD() {  
            System.out.println("加载Sub的实例变量");  
            return 4;  
        }  
    }  

结果：

从结果来看，可以看到静态成员加载顺序大于普通成员加载顺序，父类大于子类，而对于某个类的成员加载顺序依次为：静态变量 > 静态块 > 普通（实例）变量 > 普通快 > 构造器。对于相同性质的成员，从上到下依次加载。

再回头看开头的两个例子：

　　例子一，先加载private static Singleton singleton = new Singleton();然后调用构造函数，由于此时counter1和counter2还未加载，此时默认先赋值默认初始值0，经过构造函数counter1++和counter2++，分别变成1,1。然后加载 public static int counter1; public static int counter2=4;此时只有counter2重新赋值，所以最终的结果为counter1=1;counter2=4。

　　例子二，先加载 public static int counter1; public static int counter2=4;此时的counter1=0;counter2=4;然后加载private static Singleton singleton = new Singleton();，调用构造方法，构造方法体为counter1++和counter2++，所以最终counter1=1;counter2=5。