How to create a static object variable of its own class?

Preface

C： In the past two weeks, I have been frantically interviewing and recruiting the project team , Last night, 10 Some more , The product director sent a question in the interview group , What is the running result , The title is as follows ：

class Singleton {
    
    private static Singleton singleton = new Singleton();
    public static int count1;
    public static int count2 = 3;

    private Singleton() {
    
        count1++;
        count2++;
    }

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

public class Test {
    
    public static void main(String[] args) {
    
        Singleton singleTon = Singleton.getInstance();
        System.out.println("count1=" + singleTon.count1);
        System.out.println("count2=" + singleTon.count2);
    }
}

This aroused our enthusiasm for doing Technology , Then analyze it .

Simple analysis

1、 After a brief look at the topic , This is not a singleton class that adopts the hungry Han singleton pattern , Next, of course, is to find the program entry .

2、 stay Test Class main In the method , First, I call Singleton Class getInstance() Method , Obviously, this is to get Singleton The only object of this singleton class （ example ） 了 .

3、 Then get the unique object （ example ） after , Output Singleton Two static member variables of class count1、count2 Value .（ Although calling static information by object name is not recommended , But it has no effect on the results ）

4、 See here , There are no other output statements in the two classes , So the final result is to see count1、count2 Output value of .

5、 The key is coming. ： Calling getInstance() Before the method , because Singleton Class not loaded , So you must load the class first , because count1、count2、Singleton The only object of （ example ） It's all static , So they will be loaded as the class is loaded . among int Type of count1 The variable does not specify an initial value , The default is 0,count2 The initial value specified is 3, Singleton Class （ example ） To create a constructor that will call , The construction method is also right count1 and count2 It's self increasing 1 Arithmetic , The result is naturally count1 yes 1,count2 yes 4.

After such a spark and lightning analysis , Confidently sent the answer to the Group .

count1=1
count2=4

In-depth analysis of

Obviously wrong , Otherwise, it will not be recorded separately . The reason for the wrong answer , Because the loading order of static information is ignored , The loading order of static information is determined by the coding order , The preconceived ones in the above analysis count1 and count2 Finished loading , But in fact, the first implementation is Singleton The only object of （ example ） Create and assign variables , Then there is implementation count1、count2.

We can go through javap -c Singleton.class Disassemble the bytecode file , After anti exchange compilation JVM The instructions are as follows ：

Compiled from "Test.java"
class org.example.Singleton {
    
  public static int count1;

  public static int count2;

  public static org.example.Singleton getInstance();
    Code:
       //  obtain  singleton  Static object variables , And push its value to the top of the stack 
       0: getstatic     #4                  // Field singleton:Lorg/example/Singleton;
       //  Return from current method  singleton  Object reference 
       3: areturn

  static {
    };
    Code:
       // 1、 establish  Singleton  Class object , And assign it to the static object variable  singleton
       // 1.1  Create objects 
       0: new           #5                  // class org/example/Singleton
       // 1.2  Copy the stack top value and push the copied value into the stack top 
       3: dup                        
       // 1.3  call  Singleton  Class constructor ,count1  and  count2  Self increasing  1, here  count1  by  1,count2  by  1
       4: invokespecial #6                  // Method "<init>":()V
       // 1.4  The object is created successfully, and the object reference is assigned to the static object variable  singleton
       7: putstatic     #4                  // Field singleTon:Lorg/example/Singleton;
      
      // 2、 take  3  Assign a value to  count2
      // 2.1  take  int  type  3  Push to top of stack 
      10: iconst_3                         
      // 2.2  by  count2  Static variable assignment 
      11: putstatic     #3                  // Field count2:I
      
      // 3、 The ending method 
      14: return                            
}

Obviously ,count2 Finally, it is assigned 3 了 .

The correct answer is ：

count1=1
count2=3

Extra extension

What if you really want to get the previous results ？

count1=1
count2=4

Only need to count1、count2 The order of the two static variables is adjusted to Singleton Class （ example ） Just above the variable .

class Singleton {
    
    public static int count1;
    public static int count2 = 3;
    private static Singleton singleton = new Singleton();

    private Singleton() {
    
        count1++;
        count2++;
    }

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

public class Test {
    
    public static void main(String[] args) {
    
        Singleton singleTon = Singleton.getInstance();
        System.out.println("count1=" + singleTon.count1);
        System.out.println("count2=" + singleTon.count2);
    }
}

We pass... Again javap -c Singleton.class Disassemble the bytecode file , After anti exchange compilation JVM The instructions are as follows ：

Compiled from "Test.java"
class org.example.Singleton {
    
  public static int count1;

  public static int count2;

  public static org.example.Singleton getInstance();
    Code:
       //  obtain  singleton  Static object variables , And push its value to the top of the stack 
       0: getstatic     #4                  // Field singleton:Lorg/example/Singleton;
       //  Return from current method  singleton  Object reference 
       3: areturn

  static {
    };
    Code:
       // 1、 take  3  Assign a value to  count2,count2  This is the case  3
       // 1.1  take  int  type  3  Push to top of stack 
       0: iconst_3
       // 1.2  by  count2  Static variable assignment 
       1: putstatic     #3                  // Field count2:I
       
       // 2、 establish  Singleton  Class object , And assign it to the static object variable  singleton
       // 2.1  Create objects 
       4: new           #5                  // class org/example/Singleton
       // 2.2  Copy the stack top value and push the copied value into the stack top 
       7: dup
       // 2.3  call  Singleton  Class constructor ,count1  and  count2  Self increasing  1,count1  This is the case  1,count2  This is the case  4
       8: invokespecial #6                  // Method "<init>":()V
      // 2.4  The object is created successfully, and the object reference is assigned to the static object variable  singleton
      11: putstatic     #4                  // Field singleTon:Lorg/example/Singleton;

      // 3、 The ending method 
      14: return
}

Obviously ,count2 Finally, it was increased to 4 了 .