Recently I meet with an issue using Spring which finally turns out that my bean is initialized multiple times although it is expected to be a singleton. As a result I look into the possible scenario that might bypass the expected singleton behavior.

This blog introduces two possible scenarios which will cause your singleton design fail to work as expected.

This is my singleton class in ABAP:

CLASS zcl_jerry_singleton DEFINITION
  PUBLIC
  FINAL
  CREATE PRIVATE .
  PUBLIC SECTION.
    INTERFACES if_serializable_object .
    CLASS-METHODS class_constructor .
    CLASS-METHODS get_instance
      RETURNING
        VALUE(ro_instance) TYPE REF TO zcl_jerry_singleton .
  PROTECTED SECTION.
  PRIVATE SECTION.
    CLASS-DATA so_instance TYPE REF TO zcl_jerry_singleton .
    DATA mv_name TYPE string .
    DATA mv_initialized TYPE abap_bool .
    METHODS constructor .
ENDCLASS.
CLASS ZCL_JERRY_SINGLETON IMPLEMENTATION.
* <SIGNATURE>---------------------------------------------------------------------------------------+
* | Static Public Method ZCL_JERRY_SINGLETON=>CLASS_CONSTRUCTOR
* +-------------------------------------------------------------------------------------------------+
* +--------------------------------------------------------------------------------------</SIGNATURE>
  METHOD class_constructor.
    so_instance = NEW zcl_jerry_singleton( ).
  ENDMETHOD.
* <SIGNATURE>---------------------------------------------------------------------------------------+
* | Instance Public Method ZCL_JERRY_SINGLETON->CONSTRUCTOR
* +-------------------------------------------------------------------------------------------------+
* +--------------------------------------------------------------------------------------</SIGNATURE>
  METHOD constructor.
    mv_name = 'Jerry'.
    IF mv_initialized = abap_false.
      mv_initialized = abap_true.
    ELSE.
      MESSAGE 'you are in trouble!' TYPE 'E' DISPLAY LIKE 'I'.
    ENDIF.
  ENDMETHOD.
* <SIGNATURE>---------------------------------------------------------------------------------------+
* | Static Public Method ZCL_JERRY_SINGLETON=>GET_INSTANCE
* +-------------------------------------------------------------------------------------------------+
* | [<-()] RO_INSTANCE                    TYPE REF TO ZCL_JERRY_SINGLETON
* +--------------------------------------------------------------------------------------</SIGNATURE>
  METHOD get_instance.
    ro_instance = so_instance.
  ENDMETHOD.
ENDCLASS.

DATA(lo_instance) = zcl_jerry_singleton=>get_instance( ).
DATA: s TYPE string.

CALL TRANSFORMATION id SOURCE model = lo_instance RESULT XML s.

DATA: lo_instance2 TYPE REF TO zcl_jerry_singleton.

CALL TRANSFORMATION id SOURCE XML s RESULT model = lo_instance2.

public class JerrySingleton {
	private String name;
    private JerrySingleton(){  
    	name = "Jerry";
    }  
    private  static class SingletonHolder{  
        private static final JerrySingleton INSTANCE = new JerrySingleton();  
    }  
    public static JerrySingleton getInstance()  
    {  
        return SingletonHolder.INSTANCE;  
    }  
}

Class<?> classType = JerrySingleton.class;  
Constructor<?> c = classType.getDeclaredConstructor(null);  
c.setAccessible(true);  
JerrySingleton e1 = (JerrySingleton)c.newInstance();  
JerrySingleton e2 = JerrySingleton.getInstance();  
System.out.println(e1 == e2);   

package singleton;

public class JerrySingletonImproved  
{  
    private static boolean flag = false;  
 
    private JerrySingletonImproved(){  
        synchronized(JerrySingletonImproved.class)  
        {  
            if(flag == false)  
            {  
                flag = !flag;  
            }  
            else 
            {  
                throw new RuntimeException("Singleton violated");  
            }  
        }  
    }  
 
    private  static class SingletonHolder{  
        private static final JerrySingletonImproved INSTANCE = new JerrySingletonImproved();  
    }  
 
    public static JerrySingletonImproved getInstance()  
    {  
        return SingletonHolder.INSTANCE;  
    }  
}

public enum JerrySingletonAnotherApproach {
	INSTANCE ;  
	
	private String name = "Jerry" ;
	 
	public String getName() {  
	    return this.name;  
	}  
}

System.out.println("Name:" + JerrySingletonAnotherApproach.INSTANCE.getName());

Exception in thread "main" java.lang.NoSuchMethodException: singleton.JerrySingletonAnotherApproach.<init>()
	at java.lang.Class.getConstructor0(Class.java:3082)
	at java.lang.Class.getDeclaredConstructor(Class.java:2178)
	at singleton.SingletonAttack.test3(SingletonAttack.java:31)
	at singleton.SingletonAttack.main(SingletonAttack.java:43)