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Java interface static field lazy initialization

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1

Given example

interface A 
 static int aInit() 
 System.out.println("Interface field");
 return 42;
 
 int a = aInit();


class B implements A 
 static int bInit() 
 System.out.println("Class field");
 return 42;
 
 static final int b = bInit();


A a = new B();

on both JDK8 and JDK10 prints just "Class field". Direct access to A.a spawns its initialization and "Interface field" output.

This shows that interface static field initialization is lazy, which is not true for final static class field.

I can see OpenJDK JEP draft about such laziness for classes, but is it a documented feature for interface? Or just a detail of JVM implementation?

java static lazy-initialization

share|improve this question

asked Mar 28 at 14:15

Alexey AdamovskiyAlexey Adamovskiy

51833 silver badges2727 bronze badges

• 
  
  
  

  
  

  
  
  
  
  
  

  
  
  No, it shows that using a class doesn't necessarily load its implemented interfaces, notably not when there is no need to.
  
  – kumesana
  Mar 28 at 14:20
  

  
  
  
  


• 
  
  
  

  
  1
  

  
  
  
  
  
  

  
  
  It is a documented behavior. The interface A will not be initialized as per docs.oracle.com/javase/specs/jls/se12/html/…. It is only initialized when either the field a or the method aInit() are called.
  
  – manouti
  Mar 28 at 14:27
  

  
  
  
  


• 
  
  
  

  
  

  
  
  
  
  
  

  
  
  @manouti indeed, T is a class and... – not applied for interfaces, thanks! Would you like to post it as an answer?
  
  – Alexey Adamovskiy
  Mar 28 at 14:38
  

  
  
  
  


• 
  
  
  

  
  

  
  
  
  
  
  

  
  
  @AlexeyAdamovskiy Sure, just posted it.
  
  – manouti
  Mar 28 at 14:47
  

  
  
  
  

add a comment
 | 

1

Given example

interface A 
 static int aInit() 
 System.out.println("Interface field");
 return 42;
 
 int a = aInit();


class B implements A 
 static int bInit() 
 System.out.println("Class field");
 return 42;
 
 static final int b = bInit();


A a = new B();

on both JDK8 and JDK10 prints just "Class field". Direct access to A.a spawns its initialization and "Interface field" output.

This shows that interface static field initialization is lazy, which is not true for final static class field.

I can see OpenJDK JEP draft about such laziness for classes, but is it a documented feature for interface? Or just a detail of JVM implementation?

java static lazy-initialization

share|improve this question

asked Mar 28 at 14:15

Alexey AdamovskiyAlexey Adamovskiy

51833 silver badges2727 bronze badges

• 
  
  
  

  
  

  
  
  
  
  
  

  
  
  No, it shows that using a class doesn't necessarily load its implemented interfaces, notably not when there is no need to.
  
  – kumesana
  Mar 28 at 14:20
  

  
  
  
  


• 
  
  
  

  
  1
  

  
  
  
  
  
  

  
  
  It is a documented behavior. The interface A will not be initialized as per docs.oracle.com/javase/specs/jls/se12/html/…. It is only initialized when either the field a or the method aInit() are called.
  
  – manouti
  Mar 28 at 14:27
  

  
  
  
  


• 
  
  
  

  
  

  
  
  
  
  
  

  
  
  @manouti indeed, T is a class and... – not applied for interfaces, thanks! Would you like to post it as an answer?
  
  – Alexey Adamovskiy
  Mar 28 at 14:38
  

  
  
  
  


• 
  
  
  

  
  

  
  
  
  
  
  

  
  
  @AlexeyAdamovskiy Sure, just posted it.
  
  – manouti
  Mar 28 at 14:47
  

  
  
  
  

add a comment
 | 

Given example

interface A 
 static int aInit() 
 System.out.println("Interface field");
 return 42;
 
 int a = aInit();


class B implements A 
 static int bInit() 
 System.out.println("Class field");
 return 42;
 
 static final int b = bInit();


A a = new B();

on both JDK8 and JDK10 prints just "Class field". Direct access to A.a spawns its initialization and "Interface field" output.

This shows that interface static field initialization is lazy, which is not true for final static class field.

I can see OpenJDK JEP draft about such laziness for classes, but is it a documented feature for interface? Or just a detail of JVM implementation?

java static lazy-initialization

share|improve this question

asked Mar 28 at 14:15

Alexey AdamovskiyAlexey Adamovskiy

51833 silver badges2727 bronze badges

interface A 
 static int aInit() 
 System.out.println("Interface field");
 return 42;
 
 int a = aInit();


class B implements A 
 static int bInit() 
 System.out.println("Class field");
 return 42;
 
 static final int b = bInit();


A a = new B();

share|improve this question

asked Mar 28 at 14:15

Alexey AdamovskiyAlexey Adamovskiy

51833 silver badges2727 bronze badges

share|improve this question

asked Mar 28 at 14:15

Alexey AdamovskiyAlexey Adamovskiy

51833 silver badges2727 bronze badges

asked Mar 28 at 14:15

Alexey AdamovskiyAlexey Adamovskiy

51833 silver badges2727 bronze badges

asked Mar 28 at 14:15

Alexey AdamovskiyAlexey Adamovskiy

51833 silver badges2727 bronze badges

1 Answer
1

active

oldest

votes

1

It is a documented behavior. The interface A will not be initialized as per https://docs.oracle.com/javase/specs/jls/se12/html/jls-12.html#jls-12.4.1. It is only initialized when either the field a or the method aInit() are called.

share|improve this answer

answered Mar 28 at 14:46

manoutimanouti

56.6k1313 gold badges9292 silver badges136136 bronze badges

add a comment
 | 

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1

It is a documented behavior. The interface A will not be initialized as per https://docs.oracle.com/javase/specs/jls/se12/html/jls-12.html#jls-12.4.1. It is only initialized when either the field a or the method aInit() are called.

share|improve this answer

answered Mar 28 at 14:46

manoutimanouti

56.6k1313 gold badges9292 silver badges136136 bronze badges

add a comment
 | 

1

It is a documented behavior. The interface A will not be initialized as per https://docs.oracle.com/javase/specs/jls/se12/html/jls-12.html#jls-12.4.1. It is only initialized when either the field a or the method aInit() are called.

share|improve this answer

answered Mar 28 at 14:46

manoutimanouti

56.6k1313 gold badges9292 silver badges136136 bronze badges

add a comment
 | 

It is a documented behavior. The interface A will not be initialized as per https://docs.oracle.com/javase/specs/jls/se12/html/jls-12.html#jls-12.4.1. It is only initialized when either the field a or the method aInit() are called.

share|improve this answer

answered Mar 28 at 14:46

manoutimanouti

56.6k1313 gold badges9292 silver badges136136 bronze badges

share|improve this answer

answered Mar 28 at 14:46

manoutimanouti

56.6k1313 gold badges9292 silver badges136136 bronze badges

answered Mar 28 at 14:46

manoutimanouti

56.6k1313 gold badges9292 silver badges136136 bronze badges

answered Mar 28 at 14:46

manoutimanouti

56.6k1313 gold badges9292 silver badges136136 bronze badges