java의 volatile

java core 2017. 4. 17. 10:08


java의 volatile 타입은 다음과 같이 정의되어 있다. 


https://docs.oracle.com/javase/specs/jls/se7/html/jls-8.html#jls-8.3.1.4



8.3.1.4. volatile Fields


The Java programming language allows threads to access shared variables (§17.1). As a rule, to ensure that shared variables are consistently and reliably updated, a thread should ensure that it has exclusive use of such variables by obtaining a lock that, conventionally, enforces mutual exclusion for those shared variables.


The Java programming language provides a second mechanism, volatile fields, that is more convenient than locking for some purposes.


A field may be declared volatile, in which case the Java Memory Model ensures that all threads see a consistent value for the variable (§17.4).





소스에서 자바의 volatile을 잘 만날 수 없는 요인 중에 하나는 자바 concurrent package의 클래스가 자바의 volatile을 감싸고 있기 때문이다. 



예를 들어  AtomicInteger의 내부를 살펴본다. value 값에 대해서 volatile라고 선언되어 있다. 


http://grepcode.com/file/repository.grepcode.com/java/root/jdk/openjdk/6-b14/java/util/concurrent/atomic/AtomicInteger.java




51 

52 public class AtomicInteger extends Number implements java.io.Serializable {

53     private static final long serialVersionUID = 6214790243416807050L;

54 

55     // setup to use Unsafe.compareAndSwapInt for updates

56     private static final Unsafe unsafe = Unsafe.getUnsafe();

57     private static final long valueOffset;

58 

59     static {

60       try {

61         valueOffset = unsafe.objectFieldOffset

62             (AtomicInteger.class.getDeclaredField("value"));

63       } catch (Exception ex) { throw new Error(ex); }

64     }

65 

66     private volatile int value;



    

72 

73     public More ...AtomicInteger(int initialValue) {

74         value = initialValue;

75     }




Posted by 김용환 '김용환'

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