The document discusses various aspects of concurrency in Java such as synchronized blocks, volatile variables, and reordering issues. It provides examples of how to properly synchronize access to shared mutable data using locks and compares the behavior of synchronized and volatile. The final sections cover atomicity in Java and how the volatile keyword prevents instruction reordering problems but not all data races.

1. ~Java ~
@hakurai

3. stopRequested
Java HotSpot

4. while(!stopRequested){ if(!stopRequested){
i++; while(true){
} i++;
}
}

5. while(!stopRequested){ if(!stopRequested){
i++; while(true){
} i++;
}
}

8. Java
synchronized
volatile

9. synchronized
synchronized
synchronized

11. private final Object lock = new Object();
private List<String> list;
public void add(String text){
synchronized (lock){
listadd(text);
}
}

12. private final Object lock = new Object();
private List<String> list;
public void add(String text){
synchronized (lock){
listadd(text);
}
}

13. private final Object lock = new Object();
private List<String> list;
public void add(String text){
synchronized (lock){
listadd(text); lock
}
}

16. private final Object lock = new Object();
private List<String> list;
public String get(int index){
synchronized (lock){
return list.get(index);
}
}

17. private final Object lock = new Object();
private List<String> list;
public String get(int index){
synchronized (lock){
return list.get(index); list
}
}

18. private final Object lock = new Object();
private List<String> list;
public void add(String text){
synchronized (lock){
list.add(text);
}
}
public String get(int index){
return list.get(index);
}

19. private final Object lock = new Object();
private List<String> list;
public void add(String text){
synchronized (lock){
list.add(text);
list
}
}
IllegalArgumentException
public String get(int index){
return list.get(index);
}

20. private final Object lock = new Object();
private List<String> list;
public void add(String text){
synchronized (lock){
list.add(text);
list
}
}
IllegalArgumentException
public String get(int index){
return list.get(index);
}

21. // Java

22. 1
2
3
Java
null ( )

24. !
1 2 N

26. 1 2 N

27. T

28. (atomic)

29. from to

30. from to

35. 1

36. 1

37. 1

38. 2

39. 2

40. 2

41. 3

42. 3

43. 3

44. 4

45. 4

46. 4

47. 1

48. 2

49. 2

50. 2

51. 3
V T A
P
V T
B
Q

52. 3
P Q

53. 3
(1)P Q
P
(2)Q
(3)Q
(4)P

55. 1

56. 2
L T L
V T
V

57. private final Object lock = new Object();
private List<String> list;
private String get(int index){
synchronized (lock){
return list.get(index);
}
}

58. private final Object lock = new Object();
private List<String> list;
private String get(int index){
synchronized (lock){
return list.get(index); list
}
}

61. volatile
volatile

62. volatile 1
V T V
T
V T V
T

63. volatile 2
V T V
T
V T V
T

64. volatile
volatile

65. volatile 3
volatile (reordering)
private int num = 0;
private volatile boolean initialized;
public void run(){
num = 100;
initialized = true;
}

66. volatile 3
volatile (reordering)
private int num = 0;
private volatile boolean initialized;
public void run(){
num = 100; num
initialized = true;
}

67. volatile
Java

68. T

70. Java
Java
Effective Java