Java并发-创建后更改可变对象

Question

I've created an Employee Object in Thread A. An Employee class is mutable . In the same thread A, I've updated employee salary and change other fields. Then I put the object into a map and I do not access Employee object from Thread A anymore.

Employee empl = new Employee("Jhon", 12000);
empl.setSalary(9000);
Map<String, Employee> employees = new ConcurrentHashMap<>();

And there is another Thread B, which constantly iterating over the map values in infinite loop and reading Employee salaries and other fields.

Is there any chances that Thread B will see the salary used when the object has been constructed (12000), not the updated one? (9000)

Please note, I do not update the same object from the different threads at the same time.

Answer 1

Is there any chances that Thread B will see the salary used when the object has been constructed (12000), not the updated one? (9000)

Given:

1. Employee employee = new Employee("John", 12000) in thread A
2. employee.setSalary(9000) in thread A
3. employees.put("someKey", employee) in thread A
4. retrieving the employee from the employees map (map is a ConcurrentMap ) in thread B
5. employee.getSalary() in thread B

thread B guaranteed to see only updated value (9000)

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From ConcurrentMap 's javadoc :

Memory consistency effects: As with other concurrent collections, actions in a thread prior to placing an object into a ConcurrentMap as a key or value happen-before actions subsequent to the access or removal of that object from the ConcurrentMap in another thread.

There is a happens-before relation between put ting an Employee into a ConcurrentMap and its subsequent retrieval by threadB .

It implies that there is also happens-before relation between setSalary action by thread A and getSalary by thread B.

So thread B will see 9000