CODEWITHSUNDEEP
javamultithreading
user1300214
user1300214

Reputation:

Protection for thread safety

I understand the very basics of the public and private keywords (I'm still not sure what protected does)

I have a question regarding "protection of data members for thread safety."

For example, say I have a class MyClass that extends Thread and belonging to this class is a private data member called MyDataMember. Suppose there are public accessor and setter functions for this data member which make sure to synchronize the data member.

Now this is all fine from an "external" point of view in that other threads can only set and retrieve the data member when synchronized properly. However, what about other functions within MyClass ? Say I have another function within MyClass called DoSomething. I know I should not attempt to access MyDataMember directly, but I might forget from time to time to call the accessor/setter methods and access/set the data member directly.

My question is: Is there a keyword I can use to declare function members exclusive access to certain data members? This way, if I "accidentally" directly access a data member from a method that does not have exclusive access, then an error would occur. I just think this would make things a bit safer during development (at least for me!)

Upvotes: 0

Views: 2473

Answers (3)

joscarsson
joscarsson

Reputation: 4859

Protected keyword

Regarding the protected key word, read more here:

Short version: methods/fields marked protected can be accessed by subclasses and by classes in the same package.

Synchronization

I think you have confused the terms (or I do not understand your question properly). There is no synchronization guaranteed just because you create a setter or a getter for a field in a class. However, if you add the synchronized keyword to the method signature (public synchronized void doSomething()), you are guaranteed there will be only one thread running the method at any given time. Other calls to doSomething() will block until the thread running the method have left the method.

The volatile keyword can be used if you want to make sure all thread sees the updated reference to a (for example) immutable object (object which cannot be changed after creation). If you have two threads modifying a public String value;, the first thread might not see that the second thread has updated the reference if you do not have the volatile keyword. Read more here: When exactly do you use the volatile keyword in Java?.

I'm not sure any of this would help in your example, as you are asking for "exclusive access" to a member variable. In general, I would try to avoid mixing active object (threads) with passive objects (data). Perhaps you could instead make the MyDataMember thread-safe (perhaps by using synchronized), and you will not need to care about how you access it?

Upvotes: 0

Tomas
Tomas

Reputation: 156

No, there isn't.

Protected allows a variable to be modified by classes that inherit the class.

If you want to protect your data put it in a separate class with private members, then you can only use setters and getters.

Upvotes: 4

huseyin tugrul buyukisik
huseyin tugrul buyukisik

Reputation: 11910

Volatile keyword hints about a variable to be accessed from different threads.

volatile int a;

You need to access using synchronized keyword to make a body(or a method) a thread-safe around the variable.

// in thread 1
sycnhronized(lockedItem)
{
       a++; // just a++ alone may not be tread-safe because being non-atomic


}

// in thread 2
sycnhronized(lockedItem)
{
       a--; // just a-- alone may not be tread-safe because being non-atomic


}

Upvotes: 1

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