Volatile boolean vs AtomicBoolean

问题:

What does AtomicBoolean do that a volatile boolean cannot achieve?

回答1:

They are just totally different. Consider this example of a volatile integer:

volatile int i = 0;
void incIBy5() {
    i += 5;
}

If two threads call the function concurrently, i might be 5 afterwards, since the compiled code will be somewhat similar to this (except you cannot synchronize on int):

void incIBy5() {
    int temp;
    synchronized(i) { temp = i }
    synchronized(i) { i = temp + 5 }
}

If a variable is volatile, every atomic access to it is synchronized, but it is not always obvious what actually qualifies as an atomic access. With an Atomic* object, it is guaranteed that every method is "atomic".

Thus, if you use an AtomicInteger and getAndAdd(int delta), you can be sure that the result will be 10. In the same way, if two threads both negate a boolean variable concurrently, with an AtomicBoolean you can be sure it has the original value afterwards, with a volatile boolean, you can't.

So whenever you have more than one thread modifying a field, you need to make it atomic or use explicit synchronization.

The purpose of volatile is a different one. Consider this example

volatile boolean stop = false;
void loop() {
    while (!stop) { ... }
}
void stop() { stop = true; }

If you have a thread running loop() and another thread calling stop(), you might run into an infinite loop if you omit volatile, since the first thread might cache the value of stop. Here, the volatile serves as a hint to the compiler to be a bit more careful with optimizations.

回答2:

I use volatile fields when said field is ONLY UPDATED by its owner thread and the value is only read by other threads, you can think of it as a publish/subscribe scenario where there are many observers but only one publisher. However if those observers must perform some logic based on the value of the field and then push back a new value then I go with Atomic* vars or locks or synchronized blocks, whatever suits me best. In many concurrent scenarios it boils down to get the value, compare it with another one and update if necessary, hence the compareAndSet and getAndSet methods present in the Atomic* classes.

Check the JavaDocs of the java.util.concurrent.atomic package for a list of Atomic classes and an excellent explanation of how they work (just learned that they are lock-free, so they have an advantage over locks or synchronized blocks)

回答3:

You can't do compareAndSet, getAndSet as atomic operation with volatile boolean (unless of course you synchronize it).

回答4:

AtomicBoolean has methods that perform their compound operations atomically and without having to use a synchronized block. On the other hand, volatile boolean can only perform compound operations if done so within a synchronized block.

The memory effects of reading/writing to volatile boolean are identical to the get and set methods of AtomicBoolean respectively.

For example the compareAndSet method will atomically perform the following (without a synchronized block):

if (value == expectedValue) {
    value = newValue;
    return true;
} else {
    return false;
}

Hence, the compareAndSet method will let you write code that is guaranteed to execute only once, even when called from multiple threads. For example:

final AtomicBoolean isJobDone = new AtomicBoolean(false);

...

if (isJobDone.compareAndSet(false, true)) {
    listener.notifyJobDone();
}

Is guaranteed to only notify the listener once (assuming no other thread sets the AtomicBoolean back to false again after it being set to true).

回答5:

volatile keyword guarantees happens-before relationship among threads sharing that variable. It doesn't guarantee you that 2 or more threads won't interrupt each other while accessing that boolean variable.

回答6:

If there are multiple threads accessing class level variable then each thread can keep copy of that variable in its threadlocal cache.

Making the variable volatile will prevent threads from keeping the copy of variable in threadlocal cache.

Atomic variables are different and they allow atomic modification of their values.

回答7:

Boolean primitive type is atomic for write and read operations, volatile guarantees the happens-before principle. So if you need a simple get() and set() then you don't need the AtomicBoolean.

On the other hand if you need to implement some check before setting the value of a variable, e.g. "if true then set to false", then you need to do this operation atomically as well, in this case use compareAndSet and other methods provided by AtomicBoolean, since if you try to implement this logic with volatile boolean you'll need some synchronization to be sure that the value has not changed between get and set.

回答8:

Volatile boolean vs AtomicBoolean

The Atomic* classes wrap a volatile primitive of the same type. From the source:

public class AtomicLong extends Number implements java.io.Serializable {
   ...
   private volatile long value;
   ...
   public final long get() {
       return value;
   }
   ...
   public final void set(long newValue) {
       value = newValue;
   }

So if all you are doing is getting and setting a Atomic* then you might as well just have a volatile field instead.

What does AtomicBoolean do that a volatile boolean cannot achieve?

What the Atomic* classes give you however, are methods that provide more advanced functionality such as incrementAndGet(), compareAndSet(), and others that implement multiple operations (get/increment/set, test/set) without locking. That's why the Atomic* classes are so powerful.

For example, if multiple threads are using the following code using ++, there will be race conditions because ++ is actually: get, increment, and set.

private volatile value;
...
// race conditions here
value++;

However, the following code will work in a multi-threaded environment safely:

private final AtomicLong value = new AtomicLong();
...
value.incrementAndGet();

It's also important to note that wrapping your volatile field using Atomic* class is a good way to encapsulate the critical shared resource from an object standpoint. This means that developers can't just deal with the field assuming it is not shared possibly injecting problems with a field++; or other code that introducing race conditions.

回答9:

Remember the IDIOM -

READ - MODIFY- WRITE this you can't achieve with volatile

回答10:

If you have only one thread modifying your boolean, you can use a volatile boolean (usually you do this to define a stop variable checked in the thread's main loop).

However, if you have multiple threads modifying the boolean, you should use an AtomicBoolean. Else, the following code is not safe:

boolean r = !myVolatileBoolean;

This operation is done in two steps:

1. The boolean value is read.
2. The boolean value is written.

If an other thread modify the value between #1 and 2#, you might got a wrong result. AtomicBoolean methods avoid this problem by doing steps #1 and #2 atomically.