【Java并发】原子类源码分析之AtomicLong
JDK1.8
// 大部分和AtomicInteger没有太多差别，只标记不同的地方
public class AtomicLong extends Number implements java.io.Serializable {
private static final long serialVersionUID = 1927816293512124184L;
// setup to use Unsafe.compareAndSwapLong for updates
// 通过unsafe来进行更新赋值
private static final Unsafe unsafe = Unsafe.getUnsafe();
private static final long valueOffset;
/**
* Records whether the underlying JVM supports lockless
* compareAndSwap for longs. While the Unsafe.compareAndSwapLong
* method works in either case, some constructions should be
* handled at Java level to avoid locking user-visible locks.
*/
// 这里VM_SUPPORTS_LONG_CAS是一个静态变量，加载时调用VMSupportsCS8方法
// VMSupportsCS8判断当前的JVM是否支持无锁的CAS，只调用一次并缓存下来
static final boolean VM_SUPPORTS_LONG_CAS = VMSupportsCS8();
/**
* Returns whether underlying JVM supports lockless CompareAndSet
* for longs. Called only once and cached in VM_SUPPORTS_LONG_CAS.
*/
private static native boolean VMSupportsCS8();
static {
try {
valueOffset = unsafe.objectFieldOffset
(AtomicLong.class.getDeclaredField("value"));
} catch (Exception ex) { throw new Error(ex); }
}
private volatile long value;
/**
* Creates a new AtomicLong with the given initial value.
*
* @param initialValue the initial value
*/
public AtomicLong(long initialValue) {
value = initialValue;
}
/**
* Creates a new AtomicLong with initial value {@code 0}.
*/
public AtomicLong() {
}
/**
* Gets the current value.
*
* @return the current value
*/
public final long get() {
return value;
}
/**
* Sets to the given value.
*
* @param newValue the new value
*/
public final void set(long newValue) {
value = newValue;
}
/**
* Eventually sets to the given value.
*
* @param newValue the new value
* @since 1.6
*/
public final void lazySet(long newValue) {
unsafe.putOrderedLong(this, valueOffset, newValue);
}
/**
* Atomically sets to the given value and returns the old value.
*
* @param newValue the new value
* @return the previous value
*/
public final long getAndSet(long newValue) {
return unsafe.getAndSetLong(this, valueOffset, newValue);
}
/**
* Atomically sets the value to the given updated value
* if the current value {@code ==} the expected value.
*
* @param expect the expected value
* @param update the new value
* @return {@code true} if successful. False return indicates that
* the actual value was not equal to the expected value.
*/
public final boolean compareAndSet(long expect, long update) {
return unsafe.compareAndSwapLong(this, valueOffset, expect, update);
}
/**
* Atomically sets the value to the given updated value
* if the current value {@code ==} the expected value.
*
* <p><a href="package-summary.html#weakCompareAndSet">May fail
* spuriously and does not provide ordering guarantees</a>, so is
* only rarely an appropriate alternative to {@code compareAndSet}.
*
* @param expect the expected value
* @param update the new value
* @return {@code true} if successful
*/
public final boolean weakCompareAndSet(long expect, long update) {
return unsafe.compareAndSwapLong(this, valueOffset, expect, update);
}
/**
* Atomically increments by one the current value.
*
* @return the previous value
*/
public final long getAndIncrement() {
return unsafe.getAndAddLong(this, valueOffset, 1L);
}
/**
* Atomically decrements by one the current value.
*
* @return the previous value
*/
public final long getAndDecrement() {
return unsafe.getAndAddLong(this, valueOffset, -1L);
}
/**
* Atomically adds the given value to the current value.
*
* @param delta the value to add
* @return the previous value
*/
public final long getAndAdd(long delta) {
return unsafe.getAndAddLong(this, valueOffset, delta);
}
/**
* Atomically increments by one the current value.
*
* @return the updated value
*/
public final long incrementAndGet() {
return unsafe.getAndAddLong(this, valueOffset, 1L) + 1L;
}
/**
* Atomically decrements by one the current value.
*
* @return the updated value
*/
public final long decrementAndGet() {
return unsafe.getAndAddLong(this, valueOffset, -1L) - 1L;
}
/**
* Atomically adds the given value to the current value.
*
* @param delta the value to add
* @return the updated value
*/
public final long addAndGet(long delta) {
return unsafe.getAndAddLong(this, valueOffset, delta) + delta;
}
/**
* Atomically updates the current value with the results of
* applying the given function, returning the previous value. The
* function should be side-effect-free, since it may be re-applied
* when attempted updates fail due to contention among threads.
*
* @param updateFunction a side-effect-free function
* @return the previous value
* @since 1.8
*/
public final long getAndUpdate(LongUnaryOperator updateFunction) {
long prev, next;
do {
prev = get();
next = updateFunction.applyAsLong(prev);
} while (!compareAndSet(prev, next));
return prev;
}
/**
* Atomically updates the current value with the results of
* applying the given function, returning the updated value. The
* function should be side-effect-free, since it may be re-applied
* when attempted updates fail due to contention among threads.
*
* @param updateFunction a side-effect-free function
* @return the updated value
* @since 1.8
*/
public final long updateAndGet(LongUnaryOperator updateFunction) {
long prev, next;
do {
prev = get();
next = updateFunction.applyAsLong(prev);
} while (!compareAndSet(prev, next));
return next;
}
/**
* Atomically updates the current value with the results of
* applying the given function to the current and given values,
* returning the previous value. The function should be
* side-effect-free, since it may be re-applied when attempted
* updates fail due to contention among threads. The function
* is applied with the current value as its first argument,
* and the given update as the second argument.
*
* @param x the update value
* @param accumulatorFunction a side-effect-free function of two arguments
* @return the previous value
* @since 1.8
*/
public final long getAndAccumulate(long x,
LongBinaryOperator accumulatorFunction) {
long prev, next;
do {
prev = get();
next = accumulatorFunction.applyAsLong(prev, x);
} while (!compareAndSet(prev, next));
return prev;
}
/**
* Atomically updates the current value with the results of
* applying the given function to the current and given values,
* returning the updated value. The function should be
* side-effect-free, since it may be re-applied when attempted
* updates fail due to contention among threads. The function
* is applied with the current value as its first argument,
* and the given update as the second argument.
*
* @param x the update value
* @param accumulatorFunction a side-effect-free function of two arguments
* @return the updated value
* @since 1.8
*/
public final long accumulateAndGet(long x,
LongBinaryOperator accumulatorFunction) {
long prev, next;
do {
prev = get();
next = accumulatorFunction.applyAsLong(prev, x);
} while (!compareAndSet(prev, next));
return next;
}
/**
* Returns the String representation of the current value.
* @return the String representation of the current value
*/
public String toString() {
return Long.toString(get());
}
/**
* Returns the value of this {@code AtomicLong} as an {@code int}
* after a narrowing primitive conversion.
* @jls 5.1.3 Narrowing Primitive Conversions
*/
public int intValue() {
return (int)get();
}
/**
* Returns the value of this {@code AtomicLong} as a {@code long}.
*/
public long longValue() {
return get();
}
/**
* Returns the value of this {@code AtomicLong} as a {@code float}
* after a widening primitive conversion.
* @jls 5.1.2 Widening Primitive Conversions
*/
public float floatValue() {
return (float)get();
}
/**
* Returns the value of this {@code AtomicLong} as a {@code double}
* after a widening primitive conversion.
* @jls 5.1.2 Widening Primitive Conversions
*/
public double doubleValue() {
return (double)get();
}
}
