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最新评论
AtomicInteger
一、总结
1.jdk 1.8.0
2.
- 基于CAS的乐观锁实现
- 基于JNI调用硬件级别的锁实现原子性操作
- CAS算法,此算法是由unsafe的底层代码实现,它是一个原子的操作,CAS(V,E,N) V 变量,E expected 预期的旧值,N new 新值,Compare And Swap 比较交换,仅当 V中存放的值与E相等时,才将V赋值为N;否则,不做任何处理
- 语言层面不做处理,我们将其交给硬件—CPU和内存,利用CPU的多处理能力,实现硬件层面的阻塞,再加上volatile变量的特性即可实现基于原子操作的线程安全。所以说,CAS并不是无阻塞,只是阻塞并非在语言、线程方面,而是在硬件层面,所以无疑这样的操作会更快更高效
二、源码分析
package java.util.concurrent.atomic; import java.util.function.IntUnaryOperator; import java.util.function.IntBinaryOperator; import sun.misc.Unsafe; /** * An {@code int} value that may be updated atomically. See the * {@link java.util.concurrent.atomic} package specification for * description of the properties of atomic variables. An * {@code AtomicInteger} is used in applications such as atomically * incremented counters, and cannot be used as a replacement for an * {@link java.lang.Integer}. However, this class does extend * {@code Number} to allow uniform access by tools and utilities that * deal with numerically-based classes. * * @since 1.5 * @author Doug Lea */ public class AtomicInteger extends Number implements java.io.Serializable { private static final long serialVersionUID = 6214790243416807050L;
- Number,作用是Integer与其他数据类型之间的转换
- Serializable,作用是序列化
// setup to use Unsafe.compareAndSwapInt for updates private static final Unsafe unsafe = Unsafe.getUnsafe(); private static final long valueOffset; static { try { valueOffset = unsafe.objectFieldOffset (AtomicInteger.class.getDeclaredField("value")); } catch (Exception ex) { throw new Error(ex); } }
- sun.misc.Unsafe的compareAndSwapInt方法获取类字段的偏移地址,进而通过对象的起始地址与偏移量地址可以读取字段的值
private volatile int value;
- value是一个volatile变量,在内存中可见,任何线程都不允许对其进行拷贝,因此JVM可以保证任何时刻任何线程总能拿到该变量的最新值。此处value的值,可以在AtomicInteger类初始化的时候传入,也可以留空,留空则自动赋值为0
/** * Creates a new AtomicInteger with the given initial value. * * @param initialValue the initial value */ public AtomicInteger(int initialValue) { value = initialValue; } /** * Creates a new AtomicInteger with initial value {@code 0}. */ public AtomicInteger() { } /** * Gets the current value. * * @return the current value */ public final int get() { return value; } /** * Sets to the given value. * * @param newValue the new value */ public final void set(int newValue) { value = newValue; } /** * Eventually sets to the given value. * * @param newValue the new value * @since 1.6 */ public final void lazySet(int newValue) { unsafe.putOrderedInt(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 int getAndSet(int newValue) { return unsafe.getAndSetInt(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(int expect, int update) { return unsafe.compareAndSwapInt(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(int expect, int update) { return unsafe.compareAndSwapInt(this, valueOffset, expect, update); } /** * Atomically increments by one the current value. * * @return the previous value */ public final int getAndIncrement() { return unsafe.getAndAddInt(this, valueOffset, 1); } /** * Atomically decrements by one the current value. * * @return the previous value */ public final int getAndDecrement() { return unsafe.getAndAddInt(this, valueOffset, -1); } /** * Atomically adds the given value to the current value. * * @param delta the value to add * @return the previous value */ public final int getAndAdd(int delta) { return unsafe.getAndAddInt(this, valueOffset, delta); } /** * Atomically increments by one the current value. * * @return the updated value */ public final int incrementAndGet() { return unsafe.getAndAddInt(this, valueOffset, 1) + 1; } /** * Atomically decrements by one the current value. * * @return the updated value */ public final int decrementAndGet() { return unsafe.getAndAddInt(this, valueOffset, -1) - 1; } /** * Atomically adds the given value to the current value. * * @param delta the value to add * @return the updated value */ public final int addAndGet(int delta) { return unsafe.getAndAddInt(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 int getAndUpdate(IntUnaryOperator updateFunction) { int prev, next; do { prev = get(); next = updateFunction.applyAsInt(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 int updateAndGet(IntUnaryOperator updateFunction) { int prev, next; do { prev = get(); next = updateFunction.applyAsInt(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 int getAndAccumulate(int x, IntBinaryOperator accumulatorFunction) { int prev, next; do { prev = get(); next = accumulatorFunction.applyAsInt(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 int accumulateAndGet(int x, IntBinaryOperator accumulatorFunction) { int prev, next; do { prev = get(); next = accumulatorFunction.applyAsInt(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 Integer.toString(get()); } /** * Returns the value of this {@code AtomicInteger} as an {@code int}. */ public int intValue() { return get(); } /** * Returns the value of this {@code AtomicInteger} as a {@code long} * after a widening primitive conversion. * @jls 5.1.2 Widening Primitive Conversions */ public long longValue() { return (long)get(); } /** * Returns the value of this {@code AtomicInteger} 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 AtomicInteger} as a {@code double} * after a widening primitive conversion. * @jls 5.1.2 Widening Primitive Conversions */ public double doubleValue() { return (double)get(); } }
博文参考:
AtomicInteger源码分析
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