多线程爬坑之路-J.U.C.atomic包下的AtomicInteger，AtomicLong等类的源码解析

Atomic原子类：为基本类型的封装类Boolean,Integer,Long,对象引用等提供原子操作.

一、Atomic包下的所有类如下表：

类摘要
AtomicBoolean	可以用原子方式更新的 boolean 值。
AtomicInteger	可以用原子方式更新的 int 值。
AtomicIntegerArray	可以用原子方式更新其元素的 int 数组。
AtomicIntegerFieldUpdater<T>	基于反射的实用工具，可以对指定类的指定 volatile int 字段进行原子更新。
AtomicLong	可以用原子方式更新的 long 值。
AtomicLongArray	可以用原子方式更新其元素的 long 数组。
AtomicLongFieldUpdater<T>	基于反射的实用工具，可以对指定类的指定 volatile long 字段进行原子更新。
AtomicMarkableReference<V>	AtomicMarkableReference 维护带有标记位的对象引用，可以原子方式对其进行更新。
AtomicReference<V>	可以用原子方式更新的对象引用。
AtomicReferenceArray<E>	可以用原子方式更新其元素的对象引用数组。
AtomicReferenceFieldUpdater<T,V>	基于反射的实用工具，可以对指定类的指定 volatile 字段进行原子更新。
AtomicStampedReference<V>	AtomicStampedReference 维护带有整数“标志”的对象引用，可以用原子方式对其进行更新。

二、AtomicInteger源码分析和基本的方法使用:

Atomicinteger类中的方法列表：

构造方法摘要
AtomicInteger()            创建具有初始值 0 的新 AtomicInteger。
AtomicInteger(int initialValue)            创建具有给定初始值的新 AtomicInteger。

方法摘要
int	addAndGet(int delta)            以原子方式将给定值与当前值相加。
boolean	compareAndSet(int expect, int update)            如果当前值 == 预期值，则以原子方式将该值设置为给定的更新值。
int	decrementAndGet()            以原子方式将当前值减 1。
double	doubleValue()            以 double 形式返回指定的数值。
float	floatValue()            以 float 形式返回指定的数值。
int	get()            获取当前值。
int	getAndAdd(int delta)            以原子方式将给定值与当前值相加。
int	getAndDecrement()            以原子方式将当前值减 1。
int	getAndIncrement()            以原子方式将当前值加 1。
int	getAndSet(int newValue)            以原子方式设置为给定值，并返回旧值。
int	incrementAndGet()            以原子方式将当前值加 1。
int	intValue()            以 int 形式返回指定的数值。
void	lazySet(int newValue)            最后设置为给定值。
long	longValue()            以 long 形式返回指定的数值。
void	set(int newValue)            设置为给定值。
String	toString()            返回当前值的字符串表示形式。
boolean	weakCompareAndSet(int expect, int update)            如果当前值 == 预期值，则以原子方式将该设置为给定的更新值。

从类 java.lang.Number 继承的方法
byteValue, shortValue

从类 java.lang.Object 继承的方法
clone, equals, finalize, getClass, hashCode, notify, notifyAll, wait, wait, wait

AtomicInteger源码：

/*
 * ORACLE PROPRIETARY/CONFIDENTIAL. Use is subject to license terms.
 *
 *
 *
 *
 *
 *
 *
 *
 *
 *
 *
 *
 *
 *
 *
 *
 *
 *
 *
 *
 */

/*
 *
 *
 *
 *
 *
 * Written by Doug Lea with assistance from members of JCP JSR-166
 * Expert Group and released to the public domain, as explained at
 * http://creativecommons.org/publicdomain/zero/1.0/
 */

package java.util.concurrent.atomic;
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;

    // 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); }
    }

    private volatile int value;

    /**
     * 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) {
        for (;;) {
            int current = get();
            if (compareAndSet(current, newValue))
                return current;
        }
    }

    /**
     * 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 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>May <a href="package-summary.html#Spurious">fail spuriously</a>
     * and does not provide ordering guarantees, so is only rarely an
     * appropriate alternative to {@code compareAndSet}.
     *
     * @param expect the expected value
     * @param update the new value
     * @return 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() {
        for (;;) {
            int current = get();
            int next = current + 1;
            if (compareAndSet(current, next))
                return current;
        }
    }

    /**
     * Atomically decrements by one the current value.
     *
     * @return the previous value
     */
    public final int getAndDecrement() {
        for (;;) {
            int current = get();
            int next = current - 1;
            if (compareAndSet(current, next))
                return current;
        }
    }

    /**
     * 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) {
        for (;;) {
            int current = get();
            int next = current + delta;
            if (compareAndSet(current, next))
                return current;
        }
    }

    /**
     * Atomically increments by one the current value.
     *
     * @return the updated value
     */
    public final int incrementAndGet() {
        for (;;) {
            int current = get();
            int next = current + 1;
            if (compareAndSet(current, next))
                return next;
        }
    }

    /**
     * Atomically decrements by one the current value.
     *
     * @return the updated value
     */
    public final int decrementAndGet() {
        for (;;) {
            int current = get();
            int next = current - 1;
            if (compareAndSet(current, next))
                return next;
        }
    }

    /**
     * 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) {
        for (;;) {
            int current = get();
            int next = current + delta;
            if (compareAndSet(current, 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());
    }

    public int intValue() {
        return get();
    }

    public long longValue() {
        return (long)get();
    }

    public float floatValue() {
        return (float)get();
    }

    public double doubleValue() {
        return (double)get();
    }

}

AtomicInteger

其中的属性：

private static final long serialVersionUID = 6214790243416807050L;

    // 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); }
    }

    private volatile int value;

unsafe ：java中的sun.misc.Unsafe包,提供了安全访问内存的方法。这些方法提供给java访问底层的JNI（java-native-interface），因为这些方法最终是调用c/c++实现。
valueOffset:指向相对于对象起始位置的偏移量（内存中）可以理解为引用指向的内存，通过这个值可以去内存中查找某个引用在内存的值。
value:引用的当前值[预期值E]

方法示例：

public class AtomTest {
    private static AtomicInteger num = new  AtomicInteger();
    public static void main(String[] args) {
        System.out.println(num.compareAndSet(, ));//比较并设置，用0与内存中的值比较，相等的话，修改值为1
        System.out.println(num.compareAndSet(, ));//比较并设置，用0与内存中的值比较，相等的话，修改值为1
        System.out.println(num.get());;//获取初值
        num.set();//设置初值，
        System.out.println(num.get());
        System.out.println(num.decrementAndGet());//获取值自减返回减1之后的值
        System.out.println(num.addAndGet());//获取值添加2然后返回添加后的值
        System.out.println(num.getAndIncrement());//获取值并且自增，返回的是自增前的值
        System.out.println(num.getAndAdd());
        System.out.println(num.getAndDecrement());
        System.out.println(num.getAndSet());
        System.out.println(num.get());
    }
}

运行结果：

true
false

初始化的时候赋值为0,调用从compareAndSwap(0,1)之后返回true，compareAndSwap比较并交换，比较结果相同则修改值并返回TRUE不通则返回FALSE。

看看这个方法的源码：

/**
     * 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 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);
    }

compareAndSwap：当前引用值和与要更新的值以及对象，和偏移量做参数去更新内存，根据对象和偏移量可以找到引用的内存值与引用值（期望值）作比较，相等则说明这个值在此对象获得后未被修改，那么我们就可以更新，则结果为true。这个方法就是传说中的CAS,CAS相较于synchonrized提供了一种无锁的线程安全保证。

示例中的前两个操作是一样的，我们可以理解为两个线程同时更新值为1,前一个快一点，发现更新的时候内存值未变则更新，等到第二个更新的时候原始值0与内存值(此时内存值变为1了)比较则失败返回FALSE并且不进行更新。这样就保证了一致性，而我们看到变量的值value使用voliate修饰的，保证了数据的内存可见性(一个线程修改其他线程可见)。所以多个线程并发的时候这个类代替Integer能够保证安全。

这个操作非常像数据库的乐观锁，给每个表添加一个version版本号，修改数据后要持久化先查询看版本号一致不，一致的话则持久化，不一致则重新获取值进行修改。

三、AtomicLong源码以及与AtomicInteger的不同：

package java.util.concurrent.atomic;
import sun.misc.Unsafe;

/**
 * A {@code long} 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 AtomicLong} is used in applications such as atomically
 * incremented sequence numbers, and cannot be used as a replacement
 * for a {@link java.lang.Long}. 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 AtomicLong extends Number implements java.io.Serializable {
    private static final long serialVersionUID = 1927816293512124184L;

    // setup to use Unsafe.compareAndSwapLong for updates
    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.
     */
    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) {
        while (true) {
            long current = get();
            if (compareAndSet(current, newValue))
                return current;
        }
    }

    /**
     * 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 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>May <a href="package-summary.html#Spurious">fail spuriously</a>
     * and does not provide ordering guarantees, so is only rarely an
     * appropriate alternative to {@code compareAndSet}.
     *
     * @param expect the expected value
     * @param update the new value
     * @return 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() {
        while (true) {
            long current = get();
            long next = current + 1;
            if (compareAndSet(current, next))
                return current;
        }
    }

    /**
     * Atomically decrements by one the current value.
     *
     * @return the previous value
     */
    public final long getAndDecrement() {
        while (true) {
            long current = get();
            long next = current - 1;
            if (compareAndSet(current, next))
                return current;
        }
    }

    /**
     * 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) {
        while (true) {
            long current = get();
            long next = current + delta;
            if (compareAndSet(current, next))
                return current;
        }
    }

    /**
     * Atomically increments by one the current value.
     *
     * @return the updated value
     */
    public final long incrementAndGet() {
        for (;;) {
            long current = get();
            long next = current + 1;
            if (compareAndSet(current, next))
                return next;
        }
    }

    /**
     * Atomically decrements by one the current value.
     *
     * @return the updated value
     */
    public final long decrementAndGet() {
        for (;;) {
            long current = get();
            long next = current - 1;
            if (compareAndSet(current, next))
                return next;
        }
    }

    /**
     * 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) {
        for (;;) {
            long current = get();
            long next = current + delta;
            if (compareAndSet(current, 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());
    }

    public int intValue() {
        return (int)get();
    }

    public long longValue() {
        return get();
    }

    public float floatValue() {
        return (float)get();
    }

    public double doubleValue() {
        return (double)get();
    }

}

AtomicLong源码

关于AtomicLong的用法和AtomicInteger类似:

不同之处在于多了下面一部分：一个静态的Boolean值VM_SUPPORTS_LONG_CAS,调用一个 native方法VMSupportCS8()返回虚拟机是否支持Long类型的无锁CAS机制

 /**
     * 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.
     */
    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();

从这里可以看出不同的VM支持的数据类型可能有所差别，因为CAS需要硬件系统的支持。

四、AtomicReference源码及使用：

/**
 * An object reference that may be updated atomically. See the {@link
 * java.util.concurrent.atomic} package specification for description
 * of the properties of atomic variables.
 * @since 1.5
 * @author Doug Lea
 * @param <V> The type of object referred to by this reference
 */
public class AtomicReference<V>  implements java.io.Serializable {
    private static final long serialVersionUID = -1848883965231344442L;

    private static final Unsafe unsafe = Unsafe.getUnsafe();
    private static final long valueOffset;

    static {
      try {
        valueOffset = unsafe.objectFieldOffset
            (AtomicReference.class.getDeclaredField("value"));
      } catch (Exception ex) { throw new Error(ex); }
    }

    private volatile V value;

    /**
     * Creates a new AtomicReference with the given initial value.
     *
     * @param initialValue the initial value
     */
    public AtomicReference(V initialValue) {
        value = initialValue;
    }

    /**
     * Creates a new AtomicReference with null initial value.
     */
    public AtomicReference() {
    }

    /**
     * Gets the current value.
     *
     * @return the current value
     */
    public final V get() {
        return value;
    }

    /**
     * Sets to the given value.
     *
     * @param newValue the new value
     */
    public final void set(V newValue) {
        value = newValue;
    }

    /**
     * Eventually sets to the given value.
     *
     * @param newValue the new value
     * @since 1.6
     */
    public final void lazySet(V newValue) {
        unsafe.putOrderedObject(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 true if successful. False return indicates that
     * the actual value was not equal to the expected value.
     */
    public final boolean compareAndSet(V expect, V update) {
        return unsafe.compareAndSwapObject(this, valueOffset, expect, update);
    }

    /**
     * Atomically sets the value to the given updated value
     * if the current value {@code ==} the expected value.
     *
     * <p>May <a href="package-summary.html#Spurious">fail spuriously</a>
     * and does not provide ordering guarantees, so is only rarely an
     * appropriate alternative to {@code compareAndSet}.
     *
     * @param expect the expected value
     * @param update the new value
     * @return true if successful.
     */
    public final boolean weakCompareAndSet(V expect, V update) {
        return unsafe.compareAndSwapObject(this, valueOffset, expect, update);
    }

    /**
     * Atomically sets to the given value and returns the old value.
     *
     * @param newValue the new value
     * @return the previous value
     */
    public final V getAndSet(V newValue) {
        while (true) {
            V x = get();
            if (compareAndSet(x, newValue))
                return x;
        }
    }

    /**
     * Returns the String representation of the current value.
     * @return the String representation of the current value.
     */
    public String toString() {
        return String.valueOf(get());
    }

}

AtomicReference源码

除了类型是个对象的之外，方法使用和AtomicInteger一模一样：

AtomicReference的使用：

import java.util.concurrent.atomic.AtomicReference;

public class AtomTest {
    private static AtomicReference<Pig> pigtest;
    public static void main(String[] args) {
        Pig pig = new Pig("猪坚强", 2);
        Pig pig2 = new Pig("猪八戒", 2);
        System.out.println("pig_hashCode:"+pig.hashCode());
        System.out.println("pig2_hashCode:"+pig2.hashCode());
        AtomTest.pigtest = new AtomicReference<Pig>(pig);
        System.out.println(pigtest.get().toString());
        System.out.println(pigtest.get().hashCode());
        System.out.println(pigtest.compareAndSet(pig, pig2));
        System.out.println(pigtest.compareAndSet(pig, pig2));
        System.out.println(pigtest.get().toString());
        System.out.println(pigtest.get().hashCode());
    }
}

运行结果：

pig_hashCode:779824645
pig2_hashCode:420110874
[猪坚强,2]
779824645
true
false
[猪八戒,2]
420110874

第一步：先获取pigTest的toString(),和hashCode,结果是【猪坚强，2】，hashCode：779824645【我们发现hashCode居然==pig_hashCode】

第二步：进行CAS修改为pig2结果为true,再进行一次结果为FALSE,

第三步：获取pigTest的toString(),和hashCode,结果是【猪八戒，2】，hashCode：420110874【我们发现hashCode居然==pig2_hashCode】

结果说明了：pigtest是一个指向对象引用的引用，利用CAS操作可以修改pigTest指向的对象引用从而改变自身指向的对象。第一次CAS成功将pigtest指向的pig修改成了pig2.第二次CAS操作失败了，可以保证多线程并发时的安全问题。

五、总结：

　　Atomic包下内容并不复杂，一句话来说就是提供了CAS无锁的安全访问机制。表现出来的是通过期望值E与内存值M作比较，相同则修改内存值M为更新值U。四个参数：当前对象this，偏移量V，期望值E，更新值U。

　　利用CAS+voliate+native的机制保证数据操作的原子性，可见性和一致性。voliate使变量可见，CAS调用unsafe中的native方法访问系统底层的实现。unsafe中的这些方法直接操作内存，运用不当可能造成很大的问题。

　　其实从Atomic包中的原子类的探索中，只是想引出CAS这个概念，CAS同样提供了一种线程安全的机制，而它不同于Synchonrized，synchonrized被称之为重量级锁，原因是因为粒度太强，加锁就代表着线程阻塞，高并发访问时带来的性能问题是硬伤。

　　为了解决这种问题出现了两种机制：一种就是CAS，另一种是锁优化。

　　CAS是将阻塞下降到了底层CPU上(纯属个人理解，因为看到有权威是说存在阻塞的，可能让别的线程知道已经更改了数据并且更新失败也是一种阻塞吧)，语言层面访问效率远远低于系统内部硬件上，尽管同样是阻塞，在系统内部加锁解锁的效率要高很多。但是需要的是硬件支持，不过现在绝大部分CPU都已经支持CAS了。

　　unsafe类：http://www.cnblogs.com/mickole/articles/3757278.html大家可以看这篇博客。