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Java多线程:“JUC原子类”02之AtomicLong原子类

2017-11-02 08:00 星期四 所属: JAVA 教程 浏览:49

副标题#e#

AtomicLong先容和函数列表

AtomicLong是浸染是对长整形举办原子操纵。

在32位操纵系统中,64位的long 和 double 变量由 于会被JVM看成两个疏散的32位来举办操纵,所以不具有原子性。而利用AtomicLong能让long的操纵保持 原子型。

AtomicLong函数列表

// 结构函数
AtomicLong()
// 建设值为initialValue的AtomicLong工具
AtomicLong(long initialValue)
// 以原子方法配置当前值为newValue。
final void set(long newValue) 
// 获取当前值
final long get() 
// 以原子方法将当前值减 1,并返回减1后的值。等价于“--num”
final long decrementAndGet() 
// 以原子方法将当前值减 1,并返回减1前的值。等价于“num--”
final long getAndDecrement() 
// 以原子方法将当前值加 1,并返回加1后的值。等价于“++num”
final long incrementAndGet() 
// 以原子方法将当前值加 1,并返回加1前的值。等价于“num++”
final long getAndIncrement()    
// 以原子方法将delta与当前值相加,并返回相加后的值。
final long addAndGet(long delta) 
// 以原子方法将delta添加到当前值,并返回相加前的值。
final long getAndAdd(long delta) 
// 假如当前值 == expect,则以原子方法将该值配置为update。乐成返回true,不然返回false,而且不

修改原值。
final boolean compareAndSet(long expect, long update)
// 以原子方法配置当前值为newValue,并返回旧值。
final long getAndSet(long newValue)
// 返回当前值对应的int值
int intValue() 
// 获取当前值对应的long值
long longValue()    
// 以 float 形式返回当前值
float floatValue()    
// 以 double 形式返回当前值
double doubleValue()    
// 最后配置为给定值。延时配置变量值,这个等价于set()要领,可是由于字段是volatile范例的,因此

次字段的修改会比普通字段(非volatile字段)有稍微的机能延时(尽量可以忽略),所以假如不是想立

即读取配置的新值,答允在“靠山”修改值,那么此要领就很有用。假如照旧难以领略,这里

就雷同于启动一个靠山线程如执行修改新值的任务,原线程就不期待修改功效当即返回(这种表明其实是

不正确的,可是可以这么领略)。
final void lazySet(long newValue)
// 假如当前值 == 预期值,则以原子方法将该配置为给定的更新值。JSR类型中说:以原子方法读取和有

条件地写入变量但不 建设任何 happen-before 排序,因此不提供与除 weakCompareAndSet 方针外任何

变量以前或后续读取或写入操纵有关的任何担保。大意就是说挪用weakCompareAndSet时并不能担保不存

在happen-before的产生(也就是大概存在指令重排序导致此操纵失败)。可是从Java源码来看,其实此

要领并没有实现JSR类型的要求,最后结果和compareAndSet是等效的,都挪用了

unsafe.compareAndSwapInt()完成操纵。
final boolean weakCompareAndSet(long expect, long update)

AtomicLong源码阐明(基于JDK1.7.0_40)


#p#副标题#e#

AtomicLong的完整源码

/*
 * 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;
    
/**
 * 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();
    }
    
}

查察本栏目

#p#副标题#e#

AtomicLong的代码很简朴,下面仅以incrementAndGet()为例,对AtomicLong的道理举办说明。

#p#分页标题#e#

incrementAndGet()源码如下:

public final long incrementAndGet() {
    for (;;) {
        // 获取AtomicLong当前对应的long值
        long current = get();
        // 将current加1
        long next = current + 1;
        // 通过CAS函数,更新current的值
        if (compareAndSet(current, next))
            return next;
    }
}

说明:

(01) incrementAndGet()首先会按照get()获取AtomicLong对应的long值。该值是volatile 范例的变量,get()的源码如下:

// value是AtomicLong对应的long值

private volatile long value;

// 返回AtomicLong对应的 long值

public final long get() {

   return value;

}

(02) incrementAndGet()接着将current加1,然后通过CAS函数,将新的值赋值给value。

compareAndSet()的源码如下:

public final boolean compareAndSet(long expect, long update) {

   return unsafe.compareAndSwapLong(this, valueOffset, expect, update);

}

compareAndSet()的浸染是更新AtomicLong对应的long值。它会较量AtomicLong的原始值是否与expect 相等,若相等的话,则配置AtomicLong的值为update。

#p#分页标题#e#

AtomicLong示例

1 // LongTest.java的源码
2 import java.util.concurrent.atomic.AtomicLong;
3
4 public class LongTest {
5
6 public static void main(String[] args) {
7
8 // 新建AtomicLong工具
9 AtomicLong mAtoLong = new AtomicLong();
10
11 mAtoLong.set(0x0123456789ABCDEFL);
12 System.out.printf("%20s : 0x%016X\n", "get()", mAtoLong.get());
13 System.out.printf("%20s : 0x%016X\n", "intValue()", mAtoLong.intValue());
14 System.out.printf("%20s : 0x%016X\n", "longValue()", mAtoLong.longValue());
15 System.out.printf("%20s : %s\n", "doubleValue()", mAtoLong.doubleValue());
16 System.out.printf("%20s : %s\n", "floatValue()", mAtoLong.floatValue());
17
18 System.out.printf("%20s : 0x%016X\n", "getAndDecrement()", mAtoLong.getAndDecrement());
19 System.out.printf("%20s : 0x%016X\n", "decrementAndGet()", mAtoLong.decrementAndGet());
20 System.out.printf("%20s : 0x%016X\n", "getAndIncrement()", mAtoLong.getAndIncrement());
21 System.out.printf("%20s : 0x%016X\n", "incrementAndGet()", mAtoLong.incrementAndGet());
22
23 System.out.printf("%20s : 0x%016X\n", "addAndGet(0x10)", mAtoLong.addAndGet(0x10));
24 System.out.printf("%20s : 0x%016X\n", "getAndAdd(0x10)", mAtoLong.getAndAdd(0x10));
25
26 System.out.printf("\n%20s : 0x%016X\n", "get()", mAtoLong.get());
27 28 System.out.printf("%20s : %s\n", "compareAndSet()", mAtoLong.compareAndSet(0x12345679L, 0xFEDCBA9876543210L));
29 System.out.printf("%20s : 0x%016X\n", "get()", mAtoLong.get());
30 }
31 }

运行功效:

get() : 0x0123456789ABCDEF
          intValue() : 0x0000000089ABCDEF
         longValue() : 0x0123456789ABCDEF
       doubleValue() : 8.1985529216486896E16
        floatValue() : 8.1985531E16
   getAndDecrement() : 0x0123456789ABCDEF
   decrementAndGet() : 0x0123456789ABCDED
   getAndIncrement() : 0x0123456789ABCDED
   incrementAndGet() : 0x0123456789ABCDEF
     addAndGet(0x10) : 0x0123456789ABCDFF
     getAndAdd(0x10) : 0x0123456789ABCDFF
    
               get() : 0x0123456789ABCE0F
     compareAndSet() : false
               get() : 0x0123456789ABCE0F

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