Java多線程系列--“JUC原子類”02之 AtomicLong原子類

 

概要

AtomicInteger, AtomicLong和AtomicBoolean這3個基本類型的原子類的原理和用法相似。本章以AtomicLong對基本類型的原子類進行介紹。內容包括：
AtomicLong介紹和函數列表
AtomicLong源碼分析(基於JDK1.7.0_40)
AtomicLong示例

轉載請注明出處：http://www.cnblogs.com/skywang12345/p/3514593.html

 

AtomicLong介紹和函數列表

AtomicLong是作用是對長整形進行原子操作。
在32位操作系統中，64位的long 和 double 變量由於會被JVM當作兩個分離的32位來進行操作，所以不具有原子性。而使用AtomicLong能讓long的操作保持原子型。

AtomicLong函數列表

// 構造函數
AtomicLong()
// 創建值為initialValue的AtomicLong對象
AtomicLong(long initialValue)
// 以原子方式設置當前值為newValue。
final void set(long newValue) 
// 獲取當前值
final long get() 
// 以原子方式將當前值減 1，並返回減1后的值。等價於“--num”
final long decrementAndGet() 
// 以原子方式將當前值減 1，並返回減1前的值。等價於“num--”
final long getAndDecrement() 
// 以原子方式將當前值加 1，並返回加1后的值。等價於“++num”
final long incrementAndGet() 
// 以原子方式將當前值加 1，並返回加1前的值。等價於“num++”
final long getAndIncrement()    
// 以原子方式將delta與當前值相加，並返回相加后的值。
final long addAndGet(long delta) 
// 以原子方式將delta添加到當前值，並返回相加前的值。
final long getAndAdd(long delta) 
// 如果當前值 == expect，則以原子方式將該值設置為update。成功返回true，否則返回false，並且不修改原值。
final boolean compareAndSet(long expect, long update)
// 以原子方式設置當前值為newValue，並返回舊值。
final long getAndSet(long newValue)
// 返回當前值對應的int值
int intValue() 
// 獲取當前值對應的long值
long longValue()    
// 以 float 形式返回當前值
float floatValue()    
// 以 double 形式返回當前值
double doubleValue()    
// 最后設置為給定值。延時設置變量值，這個等價於set()方法，但是由於字段是volatile類型的，因此次字段的修改會比普通字段（非volatile字段）有稍微的性能延時（盡管可以忽略），所以如果不是想立即讀取設置的新值，允許在“后台”修改值，那么此方法就很有用。如果還是難以理解，這里就類似於啟動一個后台線程如執行修改新值的任務，原線程就不等待修改結果立即返回（這種解釋其實是不正確的，但是可以這么理解）。
final void lazySet(long newValue)
// 如果當前值 == 預期值，則以原子方式將該設置為給定的更新值。JSR規范中說：以原子方式讀取和有條件地寫入變量但不 創建任何 happen-before 排序，因此不提供與除 weakCompareAndSet 目標外任何變量以前或后續讀取或寫入操作有關的任何保證。大意就是說調用weakCompareAndSet時並不能保證不存在happen-before的發生（也就是可能存在指令重排序導致此操作失敗）。但是從Java源碼來看，其實此方法並沒有實現JSR規范的要求，最后效果和compareAndSet是等效的，都調用了unsafe.compareAndSwapInt()完成操作。
final boolean weakCompareAndSet(long expect, long update)

 

AtomicLong源碼分析(基於JDK1.7.0_40)

AtomicLong的完整源碼

/*
 * ORACLE PROPRIETARY/CONFIDENTIAL. Use is subject to license terms.
 *
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/*
 *
 *
 *
 *
 *
 * 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();
    }

}

 

AtomicLong的代碼很簡單，下面僅以incrementAndGet()為例，對AtomicLong的原理進行說明。
incrementAndGet()源碼如下：

public final long incrementAndGet() {
    for (;;) {
        // 獲取AtomicLong當前對應的long值
        long current = get();
        // 將current加1
        long next = current + 1;
        // 通過CAS函數，更新current的值
        if (compareAndSet(current, next))
            return next;
    }
}

說明：
(01) incrementAndGet()首先會根據get()獲取AtomicLong對應的long值。該值是volatile類型的變量，get()的源碼如下：

// value是AtomicLong對應的long值
private volatile long value;
// 返回AtomicLong對應的long值
public final long get() {
    return value;
}

(02) incrementAndGet()接着將current加1,然后通過CAS函數，將新的值賦值給value。
compareAndSet()的源碼如下：

public final boolean compareAndSet(long expect, long update) {
    return unsafe.compareAndSwapLong(this, valueOffset, expect, update);
}

compareAndSet()的作用是更新AtomicLong對應的long值。它會比較AtomicLong的原始值是否與expect相等，若相等的話，則設置AtomicLong的值為update。

 

AtomicLong示例

// LongTest.java的源碼
import java.util.concurrent.atomic.AtomicLong;

public class LongTest {
    
    public static void main(String[] args){

        // 新建AtomicLong對象
        AtomicLong mAtoLong = new AtomicLong();

        mAtoLong.set(0x0123456789ABCDEFL);
        System.out.printf("%20s : 0x%016X\n", "get()", mAtoLong.get());
        System.out.printf("%20s : 0x%016X\n", "intValue()", mAtoLong.intValue());
        System.out.printf("%20s : 0x%016X\n", "longValue()", mAtoLong.longValue());
        System.out.printf("%20s : %s\n", "doubleValue()", mAtoLong.doubleValue());
        System.out.printf("%20s : %s\n", "floatValue()", mAtoLong.floatValue());

        System.out.printf("%20s : 0x%016X\n", "getAndDecrement()", mAtoLong.getAndDecrement());
        System.out.printf("%20s : 0x%016X\n", "decrementAndGet()", mAtoLong.decrementAndGet());
        System.out.printf("%20s : 0x%016X\n", "getAndIncrement()", mAtoLong.getAndIncrement());
        System.out.printf("%20s : 0x%016X\n", "incrementAndGet()", mAtoLong.incrementAndGet());

        System.out.printf("%20s : 0x%016X\n", "addAndGet(0x10)", mAtoLong.addAndGet(0x10));
        System.out.printf("%20s : 0x%016X\n", "getAndAdd(0x10)", mAtoLong.getAndAdd(0x10));

        System.out.printf("\n%20s : 0x%016X\n", "get()", mAtoLong.get());

        System.out.printf("%20s : %s\n", "compareAndSet()", mAtoLong.compareAndSet(0x12345679L, 0xFEDCBA9876543210L));
        System.out.printf("%20s : 0x%016X\n", "get()", mAtoLong.get());
    }
}

運行結果：

               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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更多內容

1. Java多線程系列--“JUC原子類”01之 框架

2. Java多線程系列目錄(共xx篇)