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Java多线程:“JUC原子类”05之AtomicLongFieldUpdater原子

2017-11-02 08:00 星期四 所属: JAVA 教程 浏览:79

副标题#e#

AtomicLongFieldUpdater先容和函数列表

AtomicLongFieldUpdater可以对指定"类的 ‘volatile long’范例的成员"举办原子更新。 它是基于反射道理实现的。

AtomicLongFieldUpdater函数列表

// 受掩护的无操纵结构要领,供子类利用。
protected AtomicLongFieldUpdater()
    
// 以原子方法将给定值添加到此更新器打点的给定工具的字段的当前值。
long addAndGet(T obj, long delta)
// 假如当前值 == 预期值,则以原子方法将此更新器所打点的给定工具的字段配置为给定的更新值。
abstract boolean compareAndSet(T obj, long expect, long update)
// 以原子方法将此更新器打点的给定工具字段当前值减 1。
long decrementAndGet(T obj)
// 获取此更新器打点的在给定工具的字段中保持的当前值。
abstract long get(T obj)
// 以原子方法将给定值添加到此更新器打点的给定工具的字段的当前值。
long getAndAdd(T obj, long delta)
// 以原子方法将此更新器打点的给定工具字段当前值减 1。
long getAndDecrement(T obj)
// 以原子方法将此更新器打点的给定工具字段的当前值加 1。
long getAndIncrement(T obj)
// 将此更新器打点的给定工具的字段以原子方法配置为给定值,并返回旧值。
long getAndSet(T obj, long newValue)
// 以原子方法将此更新器打点的给定工具字段当前值加 1。
long incrementAndGet(T obj)
// 最后将此更新器打点的给定工具的字段配置为给定更新值。
abstract void lazySet(T obj, long newValue)
// 为工具建设并返回一个具有给定字段的更新器。
static <U> AtomicLongFieldUpdater<U> newUpdater(Class<U> tclass, String 

fieldName)
// 将此更新器打点的给定工具的字段配置为给定更新值。
abstract void set(T obj, long newValue)
// 假如当前值 == 预期值,则以原子方法将此更新器所打点的给定工具的字段配置为给定的更新值。
abstract boolean weakCompareAndSet(T obj, long expect, long update)

AtomicLongFieldUpdater示例

// LongTest.java的源码
import java.util.concurrent.atomic.AtomicLongFieldUpdater;
    
public class LongFieldTest {
        
    public static void main(String[] args) {
    
        // 获取Person的class工具
        Class cls = Person.class; 
        // 新建AtomicLongFieldUpdater工具,通报参数是“class工具”和“long范例在类中对应的名称”
        AtomicLongFieldUpdater mAtoLong = AtomicLongFieldUpdater.newUpdater(cls, "id");
        Person person = new Person(12345678L);
    
        // 较量person的"id"属性,假如id的值为12345678L,则配置为1000。
        mAtoLong.compareAndSet(person, 12345678L, 1000);
        System.out.println("id="+person.getId());
    }
}
    
class Person {
    volatile long id;
    public Person(long id) {
        this.id = id;
    }
    public void setId(long id) {
        this.id = id;
    }
    public long getId() {
        return id;
    }
}


#p#副标题#e#

运行功效:

id=1000

AtomicLongFieldUpdater源码阐明(基于JDK1.7.0_40)

AtomicLongFieldUpdater完整源码

/*
 * 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 java.lang.reflect.*;
import sun.misc.Unsafe;
import sun.reflect.CallerSensitive;
import sun.reflect.Reflection;
    
/**
 * A reflection-based utility that enables atomic updates to
 * designated {@code volatile} reference fields of designated
 * classes.  This class is designed for use in atomic data structures
 * in which several reference fields of the same node are
 * independently subject to atomic updates. For example, a tree node
 * might be declared as
 *
 *  <pre> {@code
 * class Node {
 *   private volatile Node left, right;
 *
 *   private static final AtomicReferenceFieldUpdater<Node, Node> leftUpdater =
 *     AtomicReferenceFieldUpdater.newUpdater(Node.class, Node.class, "left");
 *   private static AtomicReferenceFieldUpdater<Node, Node> rightUpdater =
 *     AtomicReferenceFieldUpdater.newUpdater(Node.class, Node.class, "right");
 *
 *   Node getLeft() { return left;  }
 *   boolean compareAndSetLeft(Node expect, Node update) {
 *     return leftUpdater.compareAndSet(this, expect, update);
 *   }
 *   // ... and so on
 * }}</pre>
 *
 * <p>Note that the guarantees of the {@code compareAndSet}
 * method in this class are weaker than in other atomic classes.
 * Because this class cannot ensure that all uses of the field
 * are appropriate for purposes of atomic access, it can
 * guarantee atomicity only with respect to other invocations of
 * {@code compareAndSet} and {@code set} on the same updater.
 *
 * @since 1.5
 * @author Doug Lea
 * @param <T> The type of the object holding the updatable field
 * @param <V> The type of the field
 */
public abstract class AtomicReferenceFieldUpdater<T, V> {
    
    /**
     * Creates and returns an updater for objects with the given field.
     * The Class arguments are needed to check that reflective types and
     * generic types match.
     *
     * @param tclass the class of the objects holding the field.
     * @param vclass the class of the field
     * @param fieldName the name of the field to be updated.
     * @return the updater
     * @throws IllegalArgumentException if the field is not a volatile reference type.
     * @throws RuntimeException with a nested reflection-based
     * exception if the class does not hold field or is the wrong type.
     */
    @CallerSensitive
    public static <U, W> AtomicReferenceFieldUpdater<U,W> newUpdater

(Class<U> tclass, Class<W> vclass, String fieldName) {
        return new AtomicReferenceFieldUpdaterImpl<U,W>(tclass,
                                                        vclass,
                                                        fieldName,
                                                        Reflection.getCallerClass());
    }
    
    /**
     * Protected do-nothing constructor for use by subclasses.
     */
    protected AtomicReferenceFieldUpdater() {
    }
    
    /**
     * Atomically sets the field of the given object managed by this updater
     * to the given updated value if the current value {@code ==} the
     * expected value. This method is guaranteed to be atomic with respect to
     * other calls to {@code compareAndSet} and {@code set}, but not
     * necessarily with respect to other changes in the field.
     *
     * @param obj An object whose field to conditionally set
     * @param expect the expected value
     * @param update the new value
     * @return true if successful.
     */
    public abstract boolean compareAndSet(T obj, V expect, V update);
    
    /**
     * Atomically sets the field of the given object managed by this updater
     * to the given updated value if the current value {@code ==} the
     * expected value. This method is guaranteed to be atomic with respect to
     * other calls to {@code compareAndSet} and {@code set}, but not
     * necessarily with respect to other changes in the field.
     *
     * <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 obj An object whose field to conditionally set
     * @param expect the expected value
     * @param update the new value
     * @return true if successful.
     */
    public abstract boolean weakCompareAndSet(T obj, V expect, V update);
    
    /**
     * Sets the field of the given object managed by this updater to the
     * given updated value. This operation is guaranteed to act as a volatile
     * store with respect to subsequent invocations of {@code compareAndSet}.
     *
     * @param obj An object whose field to set
     * @param newValue the new value
     */
    public abstract void set(T obj, V newValue);
    
    /**
     * Eventually sets the field of the given object managed by this
     * updater to the given updated value.
     *
     * @param obj An object whose field to set
     * @param newValue the new value
     * @since 1.6
     */
    public abstract void lazySet(T obj, V newValue);
    
    /**
     * Gets the current value held in the field of the given object managed
     * by this updater.
     *
     * @param obj An object whose field to get
     * @return the current value
     */
    public abstract V get(T obj);
    
    /**
     * Atomically sets the field of the given object managed by this updater
     * to the given value and returns the old value.
     *
     * @param obj An object whose field to get and set
     * @param newValue the new value
     * @return the previous value
     */
    public V getAndSet(T obj, V newValue) {
        for (;;) {
            V current = get(obj);
            if (compareAndSet(obj, current, newValue))
                return current;
        }
    }
    
    private static final class AtomicReferenceFieldUpdaterImpl<T,V>
        extends AtomicReferenceFieldUpdater<T,V> {
        private static final Unsafe unsafe = Unsafe.getUnsafe();
        private final long offset;
        private final Class<T> tclass;
        private final Class<V> vclass;
        private final Class cclass;
    
        /*
         * Internal type checks within all update methods contain
         * internal inlined optimizations checking for the common
         * cases where the class is final (in which case a simple
         * getClass comparison suffices) or is of type Object (in
         * which case no check is needed because all objects are
         * instances of Object). The Object case is handled simply by
         * setting vclass to null in constructor.  The targetCheck and
         * updateCheck methods are invoked when these faster
         * screenings fail.
         */
    
        AtomicReferenceFieldUpdaterImpl(Class<T> tclass,
                                        Class<V> vclass,
                                        String fieldName,
                                        Class<?> caller) {
            Field field = null;
            Class fieldClass = null;
            int modifiers = 0;
            try {
                field = tclass.getDeclaredField(fieldName);
                modifiers = field.getModifiers();
                sun.reflect.misc.ReflectUtil.ensureMemberAccess(
                    caller, tclass, null, modifiers);
                sun.reflect.misc.ReflectUtil.checkPackageAccess(tclass);
                fieldClass = field.getType();
            } catch (Exception ex) {
                throw new RuntimeException(ex);
            }
    
            if (vclass != fieldClass)
                throw new ClassCastException();
    
            if (!Modifier.isVolatile(modifiers))
                throw new IllegalArgumentException("Must be volatile type");
    
            this.cclass = (Modifier.isProtected(modifiers) &&
                           caller != tclass) ? caller : null;
            this.tclass = tclass;
            if (vclass == Object.class)
                this.vclass = null;
            else
                this.vclass = vclass;
            offset = unsafe.objectFieldOffset(field);
        }
    
        void targetCheck(T obj) {
            if (!tclass.isInstance(obj))
                throw new ClassCastException();
            if (cclass != null)
                ensureProtectedAccess(obj);
        }
    
        void updateCheck(T obj, V update) {
            if (!tclass.isInstance(obj) ||
                (update != null && vclass != null && !vclass.isInstance(update)))
                throw new ClassCastException();
            if (cclass != null)
                ensureProtectedAccess(obj);
        }
    
        public boolean compareAndSet(T obj, V expect, V update) {
            if (obj == null || obj.getClass() != tclass || cclass != null ||
                (update != null && vclass != null &&
                 vclass != update.getClass()))
                updateCheck(obj, update);
            return unsafe.compareAndSwapObject(obj, offset, expect, update);
        }
    
        public boolean weakCompareAndSet(T obj, V expect, V update) {
            // same implementation as strong form for now
            if (obj == null || obj.getClass() != tclass || cclass != null ||
                (update != null && vclass != null &&
                 vclass != update.getClass()))
                updateCheck(obj, update);
            return unsafe.compareAndSwapObject(obj, offset, expect, update);
        }
    
        public void set(T obj, V newValue) {
            if (obj == null || obj.getClass() != tclass || cclass != null ||
                (newValue != null && vclass != null &&
                 vclass != newValue.getClass()))
                updateCheck(obj, newValue);
            unsafe.putObjectVolatile(obj, offset, newValue);
        }
    
        public void lazySet(T obj, V newValue) {
            if (obj == null || obj.getClass() != tclass || cclass != null ||
                (newValue != null && vclass != null &&
                 vclass != newValue.getClass()))
                updateCheck(obj, newValue);
            unsafe.putOrderedObject(obj, offset, newValue);
        }
    
        public V get(T obj) {
            if (obj == null || obj.getClass() != tclass || cclass != null)
                targetCheck(obj);
            return (V)unsafe.getObjectVolatile(obj, offset);
        }
    
        private void ensureProtectedAccess(T obj) {
            if (cclass.isInstance(obj)) {
                return;
            }
            throw new RuntimeException(
                new IllegalAccessException("Class " +
                    cclass.getName() +
                    " can not access a protected member of class " +
                    tclass.getName() +
                    " using an instance of " +
                    obj.getClass().getName()
                )
            );
        }
    }
}

下面阐明LongFieldTest.java的流程。

1. newUpdater()

#p#分页标题#e#

newUpdater()的源码如下:

public static <U> AtomicLongFieldUpdater<U> newUpdater(Class<U> tclass, 

String fieldName) {
    Class<?> caller = Reflection.getCallerClass();
    if (AtomicLong.VM_SUPPORTS_LONG_CAS)
        return new CASUpdater<U>(tclass, fieldName, caller);
    else
        return new LockedUpdater<U>(tclass, fieldName, caller);
}

说明:newUpdater()的浸染是获取一个AtomicIntegerFieldUpdater范例的工具。

#p#分页标题#e#

它实际上返回的 是CASUpdater工具,可能LockedUpdater工具;详细返回哪一个类取决于JVM是否支持long范例的CAS函数 。CASUpdater和LockedUpdater都是AtomicIntegerFieldUpdater的子类,它们的实现雷同。下面以 CASUpdater来举办说明。

CASUpdater类的源码如下:

public boolean compareAndSet(T obj, long expect, long update) {

   if (obj == null || obj.getClass() != tclass || cclass != null) fullCheck(obj);

   return unsafe.compareAndSwapLong(obj, offset, expect, update);

}

说明:它实际上是通过CAS函数操纵。假如类的long工具的值是expect,则配置它的值为update。

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