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jeremybenn |
/*
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* Written by Doug Lea with assistance from members of JCP JSR-166
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* Expert Group and released to the public domain, as explained at
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* http://creativecommons.org/licenses/publicdomain
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*/
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package java.util.concurrent.atomic;
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import sun.misc.Unsafe;
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/**
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* A <tt>boolean</tt> value that may be updated atomically. See the
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* {@link java.util.concurrent.atomic} package specification for
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* description of the properties of atomic variables. An
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* <tt>AtomicBoolean</tt> is used in applications such as atomically
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* updated flags, and cannot be used as a replacement for a
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* {@link java.lang.Boolean}.
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*
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* @since 1.5
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* @author Doug Lea
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*/
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public class AtomicBoolean implements java.io.Serializable {
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private static final long serialVersionUID = 4654671469794556979L;
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// setup to use Unsafe.compareAndSwapInt for updates
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private static final Unsafe unsafe = Unsafe.getUnsafe();
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private static final long valueOffset;
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static {
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try {
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valueOffset = unsafe.objectFieldOffset
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(AtomicBoolean.class.getDeclaredField("value"));
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} catch (Exception ex) { throw new Error(ex); }
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}
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private volatile int value;
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/**
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* Creates a new <tt>AtomicBoolean</tt> with the given initial value.
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*
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* @param initialValue the initial value
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*/
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public AtomicBoolean(boolean initialValue) {
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value = initialValue ? 1 : 0;
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}
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/**
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* Creates a new <tt>AtomicBoolean</tt> with initial value <tt>false</tt>.
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*/
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public AtomicBoolean() {
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}
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/**
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* Returns the current value.
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*
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* @return the current value
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*/
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public final boolean get() {
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return value != 0;
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}
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/**
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* Atomically sets the value to the given updated value
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* if the current value <tt>==</tt> the expected value.
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*
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* @param expect the expected value
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* @param update the new value
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* @return true if successful. False return indicates that
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* the actual value was not equal to the expected value.
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*/
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public final boolean compareAndSet(boolean expect, boolean update) {
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int e = expect ? 1 : 0;
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int u = update ? 1 : 0;
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return unsafe.compareAndSwapInt(this, valueOffset, e, u);
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}
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/**
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* Atomically sets the value to the given updated value
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* if the current value <tt>==</tt> the expected value.
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* May fail spuriously and does not provide ordering guarantees,
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* so is only rarely an appropriate alternative to <tt>compareAndSet</tt>.
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*
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* @param expect the expected value
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* @param update the new value
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* @return true if successful.
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*/
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public boolean weakCompareAndSet(boolean expect, boolean update) {
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int e = expect ? 1 : 0;
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int u = update ? 1 : 0;
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return unsafe.compareAndSwapInt(this, valueOffset, e, u);
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}
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/**
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* Unconditionally sets to the given value.
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*
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* @param newValue the new value
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*/
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public final void set(boolean newValue) {
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value = newValue ? 1 : 0;
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}
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/**
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* Eventually sets to the given value.
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*
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* @param newValue the new value
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* @since 1.6
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*/
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public final void lazySet(boolean newValue) {
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int v = newValue ? 1 : 0;
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unsafe.putOrderedInt(this, valueOffset, v);
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}
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/**
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* Atomically sets to the given value and returns the previous value.
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*
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* @param newValue the new value
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* @return the previous value
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*/
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public final boolean getAndSet(boolean newValue) {
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for (;;) {
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boolean current = get();
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if (compareAndSet(current, newValue))
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return current;
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}
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}
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/**
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* Returns the String representation of the current value.
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* @return the String representation of the current value.
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*/
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public String toString() {
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return Boolean.toString(get());
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}
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}
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