Javaマルチスレッドシリーズ--「JUCロック」11のSemaphore信号量の原理と例
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概要
本章では,JUCパケット中の信号量Semaphoreについて学習する.内容は、Semaphoreの概要Semaphoreデータ構造Semaphoreソース分析(JDK 1.7.0_40ベース)Semaphoreの例・
転載は出典を明記してください:http://www.cnblogs.com/skywang12345/p/3534050.html
Semaphoreの概要
Semaphoreはカウント信号量であり、その本質は「共有ロック」である.
信号量は、信号量許可セットを維持します.スレッドはacquire()を呼び出すことによって信号量の許可を得ることができる.信号量に使用可能なライセンスがある場合、スレッドはそのライセンスを取得することができる.使用可能なライセンスがあるまでスレッドを待たなければなりません.スレッドはrelease()によって、その持つ信号量許可を解放することができる.
Semaphoreの関数リスト
// Semaphore。
Semaphore(int permits)
// Semaphore。
Semaphore(int permits, boolean fair)
// ， ， 。
void acquire()
// ， ， 。
void acquire(int permits)
// ， 。
void acquireUninterruptibly()
// ， 。
void acquireUninterruptibly(int permits)
// 。
int availablePermits()
// 。
int drainPermits()
// collection， 。
protected Collection<Thread> getQueuedThreads()
// 。
int getQueueLength()
// 。
boolean hasQueuedThreads()
// true， true。
boolean isFair()
// 。
protected void reducePermits(int reduction)
// ， 。
void release()
// ， 。
void release(int permits)
// ， 。
String toString()
// ， 。
boolean tryAcquire()
// ， 。
boolean tryAcquire(int permits)
// ， ， 。
boolean tryAcquire(int permits, long timeout, TimeUnit unit)
// ， ， 。
boolean tryAcquire(long timeout, TimeUnit unit)Semaphoreデータ構造
SemaphoreのUMLクラス図は次のとおりです.
図から分かるように、
(01)は「ReentrantLock」と同様に、Semaphoreにもsyncオブジェクトが含まれており、syncはSyncタイプである.また,SyncはAQSに継承された抽象クラスである.
(02)Syncは、「公平信号量」FairSyncと「非公平信号量」NonfairSyncの2つのサブクラスを含む.syncは「FairSyncのインスタンス」または「NonfairSyncのインスタンス」である.デフォルトではsyncはNonfairSync(すなわち、デフォルトでは非標準信号量)です.
Semaphoreソース分析(JDK 1.7.0_40ベース)
Semaphoreフルソース(JDK 1.7.0_40ベース)
/*
* ORACLE PROPRIETARY/CONFIDENTIAL. Use is subject to license terms.
*
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/*
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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;
import java.util.*;
import java.util.concurrent.locks.*;
import java.util.concurrent.atomic.*;
/**
* A counting semaphore. Conceptually, a semaphore maintains a set of
* permits. Each {@link #acquire} blocks if necessary until a permit is
* available, and then takes it. Each {@link #release} adds a permit,
* potentially releasing a blocking acquirer.
* However, no actual permit objects are used; the {@code Semaphore} just
* keeps a count of the number available and acts accordingly.
*
* <p>Semaphores are often used to restrict the number of threads than can
* access some (physical or logical) resource. For example, here is
* a class that uses a semaphore to control access to a pool of items:
* <pre>
* class Pool {
* private static final int MAX_AVAILABLE = 100;
* private final Semaphore available = new Semaphore(MAX_AVAILABLE, true);
*
* public Object getItem() throws InterruptedException {
* available.acquire();
* return getNextAvailableItem();
* }
*
* public void putItem(Object x) {
* if (markAsUnused(x))
* available.release();
* }
*
* // Not a particularly efficient data structure; just for demo
*
* protected Object[] items = ... whatever kinds of items being managed
* protected boolean[] used = new boolean[MAX_AVAILABLE];
*
* protected synchronized Object getNextAvailableItem() {
* for (int i = 0; i < MAX_AVAILABLE; ++i) {
* if (!used[i]) {
* used[i] = true;
* return items[i];
* }
* }
* return null; // not reached
* }
*
* protected synchronized boolean markAsUnused(Object item) {
* for (int i = 0; i < MAX_AVAILABLE; ++i) {
* if (item == items[i]) {
* if (used[i]) {
* used[i] = false;
* return true;
* } else
* return false;
* }
* }
* return false;
* }
*
* }
* </pre>
*
* <p>Before obtaining an item each thread must acquire a permit from
* the semaphore, guaranteeing that an item is available for use. When
* the thread has finished with the item it is returned back to the
* pool and a permit is returned to the semaphore, allowing another
* thread to acquire that item. Note that no synchronization lock is
* held when {@link #acquire} is called as that would prevent an item
* from being returned to the pool. The semaphore encapsulates the
* synchronization needed to restrict access to the pool, separately
* from any synchronization needed to maintain the consistency of the
* pool itself.
*
* <p>A semaphore initialized to one, and which is used such that it
* only has at most one permit available, can serve as a mutual
* exclusion lock. This is more commonly known as a <em>binary
* semaphore</em>, because it only has two states: one permit
* available, or zero permits available. When used in this way, the
* binary semaphore has the property (unlike many {@link Lock}
* implementations), that the "lock" can be released by a
* thread other than the owner (as semaphores have no notion of
* ownership). This can be useful in some specialized contexts, such
* as deadlock recovery.
*
* <p> The constructor for this class optionally accepts a
* <em>fairness</em> parameter. When set false, this class makes no
* guarantees about the order in which threads acquire permits. In
* particular, <em>barging</em> is permitted, that is, a thread
* invoking {@link #acquire} can be allocated a permit ahead of a
* thread that has been waiting - logically the new thread places itself at
* the head of the queue of waiting threads. When fairness is set true, the
* semaphore guarantees that threads invoking any of the {@link
* #acquire() acquire} methods are selected to obtain permits in the order in
* which their invocation of those methods was processed
* (first-in-first-out; FIFO). Note that FIFO ordering necessarily
* applies to specific internal points of execution within these
* methods. So, it is possible for one thread to invoke
* {@code acquire} before another, but reach the ordering point after
* the other, and similarly upon return from the method.
* Also note that the untimed {@link #tryAcquire() tryAcquire} methods do not
* honor the fairness setting, but will take any permits that are
* available.
*
* <p>Generally, semaphores used to control resource access should be
* initialized as fair, to ensure that no thread is starved out from
* accessing a resource. When using semaphores for other kinds of
* synchronization control, the throughput advantages of non-fair
* ordering often outweigh fairness considerations.
*
* <p>This class also provides convenience methods to {@link
* #acquire(int) acquire} and {@link #release(int) release} multiple
* permits at a time. Beware of the increased risk of indefinite
* postponement when these methods are used without fairness set true.
*
* <p>Memory consistency effects: Actions in a thread prior to calling
* a "release" method such as {@code release()}
* <a href="package-summary.html#MemoryVisibility"><i>happen-before</i></a>
* actions following a successful "acquire" method such as {@code acquire()}
* in another thread.
*
* @since 1.5
* @author Doug Lea
*
*/
public class Semaphore implements java.io.Serializable {
private static final long serialVersionUID = -3222578661600680210L;
/** All mechanics via AbstractQueuedSynchronizer subclass */
private final Sync sync;
/**
* Synchronization implementation for semaphore. Uses AQS state
* to represent permits. Subclassed into fair and nonfair
* versions.
*/
abstract static class Sync extends AbstractQueuedSynchronizer {
private static final long serialVersionUID = 1192457210091910933L;
Sync(int permits) {
setState(permits);
}
final int getPermits() {
return getState();
}
final int nonfairTryAcquireShared(int acquires) {
for (;;) {
int available = getState();
int remaining = available - acquires;
if (remaining < 0 ||
compareAndSetState(available, remaining))
return remaining;
}
}
protected final boolean tryReleaseShared(int releases) {
for (;;) {
int current = getState();
int next = current + releases;
if (next < current) // overflow
throw new Error("Maximum permit count exceeded");
if (compareAndSetState(current, next))
return true;
}
}
final void reducePermits(int reductions) {
for (;;) {
int current = getState();
int next = current - reductions;
if (next > current) // underflow
throw new Error("Permit count underflow");
if (compareAndSetState(current, next))
return;
}
}
final int drainPermits() {
for (;;) {
int current = getState();
if (current == 0 || compareAndSetState(current, 0))
return current;
}
}
}
/**
* NonFair version
*/
static final class NonfairSync extends Sync {
private static final long serialVersionUID = -2694183684443567898L;
NonfairSync(int permits) {
super(permits);
}
protected int tryAcquireShared(int acquires) {
return nonfairTryAcquireShared(acquires);
}
}
/**
* Fair version
*/
static final class FairSync extends Sync {
private static final long serialVersionUID = 2014338818796000944L;
FairSync(int permits) {
super(permits);
}
protected int tryAcquireShared(int acquires) {
for (;;) {
if (hasQueuedPredecessors())
return -1;
int available = getState();
int remaining = available - acquires;
if (remaining < 0 ||
compareAndSetState(available, remaining))
return remaining;
}
}
}
/**
* Creates a {@code Semaphore} with the given number of
* permits and nonfair fairness setting.
*
* @param permits the initial number of permits available.
* This value may be negative, in which case releases
* must occur before any acquires will be granted.
*/
public Semaphore(int permits) {
sync = new NonfairSync(permits);
}
/**
* Creates a {@code Semaphore} with the given number of
* permits and the given fairness setting.
*
* @param permits the initial number of permits available.
* This value may be negative, in which case releases
* must occur before any acquires will be granted.
* @param fair {@code true} if this semaphore will guarantee
* first-in first-out granting of permits under contention,
* else {@code false}
*/
public Semaphore(int permits, boolean fair) {
sync = fair ? new FairSync(permits) : new NonfairSync(permits);
}
/**
* Acquires a permit from this semaphore, blocking until one is
* available, or the thread is {@linkplain Thread#interrupt interrupted}.
*
* <p>Acquires a permit, if one is available and returns immediately,
* reducing the number of available permits by one.
*
* <p>If no permit is available then the current thread becomes
* disabled for thread scheduling purposes and lies dormant until
* one of two things happens:
* <ul>
* <li>Some other thread invokes the {@link #release} method for this
* semaphore and the current thread is next to be assigned a permit; or
* <li>Some other thread {@linkplain Thread#interrupt interrupts}
* the current thread.
* </ul>
*
* <p>If the current thread:
* <ul>
* <li>has its interrupted status set on entry to this method; or
* <li>is {@linkplain Thread#interrupt interrupted} while waiting
* for a permit,
* </ul>
* then {@link InterruptedException} is thrown and the current thread's
* interrupted status is cleared.
*
* @throws InterruptedException if the current thread is interrupted
*/
public void acquire() throws InterruptedException {
sync.acquireSharedInterruptibly(1);
}
/**
* Acquires a permit from this semaphore, blocking until one is
* available.
*
* <p>Acquires a permit, if one is available and returns immediately,
* reducing the number of available permits by one.
*
* <p>If no permit is available then the current thread becomes
* disabled for thread scheduling purposes and lies dormant until
* some other thread invokes the {@link #release} method for this
* semaphore and the current thread is next to be assigned a permit.
*
* <p>If the current thread is {@linkplain Thread#interrupt interrupted}
* while waiting for a permit then it will continue to wait, but the
* time at which the thread is assigned a permit may change compared to
* the time it would have received the permit had no interruption
* occurred. When the thread does return from this method its interrupt
* status will be set.
*/
public void acquireUninterruptibly() {
sync.acquireShared(1);
}
/**
* Acquires a permit from this semaphore, only if one is available at the
* time of invocation.
*
* <p>Acquires a permit, if one is available and returns immediately,
* with the value {@code true},
* reducing the number of available permits by one.
*
* <p>If no permit is available then this method will return
* immediately with the value {@code false}.
*
* <p>Even when this semaphore has been set to use a
* fair ordering policy, a call to {@code tryAcquire()} <em>will</em>
* immediately acquire a permit if one is available, whether or not
* other threads are currently waiting.
* This "barging" behavior can be useful in certain
* circumstances, even though it breaks fairness. If you want to honor
* the fairness setting, then use
* {@link #tryAcquire(long, TimeUnit) tryAcquire(0, TimeUnit.SECONDS) }
* which is almost equivalent (it also detects interruption).
*
* @return {@code true} if a permit was acquired and {@code false}
* otherwise
*/
public boolean tryAcquire() {
return sync.nonfairTryAcquireShared(1) >= 0;
}
/**
* Acquires a permit from this semaphore, if one becomes available
* within the given waiting time and the current thread has not
* been {@linkplain Thread#interrupt interrupted}.
*
* <p>Acquires a permit, if one is available and returns immediately,
* with the value {@code true},
* reducing the number of available permits by one.
*
* <p>If no permit is available then the current thread becomes
* disabled for thread scheduling purposes and lies dormant until
* one of three things happens:
* <ul>
* <li>Some other thread invokes the {@link #release} method for this
* semaphore and the current thread is next to be assigned a permit; or
* <li>Some other thread {@linkplain Thread#interrupt interrupts}
* the current thread; or
* <li>The specified waiting time elapses.
* </ul>
*
* <p>If a permit is acquired then the value {@code true} is returned.
*
* <p>If the current thread:
* <ul>
* <li>has its interrupted status set on entry to this method; or
* <li>is {@linkplain Thread#interrupt interrupted} while waiting
* to acquire a permit,
* </ul>
* then {@link InterruptedException} is thrown and the current thread's
* interrupted status is cleared.
*
* <p>If the specified waiting time elapses then the value {@code false}
* is returned. If the time is less than or equal to zero, the method
* will not wait at all.
*
* @param timeout the maximum time to wait for a permit
* @param unit the time unit of the {@code timeout} argument
* @return {@code true} if a permit was acquired and {@code false}
* if the waiting time elapsed before a permit was acquired
* @throws InterruptedException if the current thread is interrupted
*/
public boolean tryAcquire(long timeout, TimeUnit unit)
throws InterruptedException {
return sync.tryAcquireSharedNanos(1, unit.toNanos(timeout));
}
/**
* Releases a permit, returning it to the semaphore.
*
* <p>Releases a permit, increasing the number of available permits by
* one. If any threads are trying to acquire a permit, then one is
* selected and given the permit that was just released. That thread
* is (re)enabled for thread scheduling purposes.
*
* <p>There is no requirement that a thread that releases a permit must
* have acquired that permit by calling {@link #acquire}.
* Correct usage of a semaphore is established by programming convention
* in the application.
*/
public void release() {
sync.releaseShared(1);
}
/**
* Acquires the given number of permits from this semaphore,
* blocking until all are available,
* or the thread is {@linkplain Thread#interrupt interrupted}.
*
* <p>Acquires the given number of permits, if they are available,
* and returns immediately, reducing the number of available permits
* by the given amount.
*
* <p>If insufficient permits are available then the current thread becomes
* disabled for thread scheduling purposes and lies dormant until
* one of two things happens:
* <ul>
* <li>Some other thread invokes one of the {@link #release() release}
* methods for this semaphore, the current thread is next to be assigned
* permits and the number of available permits satisfies this request; or
* <li>Some other thread {@linkplain Thread#interrupt interrupts}
* the current thread.
* </ul>
*
* <p>If the current thread:
* <ul>
* <li>has its interrupted status set on entry to this method; or
* <li>is {@linkplain Thread#interrupt interrupted} while waiting
* for a permit,
* </ul>
* then {@link InterruptedException} is thrown and the current thread's
* interrupted status is cleared.
* Any permits that were to be assigned to this thread are instead
* assigned to other threads trying to acquire permits, as if
* permits had been made available by a call to {@link #release()}.
*
* @param permits the number of permits to acquire
* @throws InterruptedException if the current thread is interrupted
* @throws IllegalArgumentException if {@code permits} is negative
*/
public void acquire(int permits) throws InterruptedException {
if (permits < 0) throw new IllegalArgumentException();
sync.acquireSharedInterruptibly(permits);
}
/**
* Acquires the given number of permits from this semaphore,
* blocking until all are available.
*
* <p>Acquires the given number of permits, if they are available,
* and returns immediately, reducing the number of available permits
* by the given amount.
*
* <p>If insufficient permits are available then the current thread becomes
* disabled for thread scheduling purposes and lies dormant until
* some other thread invokes one of the {@link #release() release}
* methods for this semaphore, the current thread is next to be assigned
* permits and the number of available permits satisfies this request.
*
* <p>If the current thread is {@linkplain Thread#interrupt interrupted}
* while waiting for permits then it will continue to wait and its
* position in the queue is not affected. When the thread does return
* from this method its interrupt status will be set.
*
* @param permits the number of permits to acquire
* @throws IllegalArgumentException if {@code permits} is negative
*
*/
public void acquireUninterruptibly(int permits) {
if (permits < 0) throw new IllegalArgumentException();
sync.acquireShared(permits);
}
/**
* Acquires the given number of permits from this semaphore, only
* if all are available at the time of invocation.
*
* <p>Acquires the given number of permits, if they are available, and
* returns immediately, with the value {@code true},
* reducing the number of available permits by the given amount.
*
* <p>If insufficient permits are available then this method will return
* immediately with the value {@code false} and the number of available
* permits is unchanged.
*
* <p>Even when this semaphore has been set to use a fair ordering
* policy, a call to {@code tryAcquire} <em>will</em>
* immediately acquire a permit if one is available, whether or
* not other threads are currently waiting. This
* "barging" behavior can be useful in certain
* circumstances, even though it breaks fairness. If you want to
* honor the fairness setting, then use {@link #tryAcquire(int,
* long, TimeUnit) tryAcquire(permits, 0, TimeUnit.SECONDS) }
* which is almost equivalent (it also detects interruption).
*
* @param permits the number of permits to acquire
* @return {@code true} if the permits were acquired and
* {@code false} otherwise
* @throws IllegalArgumentException if {@code permits} is negative
*/
public boolean tryAcquire(int permits) {
if (permits < 0) throw new IllegalArgumentException();
return sync.nonfairTryAcquireShared(permits) >= 0;
}
/**
* Acquires the given number of permits from this semaphore, if all
* become available within the given waiting time and the current
* thread has not been {@linkplain Thread#interrupt interrupted}.
*
* <p>Acquires the given number of permits, if they are available and
* returns immediately, with the value {@code true},
* reducing the number of available permits by the given amount.
*
* <p>If insufficient permits are available then
* the current thread becomes disabled for thread scheduling
* purposes and lies dormant until one of three things happens:
* <ul>
* <li>Some other thread invokes one of the {@link #release() release}
* methods for this semaphore, the current thread is next to be assigned
* permits and the number of available permits satisfies this request; or
* <li>Some other thread {@linkplain Thread#interrupt interrupts}
* the current thread; or
* <li>The specified waiting time elapses.
* </ul>
*
* <p>If the permits are acquired then the value {@code true} is returned.
*
* <p>If the current thread:
* <ul>
* <li>has its interrupted status set on entry to this method; or
* <li>is {@linkplain Thread#interrupt interrupted} while waiting
* to acquire the permits,
* </ul>
* then {@link InterruptedException} is thrown and the current thread's
* interrupted status is cleared.
* Any permits that were to be assigned to this thread, are instead
* assigned to other threads trying to acquire permits, as if
* the permits had been made available by a call to {@link #release()}.
*
* <p>If the specified waiting time elapses then the value {@code false}
* is returned. If the time is less than or equal to zero, the method
* will not wait at all. Any permits that were to be assigned to this
* thread, are instead assigned to other threads trying to acquire
* permits, as if the permits had been made available by a call to
* {@link #release()}.
*
* @param permits the number of permits to acquire
* @param timeout the maximum time to wait for the permits
* @param unit the time unit of the {@code timeout} argument
* @return {@code true} if all permits were acquired and {@code false}
* if the waiting time elapsed before all permits were acquired
* @throws InterruptedException if the current thread is interrupted
* @throws IllegalArgumentException if {@code permits} is negative
*/
public boolean tryAcquire(int permits, long timeout, TimeUnit unit)
throws InterruptedException {
if (permits < 0) throw new IllegalArgumentException();
return sync.tryAcquireSharedNanos(permits, unit.toNanos(timeout));
}
/**
* Releases the given number of permits, returning them to the semaphore.
*
* <p>Releases the given number of permits, increasing the number of
* available permits by that amount.
* If any threads are trying to acquire permits, then one
* is selected and given the permits that were just released.
* If the number of available permits satisfies that thread's request
* then that thread is (re)enabled for thread scheduling purposes;
* otherwise the thread will wait until sufficient permits are available.
* If there are still permits available
* after this thread's request has been satisfied, then those permits
* are assigned in turn to other threads trying to acquire permits.
*
* <p>There is no requirement that a thread that releases a permit must
* have acquired that permit by calling {@link Semaphore#acquire acquire}.
* Correct usage of a semaphore is established by programming convention
* in the application.
*
* @param permits the number of permits to release
* @throws IllegalArgumentException if {@code permits} is negative
*/
public void release(int permits) {
if (permits < 0) throw new IllegalArgumentException();
sync.releaseShared(permits);
}
/**
* Returns the current number of permits available in this semaphore.
*
* <p>This method is typically used for debugging and testing purposes.
*
* @return the number of permits available in this semaphore
*/
public int availablePermits() {
return sync.getPermits();
}
/**
* Acquires and returns all permits that are immediately available.
*
* @return the number of permits acquired
*/
public int drainPermits() {
return sync.drainPermits();
}
/**
* Shrinks the number of available permits by the indicated
* reduction. This method can be useful in subclasses that use
* semaphores to track resources that become unavailable. This
* method differs from {@code acquire} in that it does not block
* waiting for permits to become available.
*
* @param reduction the number of permits to remove
* @throws IllegalArgumentException if {@code reduction} is negative
*/
protected void reducePermits(int reduction) {
if (reduction < 0) throw new IllegalArgumentException();
sync.reducePermits(reduction);
}
/**
* Returns {@code true} if this semaphore has fairness set true.
*
* @return {@code true} if this semaphore has fairness set true
*/
public boolean isFair() {
return sync instanceof FairSync;
}
/**
* Queries whether any threads are waiting to acquire. Note that
* because cancellations may occur at any time, a {@code true}
* return does not guarantee that any other thread will ever
* acquire. This method is designed primarily for use in
* monitoring of the system state.
*
* @return {@code true} if there may be other threads waiting to
* acquire the lock
*/
public final boolean hasQueuedThreads() {
return sync.hasQueuedThreads();
}
/**
* Returns an estimate of the number of threads waiting to acquire.
* The value is only an estimate because the number of threads may
* change dynamically while this method traverses internal data
* structures. This method is designed for use in monitoring of the
* system state, not for synchronization control.
*
* @return the estimated number of threads waiting for this lock
*/
public final int getQueueLength() {
return sync.getQueueLength();
}
/**
* Returns a collection containing threads that may be waiting to acquire.
* Because the actual set of threads may change dynamically while
* constructing this result, the returned collection is only a best-effort
* estimate. The elements of the returned collection are in no particular
* order. This method is designed to facilitate construction of
* subclasses that provide more extensive monitoring facilities.
*
* @return the collection of threads
*/
protected Collection<Thread> getQueuedThreads() {
return sync.getQueuedThreads();
}
/**
* Returns a string identifying this semaphore, as well as its state.
* The state, in brackets, includes the String {@code "Permits ="}
* followed by the number of permits.
*
* @return a string identifying this semaphore, as well as its state
*/
public String toString() {
return super.toString() + "[Permits = " + sync.getPermits() + "]";
}
}View Code
Semaphoreは共有ロックで実現している.共有ロックの取得の原則に従って、Semaphoreは「公平信号量」と「非公平信号量」に分けられる.
「公平信号量」と「非公平信号量」の違い
「公平信号量」と「非公平信号量」の放出信号量のメカニズムは同じです!異なるのは、信号量を取得するメカニズムである.スレッドは、信号量許可を取得しようとすると、公平な信号量の場合、現在のスレッドがCLHキューのヘッダにない場合、キューに並んで待つ.一方、非公開フラット信号量では、現在のスレッドがCLHキューのヘッダにあるかどうかにかかわらず、信号量が直接取得される.この違いは,それらのtryAcquireShared()関数の実装が異なることに具体的に現れる.
「公平信号量」クラス
static final class FairSync extends Sync {
private static final long serialVersionUID = 2014338818796000944L;
FairSync(int permits) {
super(permits);
}
protected int tryAcquireShared(int acquires) {
for (;;) {
if (hasQueuedPredecessors())
return -1;
int available = getState();
int remaining = available - acquires;
if (remaining < 0 ||
compareAndSetState(available, remaining))
return remaining;
}
}
}View Code
「非公開フラット信号量」クラス
static final class NonfairSync extends Sync {
private static final long serialVersionUID = -2694183684443567898L;
NonfairSync(int permits) {
super(permits);
}
protected int tryAcquireShared(int acquires) {
return nonfairTryAcquireShared(acquires);
}
}View Code
次に,それらのソースコードを逐次解析する.
1.信号量構築関数
public Semaphore(int permits) {
sync = new NonfairSync(permits);
}
public Semaphore(int permits, boolean fair) {
sync = fair ? new FairSync(permits) : new NonfairSync(permits);
}ここで,信号量を「フェア信号量(FairSync)」と「ノンフェア信号量(NonfairSync)」に分けることができる.Semaphore(int permits)関数では、デフォルトで「非標準信号量」が作成されます.
2.公平信号量の取得と解放
2.1公平信号量の取得Semaphoreにおける公平信号量はFairSyncである.その取得APIは以下の通りである.
public void acquire() throws InterruptedException {
sync.acquireSharedInterruptibly(1);
}
public void acquire(int permits) throws InterruptedException {
if (permits < 0) throw new IllegalArgumentException();
sync.acquireSharedInterruptibly(permits);
}信号量のacquire()取得関数は,実際には呼び出されたAQSのacquireSharedInterruptibly()である.
acquireSharedInterruptibly()のソースコードは次のとおりです.
public final void acquireSharedInterruptibly(int arg)
throws InterruptedException {
// ， 。
if (Thread.interrupted())
throw new InterruptedException();
// ， “ ”； ， ， doAcquireSharedInterruptibly() 。
if (tryAcquireShared(arg) < 0)
doAcquireSharedInterruptibly(arg);
}Semaphoreの「フェアロック」に対応するtryAcquireShared()は、次のように実現されます.
protected int tryAcquireShared(int acquires) {
for (;;) {
// “ ” CLH ，
// ， -1。
if (hasQueuedPredecessors())
return -1;
// “ ”
int available = getState();
// “ acquires ， ”
int remaining = available - acquires;
// “ >=0”， “ ” remaining。
if (remaining < 0 ||
compareAndSetState(available, remaining))
return remaining;
}
}説明:tryAcquireShared()の役割は、acquires個の信号量許可数を取得しようとすることです.Semaphoreの場合、stateは「現在入手可能な信号量許可数」を表す.
AQSのdoAcquireSharedInterruptibly()の実装を見てみましょう.
private void doAcquireSharedInterruptibly(long arg)
throws InterruptedException {
// ” “ Node ， Node ” “ ； CLH 。
final Node node = addWaiter(Node.SHARED);
boolean failed = true;
try {
for (;;) {
// 。
// CLH ， ” “。
final Node p = node.predecessor();
if (p == head) {
long r = tryAcquireShared(arg);
if (r >= 0) {
setHeadAndPropagate(node, r);
p.next = null; // help GC
failed = false;
return;
}
}
// ， 。
// ， ( )。
if (shouldParkAfterFailedAcquire(p, node) &&
parkAndCheckInterrupt())
throw new InterruptedException();
}
} finally {
if (failed)
cancelAcquire(node);
}
}説明:doAcquireSharedInterruptibly()は、現在のスレッドが共有ロック(または中断)を取得するまで待機します.(01)addWaiter(Node.SHARED)の役割は、「現在のスレッド」のノードを作成し、ノードに記録されているロックのタイプは「共有ロック」(Node.SHARED)であり、このノードをCLHキューの末尾に追加する.ノードとCLHについては「Javaマルチスレッドシリーズ--「JUCロック」03のフェアロック(一)」で詳しく説明したが、ここでは説明を繰り返さない.(02)node.predcessor()の役割は、前のノードを取得することである.前のノードがCLHキューのヘッダである場合共有ロックの取得を試みる.(03)shouldParkAfterFailedAcquire()の役割は、その名前と同様であり、ロックの取得に失敗した後、スレッドが待機する必要がある場合はtrueを返す.そうでなければfalseを返す.(04)shouldParkAfterFailedAcquire()がtureを返すとparkAndCheckInterrupt()を呼び出すを選択すると、現在のスレッドは共有ロックが取得されるまで待機状態になります.doAcquireSharedInterruptibly()のshouldParkAfterFailedAcquire()、parkAndCheckInterruptなどの関数は「Javaマルチスレッドシリーズ--「JUCロック」03のフェアロック(一)」で紹介されていますが、ここでは詳しく説明しません.
2.2公平信号量の解放
Semaphoreにおけるフェアシグナル量(FairSync)の解放APIは以下の通りである.
public void release() {
sync.releaseShared(1);
}
public void release(int permits) {
if (permits < 0) throw new IllegalArgumentException();
sync.releaseShared(permits);
}信号量のreleases()解放関数は,実際には呼び出されたAQSにおけるreleaseShared()である.
releaseShared()はAQSで実装され、ソースコードは以下の通りである.
public final boolean releaseShared(int arg) {
if (tryReleaseShared(arg)) {
doReleaseShared();
return true;
}
return false;
}説明:releaseShared()の目的は、現在のスレッドが保持している共有ロックを解放することです.まずtryReleaseShared()を介して共有ロックを解放しようとします.試行に成功すると、直接戻ります.試行に失敗すると、doReleaseShared()を使用して共有ロックを解放します.
SemaphoreはtryReleaseShared()を書き換え、そのソースコードは以下の通りである.
protected final boolean tryReleaseShared(int releases) {
for (;;) {
// “ ”
int current = getState();
// “ releases ， ”
int next = current + releases;
if (next < current) // overflow
throw new Error("Maximum permit count exceeded");
// “ ” next。
if (compareAndSetState(current, next))
return true;
}
}tryReleaseShared()が共有ロックの解放に失敗した場合、doReleaseShared()が呼び出されて共有ロックが解放されます.doReleaseShared()のソースコードは次のとおりです.
private void doReleaseShared() {
for (;;) {
// CLH
Node h = head;
// null， tail 。
if (h != null && h != tail) {
//
int ws = h.waitStatus;
// SIGNAL ， “ ” unpark 。
if (ws == Node.SIGNAL) {
// “ ” 。 ， 。
if (!compareAndSetWaitStatus(h, Node.SIGNAL, 0))
continue;
// “ ”。
unparkSuccessor(h);
}
// ， “ ” 。
else if (ws == 0 &&
!compareAndSetWaitStatus(h, 0, Node.PROPAGATE))
continue; // loop on failed CAS
}
// ， 。 ， 。
if (h == head) // loop if head changed
break;
}
}説明:doReleaseShared()は共有ロックを解放します.その後のCLHキューを巡回し、キュー内の各ノードに対応するスレッドを「起動」し、「実行」します.最終的な目的は、これらのスレッドが保有する信号量を解放することである.
3非公平信号量の取得と解放
Semaphoreにおける非公開フラット信号量はNonFairSyncである.Semaphoreでは、「非公平信号量許可の解放(release)」は、「公平信号量許可の解放(release)」と同じである.異なるのは、それらが「信号量許可」を取得するメカニズムが異なり、以下は非公平信号量が信号量許可を取得するコードである.
非公開フラット信号量のtryAcquireShared()は以下のように実現される.
protected int tryAcquireShared(int acquires) {
return nonfairTryAcquireShared(acquires);
}nonfairTryAcquireShared()の実装は次のとおりです.
final int nonfairTryAcquireShared(int acquires) {
for (;;) {
// “ ”
int available = getState();
// “ acquires ， ”
int remaining = available - acquires;
// “ >=0”， “ ” remaining。
if (remaining < 0 ||
compareAndSetState(available, remaining))
return remaining;
}
}説明:非公開フラット信号量のtryAcquireShared()は、AQSのnonfairTryAcquireShared()を呼び出す.nonfairTryAcquireShared()のforサイクルでは、「現在の残りの信号量許可数」が十分かどうかを直接判断します.十分であれば、直接「取得可能な信号量許可数を設定」し、さらに信号量を取得する.一方、公平信号量のtryAcquireShared()では、信号量を取得する前にif(hasQueuedPredecessors()で「現在のスレッドがCLHキューのヘッダにあるかどうか」を判断し、そうであれば-1を返す.
Semaphoreの例
1 import java.util.concurrent.ExecutorService;
2 import java.util.concurrent.Executors;
3 import java.util.concurrent.Semaphore;
4
5 public class SemaphoreTest1 {
6 private static final int SEM_MAX = 10;
7 public static void main(String[] args) {
8 Semaphore sem = new Semaphore(SEM_MAX);
9 //
10 ExecutorService threadPool = Executors.newFixedThreadPool(3);
11 //
12 threadPool.execute(new MyThread(sem, 5));
13 threadPool.execute(new MyThread(sem, 4));
14 threadPool.execute(new MyThread(sem, 7));
15 //
16 threadPool.shutdown();
17 }
18 }
19
20 class MyThread extends Thread {
21 private volatile Semaphore sem; //
22 private int count; //
23
24 MyThread(Semaphore sem, int count) {
25 this.sem = sem;
26 this.count = count;
27 }
28
29 public void run() {
30 try {
31 // count
32 sem.acquire(count);
33
34 Thread.sleep(2000);
35 System.out.println(Thread.currentThread().getName() + " acquire count="+count);
36 } catch (InterruptedException e) {
37 e.printStackTrace();
38 } finally {
39 // ， 。
40 sem.release(count);
41 System.out.println(Thread.currentThread().getName() + " release " + count + "");
42 }
43 }
44 }実行結果:
pool-1-thread-1 acquire count=5
pool-1-thread-2 acquire count=4
pool-1-thread-1 release 5
pool-1-thread-2 release 4
pool-1-thread-3 acquire count=7
pool-1-thread-3 release 7結果説明:信号量semのライセンス総数は10個である.合計3スレッドで,それぞれ取得する信号量許可数は5,4,7である.前の2つのスレッドが信号量の許可を取得した後、semの残りの利用可能な許可数は1である.したがって、最後のスレッドは、7つの信号量ライセンスを取得するには、最初の2つのスレッドが保有する信号量ライセンスを解放するのを待たなければならない.
その他
1. Javaマルチスレッドシリーズ--「JUCロック」01のフレームワーク
2. Javaマルチスレッドシリーズ--「JUCロック」02の反発ロックReentrantLock
3. Javaマルチスレッドシリーズ--「JUCロック」03のフェアロック(一)
4. Javaマルチスレッドシリーズ--「JUCロック」04のフェアロック(二)
5. Javaマルチスレッドシリーズ--「JUCロック」05の非公平ロック
6. Javaマルチスレッドシリーズ--「JUCロック」06のCondition条件
7. Javaマルチスレッドシリーズ--「JUCロック」07のLockSupport
8. Javaマルチスレッドシリーズ--「JUCロック」08の共有ロックとReentrantReadWriteLock
9. Javaマルチスレッドシリーズ--「JUCロック」09のCountDownLatchの原理と例
10. Javaマルチスレッドシリーズ--「JUCロック」10のCyclicBarrierの原理と例
11. Javaマルチスレッドシリーズディレクトリ(共にxx編)