受comment to an given answer的启发我尝试创建多线程模式的线程安全实现，它依赖于唯一键并对它们执行锁定（我从 JB Nizet 获得了这个想法这是question）的回答。
问题
我提供的实施是否可行？
我对Multiton（或Singleton）是否总体良好的模式不感兴趣，它会导致讨论。我只想要一个干净而有效的实施方案。
反政府：
赞成
getInstance。int或String，因此您可以确保在方法调用后获得非null实例。public class Multiton
{
private static final Map<Enum<?>, Multiton> instances = new HashMap<Enum<?>, Multiton>();
private Multiton() {System.out.println("Created instance."); }
/* Can be called concurrently, since it only synchronizes on id */
public static <KEY extends Enum<?> & MultitionKey> Multiton getInstance(KEY id)
{
synchronized (id)
{
if (instances.get(id) == null)
instances.put(id, new Multiton());
}
System.out.println("Retrieved instance.");
return instances.get(id);
}
public interface MultitionKey { /* */ }
public static void main(String[] args) throws InterruptedException
{
//getInstance(Keys.KEY_1);
getInstance(OtherKeys.KEY_A);
Runnable r = new Runnable() {
@Override
public void run() { getInstance(Keys.KEY_1); }
};
int size = 100;
List<Thread> threads = new ArrayList<Thread>();
for (int i = 0; i < size; i++)
threads.add(new Thread(r));
for (Thread t : threads)
t.start();
for (Thread t : threads)
t.join();
}
enum Keys implements MultitionKey
{
KEY_1;
/* define more keys */
}
enum OtherKeys implements MultitionKey
{
KEY_A;
/* define more keys */
}
}
我试图阻止调整地图大小和滥用我同步的枚举。 在我完成它之前，它更像是一个概念证明！ ：）
public class Multiton
{
private static final Map<MultitionKey, Multiton> instances = new HashMap<MultitionKey, Multiton>((int) (Key.values().length/0.75f) + 1);
private static final Map<Key, MultitionKey> keyMap;
static
{
Map<Key, MultitionKey> map = new HashMap<Key, MultitionKey>();
map.put(Key.KEY_1, Keys.KEY_1);
map.put(Key.KEY_2, OtherKeys.KEY_A);
keyMap = Collections.unmodifiableMap(map);
}
public enum Key {
KEY_1, KEY_2;
}
private Multiton() {System.out.println("Created instance."); }
/* Can be called concurrently, since it only synchronizes on KEY */
public static <KEY extends Enum<?> & MultitionKey> Multiton getInstance(Key id)
{
@SuppressWarnings ("unchecked")
KEY key = (KEY) keyMap.get(id);
synchronized (keyMap.get(id))
{
if (instances.get(key) == null)
instances.put(key, new Multiton());
}
System.out.println("Retrieved instance.");
return instances.get(key);
}
private interface MultitionKey { /* */ }
private enum Keys implements MultitionKey
{
KEY_1;
/* define more keys */
}
private enum OtherKeys implements MultitionKey
{
KEY_A;
/* define more keys */
}
}
答案 0 :(得分：3)
绝对不是线程安全的。这是许多可能出错的事情的简单例子。
线程A正试图放入密钥id1。由于放置在id2，线程B正在调整存储桶表的大小。因为它们具有不同的同步监视器，所以它们可以并行进行比赛。
Thread A Thread B
-------- --------
b = key.hash % map.buckets.size
copy map.buckets reference to local var
set map.buckets = new Bucket[newSize]
insert keys from old buckets into new buckets
insert into map.buckets[b]
在此示例中，假设Thread A看到了map.buckets = new Bucket[newSize]修改。它不能保证（因为没有发生 - 在边缘之前），但它可能。在这种情况下，它会将（key，value）对插入错误的桶中。没有人会找到它。
作为一个轻微的变体，如果Thread A将map.buckets引用复制到本地var并完成所有工作，那么它将插入到正确的存储桶中，但错误的存储桶表;它不会插入Thread B即将安装的新表中，供大家查看。如果key 1上的下一个操作恰好看到新表（再次，不保证但可能会这样），那么它将不会看到Thread A's个动作，因为它们是在长期被遗忘的存储桶阵列上完成的
答案 1 :(得分：2)
我说不可行。
同步id参数充满了危险 - 如果他们将此enum用于另一个同步机制会怎么样？当然，HashMap并不像评论所指出的那样是并发的。
要演示 - 试试这个：
Runnable r = new Runnable() {
@Override
public void run() {
// Added to demonstrate the problem.
synchronized(Keys.KEY_1) {
getInstance(Keys.KEY_1);
}
}
};
这是一个使用原子而不是同步的实现，因此应该更有效。它比你的复杂得多，但处理Miltiton IS中的所有边缘情况都很复杂。
public class Multiton {
// The static instances.
private static final AtomicReferenceArray<Multiton> instances = new AtomicReferenceArray<>(1000);
// Ready for use - set to false while initialising.
private final AtomicBoolean ready = new AtomicBoolean();
// Everyone who is waiting for me to initialise.
private final Queue<Thread> waiters = new ConcurrentLinkedQueue<>();
// For logging (and a bit of linguistic fun).
private final int forInstance;
// We need a simple constructor.
private Multiton(int forInstance) {
this.forInstance = forInstance;
log(forInstance, "New");
}
// The expensive initialiser.
public void init() throws InterruptedException {
log(forInstance, "Init");
// ... presumably heavy stuff.
Thread.sleep(1000);
// We are now ready.
ready();
}
private void ready() {
log(forInstance, "Ready");
// I am now ready.
ready.getAndSet(true);
// Unpark everyone waiting for me.
for (Thread t : waiters) {
LockSupport.unpark(t);
}
}
// Get the instance for that one.
public static Multiton getInstance(int which) throws InterruptedException {
// One there already?
Multiton it = instances.get(which);
if (it == null) {
// Lazy make.
Multiton newIt = new Multiton(which);
// Successful put?
if (instances.compareAndSet(which, null, newIt)) {
// Yes!
it = newIt;
// Initialise it.
it.init();
} else {
// One appeared as if by magic (another thread got there first).
it = instances.get(which);
// Wait for it to finish initialisation.
// Put me in its queue of waiters.
it.waiters.add(Thread.currentThread());
log(which, "Parking");
while (!it.ready.get()) {
// Park me.
LockSupport.park();
}
// I'm not waiting any more.
it.waiters.remove(Thread.currentThread());
log(which, "Unparked");
}
}
return it;
}
// Some simple logging.
static void log(int which, String s) {
log(new Date(), "Thread " + Thread.currentThread().getId() + " for Multiton " + which + " " + s);
}
static final DateFormat dateFormat = new SimpleDateFormat("yyyy-MM-dd HH:mm:ss.SSS");
// synchronized so I don't need to make the DateFormat ThreadLocal.
static synchronized void log(Date d, String s) {
System.out.println(dateFormat.format(d) + " " + s);
}
// The tester class.
static class MultitonTester implements Runnable {
int which;
private MultitonTester(int which) {
this.which = which;
}
@Override
public void run() {
try {
Multiton.log(which, "Waiting");
Multiton m = Multiton.getInstance(which);
Multiton.log(which, "Got");
} catch (InterruptedException ex) {
Multiton.log(which, "Interrupted");
}
}
}
public static void main(String[] args) throws InterruptedException {
int testers = 50;
int multitons = 50;
// Do a number of them. Makes n testers for each Multiton.
for (int i = 1; i < testers * multitons; i++) {
// Which one to create.
int which = i / testers;
//System.out.println("Requesting Multiton " + i);
new Thread(new MultitonTester(which+1)).start();
}
}
}
答案 2 :(得分：0)
我不是Java程序员，但是：HashMap对于并发访问是不安全的。我可能会推荐ConcurrentHashMap。
private static final ConcurrentHashMap<Object, Multiton> instances = new ConcurrentHashMap<Object, Multiton>();
public static <TYPE extends Object, KEY extends Enum<Keys> & MultitionKey<TYPE>> Multiton getInstance(KEY id)
{
Multiton result;
synchronized (id)
{
result = instances.get(id);
if(result == null)
{
result = new Multiton();
instances.put(id, result);
}
}
System.out.println("Retrieved instance.");
return result;
}