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本文主要研究一下Elasticsearch的LazyInitializable
LazyInitializable
elasticsearch-7.0.1/server/src/main/java/org/elasticsearch/common/util/LazyInitializable.java
public final class LazyInitializable<T, E extends Exception> { private final CheckedSupplier<T, E> supplier; private final Consumer<T> onGet; private final Consumer<T> onReset; private volatile T value; /** * Creates the simple LazyInitializable instance. * * @param supplier * The {@code CheckedSupplier} to generate values which will be * served on {@code #getOrCompute()} invocations. */ public LazyInitializable(CheckedSupplier<T, E> supplier) { this(supplier, v -> {}, v -> {}); } /** * Creates the complete LazyInitializable instance. * * @param supplier * The {@code CheckedSupplier} to generate values which will be * served on {@code #getOrCompute()} invocations. * @param onGet * A {@code Consumer} which is called on each value, newly forged or * stale, that is returned by {@code #getOrCompute()} * @param onReset * A {@code Consumer} which is invoked on the value that will be * erased when calling {@code #reset()} */ public LazyInitializable(CheckedSupplier<T, E> supplier, Consumer<T> onGet, Consumer<T> onReset) { this.supplier = supplier; this.onGet = onGet; this.onReset = onReset; } /** * Returns a value that was created by <code>supplier</code>. The value might * have been previously created, if not it will be created now, thread safe of * course. */ public T getOrCompute() throws E { final T readOnce = value; // Read volatile just once... final T result = readOnce == null ? maybeCompute(supplier) : readOnce; onGet.accept(result); return result; } /** * Clears the value, if it has been previously created by calling * {@code #getOrCompute()}. The <code>onReset</code> will be called on this * value. The next call to {@code #getOrCompute()} will recreate the value. */ public synchronized void reset() { if (value != null) { onReset.accept(value); value = null; } } /** * Creates a new value thread safely. */ private synchronized T maybeCompute(CheckedSupplier<T, E> supplier) throws E { if (value == null) { value = Objects.requireNonNull(supplier.get()); } return value; }}- LazyInitializable封装了CheckedSupplier，类似CachedSupplier，不过它提供了reset方法可以重置以反复使用，另外还支持了onGet、onReset的回调
实例
elasticsearch-7.0.1/plugins/discovery-ec2/src/main/java/org/elasticsearch/discovery/ec2/AwsEc2ServiceImpl.java
class AwsEc2ServiceImpl implements AwsEc2Service { private static final Logger logger = LogManager.getLogger(AwsEc2ServiceImpl.class); private final AtomicReference<LazyInitializable<AmazonEc2Reference, ElasticsearchException>> lazyClientReference = new AtomicReference<>(); //...... public void refreshAndClearCache(Ec2ClientSettings clientSettings) { final LazyInitializable<AmazonEc2Reference, ElasticsearchException> newClient = new LazyInitializable<>( () -> new AmazonEc2Reference(buildClient(clientSettings)), clientReference -> clientReference.incRef(), clientReference -> clientReference.decRef()); final LazyInitializable<AmazonEc2Reference, ElasticsearchException> oldClient = this.lazyClientReference.getAndSet(newClient); if (oldClient != null) { oldClient.reset(); } } //......} - AwsEc2ServiceImpl的refreshAndClearCache方法会根据clientSettings创建新的LazyInitializable，之后更新lazyClientReference，如果oldClient不为null则调用其reset方法重置value
小结
LazyInitializable封装了CheckedSupplier，类似CachedSupplier，不过它提供了reset方法可以重置以反复使用，另外还支持了onGet、onReset的回调
doc
- LazyInitializable