123 lines
4.0 KiB
Java
123 lines
4.0 KiB
Java
package org.bukkit.metadata;
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import java.lang.ref.SoftReference;
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import java.util.concurrent.Callable;
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import org.apache.commons.lang.Validate;
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import org.bukkit.plugin.Plugin;
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/**
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* The LazyMetadataValue class implements a type of metadata that is not
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* computed until another plugin asks for it.
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* <p>
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* By making metadata values lazy, no computation is done by the providing
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* plugin until absolutely necessary (if ever). Additionally,
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* LazyMetadataValue objects cache their values internally unless overridden
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* by a {@link CacheStrategy} or invalidated at the individual or plugin
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* level. Once invalidated, the LazyMetadataValue will recompute its value
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* when asked.
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*/
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public class LazyMetadataValue extends MetadataValueAdapter {
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private Callable<Object> lazyValue;
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private CacheStrategy cacheStrategy;
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private SoftReference<Object> internalValue;
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private static final Object ACTUALLY_NULL = new Object();
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/**
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* Initialized a LazyMetadataValue object with the default
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* CACHE_AFTER_FIRST_EVAL cache strategy.
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*
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* @param owningPlugin the {@link Plugin} that created this metadata
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* value.
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* @param lazyValue the lazy value assigned to this metadata value.
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*/
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public LazyMetadataValue(Plugin owningPlugin, Callable<Object> lazyValue) {
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this(owningPlugin, CacheStrategy.CACHE_AFTER_FIRST_EVAL, lazyValue);
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}
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/**
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* Initializes a LazyMetadataValue object with a specific cache strategy.
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*
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* @param owningPlugin the {@link Plugin} that created this metadata
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* value.
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* @param cacheStrategy determines the rules for caching this metadata
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* value.
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* @param lazyValue the lazy value assigned to this metadata value.
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*/
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public LazyMetadataValue(Plugin owningPlugin, CacheStrategy cacheStrategy, Callable<Object> lazyValue) {
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super(owningPlugin);
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Validate.notNull(cacheStrategy, "cacheStrategy cannot be null");
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Validate.notNull(lazyValue, "lazyValue cannot be null");
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this.internalValue = new SoftReference<Object>(null);
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this.lazyValue = lazyValue;
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this.cacheStrategy = cacheStrategy;
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}
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/**
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* Protected special constructor used by FixedMetadataValue to bypass
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* standard setup.
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*
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* @param owningPlugin the owning plugin
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*/
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protected LazyMetadataValue(Plugin owningPlugin) {
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super(owningPlugin);
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}
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public Object value() {
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eval();
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Object value = internalValue.get();
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if (value == ACTUALLY_NULL) {
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return null;
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}
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return value;
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}
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/**
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* Lazily evaluates the value of this metadata item.
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*
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* @throws MetadataEvaluationException if computing the metadata value
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* fails.
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*/
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private synchronized void eval() throws MetadataEvaluationException {
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if (cacheStrategy == CacheStrategy.NEVER_CACHE || internalValue.get() == null) {
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try {
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Object value = lazyValue.call();
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if (value == null) {
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value = ACTUALLY_NULL;
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}
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internalValue = new SoftReference<Object>(value);
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} catch (Exception e) {
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throw new MetadataEvaluationException(e);
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}
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}
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}
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public synchronized void invalidate() {
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if (cacheStrategy != CacheStrategy.CACHE_ETERNALLY) {
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internalValue.clear();
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}
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}
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/**
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* Describes possible caching strategies for metadata.
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*/
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public enum CacheStrategy {
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/**
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* Once the metadata value has been evaluated, do not re-evaluate the
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* value until it is manually invalidated.
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*/
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CACHE_AFTER_FIRST_EVAL,
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/**
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* Re-evaluate the metadata item every time it is requested
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*/
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NEVER_CACHE,
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/**
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* Once the metadata value has been evaluated, do not re-evaluate the
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* value in spite of manual invalidation.
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*/
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CACHE_ETERNALLY
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}
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}
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