设计模式学习心得<享元模式 Flyweight>

享元模式（Flyweight Pattern）主要用于减小建立对象的数量，以减小内存占用和提升性能。这种类型的设计模式属于结构型模式，它提供了减小对象数量从而改善应用所需的对象结构的方式。

享元模式尝试重用现有的同类对象，若是未找到匹配的对象，则建立新对象。

概述

---

• 意图
  运用共享技术有效地支持大量细粒度的对象。

• 主要解决
  在有大量对象时，有可能会形成内存溢出，咱们把其中共同的部分抽象出来，若是有相同的业务请求，直接返回在内存中已有的对象，避免从新建立。

• 什么时候使用

1. 系统中有大量对象。
2. 这些对象消耗大量内存。
3. 这些对象的状态大部分能够外部化。
4. 这些对象能够按照内蕴状态分为不少组，当把外蕴对象从对象中剔除出来时，每一组对象均可以用一个对象来代替。
5. 系统不依赖于这些对象身份，这些对象是不可分辨的。

• 如何解决
  用惟一标识码判断，若是在内存中有，则返回这个惟一标识码所标识的对象。

• 关键代码
  用 HashMap 存储这些对象。

• 应用实例

1. JAVA 中的 String，若是有则返回，若是没有则建立一个字符串保存在字符串缓存池里面。
2. 数据库的数据池。
3. hibernate-validate自定义了基于弱引用的缓存容器ConcurrentReferenceHashMap

• 优势
  大大减小对象的建立，下降系统的内存，使效率提升。

• 缺点
  提升了系统的复杂度，须要分离出外部状态和内部状态，并且外部状态具备固有化的性质，不该该随着内部状态的变化而变化，不然会形成系统的混乱。

• 使用场景

1. 系统有大量类似对象。
2. 须要缓冲池的场景。

• 注意事项

1. 注意划分外部状态和内部状态，不然可能会引发线程安全问题。
2. 这些类必须有一个工厂对象加以控制。

实现

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摘抄hibernate-validate源码

容器：ConcurrentReferenceHashMap

调用摘抄

/**
 * This manager is in charge of providing all constraint related meta data
 * required by the validation engine.
 * <p>
 * Actual retrieval of meta data is delegated to {@link MetaDataProvider}
 * implementations which load meta-data based e.g. based on annotations or XML.
 * </p>
 * <p>
 * For performance reasons a cache is used which stores all meta data once
 * loaded for repeated retrieval. Upon initialization this cache is populated
 * with meta data provided by the given <i>eager</i> providers. If the cache
 * doesn't contain the meta data for a requested type it will be retrieved on
 * demand using the annotation based provider.
 * </p>
 *
 * @author Gunnar Morling
 * @author Chris Beckey &lt;cbeckey@paypal.com&gt;
 * @author Guillaume Smet
*/
public class BeanMetaDataManager {
    /**
     * The default initial capacity for this cache.
     */
    private static final int DEFAULT_INITIAL_CAPACITY = 16;

    /**
     * The default load factor for this cache.
     */
    private static final float DEFAULT_LOAD_FACTOR = 0.75f;

    /**
     * The default concurrency level for this cache.
     */
    private static final int DEFAULT_CONCURRENCY_LEVEL = 16;

    /**
     * Additional metadata providers used for meta data retrieval if
     * the XML and/or programmatic configuration is used.
     */
    @Immutable
    private final List<MetaDataProvider> metaDataProviders;

    /**
     * Helper for builtin constraints and their validator implementations
     */
    private final ConstraintHelper constraintHelper;

    /**
     * Used for resolving generic type information.
     */
    private final TypeResolutionHelper typeResolutionHelper;

    /**
     * The {@link ValueExtractor} manager.
     */
    private final ValueExtractorManager valueExtractorManager;

    private final ExecutableParameterNameProvider parameterNameProvider;

    /**
     * Used to cache the constraint meta data for validated entities
     */
    private final ConcurrentReferenceHashMap<Class<?>, BeanMetaData<?>> beanMetaDataCache;

    /**
     * Used for resolving type parameters. Thread-safe.
     */
    private final ExecutableHelper executableHelper;

    private final ValidationOrderGenerator validationOrderGenerator;

    /**
     * the three properties in this field affect the invocation of rules associated to section 4.5.5
     * of the specification.  By default they are all false, if true they allow
     * for relaxation of the Liskov Substitution Principal.
     */
    private final MethodValidationConfiguration methodValidationConfiguration;

    public BeanMetaDataManager(ConstraintHelper constraintHelper,
            ExecutableHelper executableHelper,
            TypeResolutionHelper typeResolutionHelper,
            ExecutableParameterNameProvider parameterNameProvider,
            ValueExtractorManager valueExtractorManager,
            ValidationOrderGenerator validationOrderGenerator,
            List<MetaDataProvider> optionalMetaDataProviders,
            MethodValidationConfiguration methodValidationConfiguration) {
        this.constraintHelper = constraintHelper;
        this.executableHelper = executableHelper;
        this.typeResolutionHelper = typeResolutionHelper;
        this.valueExtractorManager = valueExtractorManager;
        this.parameterNameProvider = parameterNameProvider;
        this.validationOrderGenerator = validationOrderGenerator;

        this.methodValidationConfiguration = methodValidationConfiguration;

        this.beanMetaDataCache = new ConcurrentReferenceHashMap<>(
                DEFAULT_INITIAL_CAPACITY,
                DEFAULT_LOAD_FACTOR,
                DEFAULT_CONCURRENCY_LEVEL,
                SOFT,
                SOFT,
                EnumSet.of( IDENTITY_COMPARISONS )
        );

        AnnotationProcessingOptions annotationProcessingOptions = getAnnotationProcessingOptionsFromNonDefaultProviders( optionalMetaDataProviders );
        AnnotationMetaDataProvider defaultProvider = new AnnotationMetaDataProvider(
                constraintHelper,
                typeResolutionHelper,
                valueExtractorManager,
                annotationProcessingOptions
        );
        List<MetaDataProvider> tmpMetaDataProviders = new ArrayList<>( optionalMetaDataProviders.size() + 1 );
        // We add the annotation based metadata provider at the first position so that the entire metadata model is assembled
        // first.
        // The other optional metadata providers will then contribute their additional metadata to the preexisting model.
        // This helps to mitigate issues like HV-1450.
        tmpMetaDataProviders.add( defaultProvider );
        tmpMetaDataProviders.addAll( optionalMetaDataProviders );

        this.metaDataProviders = CollectionHelper.toImmutableList( tmpMetaDataProviders );
    }

    @SuppressWarnings("unchecked")
    public <T> BeanMetaData<T> getBeanMetaData(Class<T> beanClass) {
        Contracts.assertNotNull( beanClass, MESSAGES.beanTypeCannotBeNull() );

        BeanMetaData<T> beanMetaData = (BeanMetaData<T>) beanMetaDataCache.computeIfAbsent( beanClass,
                bc -> createBeanMetaData( bc ) );

        return beanMetaData;
    }

    public void clear() {
        beanMetaDataCache.clear();
    }

    public int numberOfCachedBeanMetaDataInstances() {
        return beanMetaDataCache.size();
    }
}