• 设计模式学习心得<享元模式 Flyweight>


    享元模式(Flyweight Pattern)主要用于减少创建对象的数量,以减少内存占用和提高性能。这种类型的设计模式属于结构型模式,它提供了减少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,如果未找到匹配的对象,则创建新对象。

    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,有可能会造成内存溢出,我们把其中共同的部分抽象出来,如果有相同的业务请求,直接返回在内存中已有的对象,避免重新创建。

    • 何时使用

    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可以外部化。
    4. 这些对象可以按照内蕴状态分为很多组,当把外蕴对象从对象中剔除出来时,每一组对象都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
      用唯一标识码判断,如果在内存中有,则返回这个唯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,如果有则返回,如果没有则创建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
    • 优点
      大大减少对象的创建,降低系统的内存,使效率提高。

    • 缺点
      提高了系统的复杂度,需要分离出外部状态和内部状态,而且外部状态具有固有化的性质,不应该随着内部状态的变化而变化,否则会造成系统的混乱。

    • 使用场景

    1. 系统有大量相似对象。
    2. 需要缓冲池的场景。
    • 注意事项
    1. 注意划分外部状态和内部状态,否则可能会引起线程安全问题。
    2. 这些类必须有一个工厂对象加以控制。

    实现


    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();
    	}
    }
    
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  • 原文地址:https://www.cnblogs.com/snifferhu/p/9201862.html
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