Spring 源碼學習之巧用 內存緩存

在平常的開發中，咱們常常會用到相似的緩存。一般的想法是：

先查詢緩存，若是緩存中存在，返回緩存中的值，若是緩存不存在，調用相應的業務系統操做查詢（這裏業務操做查詢一般是須要費點時間的）並返回。如有數據，放入緩存中供下次使用。

這句話第一反應就是，若是緩存中沒有值，且業務查詢也沒有返回值。這樣，就須要設定一個特別的標識，避免這種沒值的屢次查詢。

sping中處處都是這樣的例子，拿 AbstractFallbackTransactionAttributeSource 參考，getTransactionAttribute就是很好的例子：

/*
 * Copyright 2002-2015 the original author or authors.
 *
 * Licensed under the Apache License, Version 2.0 (the "License");
 * you may not use this file except in compliance with the License.
 * You may obtain a copy of the License at
 *
 *      http://www.apache.org/licenses/LICENSE-2.0
 *
 * Unless required by applicable law or agreed to in writing, software
 * distributed under the License is distributed on an "AS IS" BASIS,
 * WITHOUT WARRANTIES OR CONDITIONS OF ANY KIND, either express or implied.
 * See the License for the specific language governing permissions and
 * limitations under the License.
 */

package org.springframework.transaction.interceptor;

import java.lang.reflect.Method;
import java.lang.reflect.Modifier;
import java.util.Map;
import java.util.concurrent.ConcurrentHashMap;

import org.apache.commons.logging.Log;
import org.apache.commons.logging.LogFactory;

import org.springframework.core.BridgeMethodResolver;
import org.springframework.util.ClassUtils;
import org.springframework.util.ObjectUtils;

/**
 * Abstract implementation of {@link TransactionAttributeSource} that caches
 * attributes for methods and implements a fallback policy: 1. specific target
 * method; 2. target class; 3. declaring method; 4. declaring class/interface.
 *
 * <p>Defaults to using the target class's transaction attribute if none is
 * associated with the target method. Any transaction attribute associated with
 * the target method completely overrides a class transaction attribute.
 * If none found on the target class, the interface that the invoked method
 * has been called through (in case of a JDK proxy) will be checked.
 *
 * <p>This implementation caches attributes by method after they are first used.
 * If it is ever desirable to allow dynamic changing of transaction attributes
 * (which is very unlikely), caching could be made configurable. Caching is
 * desirable because of the cost of evaluating rollback rules.
 *
 * @author Rod Johnson
 * @author Juergen Hoeller
 * @since 1.1
 */
public abstract class AbstractFallbackTransactionAttributeSource implements TransactionAttributeSource {

	/**
	 * Canonical value held in cache to indicate no transaction attribute was
	 * found for this method, and we don't need to look again.
	 */
	private final static TransactionAttribute NULL_TRANSACTION_ATTRIBUTE = new DefaultTransactionAttribute();


	/**
	 * Logger available to subclasses.
	 * <p>As this base class is not marked Serializable, the logger will be recreated
	 * after serialization - provided that the concrete subclass is Serializable.
	 */
	protected final Log logger = LogFactory.getLog(getClass());

	/**
	 * Cache of TransactionAttributes, keyed by DefaultCacheKey (Method + target Class).
	 * <p>As this base class is not marked Serializable, the cache will be recreated
	 * after serialization - provided that the concrete subclass is Serializable.
	 */
	final Map<Object, TransactionAttribute> attributeCache = new ConcurrentHashMap<Object, TransactionAttribute>(1024);


	/**
	 * Determine the transaction attribute for this method invocation.
	 * <p>Defaults to the class's transaction attribute if no method attribute is found.
	 * @param method the method for the current invocation (never {@code null})
	 * @param targetClass the target class for this invocation (may be {@code null})
	 * @return TransactionAttribute for this method, or {@code null} if the method
	 * is not transactional
	 */
	@Override
	public TransactionAttribute getTransactionAttribute(Method method, Class<?> targetClass) {
		// First, see if we have a cached value.
		Object cacheKey = getCacheKey(method, targetClass);
		Object cached = this.attributeCache.get(cacheKey);
		if (cached != null) {
			// Value will either be canonical value indicating there is no transaction attribute,
			// or an actual transaction attribute.
			if (cached == NULL_TRANSACTION_ATTRIBUTE) {
				return null;
			}
			else {
				return (TransactionAttribute) cached;
			}
		}
		else {
			// We need to work it out.
			TransactionAttribute txAtt = computeTransactionAttribute(method, targetClass);
			// Put it in the cache.
			if (txAtt == null) {
				this.attributeCache.put(cacheKey, NULL_TRANSACTION_ATTRIBUTE);
			}
			else {
				if (logger.isDebugEnabled()) {
					Class<?> classToLog = (targetClass != null ? targetClass : method.getDeclaringClass());
					logger.debug("Adding transactional method '" + classToLog.getSimpleName() + "." +
							method.getName() + "' with attribute: " + txAtt);
				}
				this.attributeCache.put(cacheKey, txAtt);
			}
			return txAtt;
		}
	}

	/**
	 * Determine a cache key for the given method and target class.
	 * <p>Must not produce same key for overloaded methods.
	 * Must produce same key for different instances of the same method.
	 * @param method the method (never {@code null})
	 * @param targetClass the target class (may be {@code null})
	 * @return the cache key (never {@code null})
	 */
	protected Object getCacheKey(Method method, Class<?> targetClass) {
		return new DefaultCacheKey(method, targetClass);
	}

	/**
	 * Same signature as {@link #getTransactionAttribute}, but doesn't cache the result.
	 * {@link #getTransactionAttribute} is effectively a caching decorator for this method.
	 * <p>As of 4.1.8, this method can be overridden.
	 * @since 4.1.8
	 * @see #getTransactionAttribute
	 */
	protected TransactionAttribute computeTransactionAttribute(Method method, Class<?> targetClass) {
		// Don't allow no-public methods as required.
		if (allowPublicMethodsOnly() && !Modifier.isPublic(method.getModifiers())) {
			return null;
		}

		// Ignore CGLIB subclasses - introspect the actual user class.
		Class<?> userClass = ClassUtils.getUserClass(targetClass);
		// The method may be on an interface, but we need attributes from the target class.
		// If the target class is null, the method will be unchanged.
		Method specificMethod = ClassUtils.getMostSpecificMethod(method, userClass);
		// If we are dealing with method with generic parameters, find the original method.
		specificMethod = BridgeMethodResolver.findBridgedMethod(specificMethod);

		// First try is the method in the target class.
		TransactionAttribute txAtt = findTransactionAttribute(specificMethod);
		if (txAtt != null) {
			return txAtt;
		}

		// Second try is the transaction attribute on the target class.
		txAtt = findTransactionAttribute(specificMethod.getDeclaringClass());
		if (txAtt != null) {
			return txAtt;
		}

		if (specificMethod != method) {
			// Fallback is to look at the original method.
			txAtt = findTransactionAttribute(method);
			if (txAtt != null) {
				return txAtt;
			}
			// Last fallback is the class of the original method.
			return findTransactionAttribute(method.getDeclaringClass());
		}
		return null;
	}


	/**
	 * Subclasses need to implement this to return the transaction attribute
	 * for the given method, if any.
	 * @param method the method to retrieve the attribute for
	 * @return all transaction attribute associated with this method
	 * (or {@code null} if none)
	 */
	protected abstract TransactionAttribute findTransactionAttribute(Method method);

	/**
	 * Subclasses need to implement this to return the transaction attribute
	 * for the given class, if any.
	 * @param clazz the class to retrieve the attribute for
	 * @return all transaction attribute associated with this class
	 * (or {@code null} if none)
	 */
	protected abstract TransactionAttribute findTransactionAttribute(Class<?> clazz);


	/**
	 * Should only public methods be allowed to have transactional semantics?
	 * <p>The default implementation returns {@code false}.
	 */
	protected boolean allowPublicMethodsOnly() {
		return false;
	}


	/**
	 * Default cache key for the TransactionAttribute cache.
	 */
	private static class DefaultCacheKey {

		private final Method method;

		private final Class<?> targetClass;

		public DefaultCacheKey(Method method, Class<?> targetClass) {
			this.method = method;
			this.targetClass = targetClass;
		}

		@Override
		public boolean equals(Object other) {
			if (this == other) {
				return true;
			}
			if (!(other instanceof DefaultCacheKey)) {
				return false;
			}
			DefaultCacheKey otherKey = (DefaultCacheKey) other;
			return (this.method.equals(otherKey.method) &&
					ObjectUtils.nullSafeEquals(this.targetClass, otherKey.targetClass));
		}

		@Override
		public int hashCode() {
			return this.method.hashCode() + (this.targetClass != null ? this.targetClass.hashCode() * 29 : 0);
		}
	}

}