Mybatis源码剖析：二级缓存源码剖析

Posted 2021-06-06 丿涛哥哥

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Mybatis源码剖析：二级缓存源码剖析

二级缓存构建在一级缓存之上，在收到查询请求时，MyBatis 首先会查询二级缓存，若二级缓存未命中，再去查询一级缓存，一级缓存没有，再查询数据库。

二级缓存------》一级缓存------》数据库

与一级缓存不同，二级缓存和具体的命名空间绑定，一个Mapper中有一个Cache，相同Mapper中的 MappedStatement共用一个Cache，一级缓存则是和 SqlSession 绑定。

1、启用二级缓存

分为三步：

1、开启全局二级缓存配置：

<settings>
	<setting name="cacheEnabled" value="true"/>
</settings>

2、在需要使用二级缓存的Mapper配置文件中配置标签

<cache></cache>

3、在具体CURD标签上配置 useCache=true

<select id="findById" resultType="com.tao.pojo.User" useCache="true">
	select * from user where id = #{id}
</select>

2、标签 < cache/> 的解析

根据之前的mybatis源码剖析，xml的解析工作主要交给XMLConfigBuilder.parse()方法来实现

// XMLConfigBuilder.parse()
public Configuration parse() {
	if (parsed) {
		throw new BuilderException("Each XMLConfigBuilder can only be used once.");
	}
	parsed = true;
	parseConfiguration(parser.evalNode("/configuration"));// 在这里
	return configuration;
}

// parseConfiguration()
// 既然是在xml中添加的，那么我们就直接看关于mappers标签的解析
private void parseConfiguration(XNode root) {
	try {
		Properties settings = settingsAsPropertiess(root.evalNode("settings"));
		propertiesElement(root.evalNode("properties"));
		loadCustomVfs(settings);
		typeAliasesElement(root.evalNode("typeAliases"));
		pluginElement(root.evalNode("plugins"));
		objectFactoryElement(root.evalNode("objectFactory"));
		objectWrapperFactoryElement(root.evalNode("objectWrapperFactory"));
		reflectionFactoryElement(root.evalNode("reflectionFactory"));
		settingsElement(settings);
		// read it after objectFactory and objectWrapperFactory issue #631
		environmentsElement(root.evalNode("environments"));
		databaseIdProviderElement(root.evalNode("databaseIdProvider"));
		typeHandlerElement(root.evalNode("typeHandlers"));
		// 就是这里
		mapperElement(root.evalNode("mappers"));
	} catch (Exception e) {
		throw new BuilderException("Error parsing SQL Mapper Configuration.Cause: " + e, e);
	}
}

                           
// mapperElement()
private void mapperElement(XNode parent) throws Exception {
	if (parent != null) {
		for (XNode child : parent.getChildren()) {
			if ("package".equals(child.getName())) {
				String mapperPackage = child.getStringAttribute("name");
				configuration.addMappers(mapperPackage);
			} else {
				String resource = child.getStringAttribute("resource");
				String url = child.getStringAttribute("url");
				String mapperClass = child.getStringAttribute("class");
				// 按照我们本例的配置，则直接走该if判断
				if (resource != null && url == null && mapperClass == null) {
					ErrorContext.instance().resource(resource);
					InputStream inputStream = Resources.getResourceAsStream(resource);
					XMLMapperBuilder mapperParser = new XMLMapperBuilder(inputStream, configuration, resource, configuration.getSqlFragments());
					// 生成XMLMapperBuilder，并执行其parse方法
					mapperParser.parse();
				} else if (resource == null && url != null && mapperClass == null) {
					ErrorContext.instance().resource(url);
					InputStream inputStream = Resources.getUrlAsStream(url);
					XMLMapperBuilder mapperParser = new XMLMapperBuilder(inputStream, configuration, url, configuration.getSqlFragments());
					mapperParser.parse();
				} else if (resource == null && url == null && mapperClass != null) {
					Class<?> mapperInterface = Resources.classForName(mapperClass);
					configuration.addMapper(mapperInterface);
				} else {
					throw new BuilderException("A mapper element may only specify a url, resource or class, but not more than one.");
				}
			}
		}
	}
}

我们来看看解析Mapper.xml

// XMLMapperBuilder.parse()
public void parse() {
	if (!configuration.isResourceLoaded(resource)) {
		// 解析mapper属性
		configurationElement(parser.evalNode("/mapper"));
		configuration.addLoadedResource(resource);
		bindMapperForNamespace();
	}
	parsePendingResultMaps();
	parsePendingChacheRefs();
	parsePendingStatements();
}

// configurationElement()
private void configurationElement(XNode context) {
	try {
		String namespace = context.getStringAttribute("namespace");
		if (namespace == null || namespace.equals("")) {
			throw new BuilderException("Mapper's namespace cannot be empty");
		}
		builderAssistant.setCurrentNamespace(namespace);
		cacheRefElement(context.evalNode("cache-ref"));
		// 最终在这里看到了关于cache属性的处理
		cacheElement(context.evalNode("cache"));
		parameterMapElement(context.evalNodes("/mapper/parameterMap"));
		resultMapElements(context.evalNodes("/mapper/resultMap"));
		sqlElement(context.evalNodes("/mapper/sql"));
		// 这里会将生成的Cache包装到对应的MappedStatement
		buildStatementFromContext(context.evalNodes("select|insert|update|delete"));
	} catch (Exception e) {
		throw new BuilderException("Error parsing Mapper XML. Cause: " + e, e);
	}
}

// cacheElement()
private void cacheElement(XNode context) throws Exception {
	if (context != null) {
		//解析<cache/>标签的type属性，这里我们可以自定义cache的实现类，比如redisCache，如果没有自定义，这里使用和一级缓存相同的PERPETUAL
		String type = context.getStringAttribute("type", "PERPETUAL");
		Class<? extends Cache> typeClass = typeAliasRegistry.resolveAlias(type);
		String eviction = context.getStringAttribute("eviction", "LRU");
		Class<? extends Cache> evictionClass = typeAliasRegistry.resolveAlias(eviction);
		Long flushInterval = context.getLongAttribute("flushInterval");
		Integer size = context.getIntAttribute("size");
		boolean readWrite = !context.getBooleanAttribute("readOnly", false);
		boolean blocking = context.getBooleanAttribute("blocking", false);
		Properties props = context.getChildrenAsProperties();
		// 构建Cache对象
		builderAssistant.useNewCache(typeClass, evictionClass, flushInterval, size, readWrite, blocking, props);
	}
}

先来看看是如何构建Cache对象的

MapperBuilderAssistant.useNewCache()

public Cache useNewCache(Class<? extends Cache> typeClass, 
                         Class<? extends Cache> evictionClass,
						Long flushInterval,
						Integer size,
						boolean readWrite,
						boolean blocking,
						Properties props) {
	// 1.生成Cache对象
	Cache cache = new CacheBuilder(currentNamespace)
		//这里如果我们定义了<cache/>中的type，就使用自定义的Cache,否则使用和一级缓存相同的PerpetualCache
		.implementation(valueOrDefault(typeClass, PerpetualCache.class))
		.addDecorator(valueOrDefault(evictionClass, LruCache.class))
		.clearInterval(flushInterval)
		.size(size)
		.readWrite(readWrite)
		.blocking(blocking)
		.properties(props)
		.build();
	// 2.添加到Configuration中
	configuration.addCache(cache);
	// 3.并将cache赋值给MapperBuilderAssistant.currentCache
	currentCache = cache;
	return cache;
}

我们看到一个Mapper.xml只会解析一次标签，也就是只创建一次Cache对象，放进configuration中，并将cache赋值给MapperBuilderAssistant.currentCache

buildStatementFromContext(context.evalNodes(“select|insert|update|delete”));将Cache包装到MappedStatement

// buildStatementFromContext()
private void buildStatementFromContext(List<XNode> list) {
	if (configuration.getDatabaseId() != null) {
		buildStatementFromContext(list, configuration.getDatabaseId());
	}
	buildStatementFromContext(list, null);
}

//buildStatementFromContext()
private void buildStatementFromContext(List<XNode> list, String requiredDatabaseId) {
	for (XNode context : list) {
		final XMLStatementBuilder statementParser = new XMLStatementBuilder(configuration, builderAssistant, context, requiredDatabaseId);
		try {
			// 每一条执行语句转换成一个MappedStatement
			statementParser.parseStatementNode();
		} catch (IncompleteElementException e) {
			configuration.addIncompleteStatement(statementParser);
		}
	}
}

// XMLStatementBuilder.parseStatementNode();
public void parseStatementNode() {
	String id = context.getStringAttribute("id");
	String databaseId = context.getStringAttribute("databaseId");
	...
	Integer fetchSize = context.getIntAttribute("fetchSize");
	Integer timeout = context.getIntAttribute("timeout");
	String parameterMap = context.getStringAttribute("parameterMap");
	String parameterType = context.getStringAttribute("parameterType");
	Class<?> parameterTypeClass = resolveClass(parameterType);
	String resultMap = context.getStringAttribute("resultMap");
	String resultType = context.getStringAttribute("resultType");
	String lang = context.getStringAttribute("lang");
	LanguageDriver langDriver = getLanguageDriver(lang);
	...
	// 创建MappedStatement对象
	builderAssistant.addMappedStatement(id, sqlSource, statementType, sqlCommandType, fetchSize, timeout, parameterMap,
					parameterTypeClass, resultMap, resultTypeClass,
					resultSetTypeEnum, flushCache, useCache,
					resultOrdered,
					keyGenerator, keyProperty, keyColumn,
					databaseId, langDriver, resultSets);
}

// builderAssistant.addMappedStatement()
public MappedStatement addMappedStatement(String id, ...) {
	if (unresolvedCacheRef) {
		throw new IncompleteElementException("Cache-ref not yet resolved");
	}
	id = applyCurrentNamespace(id, false);
	boolean isSelect = sqlCommandType == SqlCommandType.SELECT;
	//创建MappedStatement对象
	MappedStatement.Builder statementBuilder = new MappedStatement.Builder(configuration, id, sqlSource, sqlCommandType)
		...
		.flushCacheRequired(valueOrDefault(flushCache, !isSelect))
		.useCache(valueOrDefault(useCache, isSelect))
		.cache(currentCache);// 在这里将之前生成的Cache封装到MappedStatement
	ParameterMap statementParameterMap = getStatementParameterMap(parameterMap,
	parameterType, id);
	if (statementParameterMap != null) {
		statementBuilder.parameterMap(statementParameterMap);
	}
	MappedStatement statement = statementBuilder.build();
	configuration.addMappedStatement(statement);
	return statement;
}

我们看到将Mapper中创建的Cache对象，加入到了每个MappedStatement对象中。

有关于标签的解析就到这了。

3、查询源码分析

CachingExecutor

// CachingExecutor
public <E> List<E> query(MappedStatement ms, Object parameterObject, RowBounds rowBounds, ResultHandler resultHandler) throws SQLException {
	BoundSql boundSql = ms.getBoundSql(parameterObject);
	// 创建 CacheKey
	CacheKey key = createCacheKey(ms, parameterObject, rowBounds, boundSql);
	return query(ms, parameterObject, rowBounds, resultHandler, key, boundSql);
}

public <E> List<E> query(MappedStatement ms, Object parameterObject, RowBounds rowBounds, ResultHandler resultHandler, CacheKey key, BoundSql boundSql) throws SQLException {
	// 从 MappedStatement 中获取 Cache，注意这里的 Cache 是从MappedStatement中获取的
	// 也就是我们上面解析Mapper中<cache/>标签中创建的，它保存在Configration中
	// 我们在上面解析blog.xml时分析过每一个MappedStatement都有一个Cache对象，就是这里
	Cache cache = ms.getCache();
	// 如果配置文件中没有配置 <cache>，则 cache 为空
	if (cache != null) {
		//如果需要刷新缓存的话就刷新：flushCache="true"
		flushCacheIfRequired(ms);
		if (ms.isUseCache() && resultHandler == null) {
			ensureNoOutParams(ms, boundSql);
			// 访问二级缓存
			List<E> list = (List<E>) tcm.getObject(cache, key);
			// 缓存未命中
			if (list == null) {
				// 如果没有值，则执行查询，这个查询实际也是先走一级缓存查询，一级缓存也没有的话，则进行DB查询
				list = delegate.<E>query(ms, parameterObject, rowBounds, resultHandler, key, boundSql);
				// 缓存查询结果
				tcm.putObject(cache, key, list);
			}
			return list;
		}
	}
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Mybatis源码剖析：二级缓存源码剖析

Mybatis源码剖析：二级缓存源码剖析

1、 启用二级缓存

2、 标签 < cache/> 的解析

3、查询源码分析

1、启用二级缓存

2、标签 < cache/> 的解析