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源码分析Mybatis MappedStatement的创建流程

2019-05-23 21:30 591 查看
版权声明:本文为博主原创文章,未经博主允许不得转载。 https://blog.csdn.net/prestigeding/article/details/90488395

上文源码分析Mybatis MapperProxy创建流程重点阐述MapperProxy的创建流程,但并没有介绍*.Mapper.java(UserMapper.java)是如何与*Mapper.xml文件中的SQL语句是如何建立关联的。本文将重点接开这个谜团。

接下来重点从源码的角度分析Mybatis MappedStatement的创建流程。

源码分析Mybatis MappedStatement的创建流程

  • 2.2 XMLMapperBuilder
  • 1、上节回顾

    我们注意到这里有两三个与Mapper相关的配置:

    1. SqlSessionFactory#mapperLocations,指定xml文件的配置路径。
    2. SqlSessionFactory#configLocation,指定mybaits的配置文件,该配置文件也可以配置mapper.xml的配置路径信息。
    3. MapperScannerConfigurer,扫描Mapper的java类(DAO)。

    我们已经详细介绍了Mybatis Mapper对象的扫描与构建,那接下来我们将重点介绍MaperProxy与mapper.xml文件是如何建立关联关系的。

    根据上面的罗列以及上文的讲述,Mapper.xml与Mapper建立联系主要的入口有三:
    1)MapperScannerConfigurer扫描Bean流程中,在调用MapperReigistry#addMapper时如果Mapper对应的映射文件(Mapper.xml)未加载到内存,会触发加载。
    2)实例化SqlSessionFactory时,如果配置了mapperLocations。
    3)示例化SqlSessionFactory时,如果配置了configLocation。

    本节的行文思路:从SqlSessionFacotry的初始化开始讲起,因为mapperLocations、configLocation都是是SqlSessionFactory的属性。

    温馨提示:下面开始从源码的角度对其进行介绍,大家可以先跳到文末看看其调用序列图。

    2、SqlSessionFacotry

    if (xmlConfigBuilder != null) {  // XMLConfigBuilder   // @1
    try {
    xmlConfigBuilder.parse();
    
    if (logger.isDebugEnabled()) {
    logger.debug("Parsed configuration file: '" + this.configLocation + "'");
    }
    } catch (Exception ex) {
    throw new NestedIOException("Failed to parse config resource: " + this.configLocation, ex);
    } finally {
    ErrorContext.instance().reset();
    }
    }
    
    if (!isEmpty(this.mapperLocations)) {   // @2
    for (Resource mapperLocation : this.mapperLocations) {
    if (mapperLocation == null) {
    continue;
    }
    
    try {
    XMLMapperBuilder xmlMapperBuilder = new XMLMapperBuilder(mapperLocation.getInputStream(),
    configuration, mapperLocation.toString(), configuration.getSqlFragments());
    xmlMapperBuilder.parse();
    } catch (Exception e) {
    throw new NestedIOException("Failed to parse mapping resource: '" + mapperLocation + "'", e);
    } finally {
    ErrorContext.instance().reset();
    }
    
    if (logger.isDebugEnabled()) {
    logger.debug("Parsed mapper file: '" + mapperLocation + "'");
    }
    }
    } else {
    if (logger.isDebugEnabled()) {
    logger.debug("Property 'mapperLocations' was not specified or no matching resources found");
    }
    }

    上文有两个入口:
    代码@1:处理configLocation属性。
    代码@2:处理mapperLocations属性。

    我们先从XMLConfigBuilder#parse开始进行追踪。该方法主要是解析configLocation指定的配置路径,对其进行解析,具体调用parseConfiguration方法。

    2.1 XMLConfigBuilder

    我们直接查看其parseConfiguration方法。

    private void parseConfiguration(XNode root) {
    try {
    propertiesElement(root.evalNode("properties")); //issue #117 read properties first
    typeAliasesElement(root.evalNode("typeAliases"));
    pluginElement(root.evalNode("plugins"));
    objectFactoryElement(root.evalNode("objectFactory"));
    objectWrapperFactoryElement(root.evalNode("objectWrapperFactory"));
    settingsElement(root.evalNode("settings"));
    environmentsElement(root.evalNode("environments")); // read it after objectFactory and objectWrapperFactory issue #631
    databaseIdProviderElement(root.evalNode("databaseIdProvider"));
    typeHandlerElement(root.evalNode("typeHandlers"));
    mapperElement(root.evalNode("mappers"));   // @1
    } catch (Exception e) {
    throw new BuilderException("Error parsing SQL Mapper Configuration. Cause: " + e, e);
    }
    }

    重点关注mapperElement,从名称与参数即可以看出,该方法主要是处理中mappers的定义,即mapper sql语句的解析与处理。如果使用过Mapper的人应该不难知道,我们使用mapper节点,通过resource标签定义具体xml文件的位置。

    2.1.1XMLConfigBuilder#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");
    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());    // @1
    mapperParser.parse();
    } else if (resource == null && url != null && mapperClass == null) {
    ErrorContext.instance().resource(url);
    InputStream inputStream = Resources.getUrlAsStream(url);
    XMLMapperBuilder mapper
    8000
    Parser = 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.");
    }
    }
    }
    }
    }

    上面的代码比较简单,不难看出,解析出Mapper标签,解析出resource标签的属性,创建对应的文件流,通过构建XMLMapperBuilder来解析对应的mapper.xml文件。此时大家会惊讶的发现,在SqlSessionFacotry的初始化代码中,处理mapperLocations时就是通过构建XMLMapperBuilder来解析mapper文件,其实也不难理解,因为这是mybatis支持的两个地方可以使用mapper标签来定义mapper映射文件,具体解析代码当然是一样的逻辑。那我们解析来重点把目光投向XMLMapperBuilder。

    2.2 XMLMapperBuilder

    XMLMapperBuilder#parse
    public void parse() {
    if (!configuration.isResourceLoaded(resource)) {     // @1
    configurationElement(parser.evalNode("/mapper"));
    configuration.addLoadedResource(resource);
    bindMapperForNamespace();
    }
    
    parsePendingResultMaps();                                    // @2
    parsePendingChacheRefs();                                   // @3
    parsePendingStatements();                                     // @4
    }

    代码@1:如果该映射文件(*.Mapper.xml)文件未加载,则首先先加载,完成xml文件的解析,提取xml中与mybatis相关的数据,例如sql、resultMap等等。
    代码@2:处理mybatis xml中ResultMap。
    代码@3:处理mybatis缓存相关的配置。
    代码@4:处理mybatis statment相关配置,这里就是本篇关注的,Sql语句如何与Mapper进行关联的核心实现。

    接下来我们重点探讨parsePendingStatements()方法,解析statement(对应SQL语句)。

    2.2.1 XMLMapperBuilder#parsePendingStatements

    private void parsePendingStatements() {
    Collection<XMLStatementBuilder> incompleteStatements = configuration.getIncompleteStatements();
    synchronized (incompleteStatements) {
    Iterator<XMLStatementBuilder> iter = incompleteStatements.iterator();    // @1
    while (iter.hasNext()) {
    try {
    iter.next().parseStatementNode();   // @2
    iter.remove();
    } catch (IncompleteElementException e) {
    // Statement is still missing a resource...
    }
    }
    }
    }

    代码@1:遍历解析出来的所有SQL语句,用的是XMLStatementBuilder对象封装的,故接下来重点看一下代码@2,如果解析statmentNode。

    2.2.2 XMLStatementBuilder#parseStatementNode

    public void parseStatementNode() {
    String id = context.getStringAttribute("id");                                                                  // @1 start
    String databaseId = context.getStringAttribute("databaseId");
    
    if (!databaseIdMatchesCurrent(id, databaseId, this.requiredDatabaseId)) return;
    
    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);
    
    Class<?> resultTypeClass = resolveClass(resultType);
    String resultSetType = context.getStringAttribute("resultSetType");
    StatementType statementType = StatementType.valueOf(context.getStringAttribute("statementType", StatementType.PREPARED.toString()));
    ResultSetType resultSetTypeEnum = resolveResultSetType(resultSetType);
    
    String nodeName = context.getNode().getNodeName();
    SqlCommandType sqlCommandType = SqlCommandType.valueOf(nodeName.toUpperCase(Locale.ENGLISH));
    boolean isSelect = sqlCommandType == SqlCommandType.SELECT;
    boolean flushCache = context.getBooleanAttribute("flushCache", !isSelect);
    boolean useCache = context.getBooleanAttribute("useCache", isSelect);
    boolean resultOrdered = context.getBooleanAttribute("resultOrdered", false);
    
    // Include Fragments before parsing
    XMLIncludeTransformer includeParser = new XMLIncludeTransformer(configuration, builderAssistant);
    includeParser.applyIncludes(context.getNode());
    
    // Parse selectKey after includes and remove them.
    processSelectKeyNodes(id, parameterTypeClass, langDriver);             // @1 end
    
    // Parse the SQL (pre: <selectKey> and <include> were parsed and removed)
    SqlSource sqlSource = langDriver.createSqlSource(configuration, context, parameterTypeClass);                // @2
    String resultSets = context.getStringAttribute("resultSets");
    String keyProperty = context.getStringAttribute("keyProperty");
    String keyColumn = context.getStringAttribute("keyColumn");
    KeyGenerator keyGenerator;
    String keyStatementId = id + SelectKeyGenerator.SELECT_KEY_SUFFIX;
    keyStatementId = builderAssistant.applyCurrentNamespace(keyStatementId, true);
    if (configuration.hasKeyGenerator(keyStatementId)) {
    keyGenerator = configuration.getKeyGenerator(keyStatementId);
    } else {
    keyGenerator = context.getBooleanAttribute("useGeneratedKeys",
    configuration.isUseGeneratedKeys() && SqlCommandType.INSERT.equals(sqlCommandType))
    ? new Jdbc3KeyGenerator() : new NoKeyGenerator();
    }
    
    builderAssistant.addMappedStatement(id, sqlSource, statementType, sqlCommandType,                             // @3
    fetchSize, timeout, parameterMap, parameterTypeClass, resultMap, resultTypeClass,
    resultSetTypeEnum, flushCache, useCache, resultOrdered,
    keyGenerator, keyProperty, keyColumn, databaseId, langDriver, resultSets);
    }

    这个方法有点长,其关注点主要有3个:
    代码@1:构建基本属性,其实就是构建MappedStatement的属性,因为MappedStatement对象就是用来描述Mapper-SQL映射的对象。

    代码@2:根据xml配置的内容,解析出实际的SQL语句,使用SqlSource对象来表示。

    代码@3:使用MapperBuilderAssistant对象,根据准备好的属性,构建MappedStatement对象,最终将其存储在Configuration中。

    2.2.3 Configuration#addMappedStatement

    public void addMappedStatement(MappedStatement ms) {
    mappedStatements.put(ms.getId(), ms);
    }

    MappedStatement的id为:mapperInterface + methodName,例如com.demo.dao.UserMapper.findUser。

    即上述流程完成了xml的解析与初始化,对终极目标是创建MappedStatement对象,上一篇文章介绍了mapperInterface的初始化,最终会初始化为MapperProxy对象,那这两个对象如何关联起来呢?

    从下文可知,MapperProxy与MappedStatement是在调用具Mapper方法时,可以根据mapperInterface.getName + methodName构建出MappedStatement的id,然后就可以从Configuration的mappedStatements容器中根据id获取到对应的MappedStatement对象,这样就建立起联系了。

    其对应的代码:

    // MapperMethod 构造器
    public MapperMethod(Class<?> mapperInterface, Method method, Configuration config) {
    this.command = new SqlCommand(config, mapperInterface, method);
    this.method = new MethodSignature(config, method);
    }
    
    // SqlCommand 构造器
    public SqlCommand(Configuration configuration, Class<?> mapperInterface, Method method) throws BindingException {
    String statementName = mapperInterface.getName() + "." + method.getName();
    MappedStatement ms = null;
    if (configuration.hasStatement(statementName)) {
    ms = configuration.getMappedStatement(statementName);
    } else if (!mapperInterface.equals(method.getDeclaringClass().getName())) { // issue #35
    String parentStatementName = method.getDeclaringClass().getName() + "." + method.getName();
    if (configuration.
    8000
    hasStatement(parentStatementName)) {
    ms = configuration.getMappedStatement(parentStatementName);
    }
    }
    if (ms == null) {
    throw new BindingException("Invalid bound statement (not found): " + statementName);
    }
    name = ms.getId();
    type = ms.getSqlCommandType();
    if (type == SqlCommandType.UNKNOWN) {
    throw new BindingException("Unknown execution method for: " + name);
    }
    }

    怎么样,从上面的源码分析中,大家是否已经了解MapperProxy与Xml中的SQL语句是怎样建立的关系了吗?为了让大家更清晰的了解上述过程,现给出其调用时序图:

    源码分析Mybatis系列大纲:
    1、源码分析Mybatis MapperProxy创建流程 【已发布】
    2、 源码分析Mybatis MappedStatement的创建流程 【已发布】
    3、源码解析MyBatis Sharding-Jdbc SQL语句执行流程详解
    4、源码分析Mybatis插件(Plugin)机制
    5、源码分析Mybatis一二级缓存

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