MyBatis源码解析 - MyBatis初始化

我们主要从下面两个方面来分析MyBatis的初始化过程：
初始化做什么？
怎么初始化？

初始化做什么？

任何框架的初始化，无非是加载自己运行时所需要的配置信息，org.apache.ibatis.session下的Configuration类，即为Mybatis所有配置信息的容器类。
包括的配置信息有：

public class Configuration {

  protected Environment environment;

  protected boolean safeRowBoundsEnabled = false;
  protected boolean safeResultHandlerEnabled = true;
  protected boolean mapUnderscoreToCamelCase = false;
  protected boolean aggressiveLazyLoading = true;
  protected boolean multipleResultSetsEnabled = true;
  protected boolean useGeneratedKeys = false;
  protected boolean useColumnLabel = true;
  protected boolean cacheEnabled = true;
  protected boolean callSettersOnNulls = false;
  protected String logPrefix;
  protected Class <? extends Log> logImpl;
  protected LocalCacheScope localCacheScope = LocalCacheScope.SESSION;
  protected JdbcType jdbcTypeForNull = JdbcType.OTHER;
  protected Set<String> lazyLoadTriggerMethods = new HashSet<String>(Arrays.asList(new String[] { "equals", "clone", "hashCode", "toString" }));
  protected Integer defaultStatementTimeout;
  protected ExecutorType defaultExecutorType = ExecutorType.SIMPLE;
  protected AutoMappingBehavior autoMappingBehavior = AutoMappingBehavior.PARTIAL;

  protected Properties variables = new Properties();
  protected ObjectFactory objectFactory = new DefaultObjectFactory();
  protected ObjectWrapperFactory objectWrapperFactory = new DefaultObjectWrapperFactory();
  // 用于mapper接口类型注册
  protected MapperRegistry mapperRegistry = new MapperRegistry(this);

  protected boolean lazyLoadingEnabled = false;
  protected ProxyFactory proxyFactory;

  protected String databaseId;
  /**
   * Configuration factory class.
   * Used to create Configuration for loading deserialized unread properties.
   *
   * @see <a href='https://code.google.com/p/mybatis/issues/detail?id=300'>Issue 300</a> (google code)
   */
  protected Class<?> configurationFactory;

  protected final InterceptorChain interceptorChain = new InterceptorChain();
  protected final TypeHandlerRegistry typeHandlerRegistry = new TypeHandlerRegistry();
  protected final TypeAliasRegistry typeAliasRegistry = new TypeAliasRegistry();
  protected final LanguageDriverRegistry languageRegistry = new LanguageDriverRegistry();

  protected final Map<String, MappedStatement> mappedStatements = new StrictMap<MappedStatement>("Mapped Statements collection");
  protected final Map<String, Cache> caches = new StrictMap<Cache>("Caches collection");
  protected final Map<String, ResultMap> resultMaps = new StrictMap<ResultMap>("Result Maps collection");
  protected final Map<String, ParameterMap> parameterMaps = new StrictMap<ParameterMap>("Parameter Maps collection");
  protected final Map<String, KeyGenerator> keyGenerators = new StrictMap<KeyGenerator>("Key Generators collection");

  protected final Set<String> loadedResources = new HashSet<String>();
  protected final Map<String, XNode> sqlFragments = new StrictMap<XNode>("XML fragments parsed from previous mappers");

  protected final Collection<XMLStatementBuilder> incompleteStatements = new LinkedList<XMLStatementBuilder>();
  protected final Collection<CacheRefResolver> incompleteCacheRefs = new LinkedList<CacheRefResolver>();
  protected final Collection<ResultMapResolver> incompleteResultMaps = new LinkedList<ResultMapResolver>();
  protected final Collection<MethodResolver> incompleteMethods = new LinkedList<MethodResolver>();

  /*
   * A map holds cache-ref relationship. The key is the namespace that
   * references a cache bound to another namespace and the value is the
   * namespace which the actual cache is bound to.
   */
  protected final Map<String, String> cacheRefMap = new HashMap<String, String>();
  下面是一些get set方法 
  省略

MyBatis初始化的过程，就是创建 Configuration对象的过程。

怎么初始化？

MyBatis的初始化方式可以有两种方式：

• 基于XML配置文件：基于XML配置文件的方式是将MyBatis的所有配置信息放在XML文件中，MyBatis通过加载并XML配置文件，将配置文信息组装成内部的Configuration对象
• 基于Java API：这种方式不使用XML配置文件，需要MyBatis使用者在Java代码中，手动创建Configuration对象，然后将配置参数set 进入Configuration对象中

下面以基于XML配置文件进行初始化的方式来具体分析MyBatis是如何进行初始化的。
这里以MyBatis的独立配置文件为例，不是和Spring整合的配置文件。

InputStream inputStream = Resources.getResourceAsStream("mybatis-config.xml");
SqlSessionFactory sqlSessionFactory = new SqlSessionFactoryBuilder().build(inputStream);

除了可以使用InputStream读取文件流之外， 还可以使用Reader
下面的代码是Mybatis源码中关于SqlSession的测试例子

public class SqlSessionTest extends BaseDataTest {

  private static SqlSessionFactory sqlMapper;

  @BeforeClass
  public static void setup() throws Exception {
    createBlogDataSource();
    final String resource = "org/apache/ibatis/builder/MapperConfig.xml";
    final Reader reader = Resources.getResourceAsReader(resource);
    sqlMapper = new SqlSessionFactoryBuilder().build(reader);
  }
    ...
    ...
    ...
}

SqlSessionFactoryBuilder根据传入的数据流生成Configuration对象，然后根据Configuration对象创建默认的SqlSessionFactory实例。

public SqlSessionFactory build(InputStream inputStream, String environment, Properties properties) {
    try {
      XMLConfigBuilder parser = new XMLConfigBuilder(inputStream, environment, properties);
      // parser.parse() 返回Configuration
      // build(configuration) 返回DefaultSqlSessionFactory
      return build(parser.parse());
    } catch (Exception e) {
      throw ExceptionFactory.wrapException("Error building SqlSession.", e);
    } finally {
      ErrorContext.instance().reset();
      try {
        inputStream.close();
      } catch (IOException e) {
        // Intentionally ignore. Prefer previous error.
      }
    }
  }

public SqlSessionFactory build(Configuration config) {
    // 默认是DefaultSqlSessionFactory
    return new DefaultSqlSessionFactory(config);
  }

public Configuration parse() {
    // 为什么Each XMLConfigBuilder can only be used once？？？？
    if (parsed) {
      throw new BuilderException("Each XMLConfigBuilder can only be used once.");
    }
    parsed = true;
    //会从XPathParser中取出 <configuration>节点对应的XNode对象，然后解析它的子节点
    parseConfiguration(parser.evalNode("/configuration"));
    return configuration;
  }

XMLConfigBuilder parser = new XMLConfigBuilder(inputStream, environment, properties);
      // parser.parse() 返回Configuration
      // build(configuration) 返回DefaultSqlSessionFactory
      return build(parser.parse());

从以上代码也可知MyBatis默认初始化的SessionFactory是DefaultSqlSessionFactory。
（SessionFactory接口有两个实现类 ，一个是SqlSessionManager，一个是DefaultSqlSessionFactory。）

对于初始化工程中涉及到的几个类，我们再具体分析一下

SqlSessionFactoryBuilder ： SqlSessionFactory的构造器，用于创建SqlSessionFactory，采用了Builder设计模式
Configuration ：该对象是mybatis-config.xml文件中所有mybatis配置信息的容器
SqlSessionFactory：SqlSession工厂类，以工厂形式创建SqlSession对象，采用了Factory工厂设计模式
XmlConfigBuilder ：负责将mybatis-config.xml配置文件解析成Configuration对象，供SqlSessonFactoryBuilder使用，创建SqlSessionFactory

parse()方法是如何处理XML文件，生成Configuration对象的呢？

1、XMLConfigBuilder会将XML配置文件的信息转换为Document对象，而XML配置定义文件DTD转换成XMLMapperEntityResolver对象，然后将二者封装到XpathParser对象中，XpathParser的作用是提供根据Xpath表达式获取基本的DOM节点Node信息的操作。

2、之后XMLConfigBuilder调用parse()方法：会从XPathParser中取出 节点对应的Node对象，然后解析此Node节点的子Node：properties, settings, typeAliases,typeHandlers, objectFactory, objectWrapperFactory, plugins, environments,databaseIdProvider, mappers
Xml文件元素和Configuration属性映射表：

properties ：Properties variables。
settings：Integer defaultStatementTimeout、Integer defaultFetchSize、ExecutorType defaultExecutorType……
typeAliases：TypeAliasRegistry typeAliasRegistry。
typeHandlers：TypeHandlerRegistry typeHandlerRegistry。
environments：Environment environment。配置多个元素时，Mybatis只会读取默认的那一个
mappers：MapperRegistry mapperRegistry
等等 具体的可以看下面的parseConfiguration方法的代码，解析哪些内容

我们具体看下parse（）方法，看它如何根据XML文件来生成Configuration对象的

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"));
    } catch (Exception e) {
      throw new BuilderException("Error parsing SQL Mapper Configuration. Cause: " + e, e);
    }
  }

在对mappers元素进行解析的时候，用到的是mapperElement(root.evalNode(“mappers”));
关于对于mappers元素的解析后面的文章进行分享。

3、 然后将这些值解析出来设置到Configuration对象中。
4、返回Configuration对象

参考：https://blog.csdn.net/luanlouis/article/details/37744073