mybatis源码解析6---MappedStatement解析

MappedStatement类位于mybatis包的org.apache.ibatis.mapping目录下，是一个final类型也就是说实例化之后就不允许改变

MappedStatement对象对应Mapper.xml配置文件中的一个select/update/insert/delete节点，描述的就是一条SQL语句，属性如下：

 1   private String resource;//mapper配置文件名，如：UserMapper.xml
  private Configuration configuration;//全局配置
  private String id;//节点的id属性加命名空间,如：com.lucky.mybatis.dao.UserMapper.selectByExample
  private Integer fetchSize;
  private Integer timeout;//超时时间
  private StatementType statementType;//操作SQL的对象的类型
  private ResultSetType resultSetType;//结果类型
  private SqlSource sqlSource;//sql语句
  private Cache cache;//缓存
  private ParameterMap parameterMap;
  private List<ResultMap> resultMaps;
  private boolean flushCacheRequired;
  private boolean useCache;//是否使用缓存，默认为true
  private boolean resultOrdered;//结果是否排序
  private SqlCommandType sqlCommandType;//sql语句的类型，如select、update、delete、insert
  private KeyGenerator keyGenerator;
  private String[] keyProperties;
  private String[] keyColumns;
  private boolean hasNestedResultMaps;
  private String databaseId;//数据库ID
  private Log statementLog;
  private LanguageDriver lang;
  private String[] resultSets;

其中StatementType指操作SQL对象的类型，是个枚举类型，值分别为：

STATEMENT（直接操作SQL，不进行预编译），

PREPARED（预处理参数，进行预编译，获取数据），

CALLABLE（执行存储过程）

ResultSetType指返回结果集的类型，也是个枚举类型，值分别为：

FORWARD_ONLY：结果集的游标只能向下滚动

SCROLL_INSENSITIVE：结果集的游标可以上下移动，当数据库变化时当前结果集不变

SCROLL_SENSITIVE：结果集客自由滚动，数据库变化时当前结果集同步改变

言归正传，现在我们知道一个MappedStatement对象对应一个mapper.xml中的一个SQL节点，而Mapper.xml文件是初始化Configuration对象的时候进行解析加载的，则说明MappedStatement对象就是在初始化Configuration对象的时候创建的，并且是final类型不可更改。

之前我们知道Configuration对象的初始化过程，是通过XMLConfigBuilder类的parse方法进行初始化的，现在来看下是如何初始化MappedStatement对象的，Configuration对象初始化代码如下：

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

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

parseConfiguration方法是根据XNode对Configuration对象进行属性赋值，mapperElement方法即解析<mappers>标签中的内容，mapperElement方法源码如下：

private void mapperElement(XNode parent) throws Exception {
    //parent是Configuration配置文件中的<mappers>标签
    if (parent != null) {
       //遍历<mappers>标签下的所有子标签
      for (XNode child : parent.getChildren()) {
        if ("package".equals(child.getName())) {
          //加载package包下的所有mapper
          String mapperPackage = child.getStringAttribute("name");
          configuration.addMappers(mapperPackage);//加载packege包下的所有mapper
        } else {
          //按resource或url或class加载单个mapper
          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());
            mapperParser.parse();//解析xml文件流
          } 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();//解析xml文件流
          } else if (resource == null && url == null && mapperClass != null) {
            Class<?> mapperInterface = Resources.classForName(mapperClass);
            configuration.addMapper(mapperInterface);//加载指定接口的mapper
          } else {
            throw new BuilderException("A mapper element may only specify a url, resource or class, but not more than one.");
          }
        }
      }
    }
  }

可以看出共有三种方法可以加载mapper，一个是批量加载指定package下所有mapper，一个是根据mapper接口路径加载指定mapper，还有一种是解析mapper.xml文件流进行加载，接下来挨个来看下；

先来看个最简单的，根据指定接口加载mapper，也就是configuration.addMapper(mapperInterface)方法，源码如下：

Configuration的addMapper方法

public <T> void addMapper(Class<T> type) {
    mapperRegistry.addMapper(type);
  }

调用了mapperRegistry的addMapper方法。mapperRegistry是Configuration类的一个属性

protected final MapperRegistry mapperRegistry = new MapperRegistry(this);

根据MapperRegistry的名字可以理解为此类的作用是mapper的注册中心，用于注册mapper，MapperRegistry源码如下：

package org.apache.ibatis.binding;

import org.apache.ibatis.builder.annotation.MapperAnnotationBuilder;
import org.apache.ibatis.io.ResolverUtil;
import org.apache.ibatis.session.Configuration;
import org.apache.ibatis.session.SqlSession;

import java.util.Collection;
import java.util.Collections;
import java.util.HashMap;
import java.util.Map;
import java.util.Set;

public class MapperRegistry {

  private final Configuration config;//全局配置
  private final Map<Class<?>, MapperProxyFactory<?>> knownMappers = new HashMap<Class<?>, MapperProxyFactory<?>>();//已注册的mapper集合

  public MapperRegistry(Configuration config) {
    this.config = config;
  }

  @SuppressWarnings("unchecked")
  public <T> T getMapper(Class<T> type, SqlSession sqlSession) {
    final MapperProxyFactory<T> mapperProxyFactory = (MapperProxyFactory<T>) knownMappers.get(type);
    if (mapperProxyFactory == null) {
      throw new BindingException("Type " + type + " is not known to the MapperRegistry.");
    }
    try {
      return mapperProxyFactory.newInstance(sqlSession);
    } catch (Exception e) {
      throw new BindingException("Error getting mapper instance. Cause: " + e, e);
    }
  }
  //判断指定mapper是否已经存在
  public <T> boolean hasMapper(Class<T> type) {
    return knownMappers.containsKey(type);
  }

  //新增一个mapper
  public <T> void addMapper(Class<T> type) {
    if (type.isInterface()) {
      if (hasMapper(type)) {
        throw new BindingException("Type " + type + " is already known to the MapperRegistry.");
      }
      boolean loadCompleted = false;
      try {
        knownMappers.put(type, new MapperProxyFactory<T>(type));
        // It's important that the type is added before the parser is run
        // otherwise the binding may automatically be attempted by the
        // mapper parser. If the type is already known, it won't try.
        MapperAnnotationBuilder parser = new MapperAnnotationBuilder(config, type);
        parser.parse();
        loadCompleted = true;
      } finally {
        if (!loadCompleted) {
          knownMappers.remove(type);
        }
      }
    }
  }

  //获取所有mapper集合
  public Collection<Class<?>> getMappers() {
    return Collections.unmodifiableCollection(knownMappers.keySet());
  }

  
  //根据package名称加载包下所有的mapper
  public void addMappers(String packageName, Class<?> superType) {
    ResolverUtil<Class<?>> resolverUtil = new ResolverUtil<Class<?>>();
    resolverUtil.find(new ResolverUtil.IsA(superType), packageName);
    Set<Class<? extends Class<?>>> mapperSet = resolverUtil.getClasses();
    for (Class<?> mapperClass : mapperSet) {
      addMapper(mapperClass);
    }
  }

  //根据package批量加载mapper
  public void addMappers(String packageName) {
    addMappers(packageName, Object.class);
  }
  
}
  

从源码可看出MapperRegistry就是Mapper的注册中心，有两个属性一个是全局配置Configuration还有一个是已经加载过的mapper集合 knownMappers

新增一个mapper的方法就是addMapper，就是向knownsMappers集合中put一条新的mapper记录，key就是mapper的类名全称，value是这个mapper的代理工厂；

分析到这里，发现Configuration对象初始化的时候会解析所有的xml文件中配置的所有mapper接口，并添加到Configuration的mapper集合knowMappers中，但是貌似还没有MappedStatement的影子，也没有看到哪里解析了mapper.xml配置。

不用急，上面源码的第52行就是了，52到54行的意思目前还没有看源码，但是先猜测下：52行是通过Configuration对象和mapper类来构造一个MapperAnnotationBuilder对象,通过字面意思是Mapper的构建类，而第53行的parse()，应该就是解析mapper.xml文件的，第54行标记加载完成，

只有当mapper接口和mapper.xml匹配成功才能叫做是加载成功，所以下一章篇就再来看看MappedStatement是如何创建的。

总结：MappedStatement类就是对应的Mapper.xml中的一个sql语句