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*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