带你了解Mybatis注解式工做原理
Mybatis编写sql有两种方式，即经过xml和注解，我我的比较喜欢xml配置，可是注解仍是要了解下的。而且Mybatis中xml优先于注解加载，也就是若是DAO接口中的方法有对应的xml配置，再加入注解会抛异常，若是两个都没配置，在调用DAO方法时再抛异常。java
源码分析
1、XML中sql转MappedStatement
Mybatis会把编写的sql语句信息封装成一个MappedStatement对象，加载xml中sql信息从XMLMapperBuilder#buildStatementFromContext()开始，一路调用到MapperBuilderAssistant#addMappedStatement完成添加，而加载注解最终也会经过MapperBuilderAssistant类。其中 this.configuration.addMappedStatement(statement);就是最终添加到配置类中的map集合中，先无论。
sql
2、注解转MappedStatement
那么开始加载注解从什么地方开始呢？
一样是从XMLMapperBuilder类中，在bindMapperForNamespace()方法下开始，完成XML加载以后被调用。
加载注解经过代码跟踪一直在MapperRegistry下的addMapper中。此时参数是DAO的class，首先判断是否是接口，以及是否被添加过了。其次，开始new 一个MapperAnnotationBuilder对象开始处理接口中方法上的注解。mybatis
public <T> void addMapper(Class<T> type) {
//是否是接口
if (type.isInterface()) {
//是否已被添加
if (this.hasMapper(type)) {
throw new BindingException("Type " + type + " is already known to the MapperRegistry.");
}
boolean loadCompleted = false;
try {
//添加
this.knownMappers.put(type, new MapperProxyFactory(type));
//开始注解解析
MapperAnnotationBuilder parser = new MapperAnnotationBuilder(this.config, type);
parser.parse();
loadCompleted = true;
} finally {
if (!loadCompleted) {
this.knownMappers.remove(type);
}
}
}
}
MapperAnnotationBuilder的parseStatement方法中处理每个DAO层方法上的注解，其中getSqlSourceFromAnnotations返回一个SqlSource对象,为空则方法上不存在注解信息，什么也不作，不为空最终生成各类须要的信息调用MapperBuilderAssistant下的addMappedStatement方法试图添加到Configuration中的mappedStatements集合中。app
void parseStatement(Method method) {
Class<?> parameterTypeClass = this.getParameterType(method);
LanguageDriver languageDriver = this.getLanguageDriver(method);
SqlSource sqlSource = this.getSqlSourceFromAnnotations(method, parameterTypeClass, languageDriver);
if (sqlSource != null) {
Options options = (Options)method.getAnnotation(Options.class);
String mappedStatementId = this.type.getName() + "." + method.getName();
Integer fetchSize = null;
Integer timeout = null;
StatementType statementType = StatementType.PREPARED;
ResultSetType resultSetType = ResultSetType.FORWARD_ONLY;
SqlCommandType sqlCommandType = this.getSqlCommandType(method);
boolean isSelect = sqlCommandType == SqlCommandType.SELECT;
boolean flushCache = !isSelect;
boolean useCache = isSelect;
String keyProperty = "id";
String keyColumn = null;
//-----此处省略不知道多少行代码
那为何要说试图添加呢？由于最终存放MappedStatement的是一个Map，Mybatis本身实现了个静态内部类StrictMap继承HashMap，在put中判断了有没有相同的key，若是相同则抛出异常，也就是间接让你xml和注解二选一。ide
3、sqlProviderAnnotation
Mybatis还有另外4个注解，也就是SelectProvider、InsertProvider、UpdateProvider、DeleteProvider，
从getSqlSourceFromAnnotations中能够看出，原来的Insert、Select、Update、Delete和SelectProvider、InsertProvider、UpdateProvider、DeleteProvider只能二选一。源码分析
private SqlSource getSqlSourceFromAnnotations(Method method, Class<?> parameterType, LanguageDriver languageDriver) {
try {
//获取方法上Insert、Select、Update、Delete中其中一个注解
Class<? extends Annotation> sqlAnnotationType = this.getSqlAnnotationType(method);
//获取SelectProvider、InsertProvider、UpdateProvider、DeleteProvider中其中一个注解
Class<? extends Annotation> sqlProviderAnnotationType = this.getSqlProviderAnnotationType(method);
Annotation sqlProviderAnnotation;
if (sqlAnnotationType != null) {
if (sqlProviderAnnotationType != null) {
//若是两种注解都存在，则抛出异常
throw new BindingException("You cannot supply both a static SQL and SqlProvider to method named " + method.getName());
} else {
sqlProviderAnnotation = method.getAnnotation(sqlAnnotationType);
//获取值，也就是sql语句
String[] strings = (String[])((String[])sqlProviderAnnotation.getClass().getMethod("value").invoke(sqlProviderAnnotation));
//生成SqlSource对象
return this.buildSqlSourceFromStrings(strings, parameterType, languageDriver);
}
} else if (sqlProviderAnnotationType != null) {
sqlProviderAnnotation = method.getAnnotation(sqlProviderAnnotationType);
//ProviderSqlSource实现了SqlSource
return new ProviderSqlSource(this.assistant.getConfiguration(), sqlProviderAnnotation, this.type, method);
} else {
return null;
}
} catch (Exception var8) {
throw new BuilderException("Could not find value method on SQL annotation. Cause: " + var8, var8);
}
}
4、SQL ProviderAnnotation的使用
public interface IUserDao {
@ResultMap({"BaseResultMap"})
@SelectProvider(type = SelectSql.class,method = "createSelectSql")
List<UserEntity> select();
class SelectSql{
public String createSelectSql(){
return new SQL(){{
SELECT("*");
FROM("tb_user");
}}.toString();
}
}
}
测试代码测试
public static void main( String[] args )
{
String resource = "mybatis-config.xml";
try {
InputStream inputStream = Resources.getResourceAsStream(resource);
SqlSessionFactory build = new SqlSessionFactoryBuilder().build(inputStream);
SqlSession sqlSession2 = build.openSession();
IUserDao mapper = sqlSession2.getMapper(IUserDao.class);
System.out.println(mapper.select());
} catch (IOException e) {
e.printStackTrace();
}
}
end....fetch