mybatis中用户只需要提供Mapper接口,具体产生对象的工作交由mybatis代为完成,用户可以获取到相应的代理对象
代理获取
public void run(){
//获取session
SqlSession session=sqlSessionFactory.openSession();
//获取StudentMapper代理对象
StudentMapper studentMapper = session.getMapper(StudentMapper.class);
Student student = studentMapper.selectByPrimaryKey(1);
}
进入session.getMapper()
public <T> T getMapper(Class<T> type) {
return this.configuration.getMapper(type, this);
}
继续进入函数,mapperRegister为configuration中一个重要组件,里面存放着Mapper接口的动态代理对象工厂,在创建MapperProxyFactory时使用了泛型,即为每个Mapper创建相应的代理工厂
public <T> T getMapper(Class<T> type, SqlSession sqlSession) {
return this.mapperRegistry.getMapper(type, sqlSession);
}
public <T> T getMapper(Class<T> type, SqlSession sqlSession) {
//获取代理对象工厂
MapperProxyFactory<T> mapperProxyFactory = (MapperProxyFactory)this.knownMappers.get(type);
//为 null时表示配置文件中没有配置该Mapper接口
if (mapperProxyFactory == null) {
throw new BindingException("Type " + type + " is not known to the MapperRegistry.");
} else {
try {
//由代理对象工厂创建实例对象
return mapperProxyFactory.newInstance(sqlSession);
} catch (Exception var5) {
throw new BindingException("Error getting mapper instance. Cause: " + var5, var5);
}
}
}
mapperProxyFactory.newInstance()
最终调用JDK自带的方法产生一个Mapper的代理对象
protected T newInstance(MapperProxy<T> mapperProxy) {
return Proxy.newProxyInstance(this.mapperInterface.getClassLoader(), new Class[]{this.mapperInterface}, mapperProxy);
}
public T newInstance(SqlSession sqlSession) {
MapperProxy<T> mapperProxy = new MapperProxy(sqlSession, this.mapperInterface, this.methodCache);
return this.newInstance(mapperProxy);
}
执行过程(以调用selectByPrimaryKey()为例)
首先进入代理类
public Object invoke(Object proxy, Method method, Object[] args) throws Throwable {
try {
//并非所有方法都调用其代理执行,如果当前方法为Object中的方法,则直接执行
if (Object.class.equals(method.getDeclaringClass())) {
return method.invoke(this, args);
//这里invokeDefaultMethod()内部通过反射创建了新的对象,并执行了其对应的方法,但是不太理解它的作用和使用场景
} else if (isDefaultMethod(method)) {
return invokeDefaultMethod(proxy, method, args);
}
} catch (Throwable t) {
throw ExceptionUtil.unwrapThrowable(t);
}
//获取MapperMethod, 如果缓存中有就直接获取,没有就先创建在放入缓存中
final MapperMethod mapperMethod = cachedMapperMethod(method);
//方法执行
return mapperMethod.execute(sqlSession, args);
}
MapperMethod包含两个属性,SQLCommand中包含了Mapper文件中定义的SQL语句信息,MethodSignature即方法签名,但其中也包含了方法的返回信息
public MapperMethod(Class<?> mapperInterface, Method method, Configuration config) {
this.command = new SqlCommand(config, mapperInterface, method);
this.method = new MethodSignature(config, mapperInterface, method);
}
方法执行,这里对应的即为SELECT语句
public Object execute(SqlSession sqlSession, Object[] args) {
Object result;
//根据不同的语句类型执行不同的方法
switch (command.getType()) {
case INSERT: {
Object param = method.convertArgsToSqlCommandParam(args);
result = rowCountResult(sqlSession.insert(command.getName(), param));
break;
}
case UPDATE: {
Object param = method.convertArgsToSqlCommandParam(args);
result = rowCountResult(sqlSession.update(command.getName(), param));
break;
}
case DELETE: {
Object param = method.convertArgsToSqlCommandParam(args);
result = rowCountResult(sqlSession.delete(command.getName(), param));
break;
}
case SELECT:
if (method.returnsVoid() && method.hasResultHandler()) {
executeWithResultHandler(sqlSession, args);
result = null;
} else if (method.returnsMany()) {
result = executeForMany(sqlSession, args);
} else if (method.returnsMap()) {
result = executeForMap(sqlSession, args);
} else if (method.returnsCursor()) {
result = executeForCursor(sqlSession, args);
} else {
//selectByPrimaryKey() 将执行至此
//首先将参数转换成对应的格式
Object param = method.convertArgsToSqlCommandParam(args);
//
result = sqlSession.selectOne(command.getName(), param);
if (method.returnsOptional() &&
(result == null || !method.getReturnType().equals(result.getClass()))) {
result = Optional.ofNullable(result);
}
}
break;
case FLUSH:
result = sqlSession.flushStatements();
break;
default:
throw new BindingException("Unknown execution method for: " + command.getName());
}
if (result == null && method.getReturnType().isPrimitive() && !method.returnsVoid()) {
throw new BindingException("Mapper method '" + command.getName()
+ " attempted to return null from a method with a primitive return type (" + method.getReturnType() + ").");
}
return result;
}
selectOne与executeForMany最终都会调用selectList()。executor.query()执行sql语句并返回结构,至此查询流程结束。
@Override
public <E> List<E> selectList(String statement, Object parameter, RowBounds rowBounds) {
try {
MappedStatement ms = configuration.getMappedStatement(statement);
//执行相应的查询操作
return executor.query(ms, wrapCollection(parameter), rowBounds, Executor.NO_RESULT_HANDLER);
} catch (Exception e) {
throw ExceptionFactory.wrapException("Error querying database. Cause: " + e, e);
} finally {
ErrorContext.instance().reset();
}
}
在executor.query()执行会前先执行配置的拦截器,例如我在demo中使用了一个分页插件(pagehelper),它就是在这个时间点起作用,将会重新设置quert()的参数值。
分页插件在配置文件中的配置如下
<plugins>
<plugin interceptor="com.github.pagehelper.PageInterceptor">
<property name="pageSizeZero" value="true"/>
</plugin>
</plugins>