MyBatis底层实现（一）接口和XML的映射
我们学习Mybatis时学过三种方式来实现MyBatis操作数据库
一、配置mapper.xml文件然后使用专有的API
@org.junit.Test
public void testFindall() throws IOException {
//1.获取sqlSessionFactory对象
SqlSessionFactory sqlSessionFactory=getSqlSessionFactory();
//2.获取sqlSession对象
SqlSession openSession=sqlSessionFactory.openSession();
Map map=new HashMap<>();
map.put("username", "三");
try {
List list=openSession.selectList("com.mybatis.dao.UserMapper.selectAll",map);
for(User user:list) {
System.out.println(user.getUsername());
}
}finally {
openSession.close();
}
} 然后你会想为什么会这么写 底层是如何实现的
public List selectList(String statement) {
return this.selectList(statement, null);
}
@Override
public List selectList(String statement, Object parameter) {
return this.selectList(statement, parameter, RowBounds.DEFAULT);
}
@Override
public List selectList(String statement, Object parameter, RowBounds rowBounds) {
try {
MappedStatement ms = configuration.getMappedStatement(statement);//通过statement在Configuration中查找到对应的MapperStatement
return executor.query(ms, wrapCollection(parameter), rowBounds, Executor.NO_RESULT_HANDLER);//将任务委托给Execute执行器
} catch (Exception e) {
throw ExceptionFactory.wrapException("Error querying database. Cause: " + e, e);
} finally {
ErrorContext.instance().reset();
}
} 加载到内存中会生成mappedStatem对象 key为mapper中的id value为生成的mappedStatem
public List query(MappedStatement ms, Object parameter, RowBounds rowBounds, ResultHandler resultHandler) throws SQLException {
BoundSql boundSql = ms.getBoundSql(parameter);
CacheKey key = createCacheKey(ms, parameter, rowBounds, boundSql);
return query(ms, parameter, rowBounds, resultHandler, key, boundSql);
}
@SuppressWarnings("unchecked")
@Override
public List query(MappedStatement ms, Object parameter, RowBounds rowBounds, ResultHandler resultHandler, CacheKey key, BoundSql boundSql) throws SQLException {
ErrorContext.instance().resource(ms.getResource()).activity("executing a query").object(ms.getId());
if (closed) {
throw new ExecutorException("Executor was closed.");
}
if (queryStack == 0 && ms.isFlushCacheRequired()) {
clearLocalCache();
}
List list;
try {
queryStack++;
list = resultHandler == null ? (List) localCache.getObject(key) : null;
if (list != null) {
handleLocallyCachedOutputParameters(ms, key, parameter, boundSql);
} else {
list = queryFromDatabase(ms, parameter, rowBounds, resultHandler, key, boundSql);
}
} finally {
queryStack--;
}
if (queryStack == 0) {
for (DeferredLoad deferredLoad : deferredLoads) {
deferredLoad.load();
}
// issue #601
deferredLoads.clear();
if (configuration.getLocalCacheScope() == LocalCacheScope.STATEMENT) {
// issue #482
clearLocalCache();
}
}
return list;
} parameter参数的处理
public void setParameters(PreparedStatement ps) {
ErrorContext.instance().activity("setting parameters").object(mappedStatement.getParameterMap().getId());
List parameterMappings = boundSql.getParameterMappings();
//判断参数
if (parameterMappings != null) {
for (int i = 0; i < parameterMappings.size(); i++) {
ParameterMapping parameterMapping = parameterMappings.get(i);
if (parameterMapping.getMode() != ParameterMode.OUT) {
Object value;
String propertyName = parameterMapping.getProperty();
if (boundSql.hasAdditionalParameter(propertyName)) { // issue #448 ask first for additional params
value = boundSql.getAdditionalParameter(propertyName);
} else if (parameterObject == null) {
value = null;
} else if (typeHandlerRegistry.hasTypeHandler(parameterObject.getClass())) {
value = parameterObject;
} else {
MetaObject metaObject = configuration.newMetaObject(parameterObject);
value = metaObject.getValue(propertyName);
}
TypeHandler typeHandler = parameterMapping.getTypeHandler();
JdbcType jdbcType = parameterMapping.getJdbcType();
if (value == null && jdbcType == null) {
jdbcType = configuration.getJdbcTypeForNull();
}
try {
typeHandler.setParameter(ps, i + 1, value, jdbcType);
} catch (TypeException e) {
throw new TypeException("Could not set parameters for mapping: " + parameterMapping + ". Cause: " + e, e);
} catch (SQLException e) {
throw new TypeException("Could not set parameters for mapping: " + parameterMapping + ". Cause: " + e, e);
}
}
}
}
} 二、接口与xml的映射
1）注册代理接口，创建mapper代理工厂
public void addMapper(Class type) {
if (type.isInterface()) {
if (hasMapper(type)) {
throw new BindingException("Type " + type + " is already known to the MapperRegistry.");
}
boolean loadCompleted = false;
try {
/放到map中, value为创建代理的工厂
knownMappers.put(type, new MapperProxyFactory(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.
//解析接口里面的注解
MapperAnnotationBuilder parser = new MapperAnnotationBuilder(config, type);
parser.parse();
loadCompleted = true;
} finally {
if (!loadCompleted) {
knownMappers.remove(type);
}
}
}
} 2）创建接口动态类
public T getMapper(Class type, SqlSession sqlSession) {
//取出mapperProxyFactory
final MapperProxyFactory mapperProxyFactory = (MapperProxyFactory) knownMappers.get(type);
if (mapperProxyFactory == null) {
throw new BindingException("Type " + type + " is not known to the MapperRegistry.");
}
try {
//创建代理
return mapperProxyFactory.newInstance(sqlSession);
} catch (Exception e) {
throw new BindingException("Error getting mapper instance. Cause: " + e, e);
} 3）pper namespace="....">的namespace属性值，判断有没有这样一个接口的全路径与namespace属性值完全相同，如果有，就生成这个接口的动态代理类。
public void parse() {
if (!configuration.isResourceLoaded(resource)) {
//解析映射文件的根节点mapper元素
configurationElement(parser.evalNode("/mapper"));
configuration.addLoadedResource(resource);
//这个方法内部会根据namespace属性值，生成动态代理类
bindMapperForNamespace();。
}
parsePendingResultMaps();
parsePendingChacheRefs();
parsePendingStatements();4）调用代理方法 进入到 invoke
public Object invoke(Object proxy, Method method, Object[] args) throws Throwable {
try {
if (Object.class.equals(method.getDeclaringClass())) {
return method.invoke(this, args);
} else if (isDefaultMethod(method)) {
return invokeDefaultMethod(proxy, method, args);
}
} catch (Throwable t) {
throw ExceptionUtil.unwrapThrowable(t);
}
final MapperMethod mapperMethod = cachedMapperMethod(method);
//执行相应sql语句
return mapperMethod.execute(sqlSession, args);
}5）最终的拦截代码位于MapperMethod类的execute方法中，判断是使用insert|delete|update|select 还有selectOne selectList selectMap
public class MapperMethod {
private final SqlCommand command;
private final MethodSignature method;
public MapperMethod(Class mapperInterface, Method method, Configuration config) {
this.command = new SqlCommand(config, mapperInterface, method);
this.method = new MethodSignature(config, mapperInterface, method);
}
public Object execute(SqlSession sqlSession, Object[] args) {
Object result;
if (SqlCommandType.INSERT == command.getType()) {
Object param = method.convertArgsToSqlCommandParam(args);
result = rowCountResult(sqlSession.insert(command.getName(), param));
} else if (SqlCommandType.UPDATE == command.getType()) {
Object param = method.convertArgsToSqlCommandParam(args);
result = rowCountResult(sqlSession.update(command.getName(), param));
} else if (SqlCommandType.DELETE == command.getType()) {
Object param = method.convertArgsToSqlCommandParam(args);
result = rowCountResult(sqlSession.delete(command.getName(), param));
} else if (SqlCommandType.SELECT == command.getType()) {//select语句的处理逻辑
//根据调用的XxxMapper接口定义的抽象方法的返回值类型，选择SqlSession的不同的方法进行执行。
if (method.returnsVoid() && method.hasResultHandler()) {
executeWithResultHandler(sqlSession, args);
result = null;
} else if (method.returnsMany()) {//如果方法的返回值是一个集合，调用selectList方法
result = executeForMany(sqlSession, args);
} else if (method.returnsMap()) {//如果方法的返回值是一个Map，调用selectMap方法
result = executeForMap(sqlSession, args);
} else if (method.returnsCursor()) {//如果方法的返回值，调用selectCurs方法
result = executeForCursor(sqlSession, args);
} else {//否则调用sqlSession.selectOne方法
Object param = method.convertArgsToSqlCommandParam(args);
result = sqlSession.selectOne(command.getName(), param);
}
} else if (SqlCommandType.FLUSH == command.getType()) {
result = sqlSession.flushStatements();
} else {
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;
}
...
}