上文介绍了mapper的创建过程，mybatis通过动态代理的方式生成了代理，MapperProxy实现了InvocationHandler类。下文以查询流程为例，分析sql的执行过程。
MapperProxy:
@Override
public Object invoke(Object proxy, Method method, Object[] args) throws Throwable {
try {
if (Object.class.equals(method.getDeclaringClass())) {
return method.invoke(this, args); // 如果是Object类的方法，直接执行
} else {
return cachedInvoker(method).invoke(proxy, method, args, sqlSession);
}
} catch (Throwable t) {
throw ExceptionUtil.unwrapThrowable(t);
}
}
private MapperMethodInvoker cachedInvoker(Method method) throws Throwable {
try {
// 缓存method的解析流程
return MapUtil.computeIfAbsent(methodCache, method, m -> {
if (m.isDefault()) {
// 如果方法是default方法（接口可声明非abstract的default方法)，封装default方法的执行，这里不做介绍。
try {
if (privateLookupInMethod == null) {
return new DefaultMethodInvoker(getMethodHandleJava8(method));
} else {
return new DefaultMethodInvoker(getMethodHandleJava9(method));
}
} catch (IllegalAccessException | InstantiationException | InvocationTargetException
| NoSuchMethodException e) {
throw new RuntimeException(e);
}
} else {
// 非default方法，构造MapperMethod，封装在PlainMethodInvoker中
return new PlainMethodInvoker(new MapperMethod(mapperInterface, method, sqlSession.getConfiguration()));
}
});
} catch (RuntimeException re) {
Throwable cause = re.getCause();
throw cause == null ? re : cause;
}
}
MapperMethod:
public MapperMethod(Class<?> mapperInterface, Method method, Configuration config) {
// 构建SqlCommand
this.command = new SqlCommand(config, mapperInterface, method);
// 构建MethodSignature
this.method = new MethodSignature(config, mapperInterface, method);
}
SqlCommand:
public SqlCommand(Configuration configuration, Class<?> mapperInterface, Method method) {
final String methodName = method.getName();
final Class<?> declaringClass = method.getDeclaringClass();
// 根据interface、methodName、method所在的类，获取前一步解析mybatis标签时注册的mappedStatement
MappedStatement ms = resolveMappedStatement(mapperInterface, methodName, declaringClass,
configuration);
if (ms == null) {
if (method.getAnnotation(Flush.class) != null) {
name = null;
type = SqlCommandType.FLUSH;
} else {
throw new BindingException("Invalid bound statement (not found): "
+ mapperInterface.getName() + "." + methodName);
}
} else {
// 根据查询到的MappedStatemtn，给name、type赋值。
name = ms.getId();
type = ms.getSqlCommandType();
if (type == SqlCommandType.UNKNOWN) {
throw new BindingException("Unknown execution method for: " + name);
}
}
}
private MappedStatement resolveMappedStatement(Class<?> mapperInterface, String methodName,
Class<?> declaringClass, Configuration configuration) {
String statementId = mapperInterface.getName() + "." + methodName;
if (configuration.hasStatement(statementId)) { // configuration中是否包含根据statementId注册的statement
return configuration.getMappedStatement(statementId);
} else if (mapperInterface.equals(declaringClass)) {
return null;
}
// 否则，通过递归的方式查询父类是否注册了对应的mappedStatement
for (Class<?> superInterface : mapperInterface.getInterfaces()) {
if (declaringClass.isAssignableFrom(superInterface)) {
MappedStatement ms = resolveMappedStatement(superInterface, methodName,
declaringClass, configuration);
if (ms != null) {
return ms;
}
}
}
return null;
}
}
MethodSignature:
public MethodSignature(Configuration configuration, Class<?> mapperInterface, Method method) {
// 解析方法的返回类型
Type resolvedReturnType = TypeParameterResolver.resolveReturnType(method, mapperInterface);
if (resolvedReturnType instanceof Class<?>) {
this.returnType = (Class<?>) resolvedReturnType;
} else if (resolvedReturnType instanceof ParameterizedType) {
this.returnType = (Class<?>) ((ParameterizedType) resolvedReturnType).getRawType();
} else {
this.returnType = method.getReturnType();
}
// 方法是否由返回值
this.returnsVoid = void.class.equals(this.returnType);
// 方法是否返回多个
this.returnsMany = configuration.getObjectFactory().isCollection(this.returnType) || this.returnType.isArray();
this.returnsCursor = Cursor.class.equals(this.returnType);
this.returnsOptional = Optional.class.equals(this.returnType);
this.mapKey = getMapKey(method);
this.returnsMap = this.mapKey != null;
// 解析方法中RowBounds、ResultHandler参数的索引
this.rowBoundsIndex = getUniqueParamIndex(method, RowBounds.class);
this.resultHandlerIndex = getUniqueParamIndex(method, ResultHandler.class);
//
this.paramNameResolver = new ParamNameResolver(configuration, method);
}
ParamNameResolver:
public ParamNameResolver(Configuration config, Method method) {
this.useActualParamName = config.isUseActualParamName();
final Class<?>[] paramTypes = method.getParameterTypes();
// 获取方法的注解
final Annotation[][] paramAnnotations = method.getParameterAnnotations();
final SortedMap<Integer, String> map = new TreeMap<>();
int paramCount = paramAnnotations.length;
// get names from @Param annotations
for (int paramIndex = 0; paramIndex < paramCount; paramIndex++) {
// 如果参数是ResultHandler或者RowBounds类型，跳过
if (isSpecialParameter(paramTypes[paramIndex])) {
// skip special parameters
continue;
}
String name = null;
for (Annotation annotation : paramAnnotations[paramIndex]) {
if (annotation instanceof Param) { // 如果注解是Param注解
hasParamAnnotation = true;
name = ((Param) annotation).value(); // 以value属性作为参数名
break;
}
}
if (name == null) {
// @Param was not specified.
if (useActualParamName) {
name = getActualParamName(method, paramIndex); // 否则，通过放射的方式获取方法名
}
if (name == null) { // 如果name为空，以参数的索引值作为参数名。
// use the parameter index as the name ("0", "1", ...)
// gcode issue #71
name = String.valueOf(map.size());
}
}
map.put(paramIndex, name);
}
names = Collections.unmodifiableSortedMap(map);
}
PlainMethodInvoker:
@Override
public Object invoke(Object proxy, Method method, Object[] args, SqlSession sqlSession) throws Throwable {
return mapperMethod.execute(sqlSession, args);
}
MapperMethod构造完成后，代码的执行来到MapperMethod.execute()，首先分析select的执行流程：
MapperMethod:
public Object execute(SqlSession sqlSession, Object[] args) {
Object result;
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:
// 如果方法无返回，而且方法参数中包含resultHandler
if (method.returnsVoid() && method.hasResultHandler()) {
executeWithResultHandler(sqlSession, args);
result = null;
} else if (method.returnsMany()) { // 如果方法返回集合
result = executeForMany(sqlSession, args);
} else if (method.returnsMap()) { // 如果方法返回map
result = executeForMap(sqlSession, args);
} else if (method.returnsCursor()) { // 如果方法返回游标（适用于海量数据的读取，通过迭代器的方式获取数据）
result = executeForCursor(sqlSession, args);
} else {
// 如果方法返回单个元素
// 将参数返回转换成sql执行的参数
Object param = method.convertArgsToSqlCommandParam(args);
result = sqlSession.selectOne(command.getName(), param); // 获取单个结果
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;
}
首先分析查询单个元素的流程：
public Object convertArgsToSqlCommandParam(Object[] args) {
return paramNameResolver.getNamedParams(args);
}
ParamNameResolver:
public Object getNamedParams(Object[] args) {
final int paramCount = names.size(); // 方法中的参数数量
if (args == null || paramCount == 0) {
return null;
} else if (!hasParamAnnotation && paramCount == 1) { / 如果没有使用Param注解 && 参数只有一个
Object value = args[names.firstKey()];
return wrapToMapIfCollection(value, useActualParamName ? names.get(0) : null);
} else { // 如果有多个参数，或者参数使用了@Param注解：
final Map<String, Object> param = new ParamMap<>();
int i = 0;
for (Map.Entry<Integer, String> entry : names.entrySet()) {
param.put(entry.getValue(), args[entry.getKey()]);
// 添加通用参数名(param1, param2, param3 ...)
final String genericParamName = GENERIC_NAME_PREFIX + (i + 1);
// ensure not to overwrite parameter named with @Param
if (!names.containsValue(genericParamName)) {
param.put(genericParamName, args[entry.getKey()]);
}
i++;
}
return param;
}
}
// 如果参数的类型是集合或者数组，将参数包装成ParamMap类型。
public static Object wrapToMapIfCollection(Object object, String actualParamName) {
if (object instanceof Collection) {
ParamMap<Object> map = new ParamMap<>();
map.put("collection", object);
if (object instanceof List) {
map.put("list", object);
}
Optional.ofNullable(actualParamName).ifPresent(name -> map.put(name, object));
return map;
} else if (object != null && object.getClass().isArray()) {
ParamMap<Object> map = new ParamMap<>();
map.put("array", object);
Optional.ofNullable(actualParamName).ifPresent(name -> map.put(name, object));
return map;
}
return object;
}
DefaultSqSession:
@Override
public <T> T selectOne(String statement, Object parameter) {
// 该sql实际上最多只能返回一个元素，否则会抛出TooManyResultsException
List<T> list = this.selectList(statement, parameter);
if (list.size() == 1) {
return list.get(0);
} else if (list.size() > 1) {
throw new TooManyResultsException("Expected one result (or null) to be returned by selectOne(), but found: " + list.size());
} else {
return null;
}
}
@Override
public <E> List<E> selectList(String statement, Object parameter) {
return this.selectList(statement, parameter, RowBounds.DEFAULT);
}
@Override
public <E> List<E> selectList(String statement, Object parameter, RowBounds rowBounds) {
return selectList(statement, parameter, rowBounds, Executor.NO_RESULT_HANDLER);
}
private <E> List<E> selectList(String statement, Object parameter, RowBounds rowBounds, ResultHandler handler) {
try {
// 解析mybatis标签时，在configuration.mappedStatments中注册了 statement -> mappedStatment映射关系
MappedStatement ms = configuration.getMappedStatement(statement);
return executor.query(ms, wrapCollection(parameter), rowBounds, handler);
} catch (Exception e) {
throw ExceptionFactory.wrapException("Error querying database. Cause: " + e, e);
} finally {
ErrorContext.instance().reset();
}
}
上文中关于mapper创建的分析中，mapper对应的执行器executor其实是被Plugin代理过的代理对象，Plugin实现了InvocationHandler，因此executor.query()会先进入到Plugin.invoke()中：
Plugin:
public Object invoke(Object proxy, Method method, Object[] args) throws Throwable {
try {
Set<Method> methods = signatureMap.get(method.getDeclaringClass());
// 如果当前类+方法配置了拦截器，则进入到拦截器链
if (methods != null && methods.contains(method)) {
return interceptor.intercept(new Invocation(target, method, args));
}
return method.invoke(target, args);
} catch (Exception e) {
throw ExceptionUtil.unwrapThrowable(e);
}
}
拦截器可以在sql执行前后插入前置、后置操作，比如修改参数、增加sql执行(分页实现)、解析结果等，这里不做介绍，后面会通过专门的章节介绍拦截器。
在mapper创建一文中可知，如果配置了二级缓存（全局cacheEnable属性为true，mapper.xml中加入标签），sqlSession持有的executor是CacheExcetor，下一步执行的方法是CacheExecutor:
@Override
public <E> List<E> query(MappedStatement ms, Object parameterObject, RowBounds rowBounds, ResultHandler resultHandler) throws SQLException {
BoundSql boundSql = ms.getBoundSql(parameterObject); // 根据具体参数，获取mappedStatemtn对应的boundSql,boundSql内部包含了将任何动态标签处理完成之后的实际执行sql。
CacheKey key = createCacheKey(ms, parameterObject, rowBounds, boundSql);
return query(ms, parameterObject, rowBounds, resultHandler, key, boundSql);
}
MappedStatement:
public BoundSql getBoundSql(Object parameterObject) {
BoundSql boundSql = sqlSource.getBoundSql(parameterObject); // 构建boundSql
List<ParameterMapping> parameterMappings = boundSql.getParameterMappings();
if (parameterMappings == null || parameterMappings.isEmpty()) {
boundSql = new BoundSql(configuration, boundSql.getSql(), parameterMap.getParameterMappings(), parameterObject);
}
// check for nested result maps in parameter mappings (issue #30)
for (ParameterMapping pm : boundSql.getParameterMappings()) {
String rmId = pm.getResultMapId();
if (rmId != null) {
ResultMap rm = configuration.getResultMap(rmId);
if (rm != null) {
hasNestedResultMaps |= rm.hasNestedResultMaps();
}
}
}
return boundSql;
}
在Mybatis源码(1)-标签的解析文中关于MappedStatement的构建中，如果配置的sql是动态sql，那么MappedStatement持有的SqlSource（代表配置在xml或者注解中的mappedStatment内容）的实现类是DynamicSqlSource：
DynamicSqlSource:
@Override
public BoundSql getBoundSql(Object parameterObject) {
DynamicContext context = new DynamicContext(configuration, parameterObject);
rootSqlNode.apply(context); // 根据入参，执行各SqlNode的apply()，解析sql配置，将最终待执行的sql拼接在context中。“Mybatis源码(1)-标签的解析”一文以IfSqlNode为例，介绍了<if/>标签的解析过程。
SqlSourceBuilder sqlSourceParser = new SqlSourceBuilder(configuration);
Class<?> parameterType = parameterObject == null ? Object.class : parameterObject.getClass();
SqlSource sqlSource = sqlSourceParser.parse(context.getSql(), parameterType, context.getBindings());
BoundSql boundSql = sqlSource.getBoundSql(parameterObject);
context.getBindings().forEach(boundSql::setAdditionalParameter);
return boundSql;
}
获取到boundSql之后，首先会尝试从二级缓存中查询缓存，如果缓存未命中则查询DB。缓存key的构建过程如下：
BaseExecutor:
@Override
public CacheKey createCacheKey(MappedStatement ms, Object parameterObject, RowBounds rowBounds, BoundSql boundSql) {
if (closed) {
throw new ExecutorException("Executor was closed.");
}
CacheKey cacheKey = new CacheKey();
cacheKey.update(ms.getId()); // mappedStatement的id
cacheKey.update(rowBounds.getOffset()); //用于分页的offset、limit参数。
cacheKey.update(rowBounds.getLimit());
cacheKey.update(boundSql.getSql()); // 待执行的sql
List<ParameterMapping> parameterMappings = boundSql.getParameterMappings();
TypeHandlerRegistry typeHandlerRegistry = ms.getConfiguration().getTypeHandlerRegistry();
// mimic DefaultParameterHandler logic
for (ParameterMapping parameterMapping : parameterMappings) {
if (parameterMapping.getMode() != ParameterMode.OUT) { // 如果不是出参（参数用于作为返回结果）
Object value;
String propertyName = parameterMapping.getProperty();
if (boundSql.hasAdditionalParameter(propertyName)) {
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);
}
cacheKey.update(value); // 参数值
}
}
if (configuration.getEnvironment() != null) {
// issue #176
cacheKey.update(configuration.getEnvironment().getId());
}
return cacheKey;
}
CachingExecutor:
@Override
public <E> List<E> query(MappedStatement ms, Object parameterObject, RowBounds rowBounds, ResultHandler resultHandler, CacheKey key, BoundSql boundSql)
throws SQLException {
Cache cache = ms.getCache();
if (cache != null) {
flushCacheIfRequired(ms);
if (ms.isUseCache() && resultHandler == null) {
ensureNoOutParams(ms, boundSql);
@SuppressWarnings("unchecked")
List<E> list = (List<E>) tcm.getObject(cache, key); // 查询缓存
if (list == null) { // 缓存未命中，查询db，并写入缓存
list = delegate.query(ms, parameterObject, rowBounds, resultHandler, key, boundSql);
tcm.putObject(cache, key, list); // issue #578 and #116
}
return list;
}
}
return delegate.query(ms, parameterObject, rowBounds, resultHandler, key, boundSql);
}
首先会查询二级缓存，缓存未命中则会进入到cachingExecutor内部持有的executor的query()方法，查询一级缓存，如果没命中则会真正访问DB。
BaseExecutor:
public <E> List<E> 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.");
}
// 如果<select/>语句配置了flushCache为true，而且queryStack为true，刷新缓存
if (queryStack == 0 && ms.isFlushCacheRequired()) {
clearLocalCache();
}
List<E> list;
try {
queryStack++;
// 根据cacheKey，从localCache(一级缓存)查询
list = resultHandler == null ? (List<E>) localCache.getObject(key) : null;
if (list != null) {
handleLocallyCachedOutputParameters(ms, key, parameter, boundSql);
} else {
// 查询db
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;
}
private <E> List<E> queryFromDatabase(MappedStatement ms, Object parameter, RowBounds rowBounds, ResultHandler resultHandler, CacheKey key, BoundSql boundSql) throws SQLException {
List<E> list;
localCache.putObject(key, EXECUTION_PLACEHOLDER);
try {
list = doQuery(ms, parameter, rowBounds, resultHandler, boundSql);
} finally {
localCache.removeObject(key);
}
// 写缓存
localCache.putObject(key, list);
if (ms.getStatementType() == StatementType.CALLABLE) {
localOutputParameterCache.putObject(key, parameter);
}
return list;
}
BaseExecutor: BaseExecutor是抽象类，提供了查询、更新的流程，具体的实现在其子类SimpleExecutor、BatchExecutor、ReuseExecutor中，各自的功能分别是最普通的sql查询、批量sql执行、复用statement的Executor，具体的细节感兴趣可阅读代码了解，本文以SimpleExecutor作为例子讲解。
SimpleExecutor：
@Override
public <E> List<E> doQuery(MappedStatement ms, Object parameter, RowBounds rowBounds, ResultHandler resultHandler, BoundSql boundSql) throws SQLException {
Statement stmt = null;
try {
Configuration configuration = ms.getConfiguration();
// 构建 statementHandler，statementHandler用于构建jdbc statement,设置statement参数，执行statement等。
StatementHandler handler = configuration.newStatementHandler(wrapper, ms, parameter, rowBounds, resultHandler, boundSql);
// 构建 jdbc statement
stmt = prepareStatement(handler, ms.getStatementLog());
return handler.query(stmt, resultHandler);
} finally {
closeStatement(stmt);
}
}
private Statement prepareStatement(StatementHandler handler, Log statementLog) throws SQLException {
Statement stmt;
Connection connection = getConnection(statementLog); // 获取jdbc connection
stmt = handler.prepare(connection, transaction.getTimeout()); // 调用statementHandler.prepare()，构建jdbc statement
handler.parameterize(stmt); // 设置jdbc statement参数
return stmt;
}
如果statementType是PREPARED，那么对应的StatementHandler是PreparedStatementHandler。BaseStatementHandler是PreparedStatementHandler、CallableStatementHandler等的父类。
BaseStatementHandler：
@Override
public Statement prepare(Connection connection, Integer transactionTimeout) throws SQLException {
ErrorContext.instance().sql(boundSql.getSql());
Statement statement = null;
try {
statement = instantiateStatement(connection); // 构建jdbc statement，并初始化
setStatementTimeout(statement, transactionTimeout); // 设置statement的执行超时时间，如果事务超时时间<statment配置的超时时间，以事务超时时间为准。
setFetchSize(statement); // 设置fetchSize
return statement;
} catch (SQLException e) {
closeStatement(statement);
throw e;
} catch (Exception e) {
closeStatement(statement);
throw new ExecutorException("Error preparing statement. Cause: " + e, e);
}
}
PreparedStatmentHandler:
@Override
protected Statement instantiateStatement(Connection connection) throws SQLException {
// 通过connection获取preparedStatement
String sql = boundSql.getSql();
if (mappedStatement.getKeyGenerator() instanceof Jdbc3KeyGenerator) {
String[] keyColumnNames = mappedStatement.getKeyColumns();
if (keyColumnNames == null) {
return connection.prepareStatement(sql, PreparedStatement.RETURN_GENERATED_KEYS);
} else {
return connection.prepareStatement(sql, keyColumnNames);
}
} else if (mappedStatement.getResultSetType() == ResultSetType.DEFAULT) {
return connection.prepareStatement(sql);
} else {
return connection.prepareStatement(sql, mappedStatement.getResultSetType().getValue(), ResultSet.CONCUR_READ_ONLY);
}
}
PreparedStatment构建完成之后，下一步是设置参数：
PreparedStatementHandler:
@Override
public void parameterize(Statement statement) throws SQLException {
parameterHandler.setParameters((PreparedStatement) statement);
}
DefaultParameterHandler:
public void setParameters(PreparedStatement ps) {
ErrorContext.instance().activity("setting parameters").object(mappedStatement.getParameterMap().getId());
List<ParameterMapping> parameterMappings = boundSql.getParameterMappings(); // 获取入参对应的parameterMappings
if (parameterMappings != null) {
for (int i = 0; i < parameterMappings.size(); i++) {
ParameterMapping parameterMapping = parameterMappings.get(i);
if (parameterMapping.getMode() != ParameterMode.OUT) { // 如果是输出型参数，则跳过
Object value;
String propertyName = parameterMapping.getProperty(); // 获取参数名
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 {
// 根据属性名，在parameterObject中获取属性对应的值
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 {
// 根据参数类型，调用对应的typeHandler设置参数
typeHandler.setParameter(ps, i + 1, value, jdbcType);
} catch (TypeException | SQLException e) {
throw new TypeException("Could not set parameters for mapping: " + parameterMapping + ". Cause: " + e, e);
}
}
}
}
}
TypeHandler用于给statement设置参数，从ResultSet中获取值等，原生支持Integer、String、Boolean、Float、Double等多种类型的参数设置、结果获取，具体见org.apache.ibatis.type.TypeHandlerRegistry。
下一步是执行statment，并从ResultSet中解析出结果。
PreparedStatementHandler:
public <E> List<E> query(Statement statement, ResultHandler resultHandler) throws SQLException {
PreparedStatement ps = (PreparedStatement) statement;
ps.execute(); // 执行preparedStatment
return resultSetHandler.handleResultSets(ps); // 解析结果，并完成类型转换。
}
DefaultResultSetHandler:
public List<Object> handleResultSets(Statement stmt) throws SQLException {
ErrorContext.instance().activity("handling results").object(mappedStatement.getId());
final List<Object> multipleResults = new ArrayList<>();
int resultSetCount = 0;
ResultSetWrapper rsw = getFirstResultSet(stmt); // 获取第一个resultSet，除了存储过程可以返回多个值之外，一般结果集中只有一个resultSet
List<ResultMap> resultMaps = mappedStatement.getResultMaps();
int resultMapCount = resultMaps.size();
validateResultMapsCount(rsw, resultMapCount); // 执行结果、resultType/resultMap配置校验
while (rsw != null && resultMapCount > resultSetCount) {
ResultMap resultMap = resultMaps.get(resultSetCount);
handleResultSet(rsw, resultMap, multipleResults, null);// 解析resultSet，完成类型转换，并将结果放入multipleResults中
rsw = getNextResultSet(stmt);
cleanUpAfterHandlingResultSet();
resultSetCount++;
}
String[] resultSets = mappedStatement.getResultSets();
if (resultSets != null) {
while (rsw != null && resultSetCount < resultSets.length) {
ResultMapping parentMapping = nextResultMaps.get(resultSets[resultSetCount]);
if (parentMapping != null) {
String nestedResultMapId = parentMapping.getNestedResultMapId();
ResultMap resultMap = configuration.getResultMap(nestedResultMapId);
handleResultSet(rsw, resultMap, null, parentMapping);
}
rsw = getNextResultSet(stmt);
cleanUpAfterHandlingResultSet();
resultSetCount++;
}
}
return collapseSingleResultList(multipleResults);
}
private void handleResultSet(ResultSetWrapper rsw, ResultMap resultMap, List<Object> multipleResults, ResultMapping parentMapping) throws SQLException {
try {
if (parentMapping != null) {
handleRowValues(rsw, resultMap, null, RowBounds.DEFAULT, parentMapping);
} else {
if (resultHandler == null) { // 如果resultHandler为空，则使用默认的resultHandler
DefaultResultHandler defaultResultHandler = new DefaultResultHandler(objectFactory);
handleRowValues(rsw, resultMap, defaultResultHandler, rowBounds, null); // 解析resultSet，通过反射的方式，实例化resultType对应的类，并给属性赋值。
multipleResults.add(defaultResultHandler.getResultList());
} else {
handleRowValues(rsw, resultMap, resultHandler, rowBounds, null); // 解析
}
}
} finally {
// issue #228 (close resultsets)
closeResultSet(rsw.getResultSet());
}
}
public void handleRowValues(ResultSetWrapper rsw, ResultMap resultMap, ResultHandler<?> resultHandler, RowBounds rowBounds, ResultMapping parentMapping) throws SQLException {
if (resultMap.hasNestedResultMaps()) { //是否有嵌套的resultMap
ensureNoRowBounds();
checkResultHandler();
handleRowValuesForNestedResultMap(rsw, resultMap, resultHandler, rowBounds, parentMapping);
} else {
handleRowValuesForSimpleResultMap(rsw, resultMap, resultHandler, rowBounds, parentMapping); // resultMap非嵌套，解析结果集
}
}
private void handleRowValuesForSimpleResultMap(ResultSetWrapper rsw, ResultMap resultMap, ResultHandler<?> resultHandler, RowBounds rowBounds, ResultMapping parentMapping)
throws SQLException {
DefaultResultContext<Object> resultContext = new DefaultResultContext<>();
ResultSet resultSet = rsw.getResultSet();
skipRows(resultSet, rowBounds); // 如果设置了分页，则跳过指定数量的行
//遍历ResultSet
while (shouldProcessMoreRows(resultContext, rowBounds) && !resultSet.isClosed() && resultSet.next()) {
// mybatis鉴别器，本文不做介绍
ResultMap discriminatedResultMap = resolveDiscriminatedResultMap(resultSet, resultMap, null);
// 解析数据，通过反射的方式实例化对象、并完成属性赋值
Object rowValue = getRowValue(rsw, discriminatedResultMap, null);
storeObject(resultHandler, resultContext, rowValue, parentMapping, resultSet); // 将数据保存在resultHandler中
}
private Object getRowValue(ResultSetWrapper rsw, ResultMap resultMap, String columnPrefix) throws SQLException {
final ResultLoaderMap lazyLoader = new ResultLoaderMap();
// 构建结果
Object rowValue = createResultObject(rsw, resultMap, lazyLoader, columnPrefix);
// 如果这个结果是通过自定义的typeHandler生成的，那么可以直接返回；否则，只是通过反射生成的基本对象，还需要通过反射进行下面的属性填充操作。
if (rowValue != null && !hasTypeHandlerForResultObject(rsw, resultMap.getType())) {
// 元数据解析，包括反射相关的get、set field/method、constructor等
final MetaObject metaObject = configuration.newMetaObject(rowValue);
boolean foundValues = this.useConstructorMappings;
// autoMapping
if (shouldApplyAutomaticMappings(resultMap, false)) {
foundValues = applyAutomaticMappings(rsw, resultMap, metaObject, columnPrefix) || foundValues;
}
// 手动映射，autoMapping和手动映射的主要实现方式都是反射，下面只介绍通常使用的手动映射的代码实现方式。
foundValues = applyPropertyMappings(rsw, resultMap, metaObject, lazyLoader, columnPrefix) || foundValues;
foundValues = lazyLoader.size() > 0 || foundValues;
rowValue = foundValues || configuration.isReturnInstanceForEmptyRow() ? rowValue : null;
}
return rowValue;
}
private Object createResultObject(ResultSetWrapper rsw, ResultMap resultMap, ResultLoaderMap lazyLoader, String columnPrefix) throws SQLException {
this.useConstructorMappings = false; // reset previous mapping result
final List<Class<?>> constructorArgTypes = new ArrayList<>();
final List<Object> constructorArgs = new ArrayList<>();
// 创建对象
Object resultObject = createResultObject(rsw, resultMap, constructorArgTypes, constructorArgs, columnPrefix);
if (resultObject != null && !hasTypeHandlerForResultObject(rsw, resultMap.getType())) {
final List<ResultMapping> propertyMappings = resultMap.getPropertyResultMappings();
for (ResultMapping propertyMapping : propertyMappings) {
// issue gcode #109 && issue #149
// 和嵌套查询相关
if (propertyMapping.getNestedQueryId() != null && propertyMapping.isLazy()) {
resultObject = configuration.getProxyFactory().createProxy(resultObject, lazyLoader, configuration, objectFactory, constructorArgTypes, constructorArgs);
break;
}
}
}
this.useConstructorMappings = resultObject != null && !constructorArgTypes.isEmpty(); // set current mapping result
return resultObject;
}
private Object createResultObject(ResultSetWrapper rsw, ResultMap resultMap, List<Class<?>> constructorArgTypes, List<Object> constructorArgs, String columnPrefix)
throws SQLException {
final Class<?> resultType = resultMap.getType();
final MetaClass metaType = MetaClass.forClass(resultType, reflectorFactory);
final List<ResultMapping> constructorMappings = resultMap.getConstructorResultMappings();
// 如果结果类型有对应的ResultHandler，调用自定义的ResultHandler.getResult()解析结果
if (hasTypeHandlerForResultObject(rsw, resultType)) {
return createPrimitiveResultObject(rsw, resultMap, columnPrefix);
// 如果resultType配置了constructor标签，则通过反射调用对应的构造方法示例化对象
} else if (!constructorMappings.isEmpty()) {
return createParameterizedResultObject(rsw, resultType, constructorMappings, constructorArgTypes, constructorArgs, columnPrefix);
} else if (resultType.isInterface() || metaType.hasDefaultConstructor()) {
return objectFactory.create(resultType); // 通过反射调用默认构造方法，实例化对象
} else if (shouldApplyAutomaticMappings(resultMap, false)) {
return createByConstructorSignature(rsw, resultType, constructorArgTypes, constructorArgs);
}
throw new ExecutorException("Do not know how to create an instance of " + resultType);
}
如果对象是通过反射调用构造方法实例化，还没有进行属性填充，下一步是通过反射的方式进行属性填充。
DefaultResultSetHandler:
private boolean applyPropertyMappings(ResultSetWrapper rsw, ResultMap resultMap, MetaObject metaObject, ResultLoaderMap lazyLoader, String columnPrefix)
throws SQLException {
final List<String> mappedColumnNames = rsw.getMappedColumnNames(resultMap, columnPrefix); // 获取配置了映射关系的表列名
boolean foundValues = false;
final List<ResultMapping> propertyMappings = resultMap.getPropertyResultMappings(); // 配置的属性映射关系
for (ResultMapping propertyMapping : propertyMappings) {
String column = prependPrefix(propertyMapping.getColumn(), columnPrefix); // 获取列名
if (propertyMapping.getNestedResultMapId() != null) {
// the user added a column attribute to a nested result map, ignore it
column = null;
}
if (propertyMapping.isCompositeResult()
|| (column != null && mappedColumnNames.contains(column.toUpperCase(Locale.ENGLISH)))
|| propertyMapping.getResultSet() != null) {
// 根据列名，从ResultSet中获取属性对应的值
Object value = getPropertyMappingValue(rsw.getResultSet(), metaObject, propertyMapping, lazyLoader, columnPrefix);
// issue #541 make property optional
final String property = propertyMapping.getProperty();
if (property == null) {
continue;
} else if (value == DEFERRED) {
foundValues = true;
continue;
}
if (value != null) {
foundValues = true;
}
if (value != null || (configuration.isCallSettersOnNulls() && !metaObject.getSetterType(property).isPrimitive())) {
// gcode issue #377, call setter on nulls (value is not 'found')
// 使用放射的方式，调用setXX方法给属性赋值。
metaObject.setValue(property, value);
}
}
}
return foundValues;
}
private Object getPropertyMappingValue(ResultSet rs, MetaObject metaResultObject, ResultMapping propertyMapping, ResultLoaderMap lazyLoader, String columnPrefix)
throws SQLException {
if (propertyMapping.getNestedQueryId() != null) {
return getNestedQueryMappingValue(rs, metaResultObject, propertyMapping, lazyLoader, columnPrefix);
} else if (propertyMapping.getResultSet() != null) {
addPendingChildRelation(rs, metaResultObject, propertyMapping); // TODO is that OK?
return DEFERRED;
} else {
final TypeHandler<?> typeHandler = propertyMapping.getTypeHandler(); // 获取对应的typeHandler
final String column = prependPrefix(propertyMapping.getColumn(), columnPrefix); // 最终的列名
return typeHandler.getResult(rs, column); // 根据列表，底层调用ResultSet.getXX(ColomunName)获取值
}
}
至此，mybatis查询流程的实现方式已经分析完毕，其他的比如查询多个元素、带游标的查询等的实现方式都大同小异。insert/update/delete等的执行流程和查询相似，源码层面上依赖的类一样、执行流程也相似。
