Mybatis之工作原理

1.Mybatis的架构

1.1 Mybatis的框架分层

1.2 MyBatis的实现原理

mybatis底层还是采用原生jdbc来对数据库进行操作的，只是通过 SqlSessionFactory，SqlSession Executor,StatementHandler，ParameterHandler,ResultHandler和TypeHandler等几个处理器封装了这些过程

执行器：Executor (update, query, flushStatements, commit, rollback, getTransaction, close, isClosed)
参数处理器： ParameterHandler (getParameterObject, setParameters)
结构处理器 ResultSetHandler (handleResultSets, handleOutputParameters)
sql查询处理器：StatementHandler (prepare, parameterize, batch, update, query)

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其中StatementHandler用通过ParameterHandler与ResultHandler分别进行参数预编译 与结果处理。而ParameterHandler与ResultHandler都使用TypeHandler进行映射。如下图： 

2.Mybatis工作过程

通过读mybatis的源码进行分析mybatis的执行操作的整个过程，我们通过debug调试就可以知道Mybatis每一步做了什么事，我先把debug每一步结果 截图，然后在分析这个流程。 
第一步:读取配置文件，形成InputStream

2.1 创建SqlSessionFacotry的过程

 
从debug调试看出 返回的 sqlSessionFactory 是DefaultSesssionFactory类型的，但是configuration此时已经被初始化了。查看源码后画如下创建DefaultSessionFactory的时序图： 

2.2 创建SqlSession的过程

 
从debug调试 看出SqlSessinoFactory.openSession() 返回的sqlSession是 DefaultSession类型的，此SqlSession里包含一个Configuration的对象，和一个Executor对象。查看源码后画如下创建DefaultSession的时序图：

2.3 创建Mapper的过程

 
从debug调试可以看出，mapper是一个Mapper代理对象，而且初始化了Configuration对象，Executor的对象。查看源码后画如下创建Mapper的时序图： 

2.4 执行CRUD过程

2.4.1 以select为例查看各步执行的源码

1.mapper.selectEmployeeList()其实是MapperProxy执行invoke方法，此方法显示是判断Method的方法是不是Object的toString等方法如果不是就执行MapperMethod

public Object invoke(Object proxy, Method method, Object[] args) throws Throwable {
// 判断Method的方法是不是Object的toString等方法
if(Object.class.equals(method.getDeclaringClass())) {
try {
return method.invoke(this, args);
} catch (Throwable var5) {
throw ExceptionUtil.unwrapThrowable(var5);
}
} else {
//判断private final Map<Method, MapperMethod> methodCache;这个map里面有没有这个方法的一级缓存，如果没
MapperMethod mapperMethod = this.cachedMapperMethod(method);
return mapperMethod.execute(this.sqlSession, args);
}
}
//查询一级缓存和设置一级缓存

private MapperMethod cachedMapperMethod(Method method) {
MapperMethod mapperMethod = (MapperMethod)this.methodCache.get(method);
if(mapperMethod == null) {
mapperMethod = new MapperMethod(this.mapperInterface, method, this.sqlSession.getConfiguration());
this.methodCache.put(method, mapperMethod);
}

return mapperMethod;
}

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经过上面的调用后进入MapperMethod里面执行

//判断sql命令类型
public Object execute(SqlSession sqlSession, Object[] args) {
Object param;
Object result;
if(SqlCommandType.INSERT == this.command.getType()) {
param = this.method.convertArgsToSqlCommandParam(args);
result = this.rowCountResult(sqlSession.insert(this.command.getName(), param));
} else if(SqlCommandType.UPDATE == this.command.getType()) {
param = this.method.convertArgsToSqlCommandParam(args);
result = this.rowCountResult(sqlSession.update(this.command.getName(), param));
} else if(SqlCommandType.DELETE == this.command.getType()) {
param = this.method.convertArgsToSqlCommandParam(args);
result = this.rowCountResult(sqlSession.delete(this.command.getName(), param));
} else if(SqlCommandType.SELECT == this.command.getType()) {
//我们测试的是select类型，则再判断这个方法的返回类型
if(this.method.returnsVoid() && this.method.hasResultHandler()) {
this.executeWithResultHandler(sqlSession, args);
result = null;
} else if(this.method.returnsMany()) {
//我们是查询列表，此方法执行
result = this.executeForMany(sqlSession, args);
} else if(this.method.returnsMap()) {
result = this.executeForMap(sqlSession, args);
} else {
param = this.method.convertArgsToSqlCommandParam(args);
result = sqlSession.selectOne(this.command.getName(), param);
}
} else {
if(SqlCommandType.FLUSH != this.command.getType()) {
throw new BindingException("Unknown execution method for: " + this.command.getName());
}

result = sqlSession.flushStatements();
}

if(result == null && this.method.getReturnType().isPrimitive() && !this.method.returnsVoid()) {
throw new BindingException("Mapper method '" + this.command.getName() + " attempted to return null from a method with a primitive return type (" + this.method.getReturnType() + ").");
} else {
return result;
}
}

private <E> Object executeForMany(SqlSession sqlSession, Object[] args) {
//将param做处理 自动处理为param1,param2..
Object param = this.method.convertArgsToSqlCommandParam(args);
List result;
if(this.method.hasRowBounds()) {
RowBounds rowBounds = this.method.extractRowBounds(args);
//调用该对象的DefaultSqlSession的selectList方法
result = sqlSession.selectList(this.command.getName(), param, rowBounds);
} else {
result = sqlSession.selectList(this.command.getName(), param);
}

return !this.method.getReturnType().isAssignableFrom(result.getClass())?(this.method.getReturnType().isArray()?this.convertToArray(result):this.convertToDeclaredCollection(sqlSession.getConfiguration(), result)):result;
}

//处理参数方法
public Object convertArgsToSqlCommandParam(Object[] args) {
int paramCount = this.params.size();
if(args != null && paramCount != 0) {
if(!this.hasNamedParameters && paramCount == 1) {
return args[((Integer)this.params.keySet().iterator().next()).intValue()];
} else {
Map<String, Object> param = new MapperMethod.ParamMap();
int i = 0;

for(Iterator i$ = this.params.entrySet().iterator(); i$.hasNext(); ++i) {
Entry<Integer, String> entry = (Entry)i$.next();
param.put(entry.getValue(), args[((Integer)entry.getKey()).intValue()]);
String genericParamName = "param" + String.valueOf(i + 1);
if(!param.containsKey(genericParamName)) {
param.put(genericParamName, args[((Integer)entry.getKey()).intValue()]);
}
}

return param;
}
} else {
return null;
}
}

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调用DefaultSqlSession的selectList的方法

public <E> List<E> selectList(String statement, Object parameter, RowBounds rowBounds) {
List var5;
try {
//获取MappedStatement对象
MappedStatement ms = this.configuration.getMappedStatement(statement);
//调用cachingExecutor执行器的方法
var5 = this.executor.query(ms, this.wrapCollection(parameter), rowBounds, Executor.NO_RESULT_HANDLER);
} catch (Exception var9) {
throw ExceptionFactory.wrapException("Error querying database.  Cause: " + var9, var9);
} finally {
ErrorContext.instance().reset();
}

return var5;
}

//CachingExector的query方法
public <E> List<E> query(MappedStatement ms, Object parameterObject, RowBounds rowBounds, ResultHandler resultHandler) throws SQLException {
//
BoundSql boundSql = ms.getBoundSql(parameterObject);
CacheKey key = this.createCacheKey(ms, parameterObject, rowBounds, boundSql);
//调用下2代码
return this.query(ms, parameterObject, rowBounds, resultHandler, key, boundSql);
}
//2代码
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) {
this.flushCacheIfRequired(ms);
if(ms.isUseCache() && resultHandler == null) {
this.ensureNoOutParams(ms, parameterObject, boundSql);
List<E> list = (List)this.tcm.getObject(cache, key);
if(list == null) {
//这里是调用Executor里的query方法 如果开启了缓存这掉CachingExecutor的 如果没有则是调用BaseExecutor的
list = this.delegate.query(ms, parameterObject, rowBounds, resultHandler, key, boundSql);
this.tcm.putObject(cache, key, list);
}

return list;
}
}

return this.delegate.query(ms, parameterObject, rowBounds, resultHandler, key, boundSql);
}

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BaseExecutor的query方法

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(this.closed) {
throw new ExecutorException("Executor was closed.");
} else {
if(this.queryStack == 0 && ms.isFlushCacheRequired()) {
this.clearLocalCache();
}

List list;
try {
++this.queryStack;
list = resultHandler == null?(List)this.localCache.getObject(key):null;
if(list != null) {
this.handleLocallyCachedOutputParameters(ms, key, parameter, boundSql);
} else {
//如果缓存中没有就从数据库中查询
list = this.queryFromDatabase(ms, parameter, rowBounds, resultHandler, key, boundSql);
}
} finally {
--this.queryStack;
}

if(this.queryStack == 0) {
Iterator i$ = this.deferredLoads.iterator();

while(i$.hasNext()) {
BaseExecutor.DeferredLoad deferredLoad = (BaseExecutor.DeferredLoad)i$.next();
deferredLoad.load();
}

this.deferredLoads.clear();
if(this.configuration.getLocalCacheScope() == LocalCacheScope.STATEMENT) {
this.clearLocalCache();
}
}

return list;
}
}

//从数据库中查询
private <E> List<E> queryFromDatabase(MappedStatement ms, Object parameter, RowBounds rowBounds, ResultHandler resultHandler, CacheKey key, BoundSql boundSql) throws SQLException {
//放入缓存
this.localCache.putObject(key, ExecutionPlaceholder.EXECUTION_PLACEHOLDER);

List list;
try {
//此处是调用子Executor的方法，ExecutorType默认是使用的SimpleExecutor
list = this.doQuery(ms, parameter, rowBounds, resultHandler, boundSql);
} finally {
this.localCache.removeObject(key);
}

this.localCache.putObject(key, list);
if(ms.getStatementType() == StatementType.CALLABLE) {
this.localOutputParameterCache.putObject(key, parameter);
}

return list;
}

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SimpleExecutor的doQuery方法

public <E> List<E> doQuery(MappedStatement ms, Object parameter, RowBounds rowBounds, ResultHandler resultHandler, BoundSql boundSql) throws SQLException {
Statement stmt = null;

List var9;
try {
Configuration configuration = ms.getConfiguration();
//创建StateMentHandler处理器
StatementHandler handler = configuration.newStatementHandler(this.wrapper, ms, parameter, rowBounds, resultHandler, boundSql);
//调用下3的方法
stmt = this.prepareStatement(handler, ms.getStatementLog());
//调用no4的方法
var9 = handler.query(stmt, resultHandler);
} finally {
this.closeStatement(stmt);
}

return var9;
}
//下3方法
private Statement prepareStatement(StatementHandler handler, Log statementLog) throws SQLException {
Connection connection = this.getConnection(statementLog);
Statement stmt = handler.prepare(connection);
//SatementHanlder 采用PreparedStatementHandler来实现此方法，而PreparedStatementHandler调用的是父接口ParameterHandler的方法
handler.parameterize(stmt);
return stmt;
}

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ParameterHandler参数处理器的方法

public interface ParameterHandler {
Object getParameterObject();
//此方法是用DefaultParameterHandler实现的
void setParameters(PreparedStatement var1) throws SQLException;
}

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DefaultParameterHandler默认参数处理器的方法

public void setParameters(PreparedStatement ps) {
ErrorContext.instance().activity("setting parameters").object(this.mappedStatement.getParameterMap().getId());
List<ParameterMapping> parameterMappings = this.boundSql.getParameterMappings();
if(parameterMappings != null) {
for(int i = 0; i < parameterMappings.size(); ++i) {
ParameterMapping parameterMapping = (ParameterMapping)parameterMappings.get(i);
if(parameterMapping.getMode() != ParameterMode.OUT) {
String propertyName = parameterMapping.getProperty();
Object value;
if(this.boundSql.hasAdditionalParameter(propertyName)) {
value = this.boundSql.getAdditionalParameter(propertyName);
} else if(this.parameterObject == null) {
value = null;
} else if(this.typeHandlerRegistry.hasTypeHandler(this.parameterObject.getClass())) {
value = this.parameterObject;
} else {
MetaObject metaObject = this.configuration.newMetaObject(this.parameterObject);
value = metaObject.getValue(propertyName);
}
//这里用调用 TypeHandler类型映射处理器来映射
TypeHandler typeHandler = parameterMapping.getTypeHandler();
JdbcType jdbcType = parameterMapping.getJdbcType();
if(value == null && jdbcType == null) {
jdbcType = this.configuration.getJdbcTypeForNull();
}

try {
//类型处理器设置参数映射
typeHandler.setParameter(ps, i + 1, value, jdbcType);
} catch (TypeException var10) {
throw new TypeException("Could not set parameters for mapping: " + parameterMapping + ". Cause: " + var10, var10);
} catch (SQLException var11) {
throw new TypeException("Could not set parameters for mapping: " + parameterMapping + ". Cause: " + var11, var11);
}
}
}
}

}

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no4的方法

public <E> List<E> query(Statement statement, ResultHandler resultHandler) throws SQLException {
//此处调用原生sql的处理器
PreparedStatement ps = (PreparedStatement)statement;
//发出原生sql命令
ps.execute();
//采用ResultHandler结果处理器对结果集封装
return this.resultSetHandler.handleResultSets(ps);
}

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ResultHandler代码

public interface ResultSetHandler {
//此处调用的是DefaultResultSetHandler的方法
<E> List<E> handleResultSets(Statement var1) throws SQLException;

void handleOutputParameters(CallableStatement var1) throws SQLException;
}

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DefaultResultSetHandler的方法

public List<Object> handleResultSets(Statement stmt) throws SQLException {
ErrorContext.instance().activity("handling results").object(this.mappedStatement.getId());
List<Object> multipleResults = new ArrayList();
int resultSetCount = 0;
ResultSetWrapper rsw = this.getFirstResultSet(stmt);
List<ResultMap> resultMaps = this.mappedStatement.getResultMaps();
int resultMapCount = resultMaps.size();
this.validateResultMapsCount(rsw, resultMapCount);

while(rsw != null && resultMapCount > resultSetCount) {
ResultMap resultMap = (ResultMap)resultMaps.get(resultSetCount);
this.handleResultSet(rsw, resultMap, multipleResults, (ResultMapping)null);
rsw = this.getNextResultSet(stmt);
this.cleanUpAfterHandlingResultSet();
++resultSetCount;
}

String[] resultSets = this.mappedStatement.getResulSets();
if(resultSets != null) {
while(rsw != null && resultSetCount < resultSets.length) {
ResultMapping parentMapping = (ResultMapping)this.nextResultMaps.get(resultSets[resultSetCount]);
if(parentMapping != null) {
String nestedResultMapId = parentMapping.getNestedResultMapId();
ResultMap resultMap = this.configuration.getResultMap(nestedResultMapId);
this.handleResultSet(rsw, resultMap, (List)null, parentMapping);
}

rsw = this.getNextResultSet(stmt);
this.cleanUpAfterHandlingResultSet();
++resultSetCount;
}
}

return this.collapseSingleResultList(multipleResults);
}
//处理结果集
private void handleResultSet(ResultSetWrapper rsw, ResultMap resultMap, List<Object> multipleResults, ResultMapping parentMapping) throws SQLException {
try {
if(parentMapping != null) {
this.handleRowValues(rsw, resultMap, (ResultHandler)null, RowBounds.DEFAULT, parentMapping);
} else if(this.resultHandler == null) {
DefaultResultHandler defaultResultHandler = new DefaultResultHandler(this.objectFactory);
this.handleRowValues(rsw, resultMap, defaultResultHandler, this.rowBounds, (ResultMapping)null);
multipleResults.add(defaultResultHandler.getResultList());
} else {
this.handleRowValues(rsw, resultMap, this.resultHandler, this.rowBounds, (ResultMapping)null);
}
} finally {
this.closeResultSet(rsw.getResultSet());
}

}

private void handleRowValues(ResultSetWrapper rsw, ResultMap resultMap, ResultHandler<?> resultHandler, RowBounds rowBounds, ResultMapping parentMapping) throws SQLException {
if(resultMap.hasNestedResultMaps()) {
this.ensureNoRowBounds();
this.checkResultHandler();
this.handleRowValuesForNestedResultMap(rsw, resultMap, resultHandler, rowBounds, parentMapping);
} else {
this.handleRowValuesForSimpleResultMap(rsw, resultMap, resultHandler, rowBounds, parentMapping);
}

}

private void handleRowValuesForNestedResultMap(ResultSetWrapper rsw, ResultMap resultMap, ResultHandler<?> resultHandler, RowBounds rowBounds, ResultMapping parentMapping) throws SQLException {
DefaultResultContext<Object> resultContext = new DefaultResultContext();
this.skipRows(rsw.getResultSet(), rowBounds);
Object rowValue = null;

while(this.shouldProcessMoreRows(resultContext, rowBounds) && rsw.getResultSet().next()) {
ResultMap discriminatedResultMap = this.resolveDiscriminatedResultMap(rsw.getResultSet(), resultMap, (String)null);
CacheKey rowKey = this.createRowKey(discriminatedResultMap, rsw, (String)null);
Object partialObject = this.nestedResultObjects.get(rowKey);
if(this.mappedStatement.isResultOrdered()) {
if(partialObject == null && rowValue != null) {
this.nestedResultObjects.clear();
this.storeObject(resultHandler, resultContext, rowValue, parentMapping, rsw.getResultSet());
}
//获取行的值
rowValue = this.getRowValue(rsw, discriminatedResultMap, rowKey, (String)null, partialObject);
} else {
rowValue = this.getRowValue(rsw, discriminatedResultMap, rowKey, (String)null, partialObject);
if(partialObject == null) {
this.storeObject(resultHandler, resultContext, rowValue, parentMapping, rsw.getResultSet());
}
}
}

if(rowValue != null && this.mappedStatement.isResultOrdered() && this.shouldProcessMoreRows(resultContext, rowBounds)) {
this.storeObject(resultHandler, resultContext, rowValue, parentMapping, rsw.getResultSet());
}

}
String resultMapId = resultMap.getId();
Object resultObject = partialObject;
if(partialObject != null) {
MetaObject metaObject = this.configuration.newMetaObject(partialObject);
this.putAncestor(partialObject, resultMapId, columnPrefix);
this.applyNestedResultMappings(rsw, resultMap, metaObject, columnPrefix, combinedKey, false);
this.ancestorObjects.remove(resultMapId);
} else {
ResultLoaderMap lazyLoader = new ResultLoaderMap();
resultObject = this.createResultObject(rsw, resultMap, lazyLoader, columnPrefix);
if(resultObject != null && !this.typeHandlerRegistry.hasTypeHandler(resultMap.getType())) {
MetaObject metaObject = this.configuration.newMetaObject(resultObject);
boolean foundValues = !resultMap.getConstructorResultMappings().isEmpty();
if(this.shouldApplyAutomaticMappings(resultMap, true)) {
foundValues = this.applyAutomaticMappings(rsw, resultMap, metaObject, columnPrefix) || foundValues;
}

foundValues = this.applyPropertyMappings(rsw, resultMap, metaObject, lazyLoader, columnPrefix) || foundValues;
this.putAncestor(resultObject, resultMapId, columnPrefix);
foundValues = this.applyNestedResultMappings(rsw, resultMap, metaObject, columnPrefix, combinedKey, true) || foundValues;
this.ancestorObjects.remove(resultMapId);
foundValues = lazyLoader.size() > 0 || foundValues;
resultObject = foundValues?resultObject:null;
}

if(combinedKey != CacheKey.NULL_CACHE_KEY) {
this.nestedResultObjects.put(combinedKey, resultObject);
}
}

return resultObject;
}

private boolean shouldApplyAutomaticMappings(ResultMap resultMap, boolean isNested) {
return resultMap.getAutoMapping() != null?resultMap.getAutoMapping().booleanValue():(isNested?AutoMappingBehavior.FULL == this.configuration.getAutoMappingBehavior():AutoMappingBehavior.NONE != this.configuration.getAutoMappingBehavior());
}
private boolean applyAutomaticMappings(ResultSetWrapper rsw, ResultMap resultMap, MetaObject metaObject, String columnPrefix) throws SQLException {
List<DefaultResultSetHandler.UnMappedColumAutoMapping> autoMapping = this.createAutomaticMappings(rsw, resultMap, metaObject, columnPrefix);
boolean foundValues = false;
if(autoMapping.size() > 0) {
Iterator i$ = autoMapping.iterator();

while(true) {
//这里使用了内部类对参数和结果集进行映射
DefaultResultSetHandler.UnMappedColumAutoMapping mapping;
Object value;
do {
if(!i$.hasNext()) {
return foundValues;
}

mapping = (DefaultResultSetHandler.UnMappedColumAutoMapping)i$.next();
value = mapping.typeHandler.getResult(rsw.getResultSet(), mapping.column);
} while(value == null && !this.configuration.isCallSettersOnNulls());

if(value != null || !mapping.primitive) {
metaObject.setValue(mapping.property, value);
}

foundValues = true;
}
} else {
return foundValues;
}
}
private static class UnMappedColumAutoMapping {
private final String column;
private final String property;
private final TypeHandler<?> typeHandler;
private final boolean primitive;

//此处才类型器对结果进行映射
public UnMappedColumAutoMapping(String column, String property, TypeHandler<?> typeHandler, boolean primitive) {
this.column = column;
this.property = property;
this.typeHandler = typeHandler;
this.primitive = primitive;
}
}