一、前言

來看這篇文章的應該都知道，在沒有出現Hibernate和MyBatis框架時，我們要訪問數據庫底層，都得使用JDBC來連接及操作數據庫。

用過JDBC的都知道使用很繁雜，所以就誕生了Hibernate和Mybatis這種ORM（對象映射關系）框架，其實他們都是對操作數據庫底層（JDBC）的二次封裝，要使用ORM框架只需要引入對應的jar包即可。

這篇文章我們來詳細的分析一下Mybatis的底層實現原理和工作流程。

二、Mybatis核心組件

1、SqlSessionFactoryBuilder （構造器）

用Builder模式根據mybatis-config.xml配置或者代碼來生成SqISessionFactory。

2、SqlSessionFactory （工廠接口）

采用工廠模式生成SqlSession。

3、SqlSession （會話）

一個既可以發送 SQL 執行返回結果，也可以獲取MApper的接口。

4、SQL Mapper （映射器）

它由一個JAVA接口和XML文件（或注解）構成，需要給出對應的SQL和映射規則，它負責發送SQL去執行，并返回結果。

5、Executor（執行器）

三、Mybatis的工作流程

Mybatis工作流程簡述：

1、通過SqlSessionFactoryBuilder構建SqlSessionFactory工廠。

2、通過SqlSessionFactory構建SqlSession會話對象。

3、通過SqlSession拿到Mapper代理對象（用到了動態代理）。

4、通過MapperProxy調用Mapper中增刪改查的方法，然后將編譯后的sql拿到數據庫執行。

Mybatis工作流程附圖：

四、Mybatis源碼解析

1、SqlSessionFactoryBuilder創建SqlSessionFactory工廠對象

SqlSessionFactoryBuilder中的build()方法分析

public SqlSessionFactory build(InputStream inputStream) {
return build(inputStream, null, null);
}

public SqlSessionFactory build(InputStream inputStream, String environment, Properties properties) {
try {
XMLConfigBuilder parser = new XMLConfigBuilder(inputStream, environment, properties);
//解析mybatis-config.xml
return build(parser.parse());
} catch (Exception e) {
throw ExceptionFactory.wrapException("Error building SqlSession.", e);
} finally {
ErrorContext.instance().reset();
try {
inputStream.close();
} catch (IOException e) {
// Intentionally ignore. Prefer previous error.
}
}
}

//返回SqlSessionFactory，默認使用的是實現類DefaultSqlSessionFactory
public SqlSessionFactory build(Configuration config) {
return new DefaultSqlSessionFactory(config);
}

public Configuration parse() {
if (parsed) {
throw new BuilderException("Each XMLConfigBuilder can only be used once.");
}
parsed = true;
//獲取根節點configuration
parseConfiguration(parser.evalNode("/configuration"));
return configuration;
}

//開始解析mybatis-config.xml,并把解析后的數據存放到configuration中
private void parseConfiguration(XNode root) {
try {
//保存mybatis-config.xml中的標簽setting,本例中開啟全局緩存cacheEnabled，設置默認執行器defaultExecutorType=REUSE
Properties settings = settingsAsPropertiess(root.evalNode("settings"));
//issue #117 read properties first
//解析是否配置了外部properties，例如本例中配置的jdbc.propertis
propertiesElement(root.evalNode("properties"));
//查看是否配置了VFS，默認沒有，本例也沒有使用
loadCustomVfs(settings);
//查看是否用了類型別名，減少完全限定名的冗余，本例中使用了別名User代替了com.ctc.Model.User
typeAliasesElement(root.evalNode("typeAliases"));
//查看是否配置插件來攔截映射語句的執行，例如攔截Executor的Update方法，本例沒有使用
pluginElement(root.evalNode("plugins"))
//查看是否配置了ObjectFactory，默認情況下使用對象的無參構造方法或者是帶有參數的構造方法，本例沒有使用
objectFactoryElement(root.evalNode("objectFactory"));
//查看是否配置了objectWrapperFatory,這個用來或者ObjectWapper，可以訪問：對象，Collection，Map屬性。本例沒有使用
objectWrapperFactoryElement(root.evalNode("objectWrapperFactory"));
//查看是否配置了reflectorFactory,mybatis的反射工具，提供了很多反射方法。本例沒有使用
reflectorFactoryElement(root.evalNode("reflectorFactory"));
//放入參數到configuration對象中
settingsElement(settings);
// read it after objectFactory and objectWrapperFactory issue #631
//查看數據庫環境配置
environmentsElement(root.evalNode("environments"));
//查看是否使用多種數據庫，本例沒有使用
databaseIdProviderElement(root.evalNode("databaseIdProvider"));
//查看是否配置了新的類型處理器，如果跟處理的類型跟默認的一致就會覆蓋。本例沒有使用
typeHandlerElement(root.evalNode("typeHandlers"));
//查看是否配置SQL映射文件,有四種配置方式，resource，url，class以及自動掃包package。本例使用package
mapperElement(root.evalNode("mappers"));
} catch (Exception e) {
throw new BuilderException("Error parsing SQL Mapper Configuration. Cause: " + e, e);
}
}

從源碼中可以看出，最終會返回一個DefautSqlSessionFactory對象。

2、通過SqlSessionFactory構建SqlSession會話對象。

@Override
public SqlSession openSession() {//打開會話
return openSessionFromDataSource(configuration.getDefaultExecutorType(), null, false);
}


private SqlSession openSessionFromDataSource(ExecutorType execType, TransactionIsolationLevel level, boolean autoCommit) {
Transaction tx = null;
try {
//拿到從mybatis中解析到的數據庫環境配置
final Environment environment = configuration.getEnvironment();
final TransactionFactory transactionFactory = getTransactionFactoryFromEnvironment(environment);

  //拿到jdbc的事務管理器，有兩種一種是jbc,一種的managed。
tx = transactionFactory.newTransaction(environment.getDataSource(), level, autoCommit);
  
//這里返回的是ReuseExecutor并把事務傳入對象中
final Executor executor = configuration.newExecutor(tx, execType);
return new DefaultSqlSession(configuration, executor, autoCommit);
} catch (Exception e) {
closeTransaction(tx); // may have fetched a connection so lets call close()
throw ExceptionFactory.wrapException("Error opening session. Cause: " + e, e);
} finally {
ErrorContext.instance().reset();
}
}


public Executor newExecutor(Transaction transaction, ExecutorType executorType) {
executorType = executorType == null ? defaultExecutorType : executorType;
executorType = executorType == null ? ExecutorType.SIMPLE : executorType;
Executor executor;
if (ExecutorType.BATCH == executorType) {
executor = new BatchExecutor(this, transaction);
} else if (ExecutorType.REUSE == executorType) {
executor = new ReuseExecutor(this, transaction);
} else {
executor = new SimpleExecutor(this, transaction);
}
if (cacheEnabled) {
executor = new CachingExecutor(executor);
}
executor = (Executor) interceptorChain.pluginAll(executor);
return executor;
}

//返回一個SqlSession，默認使用DefaultSqlSession
public DefaultSqlSession(Configuration configuration, Executor executor, boolean autoCommit) {
this.configuration = configuration;
this.executor = executor;
this.dirty = false;
this.autoCommit = autoCommit;
}

這段源碼顯示最終會返回一個SqlSession會話對象。

3、通過SqlSession拿到Mapper代理對象（用到了動態代理）。

@Override
public <T> T getMapper(Class<T> type) {
return configuration.<T>getMapper(type, this);
}

public <T> T getMapper(Class<T> type, SqlSession sqlSession) {
return mapperRegistry.getMapper(type, sqlSession);
}

@SuppressWarnings("unchecked")
public <T> T getMapper(Class<T> type, SqlSession sqlSession) {
//knownMapper是一個HashMap在存放mapperRegistry的過程中，以每個Mapper對象的類型為Key, MapperProxyFactory 為value保存。
final MapperProxyFactory<T> mapperProxyFactory = (MapperProxyFactory<T>) knownMappers.get(type);
if (mapperProxyFactory == null) {
throw new BindingException("Type " + type + " is not known to the MapperRegistry.");
}
try {
return mapperProxyFactory.newInstance(sqlSession);
} catch (Exception e) {
throw new BindingException("Error getting mapper instance. Cause: " + e, e);
}
}

//這一塊就用到了動態代理
public T newInstance(SqlSession sqlSession) {
//生成一個mapperProxy對象，這個對象實現了InvocationHandler, Serializable。就是JDK動態代理中的方法調用處理器
final MapperProxy<T> mapperProxy = new MapperProxy<T>(sqlSession, mapperInterface, methodCache);
return newInstance(mapperProxy);
}

public MapperProxy(SqlSession sqlSession, Class<T> mapperInterface, Map<Method, MapperMethod> methodCache) {
this.sqlSession = sqlSession;
this.mapperInterface = mapperInterface;
this.methodCache = methodCache;
}

@SuppressWarnings("unchecked")
protected T newInstance(MapperProxy<T> mapperProxy) {
//通過JDK動態代理生成一個Mapper的代理，在本例中的就是UserMapper的代理類，它實現了UserMapper接口
return (T) Proxy.newProxyInstance(mapperInterface.getClassLoader(), new Class[] { mapperInterface }, mapperProxy);
}

這段源碼就釋義了怎么拿到Mapper代理對象。

4、通過MapperProxy調用Mapper中增刪改查的方法，然后將編譯后的sql拿到數據庫執行。

@Override
public Object invoke(Object proxy, Method method, Object[] args) throws Throwable {
//判斷當前調用的method是不是Object中聲明的方法，如果是的話直接執行。
if (Object.class.equals(method.getDeclaringClass())) {
try {
return method.invoke(this, args);
} catch (Throwable t) {
throw ExceptionUtil.unwrapThrowable(t);
}
}
final MapperMethod mapperMethod = cachedMapperMethod(method);
return mapperMethod.execute(sqlSession, args);
}

//把當前請求放入一個HashMap中，一旦下次還是同樣的方法進來直接返回。
private MapperMethod cachedMapperMethod(Method method) {
MapperMethod mapperMethod = methodCache.get(method);
if (mapperMethod == null) {
mapperMethod = new MapperMethod(mapperInterface, method, sqlSession.getConfiguration());
methodCache.put(method, mapperMethod);
}
return 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:
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 {
Object param = method.convertArgsToSqlCommandParam(args);
result = sqlSession.selectOne(command.getName(), param);
}
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;
}

@Override
public <T> T selectOne(String statement, Object parameter) {
// Popular vote was to return null on 0 results and throw exception on too many.
List<T> list = this.<T>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, RowBounds rowBounds) {
try {
MappedStatement ms = configuration.getMappedStatement(statement);
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();
}
}

//這邊調用的是CachingExecutor類的query
@Override
public <E> List<E> query(MappedStatement ms, Object parameterObject, RowBounds rowBounds, ResultHandler resultHandler) throws SQLException {
BoundSql boundSql = ms.getBoundSql(parameterObject);
CacheKey key = createCacheKey(ms, parameterObject, rowBounds, boundSql);
return query(ms, parameterObject, rowBounds, resultHandler, key, boundSql);
}

@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, parameterObject, boundSql);
@SuppressWarnings("unchecked")
List<E> list = (List<E>) tcm.getObject(cache, key);
if (list == null) {
//如果緩存中沒有數據則查詢數據庫
list = delegate.<E> query(ms, parameterObject, rowBounds, resultHandler, key, boundSql);
//結果集放入緩存
tcm.putObject(cache, key, list); 
}
return list;
}
}
return delegate.<E> query(ms, parameterObject, rowBounds, resultHandler, key, boundSql);
}