MyBatis原理系列(一)-手把手带你阅读MyBatis源码
MyBatis原理系列(二)-手把手带你了解MyBatis的启动流程
MyBatis原理系列(三)-手把手带你了解SqlSession,SqlSessionFactory,SqlSessionFactoryBuilder的关系
MyBatis原理系列(四)-手把手带你了解MyBatis的Executor执行器
MyBatis原理系列(五)-手把手带你了解Statement、StatementHandler、MappedStatement间的关系
MyBatis原理系列(六)-手把手带你了解BoundSql的创建过程
MyBatis原理系列(七)-手把手带你了解如何自定义插件
MyBatis原理系列(八)-手把手带你了解一级缓存和二级缓存
MyBatis原理系列(九)-手把手带你了解MyBatis事务管理机制
在上篇文章中，我们举了一个例子如何使用MyBatis，但是对其中dao层，entity层，mapper层间的关系不得而知，从此篇文章开始，笔者将从MyBatis的启动流程着手，真正的开始研究MyBatis源码了。
1. MyBatis启动代码示例
在上篇文章中，介绍了MyBatis的相关配置和各层代码编写，本文将以下代码展开描述和介绍MyBatis的启动流程，并简略的介绍各个模块的作用，各个模块的细节部分将在其它文章中呈现。
回顾下上文中使用mybatis的部分代码，包括七步。每步虽然都是一行代码，但是隐藏了很多细节。接下来我们将围绕这起步展开了解。
@Slf4j
public class MyBatisBootStrap {
public static void main(String[] args) {
try {
// 1. 读取配置
InputStream inputStream = Resources.getResourceAsStream("mybatis-config.xml");
// 2. 创建SqlSessionFactory工厂
SqlSessionFactory sqlSessionFactory = new SqlSessionFactoryBuilder().build(inputStream);
// 3. 获取sqlSession
SqlSession sqlSession = sqlSessionFactory.openSession();
// 4. 获取Mapper
TTestUserMapper userMapper = sqlSession.getMapper(TTestUserMapper.class);
// 5. 执行接口方法
TTestUser userInfo = userMapper.selectByPrimaryKey(16L);
System.out.println("userInfo = " + JSONUtil.toJsonStr(userInfo));
// 6. 提交事物
sqlSession.commit();
// 7. 关闭资源
sqlSession.close();
inputStream.close();
} catch (Exception e){
log.error(e.getMessage(), e);
}
}
}
2. 读取配置
// 1. 读取配置
InputStream inputStream = Resources.getResourceAsStream("mybatis-config.xml");
在mybatis-config.xml中我们配置了属性，环境，映射文件路径等，其实不仅可以配置以上内容，还可以配置插件，反射工厂，类型处理器等等其它内容。在启动流程中的第一步我们就需要读取这个配置文件，并获取一个输入流为下一步解析配置文件作准备。
mybatis-config.xml 内容如下
3. 创建SqlSessionFactory工厂
SqlSessionFactory sqlSessionFactory = new SqlSessionFactoryBuilder().build(inputStream);
我们在学习Java的设计模式时，会学到工厂模式，工厂模式又分为简单工厂模式，工厂方法模式，抽象工厂模式等等。工厂模式就是为了创建对象提供接口，并将创建对象的具体细节屏蔽起来，从而可以提高灵活性。
public interface SqlSessionFactory {
SqlSession openSession();
SqlSession openSession(boolean autoCommit);
SqlSession openSession(Connection connection);
SqlSession openSession(TransactionIsolationLevel level);
SqlSession openSession(ExecutorType execType);
SqlSession openSession(ExecutorType execType, boolean autoCommit);
SqlSession openSession(ExecutorType execType, TransactionIsolationLevel level);
SqlSession openSession(ExecutorType execType, Connection connection);
Configuration getConfiguration();
}
由此可知SqlSessionFactory工厂是为了创建一个对象而生的，其产出的对象就是SqlSession对象。SqlSession是MyBatis面向数据库的高级接口，其提供了执行查询sql，更新sql，提交事物，回滚事物，获取映射代理类等等方法。
在此笔者列出了主要方法，一些重载的方法就过滤掉了。
public interface SqlSession extends Closeable {
/**
* 查询一个结果对象
**/
T selectOne(String statement, Object parameter);
/**
* 查询一个结果集合
**/
List selectList(String statement, Object parameter, RowBounds rowBounds);
/**
* 查询一个map
**/
Map selectMap(String statement, Object parameter, String mapKey, RowBounds rowBounds);
/**
* 查询游标
**/
Cursor selectCursor(String statement, Object parameter, RowBounds rowBounds);
void select(String statement, Object parameter, RowBounds rowBounds, ResultHandler handler);
/**
* 插入
**/
int insert(String statement, Object parameter);
/**
* 修改
**/
int update(String statement, Object parameter);
/**
* 删除
**/
int delete(String statement, Object parameter);
/**
* 提交事物
**/
void commit(boolean force);
/**
* 回滚事物
**/
void rollback(boolean force);
List flushStatements();
void close();
void clearCache();
Configuration getConfiguration();
/**
* 获取映射代理类
**/
T getMapper(Class type);
/**
* 获取数据库连接
**/
Connection getConnection();
}
回到开始，SqlSessionFactory工厂是怎么创建的出来的呢？SqlSessionFactoryBuilder就是创建者，以Builder结尾我们很容易想到了Java设计模式中的建造者模式，一个对象的创建是由众多复杂对象组成的，建造者模式就是一个创建复杂对象的选择，它与工厂模式相比，建造者模式更加关注零件装配的顺序。
public class SqlSessionFactoryBuilder {
public SqlSessionFactory build(InputStream inputStream, String environment, Properties properties) {
try {
XMLConfigBuilder parser = new XMLConfigBuilder(inputStream, environment, properties);
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.
}
}
}
}
其中XMLConfigBuilder就是解析mybatis-config.xml中每个标签的内容，parse()方法返回的就是一个Configuration对象.Configuration也是MyBatis中一个很重要的组件，包括插件，对象工厂，反射工厂，映射文件，类型解析器等等都存储在Configuration对象中。
public Configuration parse() {
if (parsed) {
throw new BuilderException("Each XMLConfigBuilder can only be used once.");
}
parsed = true;
parseConfiguration(parser.evalNode("/configuration"));
return configuration;
}
private void parseConfiguration(XNode root) {
try {
// issue #117 read properties first
// 解析properties节点
propertiesElement(root.evalNode("properties"));
Properties settings = settingsAsProperties(root.evalNode("settings"));
loadCustomVfs(settings);
loadCustomLogImpl(settings);
typeAliasesElement(root.evalNode("typeAliases"));
pluginElement(root.evalNode("plugins"));
objectFactoryElement(root.evalNode("objectFactory"));
objectWrapperFactoryElement(root.evalNode("objectWrapperFactory"));
reflectorFactoryElement(root.evalNode("reflectorFactory"));
settingsElement(settings);
// read it after objectFactory and objectWrapperFactory issue #631
environmentsElement(root.evalNode("environments"));
databaseIdProviderElement(root.evalNode("databaseIdProvider"));
typeHandlerElement(root.evalNode("typeHandlers"));
mapperElement(root.evalNode("mappers"));
} catch (Exception e) {
throw new BuilderException("Error parsing SQL Mapper Configuration. Cause: " + e, e);
}
}
在获取到Configuration对象后，SqlSessionFactoryBuilder就会创建一个DefaultSqlSessionFactory对象，DefaultSqlSessionFactory是SqlSessionFactory的一个默认实现，还有一个实现是SqlSessionManager。
public SqlSessionFactory build(Configuration config) {
return new DefaultSqlSessionFactory(config);
}
创建SqlSessionFactory过程
4. 获取sqlSession
// 3. 获取sqlSession
SqlSession sqlSession = sqlSessionFactory.openSession();
在前面我们讲到，sqlSession是操作数据库的高级接口，我们操作数据库都是通过这个接口操作的。获取sqlSession有两种方式，一种是从数据源中获取的，还有一种是从连接中获取。
获取到的都是DefaultSqlSession对象，也就是sqlSession的默认实现。
private SqlSession openSessionFromDataSource(ExecutorType execType, TransactionIsolationLevel level, boolean autoCommit) {
Transaction tx = null;
try {
final Environment environment = configuration.getEnvironment();
final TransactionFactory transactionFactory = getTransactionFactoryFromEnvironment(environment);
tx = transactionFactory.newTransaction(environment.getDataSource(), level, autoCommit);
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();
}
}
private SqlSession openSessionFromConnection(ExecutorType execType, Connection connection) {
try {
boolean autoCommit;
try {
autoCommit = connection.getAutoCommit();
} catch (SQLException e) {
// Failover to true, as most poor drivers
// or databases won't support transactions
autoCommit = true;
}
final Environment environment = configuration.getEnvironment();
final TransactionFactory transactionFactory = getTransactionFactoryFromEnvironment(environment);
final Transaction tx = transactionFactory.newTransaction(connection);
final Executor executor = configuration.newExecutor(tx, execType);
return new DefaultSqlSession(configuration, executor, autoCommit);
} catch (Exception e) {
throw ExceptionFactory.wrapException("Error opening session. Cause: " + e, e);
} finally {
ErrorContext.instance().reset();
}
}
获取SqlSession步骤
5. 获取Mapper代理类
在上一步获取到sqlSession后，我们接下来就获取到了mapper代理类。
// 4. 获取Mapper
TTestUserMapper userMapper = sqlSession.getMapper(TTestUserMapper.class);
这个getMapper方法，我们看看DefaultSqlSession是怎么做的
DefaultSqlSession 的 getMapper 方法
public T getMapper(Class type) {
return this.configuration.getMapper(type, this);
}
Configuration 的 getMapper 方法
public T getMapper(Class type, SqlSession sqlSession) {
return this.mapperRegistry.getMapper(type, sqlSession);
}
MapperRegistry 中有个getMapper方法，实际上是从成员变量knownMappers中获取的，这个knownMappers是个key-value形式的缓存，key是mapper接口的class对象，value是MapperProxyFactory代理工厂，这个工厂就是用来创建MapperProxy代理类的。
public class MapperRegistry {
private final Configuration config;
private final Map, MapperProxyFactory> knownMappers = new HashMap();
public MapperRegistry(Configuration config) {
this.config = config;
}
public T getMapper(Class type, SqlSession sqlSession) {
MapperProxyFactory mapperProxyFactory = (MapperProxyFactory)this.knownMappers.get(type);
if (mapperProxyFactory == null) {
throw new BindingException("Type " + type + " is not known to the MapperRegistry.");
} else {
try {
return mapperProxyFactory.newInstance(sqlSession);
} catch (Exception var5) {
throw new BindingException("Error getting mapper instance. Cause: " + var5, var5);
}
}
}
}
如果对java动态代理了解的同学就知道，Proxy.newProxyInstance()方法可以创建出一个目标对象一个代理对象。由此可知每次调用getMapper方法都会创建出一个代理类出来。
public class MapperProxyFactory {
private final Class mapperInterface;
private final Map methodCache = new ConcurrentHashMap();
public MapperProxyFactory(Class mapperInterface) {
this.mapperInterface = mapperInterface;
}
public Class getMapperInterface() {
return this.mapperInterface;
}
public Map getMethodCache() {
return this.methodCache;
}
protected T newInstance(MapperProxy mapperProxy) {
return Proxy.newProxyInstance(this.mapperInterface.getClassLoader(), new Class[]{this.mapperInterface}, mapperProxy);
}
public T newInstance(SqlSession sqlSession) {
MapperProxy mapperProxy = new MapperProxy(sqlSession, this.mapperInterface, this.methodCache);
return this.newInstance(mapperProxy);
}
}
回到上面，那这个MapperProxyFactory是怎么加载到MapperRegistry的knownMappers缓存中的呢？
在上面的Configuration类的parseConfiguration方法中，我们会解析 mappers标签，mapperElement方法就会解析mapper接口。
private void parseConfiguration(XNode root) {
try {
// issue #117 read properties first
// 解析properties节点
propertiesElement(root.evalNode("properties"));
Properties settings = settingsAsProperties(root.evalNode("settings"));
loadCustomVfs(settings);
loadCustomLogImpl(settings);
typeAliasesElement(root.evalNode("typeAliases"));
pluginElement(root.evalNode("plugins"));
objectFactoryElement(root.evalNode("objectFactory"));
objectWrapperFactoryElement(root.evalNode("objectWrapperFactory"));
reflectorFactoryElement(root.evalNode("reflectorFactory"));
settingsElement(settings);
// read it after objectFactory and objectWrapperFactory issue #631
environmentsElement(root.evalNode("environments"));
databaseIdProviderElement(root.evalNode("databaseIdProvider"));
typeHandlerElement(root.evalNode("typeHandlers"));
mapperElement(root.evalNode("mappers"));
} catch (Exception e) {
throw new BuilderException("Error parsing SQL Mapper Configuration. Cause: " + e, e);
}
}
private void mapperElement(XNode parent) throws Exception {
if (parent != null) {
for (XNode child : parent.getChildren()) {
if ("package".equals(child.getName())) {
String mapperPackage = child.getStringAttribute("name");
configuration.addMappers(mapperPackage);
} else {
String resource = child.getStringAttribute("resource");
String url = child.getStringAttribute("url");
String mapperClass = child.getStringAttribute("class");
if (resource != null && url == null && mapperClass == null) {
ErrorContext.instance().resource(resource);
InputStream inputStream = Resources.getResourceAsStream(resource);
XMLMapperBuilder mapperParser = new XMLMapperBuilder(inputStream, configuration, resource, configuration.getSqlFragments());
mapperParser.parse();
} else if (resource == null && url != null && mapperClass == null) {
ErrorContext.instance().resource(url);
InputStream inputStream = Resources.getUrlAsStream(url);
XMLMapperBuilder mapperParser = new XMLMapperBuilder(inputStream, configuration, url, configuration.getSqlFragments());
mapperParser.parse();
} else if (resource == null && url == null && mapperClass != null) {
Class mapperInterface = Resources.classForName(mapperClass);
configuration.addMapper(mapperInterface);
} else {
throw new BuilderException("A mapper element may only specify a url, resource or class, but not more than one.");
}
}
}
}
}
解析完后，就讲这个mapper接口加到 mapperRegistry中，
configuration.addMapper(mapperInterface);
Configuration的addMapper方法
public void addMapper(Class type) {
mapperRegistry.addMapper(type);
}
最后还是加载到了MapperRegistry的knownMappers中去了
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 {
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.
MapperAnnotationBuilder parser = new MapperAnnotationBuilder(config, type);
parser.parse();
loadCompleted = true;
} finally {
if (!loadCompleted) {
knownMappers.remove(type);
}
}
}
}
获取mapper代理类过程
6. 执行mapper接口方法
// 5. 执行接口方法
TTestUser userInfo = userMapper.selectByPrimaryKey(16L);
selectByPrimaryKey是TTestUserMapper接口中定义的一个方法，但是我们没有编写TTestUserMapper接口的的实现类，那么Mybatis是怎么帮我们执行的呢？前面讲到，获取mapper对象时，是会获取到一个MapperProxyFactory工厂类，并创建一个MapperProxy代理类，在执行Mapper接口的方法时，会调用MapperProxy的invoke方法。
@Override
public Object invoke(Object proxy, Method method, Object[] args) throws Throwable {
try {
if (Object.class.equals(method.getDeclaringClass())) {
return method.invoke(this, args);
} else {
return cachedInvoker(method).invoke(proxy, method, args, sqlSession);
}
} catch (Throwable t) {
throw ExceptionUtil.unwrapThrowable(t);
}
}
如果是Object的方法就直接执行，否则执行cachedInvoker(method).invoke(proxy, method, args, sqlSession); 这行代码，到这里，想必有部分同学已经头晕了吧。怎么又来了个invoke方法。
cachedInvoker 是返回缓存的MapperMethodInvoker对象，MapperMethodInvoker的invoke方法会执行MapperMethod的execute方法。
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;
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);
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;
}
}
然后根据执行的接口找到mapper.xml中配置的sql，并处理参数，然后执行返回结果处理结果等步骤。
7. 提交事物
// 6. 提交事物
sqlSession.commit();
事物就是将若干数据库操作看成一个单元，要么全部成功，要么全部失败，如果失败了，则会执行执行回滚操作，恢复到开始执行的数据库状态。
8. 关闭资源
// 7. 关闭资源
sqlSession.close();
inputStream.close();
sqlSession是种共用资源，用完了要返回到池子中，以供其它地方使用。
9. 总结
至此我们已经大致了解了Mybatis启动时的大致流程，很多细节都还没有详细介绍，这是因为涉及到的层面又深又广，如果在一篇文章中介绍，反而会让读者如置云里雾里，不知所云。因此，在接下来我将每个模块的详细介绍。如果文章有什么错误或者需要改进的，希望同学们指出来，希望对大家有帮助。