MyBatis工作流程简述
原生MyBatis原理分析
初始化工作
解析配置文件
配置类方式
执行SQL
SqlSession API方式
接口方式
真正掌握一个框架源码分析是少不了的~
在讲解整合Spring的原理之前理解原生的MyBatis执行原理是非常有必要的
传统工作模式:
public static void main(String[] args) {
InputStream inputStream = Resources.getResourceAsStream("mybatis-config.xml");
SqlSessionFactory factory = new SqlSessionFactoryBuilder().build(inputStream);
SqlSession sqlSession = factory.openSession();
String name = "tom";
List list = sqlSession.selectList("com.demo.mapper.UserMapper.getUserByName",params);
}
使用Mapper接口:
由于面向接口编程的趋势,MyBatis也实现了通过接口调用mapper配置文件中的SQL语句
public static void main(String[] args) {
//前三步都相同
InputStream inputStream = Resources.getResourceAsStream("mybatis-config.xml");
SqlSessionFactory factory = new SqlSessionFactoryBuilder().build(inputStream);
SqlSession sqlSession = factory.openSession();
//这里不再调用SqlSession 的api,而是获得了接口对象,调用接口中的方法。
UserMapper mapper = sqlSession.getMapper(UserMapper.class);
List list = mapper.getUserByName("tom");
}
InputStream inputStream = Resources.getResourceAsStream("mybatis-config.xml");
//这一行代码正是初始化工作的开始。
SqlSessionFactory factory = new SqlSessionFactoryBuilder().build(inputStream);
进入源码分析:
// 1.我们最初调用的build
public SqlSessionFactory build(InputStream inputStream) {
//调用了重载方法
return build(inputStream, null, null);
}
// 2.调用的重载方法
public SqlSessionFactory build(InputStream inputStream, String environment, Properties properties) {
try {
// XMLConfigBuilder是专门解析mybatis的配置文件的类
XMLConfigBuilder parser = new XMLConfigBuilder(inputStream, environment, properties);
//这里又调用了一个重载方法。parser.parse()的返回值是Configuration对象
return build(parser.parse());
} catch (Exception e) {
throw ExceptionFactory.wrapException("Error building SqlSession.", e);
} //省略部分代码
}
下面进入对配置文件解析部分:
首先对Configuration对象进行介绍:
Configuration对象的结构和xml配置文件的对象几乎相同。
回顾一下xml中的配置标签有哪些:properties(属性),settings(设置),typeAliases(类型别名),typeHandlers(类型处理器),objectFactory(对象工厂),mappers(映射器)等
Configuration也有对应的对象属性来封装它们:
(图片来自:https://blog.csdn.net/luanlouis/article/details/37744073)
也就是说,初始化配置文件信息的本质就是创建Configuration对象,将解析的xml数据封装到Configuration内部的属性中。
//在创建XMLConfigBuilder时,它的构造方法中解析器XPathParser已经读取了配置文件
//3. 进入XMLConfigBuilder 中的 parse()方法。
public Configuration parse() {
if (parsed) {
throw new BuilderException("Each XMLConfigBuilder can only be used once.");
}
parsed = true;
//parser是XPathParser解析器对象,读取节点内数据,是MyBatis配置文件中的顶层标签
parseConfiguration(parser.evalNode("/configuration"));
//最后返回的是Configuration 对象
return configuration;
}
//4. 进入parseConfiguration方法
//此方法中读取了各个标签内容并封装到Configuration中的属性中。
private void parseConfiguration(XNode root) {
try {
//issue #117 read properties first
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);
}
}
到此对xml配置文件的解析就结束了(下文会对部分解析做详细介绍),回到步骤 2. 中调用的重载build方法。
// 5. 调用的重载方法
public SqlSessionFactory build(Configuration config) {
//创建了DefaultSqlSessionFactory对象,传入Configuration对象。
return new DefaultSqlSessionFactory(config);
}
发散一下思路,既然解析xml是对Configuration中的属性进行复制,那么我们同样可以在一个类中创建Configuration对象,手动设置其中属性的值来达到配置的效果。
先简单介绍SqlSession:
SqlSession是一个接口,它有两个实现类:DefaultSqlSession(默认)和SqlSessionManager(弃用,不做介绍)
SqlSession是MyBatis中用于和数据库交互的顶层类
,通常将它与ThreadLocal绑定,一个会话使用一个SqlSession,并且在使用完毕后需要close。
SqlSession中的两个最重要的参数,configuration与初始化时的相同,Executor为执行器,
Executor:
Executor也是一个接口,他有三个常用的实现类BatchExecutor(重用语句并执行批量更新),ReuseExecutor(重用预处理语句prepared statements),SimpleExecutor(普通的执行器,默认)。
继续分析,初始化完毕后,我们就要执行SQL了:
SqlSession sqlSession = factory.openSession();
String name = "tom";
List list = sqlSession.selectList("com.demo.mapper.UserMapper.getUserByName",params);
获得sqlSession
//6. 进入openSession方法。
public SqlSession openSession() {
//getDefaultExecutorType()传递的是SimpleExecutor
return openSessionFromDataSource(configuration.getDefaultExecutorType(), null, false);
}
//7. 进入openSessionFromDataSource。
//ExecutorType 为Executor的类型,TransactionIsolationLevel为事务隔离级别,autoCommit是否开启事务
//openSession的多个重载方法可以指定获得的SeqSession的Executor类型和事务的处理
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);
//根据参数创建指定类型的Executor
final Executor executor = configuration.newExecutor(tx, execType);
//返回的是DefaultSqlSession
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();
}
}
执行sqlsession中的api
//8.进入selectList方法,多个重载方法。
public List selectList(String statement) {
return this.selectList(statement, null);
}
public List selectList(String statement, Object parameter) {
return this.selectList(statement, parameter, RowBounds.DEFAULT);
}
public List selectList(String statement, Object parameter, RowBounds rowBounds) {
try {
//根据传入的全限定名+方法名从映射的Map中取出MappedStatement对象
MappedStatement ms = configuration.getMappedStatement(statement);
//调用Executor中的方法处理
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();
}
}
介绍一下MappedStatement :
mapper.xml文件
或者配置mapper接口
的目录。
这样的一个select标签会在初始化配置文件时
被解析封装成一个MappedStatement
对象,然后存储在Configuration对象的mappedStatements属性中,mappedStatements 是一个HashMap,存储时key = 全限定类名 + 方法名,value = 对应的MappedStatement对象
。
Map mappedStatements = new StrictMap("Mapped Statements collection")
private void parseConfiguration(XNode root) {
try {
// 省略其他标签的处理
mapperElement(root.evalNode("mappers"));
} catch (Exception e) {
throw new BuilderException("Error parsing SQL Mapper Configuration. Cause: " + e, e);
}
}
继续源码中的步骤,进入 executor.query()
//此方法在SimpleExecutor的父类BaseExecutor中实现
public List query(MappedStatement ms, Object parameter, RowBounds rowBounds, ResultHandler resultHandler) throws SQLException {
//根据传入的参数动态获得SQL语句,最后返回用BoundSql对象表示
BoundSql boundSql = ms.getBoundSql(parameter);
//为本次查询创建缓存的Key
CacheKey key = createCacheKey(ms, parameter, rowBounds, boundSql);
return query(ms, parameter, rowBounds, resultHandler, key, boundSql);
}
//进入query的重载方法中
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 {
// 如果缓存中没有本次查找的值,那么从数据库中查询
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;
}
//从数据库查询
private List queryFromDatabase(MappedStatement ms, Object parameter, RowBounds rowBounds, ResultHandler resultHandler, CacheKey key, BoundSql boundSql) throws SQLException {
List list;
localCache.putObject(key, EXECUTION_PLACEHOLDER);
try {
// 查询的方法
list = doQuery(ms, parameter, rowBounds, resultHandler, boundSql);
} finally {
localCache.removeObject(key);
}
// 将查询结果放入缓存
localCache.putObject(key, list);
if (ms.getStatementType() == StatementType.CALLABLE) {
localOutputParameterCache.putObject(key, parameter);
}
return list;
}
// SimpleExecutor中实现父类的doQuery抽象方法
public List doQuery(MappedStatement ms, Object parameter, RowBounds rowBounds, ResultHandler resultHandler, BoundSql boundSql) throws SQLException {
Statement stmt = null;
try {
Configuration configuration = ms.getConfiguration();
// 传入参数创建StatementHanlder对象来执行查询
StatementHandler handler = configuration.newStatementHandler(wrapper, ms, parameter, rowBounds, resultHandler, boundSql);
// 创建jdbc中的statement对象
stmt = prepareStatement(handler, ms.getStatementLog());
// StatementHandler进行处理
return handler.query(stmt, resultHandler);
} finally {
closeStatement(stmt);
}
}
// 创建Statement的方法
private Statement prepareStatement(StatementHandler handler, Log statementLog) throws SQLException {
Statement stmt;
//条代码中的getConnection方法经过重重调用最后会调用openConnection方法,从连接池中获得连接。
Connection connection = getConnection(statementLog);
stmt = handler.prepare(connection, transaction.getTimeout());
handler.parameterize(stmt);
return stmt;
}
//从连接池获得连接的方法
protected void openConnection() throws SQLException {
if (log.isDebugEnabled()) {
log.debug("Opening JDBC Connection");
}
//从连接池获得连接
connection = dataSource.getConnection();
if (level != null) {
connection.setTransactionIsolation(level.getLevel());
}
setDesiredAutoCommit(autoCommit);
}
//进入StatementHandler进行处理的query,StatementHandler中默认的是PreparedStatementHandler
public List query(Statement statement, ResultHandler resultHandler) throws SQLException {
PreparedStatement ps = (PreparedStatement) statement;
//原生jdbc的执行
ps.execute();
//处理结果返回。
return resultSetHandler.handleResultSets(ps);
}
回顾一下写法:
public static void main(String[] args) {
//前三步都相同
InputStream inputStream = Resources.getResourceAsStream("mybatis-config.xml");
SqlSessionFactory factory = new SqlSessionFactoryBuilder().build(inputStream);
SqlSession sqlSession = factory.openSession();
//这里不再调用SqlSession 的api,而是获得了接口对象,调用接口中的方法。
UserMapper mapper = sqlSession.getMapper(UserMapper.class);
List list = mapper.getUserByName("tom");
}
思考一个问题,通常的Mapper接口我们都没有实现的方法却可以使用,是为什么呢?答案很简单 动态代理
开始之前介绍一下MyBatis初始化时对接口的处理:MapperRegistry是Configuration中的一个属性,它内部维护一个HashMap用于存放mapper接口的工厂类
,每个接口对应一个工厂类。mappers中可以配置接口的包路径,或者某个具体的接口类。
//MapperRegistry类
public class MapperRegistry {
private final Configuration config;
//这个类中维护一个HashMap存放MapperProxyFactory
private final Map, MapperProxyFactory>> knownMappers = new HashMap<>();
//解析到接口时添加接口工厂类的方法
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 {
//重点在这行,以接口类的class对象为key,value为其对应的工厂对象,构造方法中指定了接口对象
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);
}
}
}
}
}
正文:
进入sqlSession.getMapper(UserMapper.class)中
//DefaultSqlSession中的getMapper
public T getMapper(Class type) {
return configuration.getMapper(type, this);
}
//configuration中的给getMapper
public T getMapper(Class type, SqlSession sqlSession) {
return mapperRegistry.getMapper(type, sqlSession);
}
//MapperRegistry中的getMapper
public T getMapper(Class type, SqlSession sqlSession) {
//从MapperRegistry中的HashMap中拿MapperProxyFactory
final MapperProxyFactory mapperProxyFactory = (MapperProxyFactory) knownMappers.get(type);
if (mapperProxyFactory == null) {
throw new BindingException("Type " + type + " is not known to the MapperRegistry.");
}
try {
// 通过动态代理工厂生成示例。
return mapperProxyFactory.newInstance(sqlSession);
} catch (Exception e) {
throw new BindingException("Error getting mapper instance. Cause: " + e, e);
}
}
//MapperProxyFactory类中的newInstance方法
public T newInstance(SqlSession sqlSession) {
// 创建了JDK动态代理的Handler类
final MapperProxy mapperProxy = new MapperProxy<>(sqlSession, mapperInterface, methodCache);
// 调用了重载方法
return newInstance(mapperProxy);
}
//MapperProxy类,实现了InvocationHandler接口
public class MapperProxy implements InvocationHandler, Serializable {
//省略部分源码
private final SqlSession sqlSession;
private final Class mapperInterface;
private final Map methodCache;
// 构造,传入了SqlSession,说明每个session中的代理对象的不同的!
public MapperProxy(SqlSession sqlSession, Class mapperInterface, Map methodCache) {
this.sqlSession = sqlSession;
this.mapperInterface = mapperInterface;
this.methodCache = methodCache;
}
//省略部分源码
}
//重载的方法,由动态代理创建新示例返回。
protected T newInstance(MapperProxy mapperProxy) {
return (T) Proxy.newProxyInstance(mapperInterface.getClassLoader(), new Class[] { mapperInterface }, mapperProxy);
}
在动态代理返回了示例后,我们就可以直接调用mapper类中的方法了,说明在MapperProxy中的invoke方法中已经为我们实现了方法。
public Object invoke(Object proxy, Method method, Object[] args) throws Throwable {
try {
//判断调用是是不是Object中定义的方法,toString,hashCode这类非。是的话直接放行。
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);
// 重点在这:MapperMethod最终调用了执行的方法
return mapperMethod.execute(sqlSession, args);
}
public Object execute(SqlSession sqlSession, Object[] args) {
Object result;
//判断mapper中的方法类型,最终调用的还是SqlSession中的方法
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;
}