Mybatis簡介：
MyBatis 是一款優(yōu)秀的持久層框架笙什，它支持定制化 SQL、存儲過程以及高級映射睦擂。MyBatis 避免了幾乎所有的 JDBC 代碼和手動設置參數(shù)以及獲取結果集得湘。MyBatis 可以使用簡單的 XML 或注解來配置和映射原生信息，將接口和 Java 的 POJOs(Plain Old Java Objects,普通的 Java對象)映射成數(shù)據(jù)庫中的記錄顿仇。本文將通過debug的方式來了解其工作原理淘正。
Mybatis核心類：
SqlSessionFactory：每個基于 MyBatis 的應用都是以一個 SqlSessionFactory 的實例為中心的摆马。SqlSessionFactory 的實例可以通過 SqlSessionFactoryBuilder 獲得。而 SqlSessionFactoryBuilder 則可以從 XML 配置文件或通過Java的方式構建出 SqlSessionFactory 的實例鸿吆。SqlSessionFactory 一旦被創(chuàng)建就應該在應用的運行期間一直存在囤采，建議使用單例模式或者靜態(tài)單例模式。一個SqlSessionFactory對應配置文件中的一個環(huán)境（environment）惩淳，如果你要使用多個數(shù)據(jù)庫就配置多個環(huán)境分別對應一個SqlSessionFactory蕉毯。
SqlSession：SqlSession是一個接口，它有2個實現(xiàn)類思犁，分別是DefaultSqlSession(默認使用)以及SqlSessionManager代虾。SqlSession通過內(nèi)部存放的執(zhí)行器（Executor）來對數(shù)據(jù)進行CRUD。此外SqlSession不是線程安全的激蹲，因為每一次操作完數(shù)據(jù)庫后都要調(diào)用close對其進行關閉棉磨，官方建議通過try-finally來保證總是關閉SqlSession。
Executor：Executor（執(zhí)行器）接口有兩個實現(xiàn)類学辱，其中BaseExecutor有三個繼承類分別是BatchExecutor（重用語句并執(zhí)行批量更新）乘瓤，ReuseExecutor（重用預處理語句prepared statements），SimpleExecutor（普通的執(zhí)行器）策泣。以上三個就是主要的Executor衙傀。通過下圖可以看到Mybatis在Executor的設計上面使用了裝飾者模式，我們可以用CachingExecutor來裝飾前面的三個執(zhí)行器目的就是用來實現(xiàn)緩存萨咕。
MappedStatement：MappedStatement就是用來存放我們SQL映射文件中的信息包括sql語句统抬，輸入?yún)?shù)，輸出參數(shù)等等任洞。一個SQL節(jié)點對應一個MappedStatement對象蓄喇。
Mybatis工作流程：
閱讀全文有驚喜哦７⑶帧＝惶汀！
下面將通過debug方式對Mybatis進行一步步解析刃鳄。首先貼出我的mybatis-config.xml文件以及Mapper.xml文件盅弛。
PUBLIC "-//mybatis.org//DTD Config 3.0//EN"
"http://mybatis.org/dtd/mybatis-3-config.dtd">
PUBLIC "-//mybatis.org//DTD Mapper 3.0//EN"
"http://mybatis.org/dtd/mybatis-3-mapper.dtd">
select * from user
User where id = #{id}
insert into User (username,birthday,sex,address)
values (#{name},#{birthday},#{sex},#{address})
update User set username = #{username},birthday = #{birthday},
sex = #{sex},address = #{address} where id = #{id}
delete from User where id = #{id}
select * from User where sex = #{param1}
and username like #{param2}
and address = #{parma3}
select count(*) from user where username like #{username}
username like #{pattern}
and sex = #{sex}
and address = #{address}
where id in
{id}
第一步通過SqlSessionFactoryBuilder創(chuàng)建SqlSessionFactory：
首先在SqlSessionFactoryBuilder的build（）方法中可以看到MyBatis內(nèi)部定義了一個類XMLConfigBuilder用來解析配置文件mybatis-config.xml。針對配置文件中的每一個節(jié)點進行解析并將數(shù)據(jù)存放到Configuration這個對象中叔锐，緊接著使用帶有Configuration的構造方法發(fā)返回一個DefautSqlSessionFactory挪鹏。
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(xiàn)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(jié)點configuration
parseConfiguration(parser.evalNode("/configuration"));
return configuration;
}
//開始解析mybatis-config.xml,并把解析后的數(shù)據(jù)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愉烙，設置默認執(zhí)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讨盒，例如本例中配置的jdbc.propertis
propertiesElement(root.evalNode("properties"));
//查看是否配置了VFS，默認沒有步责，本例也沒有使用
loadCustomVfs(settings);
//查看是否用了類型別名返顺，減少完全限定名的冗余禀苦，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(zhí)行，例如攔截Executor的Update方法遂鹊，本例沒有使用
pluginElement(root.evalNode("plugins"))
//查看是否配置了ObjectFactory振乏，默認情況下使用對象的無參構造方法或者是帶有參數(shù)的構造方法峡钓，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怨愤，可以訪問：對象，Collection涂召，Map屬性舟陆。本例沒有使用
objectWrapperFactoryElement(root.evalNode("objectWrapperFactory"));
//查看是否配置了reflectorFactory,mybatis的反射工具误澳，提供了很多反射方法。本例沒有使用
reflectorFactoryElement(root.evalNode("reflectorFactory"));
//放入?yún)?shù)到configuration對象中
settingsElement(settings);
// read it after objectFactory and objectWrapperFactory issue #631
//查看數(shù)據(jù)庫環(huán)境配置
environmentsElement(root.evalNode("environments"));
//查看是否使用多種數(shù)據(jù)庫秦躯，本例沒有使用
databaseIdProviderElement(root.evalNode("databaseIdProvider"));
//查看是否配置了新的類型處理器脓匿，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,有四種配置方式宦赠，resource陪毡，url，class以及自動掃包package勾扭。本例使用package
mapperElement(root.evalNode("mappers"));
} catch (Exception e) {
throw new BuilderException("Error parsing SQL Mapper Configuration. Cause: " + e, e);
}
}
第二步通過SqlSessionFactory創(chuàng)建SqlSession：
@Override
public SqlSession openSession() {
return openSessionFromDataSource(configuration.getDefaultExecutorType(), null, false);
}
private SqlSession openSessionFromDataSource(ExecutorType execType, TransactionIsolationLevel level, boolean autoCommit) {
Transaction tx = null;
try {
//拿到前文從mybatis中解析到的數(shù)據(jù)庫環(huán)境配置
final Environment environment = configuration.getEnvironment();
final TransactionFactory transactionFactory = getTransactionFactoryFromEnvironment(environment);
//拿到jdbc的事務管理器毡琉，有兩種一種是jbc,一種的managed。本例使用的是JdbcTransaction
tx = transactionFactory.newTransaction(environment.getDataSource(), level, autoCommit);
//從mybatis配置文件可以看到本例使用了REUSE妙色，因此返回的是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桅滋，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;
}
第三步通過SqlSession拿到Mapper對象的代理：
@Override
public T getMapper(Class type) {
return configuration.getMapper(type, this);
}
public T getMapper(Class type, SqlSession sqlSession) {
//前文解析Mybatis-config.xml的時候，在解析標簽mapper就是用configuration對象的mapperRegistry存放數(shù)據(jù)
return mapperRegistry.getMapper(type, sqlSession);
}
@SuppressWarnings("unchecked")
public T getMapper(Class type, SqlSession sqlSession) {
//knownMapper是一個HashMap在存放mapperRegistry的過程中身辨，以每個Mapper對象的類型為Key, MapperProxyFactory 為value保存丐谋。
//例如本例中保存的就是Key:com.ctc.mapper.UserMapper,value就是保存了key的MapperProxyFactory對象
final MapperProxyFactory mapperProxyFactory = (MapperProxyFactory) 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);
}
}
public T newInstance(SqlSession sqlSession) {
//生成一個mapperProxy對象，這個對象實現(xiàn)了InvocationHandler, Serializable煌珊。就是JDK動態(tài)代理中的方法調(diào)用處理器
final MapperProxy mapperProxy = new MapperProxy(sqlSession, mapperInterface, methodCache);
return newInstance(mapperProxy);
}
public MapperProxy(SqlSession sqlSession, Class mapperInterface, Map methodCache) {
this.sqlSession = sqlSession;
this.mapperInterface = mapperInterface;
this.methodCache = methodCache;
}
@SuppressWarnings("unchecked")
protected T newInstance(MapperProxy mapperProxy) {
//通過JDK動態(tài)代理生成一個Mapper的代理号俐，在本例中的就是UserMapper的代理類，它實現(xiàn)了UserMapper接口
return (T) Proxy.newProxyInstance(mapperInterface.getClassLoader(), new Class[] { mapperInterface }, mapperProxy);
}
第四步通過MapperProxy調(diào)用Maper中相應的方法：
@Override
public Object invoke(Object proxy, Method method, Object[] args) throws Throwable {
//判斷當前調(diào)用的method是不是Object中聲明的方法定庵，如果是的話直接執(zhí)行吏饿。
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蔬浙。
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 {
//本次案例會執(zhí)行selectOne
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(diào)用的是CachingExecutor類的query,還記得前文解析mybatis-config.xml的時候我們指定了REUSE但是因為在配置文件中開啟了緩存
//所以ReuseExecutor被CachingExecotur裝飾猪落，新增了緩存的判斷，最后還是會調(diào)用ReuseExecutor
@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(shù)據(jù)則查詢數(shù)據(jù)庫
list = delegate.<E> query(ms, parameterObject, rowBounds, resultHandler, key, boundSql);
//結果集放入緩存
tcm.putObject(cache, key, list); // issue #578 and #116
}
return list;
}
}
return delegate.<E> query(ms, parameterObject, rowBounds, resultHandler, key, boundSql);
}
MyBatis 是一款優(yōu)秀的持久層框架畴博，它支持定制化 SQL笨忌、存儲過程以及高級映射。MyBatis 避免了幾乎所有的 JDBC 代碼和手動設置參數(shù)以及獲取結果集俱病。MyBatis 可以使用簡單的 XML 或注解來配置和映射原生信息官疲，將接口和 Java 的 POJOs(Plain Old Java Objects,普通的 Java對象)映射成數(shù)據(jù)庫中的記錄杂曲。本文將通過debug的方式來了解其工作原理。
Mybatis核心類：
SqlSessionFactory：每個基于 MyBatis 的應用都是以一個 SqlSessionFactory 的實例為中心的袁余。SqlSessionFactory 的實例可以通過 SqlSessionFactoryBuilder 獲得擎勘。而 SqlSessionFactoryBuilder 則可以從 XML 配置文件或通過Java的方式構建出 SqlSessionFactory 的實例。SqlSessionFactory 一旦被創(chuàng)建就應該在應用的運行期間一直存在颖榜，建議使用單例模式或者靜態(tài)單例模式棚饵。一個SqlSessionFactory對應配置文件中的一個環(huán)境（environment），如果你要使用多個數(shù)據(jù)庫就配置多個環(huán)境分別對應一個SqlSessionFactory掩完。
SqlSession：SqlSession是一個接口噪漾，它有2個實現(xiàn)類，分別是DefaultSqlSession(默認使用)以及SqlSessionManager且蓬。SqlSession通過內(nèi)部存放的執(zhí)行器（Executor）來對數(shù)據(jù)進行CRUD欣硼。此外SqlSession不是線程安全的，因為每一次操作完數(shù)據(jù)庫后都要調(diào)用close對其進行關閉恶阴，官方建議通過try-finally來保證總是關閉SqlSession诈胜。
Executor：Executor（執(zhí)行器）接口有兩個實現(xiàn)類，其中BaseExecutor有三個繼承類分別是BatchExecutor（重用語句并執(zhí)行批量更新）冯事，ReuseExecutor（重用預處理語句prepared statements）焦匈，SimpleExecutor（普通的執(zhí)行器）。以上三個就是主要的Executor昵仅。通過下圖可以看到Mybatis在Executor的設計上面使用了裝飾者模式缓熟，我們可以用CachingExecutor來裝飾前面的三個執(zhí)行器目的就是用來實現(xiàn)緩存。
MappedStatement：MappedStatement就是用來存放我們SQL映射文件中的信息包括sql語句摔笤，輸入?yún)?shù)够滑，輸出參數(shù)等等。一個SQL節(jié)點對應一個MappedStatement對象吕世。
Mybatis工作流程：
下面將通過debug方式對Mybatis進行一步步解析彰触。首先貼出我的mybatis-config.xml文件以及Mapper.xml文件。
PUBLIC "-//mybatis.org//DTD Config 3.0//EN"
"http://mybatis.org/dtd/mybatis-3-config.dtd">
PUBLIC "-//mybatis.org//DTD Mapper 3.0//EN"
"http://mybatis.org/dtd/mybatis-3-mapper.dtd">
select * from user
User where id = #{id}
insert into User (username,birthday,sex,address)
values (#{name},#{birthday},#{sex},#{address})
update User set username = #{username},birthday = #{birthday},
sex = #{sex},address = #{address} where id = #{id}
delete from User where id = #{id}
select * from User where sex = #{param1}
and username like #{param2}
and address = #{parma3}
select count(*) from user where username like #{username}
username like #{pattern}
and sex = #{sex}
and address = #{address}
where id in
{id}
第一步通過SqlSessionFactoryBuilder創(chuàng)建SqlSessionFactory：
首先在SqlSessionFactoryBuilder的build（）方法中可以看到MyBatis內(nèi)部定義了一個類XMLConfigBuilder用來解析配置文件mybatis-config.xml寞冯。針對配置文件中的每一個節(jié)點進行解析并將數(shù)據(jù)存放到Configuration這個對象中渴析，緊接著使用帶有Configuration的構造方法發(fā)返回一個DefautSqlSessionFactory。
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吮龄，默認使用的是實現(xiàn)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(jié)點configuration
parseConfiguration(parser.evalNode("/configuration"));
return configuration;
}
//開始解析mybatis-config.xml,并把解析后的數(shù)據(jù)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(zhí)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咆疗，例如本例中配置的jdbc.propertis
propertiesElement(root.evalNode("properties"));
//查看是否配置了VFS漓帚，默認沒有，本例也沒有使用
loadCustomVfs(settings);
//查看是否用了類型別名午磁，減少完全限定名的冗余尝抖，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(zhí)行毡们，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昧辽，默認情況下使用對象的無參構造方法或者是帶有參數(shù)的構造方法衙熔，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：對象搅荞，Collection红氯，Map屬性。本例沒有使用
objectWrapperFactoryElement(root.evalNode("objectWrapperFactory"));
//查看是否配置了reflectorFactory,mybatis的反射工具咕痛，提供了很多反射方法痢甘。本例沒有使用
reflectorFactoryElement(root.evalNode("reflectorFactory"));
//放入?yún)?shù)到configuration對象中
settingsElement(settings);
// read it after objectFactory and objectWrapperFactory issue #631
//查看數(shù)據(jù)庫環(huán)境配置
environmentsElement(root.evalNode("environments"));
//查看是否使用多種數(shù)據(jù)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茉贡，如果跟處理的類型跟默認的一致就會覆蓋塞栅。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腔丧，url放椰，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);
}
}
第二步通過SqlSessionFactory創(chuàng)建SqlSession：
@Override
public SqlSession openSession() {
return openSessionFromDataSource(configuration.getDefaultExecutorType(), null, false);
}
private SqlSession openSessionFromDataSource(ExecutorType execType, TransactionIsolationLevel level, boolean autoCommit) {
Transaction tx = null;
try {
//拿到前文從mybatis中解析到的數(shù)據(jù)庫環(huán)境配置
final Environment environment = configuration.getEnvironment();
final TransactionFactory transactionFactory = getTransactionFactoryFromEnvironment(environment);
//拿到jdbc的事務管理器愉粤，有兩種一種是jbc,一種的managed庄敛。本例使用的是JdbcTransaction
tx = transactionFactory.newTransaction(environment.getDataSource(), level, autoCommit);
//從mybatis配置文件可以看到本例使用了REUSE，因此返回的是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科汗，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;
}
第三步通過SqlSession拿到Mapper對象的代理：
@Override
public T getMapper(Class type) {
return configuration.getMapper(type, this);
}
public T getMapper(Class type, SqlSession sqlSession) {
//前文解析Mybatis-config.xml的時候藻烤，在解析標簽mapper就是用configuration對象的mapperRegistry存放數(shù)據(jù)
return mapperRegistry.getMapper(type, sqlSession);
}
@SuppressWarnings("unchecked")
public T getMapper(Class 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保存头滔。
//例如本例中保存的就是Key:com.ctc.mapper.UserMapper,value就是保存了key的MapperProxyFactory對象
final MapperProxyFactory mapperProxyFactory = (MapperProxyFactory) 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);
}
}
public T newInstance(SqlSession sqlSession) {
//生成一個mapperProxy對象怖亭，這個對象實現(xiàn)了InvocationHandler, Serializable。就是JDK動態(tài)代理中的方法調(diào)用處理器
final MapperProxy mapperProxy = new MapperProxy(sqlSession, mapperInterface, methodCache);
return newInstance(mapperProxy);
}
public MapperProxy(SqlSession sqlSession, Class mapperInterface, Map methodCache) {
this.sqlSession = sqlSession;
this.mapperInterface = mapperInterface;
this.methodCache = methodCache;
}
@SuppressWarnings("unchecked")
protected T newInstance(MapperProxy mapperProxy) {
//通過JDK動態(tài)代理生成一個Mapper的代理坤检，在本例中的就是UserMapper的代理類兴猩，它實現(xiàn)了UserMapper接口
return (T) Proxy.newProxyInstance(mapperInterface.getClassLoader(), new Class[] { mapperInterface }, mapperProxy);
}
第四步通過MapperProxy調(diào)用Maper中相應的方法：
@Override
public Object invoke(Object proxy, Method method, Object[] args) throws Throwable {
//判斷當前調(diào)用的method是不是Object中聲明的方法，如果是的話直接執(zhí)行早歇。
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中倾芝，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 {
//本次案例會執(zhí)行selectOne
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(diào)用的是CachingExecutor類的query,還記得前文解析mybatis-config.xml的時候我們指定了REUSE但是因為在配置文件中開啟了緩存
//所以ReuseExecutor被CachingExecotur裝飾箭跳，新增了緩存的判斷晨另，最后還是會調(diào)用ReuseExecutor
@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(shù)據(jù)則查詢數(shù)據(jù)庫
list = delegate.<E> query(ms, parameterObject, rowBounds, resultHandler, key, boundSql);
//結果集放入緩存
tcm.putObject(cache, key, list); // issue #578 and #116
}
return list;
}
}
return delegate.<E> query(ms, parameterObject, rowBounds, resultHandler, key, boundSql);
}
