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Mybatis简介:
MyBatis 是一款优秀的持久层框架,它支持定制化 SQL、存储过程以及高级映射。MyBatis 避免了几乎所有的 JDBC 代码和手动设置参数以及获取结果集。MyBatis 可以使用简单的 XML 或注解来配置和映射原生信息,将接口和 Java 的 POJOs(Plain Old Java Objects,普通的 Java对象)映射成数据库中的记录。本文将通过debug的方式来了解其工作原理。
Mybatis核心类:
SqlSessionFactory:每个基于 MyBatis 的应用都是以一个 SqlSessionFactory 的实例为中心的。SqlSessionFactory 的实例可以通过 SqlSessionFactoryBuilder 获得。而 SqlSessionFactoryBuilder 则可以从 XML 配置文件或通过Java的方式构建出 SqlSessionFactory 的实例。SqlSessionFactory 一旦被创建就应该在应用的运行期间一直存在,建议使用单例模式或者静态单例模式。一个SqlSessionFactory对应配置文件中的一个环境(environment),如果你要使用多个数据库就配置多个环境分别对应一个SqlSessionFactory。
SqlSession:SqlSession是一个接口,它有2个实现类,分别是DefaultSqlSession(默认使用)以及SqlSessionManager。SqlSession通过内部存放的执行器(Executor)来对数据进行CRUD。此外SqlSession不是线程安全的,因为每一次操作完数据库后都要调用close对其进行关闭,官方建议通过try-finally来保证总是关闭SqlSession。
Executor:Executor(执行器)接口有两个实现类,其中BaseExecutor有三个继承类分别是BatchExecutor(重用语句并执行批量更新),ReuseExecutor(重用预处理语句prepared statement,跟Simple的唯一区别就是内部缓存statement),SimpleExecutor(默认,每次都会创建新的statement)。以上三个就是主要的Executor。通过下图可以看到Mybatis在Executor的设计上面使用了装饰器模式,我们可以用CachingExecutor来装饰前面的三个执行器目的就是用来实现缓存。
MappedStatement:MappedStatement就是用来存放我们SQL映射文件中的信息包括sql语句,输入参数,输出参数等等。一个SQL节点对应一个MappedStatement对象。
Mybatis工作流程:
下面将通过debug方式对Mybatis进行一步步解析。首先贴出我的mybatis-config.xml文件以及Mapper.xml文件。
select * from user 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}
第一步通过SqlSessionFactoryBuilder创建SqlSessionFactory:
首先在SqlSessionFactoryBuilder的build()方法中可以看到MyBatis内部定义了一个类XMLConfigBuilder用来解析配置文件mybatis-config.xml。针对配置文件中的每一个节点进行解析并将数据存放到Configuration这个对象中,紧接着使用带有Configuration的构造方法发返回一个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,默认使用的是实现类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; //获取根节点configuration parseConfiguration(parser.evalNode("/configuration")); return configuration; } //开始解析mybatis-config.xml,并把解析后的数据存放到configuration中 private void parseConfiguration(XNode root) { try { //保存mybatis-config.xml中的标签setting,本例中开启全局缓存cacheEnabled,设置默认执行器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,默认没有,本例也没有使用 loadCustomVfs(settings); //查看是否用了类型别名,减少完全限定名的冗余,本例中使用了别名User代替了com.ctc.Model.User typeAliasesElement(root.evalNode("typeAliases")); //查看是否配置插件来拦截映射语句的执行,例如拦截Executor的Update方法,本例没有使用 pluginElement(root.evalNode("plugins")) //查看是否配置了ObjectFactory,默认情况下使用对象的无参构造方法或者是带有参数的构造方法,本例没有使用 objectFactoryElement(root.evalNode("objectFactory")); //查看是否配置了objectWrapperFatory,这个用来或者ObjectWapper,可以访问:对象,Collection,Map属性。本例没有使用 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。本例没有使用 reflectorFactoryElement(root.evalNode("reflectorFactory")); //放入参数到configuration对象中 settingsElement(settings); // read it after objectFactory and objectWrapperFactory issue #631 //查看数据库环境配置 environmentsElement(root.evalNode("environments")); //查看是否使用多种数据库,本例没有使用 databaseIdProviderElement(root.evalNode("databaseIdProvider")); //查看是否配置了新的类型处理器,如果跟处理的类型跟默认的一致就会覆盖。本例没有使用 typeHandlerElement(root.evalNode("typeHandlers")); //查看是否配置SQL映射文件,有四种配置方式,resource,url,class以及自动扫包package。本例使用package mapperElement(root.evalNode("mappers")); } catch (Exception e) { throw new BuilderException("Error parsing SQL Mapper Configuration. Cause: " + e, e); } }
第二步通过SqlSessionFactory创建SqlSession:
@Override public SqlSession openSession() { return openSessionFromDataSource(configuration.getDefaultExecutorType(), null, false); } private SqlSession openSessionFromDataSource(ExecutorType execType, TransactionIsolationLevel level, boolean autoCommit) { Transaction tx = null; try { //拿到前文从mybatis中解析到的数据库环境配置 final Environment environment = configuration.getEnvironment(); final TransactionFactory transactionFactory = getTransactionFactoryFromEnvironment(environment); //拿到jdbc的事务管理器,有两种一种是jbc,一种的managed。本例使用的是JdbcTransaction tx = transactionFactory.newTransaction(environment.getDataSource(), level, autoCommit); //从mybatis配置文件可以看到本例使用了REUSE,因此返回的是ReuseExecutor并把事务传入对象中 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; } //返回一个SqlSession,默认使用DefaultSqlSession public DefaultSqlSession(Configuration configuration, Executor executor, boolean autoCommit) { this.configuration = configuration; this.executor = executor; this.dirty = false; this.autoCommit = autoCommit; }
第三步通过SqlSession拿到Mapper对象的代理:
@Override publicT getMapper(Class type) { return configuration. getMapper(type, this); } public T getMapper(Class type, SqlSession sqlSession) { //前文解析Mybatis-config.xml的时候,在解析标签mapper就是用configuration对象的mapperRegistry存放数据 return mapperRegistry.getMapper(type, sqlSession); } @SuppressWarnings("unchecked") public T getMapper(Class type, SqlSession sqlSession) { //knownMapper是一个HashMap在存放mapperRegistry的过程中,以每个Mapper对象的类型为Key, MapperProxyFactory 为value保存。 //例如本例中保存的就是Key:com.ctc.mapper.UserMapper,value就是保存了key的MapperProxyFactory对象 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) { //生成一个mapperProxy对象,这个对象实现了InvocationHandler, Serializable。就是JDK动态代理中的方法调用处理器 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) { //通过JDK动态代理生成一个Mapper的代理,在本例中的就是UserMapper的代理类,它实现了UserMapper接口 return (T) Proxy.newProxyInstance(mapperInterface.getClassLoader(), new Class[] { mapperInterface }, mapperProxy); }
第四步通过MapperProxy调用Maper中相应的方法:
@Override public Object invoke(Object proxy, Method method, Object[] args) throws Throwable { //判断当前调用的method是不是Object中声明的方法,如果是的话直接执行。 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); } //把当前请求放入一个HashMap中,一旦下次还是同样的方法进来直接返回。 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 { //本次案例会执行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 publicT selectOne(String statement, Object parameter) { // Popular vote was to return null on 0 results and throw exception on too many. List list = this. 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 List 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(); } } //这边调用的是CachingExecutor类的query,还记得前文解析mybatis-config.xml的时候我们指定了REUSE但是因为在配置文件中开启了缓存 //所以ReuseExecutor被CachingExecotur装饰,新增了缓存的判断,最后还是会调用ReuseExecutor @Override public List 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 List 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 list = (List ) tcm.getObject(cache, key); if (list == null) { //如果缓存中没有数据则查询数据库 list = delegate. query(ms, parameterObject, rowBounds, resultHandler, key, boundSql); //结果集放入缓存 tcm.putObject(cache, key, list); // issue #578 and #116 } return list; } } return delegate. query(ms, parameterObject, rowBounds, resultHandler, key, boundSql); }
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