（一）MyBatis简介

MyBatis 是支持定制化 SQL、存储过程以及高级映射的优秀的持久层框架。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 可以对配置和原生Map使用简单的 XML 或注解，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。

（二）源码分析

项目是用映射器(dao)和映射文件(xxx.xml)的方式配置mybatis

先以下面的更新方法为例

dao接口的方法如下:int updateSubjectById(Subject subject)throws Exception;

代码执行dao方法时，会调用MapperProxy类中的invoke()方法，往下执行会依次会调用

MapperMethod中的public Object execute(SqlSession sqlSession, Object[] args)，

public Object execute(SqlSession sqlSession, Object[] args) {    Object result;    if (SqlCommandType.INSERT == command.getType()) {        Object param = method.convertArgsToSqlCommandParam(args);        result = rowCountResult(sqlSession.insert(command.getName(), param));    } else if (SqlCommandType.UPDATE == command.getType()) {        Object param = method.convertArgsToSqlCommandParam(args);        result = rowCountResult(sqlSession.update(command.getName(), param));    } else if (SqlCommandType.DELETE == command.getType()) {        Object param = method.convertArgsToSqlCommandParam(args);        result = rowCountResult(sqlSession.delete(command.getName(), param));    } else if (SqlCommandType.SELECT == command.getType()) {        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 {            Object param = method.convertArgsToSqlCommandParam(args);            result = sqlSession.selectOne(command.getName(), param);        }    } else if (SqlCommandType.FLUSH == command.getType()) {        result = sqlSession.flushStatements();    } else {        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;}

MethodSignature中的public Object convertArgsToSqlCommandParam(Object[] args)，

public Object convertArgsToSqlCommandParam(Object[] args) {    final int paramCount = params.size();    if (args == null || paramCount == 0) {        return null;    } else if (!hasNamedParameters && paramCount == 1) {        return args[params.keySet().iterator().next().intValue()];    } else {        final Map
    
      param = new ParamMap
     
    

因为dao中方法的参数为Subject subject，所以这里args参数为

hasNamedParameters意思是是否使用了注解参数，这里没有用到注解，值为false。

这里params.size()的值为1，如图

所以上面的方法返回的是subject，excute()中的param就是subject。

convertArgsToSqlCommandParam()方法执行完后，依次进入rowCountResult(sqlSession.update(command.getName(), param))，SqlSessionTemplate中update()方法，

public int update(String statement, Object parameter) {    return this.sqlSessionProxy.update(statement, parameter);}

这时SqlSessionInterceptor拦截器(SqlSessiontemplate中的私有类)会拦截到。

private class SqlSessionInterceptor implements InvocationHandler {    ...    Object result=method.invoke(sqlSession, args);    ...}

其中arg[0]的值为com.services.forum.dao.SubjectDAO.updateSubjectById()方法，args[1]的值为SqlSessionTemplate中public int update(String statement, Object parameter)中的parameter，即dao层方法中传的subject对象。

this.sqlSessionProxy的类型为DefaultSqlSession，其中update(...)方法return executor.update(ms, wrapCollection(parameter)); ，wrapCollection(parameter)对参数进行包装，该方法的实现如下：

private Object wrapCollection(final Object object) {    if (object instanceof Collection) {        StrictMap

此方法首先定义一个StrictMap，

• 如果参数类型是Collection的子类，则在map中加入一条entry:("collection", object)，返回map

• 如果参数类型是List的子类，则在map中加入一条entry:("list", object)，返回map

• 如果参数是数组，则在map中加入一条entry:("array", object)，返回map

• 否则返回参数object

随后会执行以下方法：SimpleExecutorpublic int doUpdate(MappedStatement ms, Object parameter) throws SQLException    StatementHandler handler = configuration.newStatementHandler(this, ms, parameter, RowBounds.DEFAULT, null, null);

ConfigurationStatementHandler statementHandler = new RoutingStatementHandler(executor, mappedStatement, parameterObject, rowBounds, resultHandler, boundSql);

RoutingStatementHandlerpublic RoutingStatementHandler(Executor executor, MappedStatement ms, Object parameter, RowBounds rowBounds, ResultHandl    er resultHandler, BoundSql boundSql){    ...    delegate = new PreparedStatementHandler(executor, ms, parameter, rowBounds, resultHandler, boundSql);    ...}

PreparedStatementHandlerpublic PreparedStatementHandler(Executor executor, MappedStatement mappedStatement, Object parameter, RowBounds rowBound    s, ResultHandler resultHandler, BoundSql boundSql) {      super(executor, mappedStatement, parameter, rowBounds, resultHandler, boundSql);}

BaseStatementHandlerprotected BaseStatementHandler(Executor executor, MappedStatement mappedStatement, Object parameterObject, RowBounds row    Bounds, ResultHandler resultHandler, BoundSql boundSql) {    ...    boundSql = mappedStatement.getBoundSql(parameterObject);    ...}

MappedStatementpublic BoundSql getBoundSql(Object parameterObject) {    BoundSql boundSql = sqlSource.getBoundSql(parameterObject);    ...}

再跟几步会进入DynamicSqlSource类中的getBoundSql()方法

public BoundSql getBoundSql(Object parameterObject) {    DynamicContext context = new DynamicContext(configuration, parameterObject);    rootSqlNode.apply(context);    SqlSourceBuilder sqlSourceParser = new SqlSourceBuilder(configuration);    Class
     parameterType = parameterObject == null ? Object.class : parameterObject.getClass();    SqlSource sqlSource = sqlSourceParser.parse(context.getSql(), parameterType, context.getBindings());    BoundSql boundSql = sqlSource.getBoundSql(parameterObject);    for (Map.Entry
    
      entry : context.getBindings().entrySet()) {        boundSql.setAdditionalParameter(entry.getKey(), entry.getValue());    }  return boundSql;}
    

这个方法是生成sql语句的方法

sqlSourceParser中是设置和一些注册的类型[处理机的]别名，如下图

再跟到SqlSourceBuilder中的parse()方法，

public SqlSource parse(String originalSql, Class
     parameterType, Map
    
      additionalParameters) {    ParameterMappingTokenHandler handler = new ParameterMappingTokenHandler(configuration, parameterType, additionalPara        meters);    GenericTokenParser parser = new GenericTokenParser("#{", "}", handler);    String sql = parser.parse(originalSql);    return new StaticSqlSource(configuration, sql, handler.getParameterMappings());}
    

该方法中有个 originalSql参数，这是sql映射文件中的原始sql语句，

update t_forum_subject SET title=#{title}, brief=#{brief}, is_top=#{isTop}, is_open=#{isOpen}, last_modify_time=#{lastModifyTime}, last_modify_userid=#{lastModifyUserId} where id=#{id} and enable=1

GenericTockenParser类中的public String parse(String text)方法将originalSql中的#{}转为了?，并且在handler的属性parameterMappings中添加相应的映射

public String parse(String text) {    ...    builder.append(handler.handleToken(content));    ...}

SqlSourceBuilder@Overridepublic String handleToken(String content) {    parameterMappings.add(buildParameterMapping(content));    return "?";}

再回到DynamicSqlSource类中getBoundSql方法中，现在sqlSource中sql属性值变为

update t_forum_subject SET title=?, brief=?, is_top=?, is_open=?, last_modify_time=?, last_modify_userid=? where id=? and enable=1

parmeterMappings属性的值如上图所示boundSql的值如上图

SimpleExecutorprivate Statement prepareStatement(StatementHandler handler, Log statementLog) throws SQLException {    Statement stmt;    Connection connection = getConnection(statementLog);    stmt = handler.prepare(connection);    handler.parameterize(stmt);    return stmt;}

最后进入DefaultParameterhandler类中的

public void setParameters(PreparedStatement ps) {  ErrorContext.instance().activity("setting parameters").object(mappedStatement.getParameterMap().getId());  List
    
      parameterMappings = boundSql.getParameterMappings();  if (parameterMappings != null) {    for (int i = 0; i < parameterMappings.size(); i++) {      ParameterMapping parameterMapping = parameterMappings.get(i);      if (parameterMapping.getMode() != ParameterMode.OUT) {        Object value;        String propertyName = parameterMapping.getProperty();        if (boundSql.hasAdditionalParameter(propertyName)) { // issue #448 ask first for additional params          value = boundSql.getAdditionalParameter(propertyName);        } else if (parameterObject == null) {          value = null;        } else if (typeHandlerRegistry.hasTypeHandler(parameterObject.getClass())) {          value = parameterObject;        } else {          MetaObject metaObject = configuration.newMetaObject(parameterObject);          value = metaObject.getValue(propertyName);        }        TypeHandler typeHandler = parameterMapping.getTypeHandler();        JdbcType jdbcType = parameterMapping.getJdbcType();        if (value == null && jdbcType == null) {          jdbcType = configuration.getJdbcTypeForNull();        }        try {          typeHandler.setParameter(ps, i + 1, value, jdbcType);        } catch (TypeException e) {          throw new TypeException("Could not set parameters for mapping: " + parameterMapping + ". Cause: " + e, e);        } catch (SQLException e) {          throw new TypeException("Could not set parameters for mapping: " + parameterMapping + ". Cause: " + e, e);        }      }    }  }}
    

在这个方法中遍历parameterMappings，将parameterMapping中property的值赋给propertyName，再通过

MetaObject metaObject = configuration.newMetaObject(parameterObject);value = metaObject.getValue(propertyName);

将subject对象中的值赋给value，然后typeHandler.setParameter(ps, i + 1, value, jdbcType);将value值加入到ps的ColumnMap, ColumnNames, ColumnValues，再执行SimpleExecutor的doUpdate()中的return handler.update(stmt);

然后，就没有然后了

这次更新操作就完成了。

（三）其他类型的参数

我懒，不想写了

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我的心愿是世界和平！~( ゜▽゜)つロ