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mybatis关于ORM的使用以及设计(三)[参数对象转换为SQL语言]

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标签:extends   space   ping   处理对象   mic   app   orm   解析   error   

上节分析了Mapper对象的创建。

在ORM的定义中可以理解为Object->SQLMapper抽象层(这一层并不负责具体的SQL执行。这一层可以理解为SQL代理层)

本节分析以下内容:

①SqlSession在具体执行SQL时,如果通过namespace+sqlid定位到具体的MappedStatement(sql的对象化表现形式)

②参数(Object) 如何填充到具体的SQL

③SQL是如何执行的

  • 获取StateMentMapper.前面讲到初始化时,会缓存MappedStatement,MappedStatement被保存在StrictMap中.

    StrictMap是Mybatis实现HashMap子类。Key重复放入的时候会报错。

      技术分享图片

  

  • 参数(Object) 如何填充到具体的SQL(Param-SQL的Orm转换)

  1、通过Executor执行SQL

  @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();
    }
  }

 2、Executor是如何获取的?

  SqlSessionFactory工厂中代码如下:

   从Configuration中获得Excecutor,默认的执行器类型为configuration.getDefaultExecutorType()在configuration类中定义为

  protected ExecutorType defaultExecutorType = ExecutorType.SIMPLE;


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(); } }

  ExecutorType.SIMPLE会创建何种执行器?

  来看Configuraiton的获得执行器的方法

  默认的执行器为:SimpleExecutor,而cacheEnabled默认值为true.所以实际是CachingExecutor,使用了装饰器模式。

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;
  }
  •   Excecutor是如何将参数成功映射到具体SQL的参数?

   先看下MappedStatement中的类成员构成。sqlSource是具体获取待执行SQL的对象。

      技术分享图片

  •   sqlSource接口定义:
    BoundSql getBoundSql(Object parameterObject)的方法,改方调用实际的SqlSource的实现类,来获取真正执行的SQL

    技术分享图片

  先说说几个处理类的区别:

  DynamicSqlSource:sql中包含<where><if><choose>等条件是,会被定义为.通过具体的Node处理对象,拼接SQL。

  看一段代码,我们关注rootSqlNode变量,以及getBoundSql()方法的执行。

  

public class DynamicSqlSource implements SqlSource {

  private final Configuration configuration;
  private final SqlNode rootSqlNode;

  public DynamicSqlSource(Configuration configuration, SqlNode rootSqlNode) {
    this.configuration = configuration;
    this.rootSqlNode = rootSqlNode;
  }

  @Override
  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<String, Object> entry : context.getBindings().entrySet()) {
      boundSql.setAdditionalParameter(entry.getKey(), entry.getValue());
    }
    return boundSql;
  }

 

  解析过程描述:

  ①rootSqlNode为从XML解析的具体SQL节点,每一行作为一个node。对于<if><choose>等。每一个节点是一个node

  ②<if></if>的判断是在具体的node中执行的。SQLNode有以下几种类型。

  技术分享图片

  看一段IfSqlNode的代码:apply方法中的evaluateBoolean方法,将对<if>语句进行判断,返回结果。如果结果为真,则把条件添加到contents 

/**
 * @author Clinton Begin
 */
public class IfSqlNode implements SqlNode {
  private final ExpressionEvaluator evaluator;
  private final String test;
  private final SqlNode contents;

  public IfSqlNode(SqlNode contents, String test) {
    this.test = test;
    this.contents = contents;
    this.evaluator = new ExpressionEvaluator();
  }

  @Override
  public boolean apply(DynamicContext context) {
    if (evaluator.evaluateBoolean(test, context.getBindings())) {
      contents.apply(context);
      return true;
    }
    return false;
  }

}

 

  mybatis中定义的SQL节点如下。

  技术分享图片

  RawSqlSource:对于不包含<if>等条件判断,替换#{}变为? 在创建RawSqlSource对象时执行这项操作

  DynamicSqlSource: 对sql中包含${}参数的会转换为该对象

  •   SqlSource的转换是在初始化加载时完成。那真正的参数是何时转换为SQL?

  SimpleExecutor中,创建PrepareStateMent的过程。

  

  public <E> List<E> doQuery(MappedStatement ms, Object parameter, RowBounds rowBounds, ResultHandler resultHandler, BoundSql boundSql) throws SQLException {
    Statement stmt = null;
    try {
      Configuration configuration = ms.getConfiguration();
      StatementHandler handler = configuration.newStatementHandler(wrapper, ms, parameter, rowBounds, resultHandler, boundSql);
      stmt = prepareStatement(handler, ms.getStatementLog());
      return handler.<E>query(stmt, resultHandler);
    } finally {
      closeStatement(stmt);
    }
  }
  private Statement prepareStatement(StatementHandler handler, Log statementLog) throws SQLException {
    Statement stmt;
    Connection connection = getConnection(statementLog);
   //创建、初始化PreparedStatement stmt
= handler.prepare(connection, transaction.getTimeout());
  //设置参数 handler.parameterize(stmt);
return stmt; }

 

  离我们想知道的真相越来越近了,来看具体的参数化过程

  StateMentHandler.parameterize

技术分享图片

  RoutingStatementHandler用来做路由器:根据实际的StatementType做路由

  我们来看PreparedStatementHadler

    public void parameterize(Statement statement) throws SQLException {
        this.parameterHandler.setParameters((PreparedStatement)statement);
    }

 

  

                    
public void setParameters(PreparedStatement ps) {       
  ErrorContext.instance().activity("setting parameters").object(this.mappedStatement.getParameterMap().getId());     
  //获得所有的参数        
List
<ParameterMapping> parameterMappings = this.boundSql.getParameterMappings();
        
  
if (parameterMappings != null) {
            
    for(int i = 0; i < parameterMappings.size(); ++i) {
               
      ParameterMapping parameterMapping = (ParameterMapping)parameterMappings.get(i);
                
      if (parameterMapping.getMode() != ParameterMode.OUT) {

        String propertyName
= parameterMapping.getProperty(); Object value; if (this.boundSql.hasAdditionalParameter(propertyName)) { value = this.boundSql.getAdditionalParameter(propertyName); } else if (this.parameterObject == null) { value = null;
            //如果有typeHandler则用TypeHandler处理参数
            //一些基础类型是有typeHandler的
}
else if (this.typeHandlerRegistry.hasTypeHandler(this.parameterObject.getClass())) { value = this.parameterObject; } else {
            //如果没有typeHandler,通过反射,获得Bean参数中的值 MetaObject metaObject
= this.configuration.newMetaObject(this.parameterObject); value = metaObject.getValue(propertyName); }             //根据参数的JDBCtype找到TypeHandler,设置到PrePareStatement中 TypeHandler typeHandler = parameterMapping.getTypeHandler(); JdbcType jdbcType = parameterMapping.getJdbcType(); if (value == null && jdbcType == null) { jdbcType = this.configuration.getJdbcTypeForNull(); } try { typeHandler.setParameter(ps, i + 1, value, jdbcType); } catch (TypeException var10) { throw new TypeException("Could not set parameters for mapping: " + parameterMapping + ". Cause: " + var10, var10); } catch (SQLException var11) { throw new TypeException("Could not set parameters for mapping: " + parameterMapping + ". Cause: " + var11, var11); } } } } }

 

  TypeHandler的继承关系如下:

  技术分享图片

   技术分享图片

  我们查看其中的BigDecimalTypeHandler的源码

  

public class BigDecimalTypeHandler extends BaseTypeHandler<BigDecimal> {

  @Override
  public void setNonNullParameter(PreparedStatement ps, int i, BigDecimal parameter, JdbcType jdbcType)
      throws SQLException {
    ps.setBigDecimal(i, parameter);
  }

  @Override
  public BigDecimal getNullableResult(ResultSet rs, String columnName)
      throws SQLException {
    return rs.getBigDecimal(columnName);
  }

  @Override
  public BigDecimal getNullableResult(ResultSet rs, int columnIndex)
      throws SQLException {
    return rs.getBigDecimal(columnIndex);
  }

  @Override
  public BigDecimal getNullableResult(CallableStatement cs, int columnIndex)
      throws SQLException {
    return cs.getBigDecimal(columnIndex);
  }
}

  会调用JDBCAPI中的相应方法获得正确的值

  

 

mybatis关于ORM的使用以及设计(三)[参数对象转换为SQL语言]

标签:extends   space   ping   处理对象   mic   app   orm   解析   error   

原文地址:https://www.cnblogs.com/marioS/p/10306441.html

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