关于java:MyBatis中StatementHandler

StatementHandler负责操作Statement对象和数据库进行交换,其中还会用到ParameterHandler和ResultSetHandler进行参数的赋值和后果集的映射

1.StatementHandler中次要的办法

 Statement prepare(Connection connection)throws SQLException;
 void parameterize(Statement statement)throws SQLException;
 int update(Statement statement) throws SQLException;
 <E> List<E> query(Statement statement,ResultHandler resultHandler)throws SQLException;

prepare: 用于创立一个Statement实例
parametersize: 用于初始化Statement对象以及对sql的占位符进行赋值
update: 用于告诉Statement对象将 insert、update、delete 操作推送到数据库
query: 用于告诉Statement对象将 select 操作推送数据库并返回对应的查问后果

2.StatementHandler的继承机构

StatementHandler的两个实现类
BaseStatementHandler:抽象类,提供了一些公共的实现,简化StatementHandler接口的实现难度
RoutingStatementHandler:StatementHandler接口的默认实现, 依据StatementType获取到真正的StatementHandler,而后赋值给属性delegate

 public RoutingStatementHandler(Executor executor, MappedStatement ms, Object parameter, RowBounds rowBounds, ResultHandler resultHandler, BoundSql boundSql) {    

  switch (ms.getStatementType()) {    
    case STATEMENT:    
      delegate = new SimpleStatementHandler(executor, ms, parameter, rowBounds, resultHandler, boundSql);    
      break;    
    case PREPARED:    
      delegate = new PreparedStatementHandler(executor, ms, parameter, rowBounds, resultHandler, boundSql);    
      break;    
    case CALLABLE:    
      delegate = new CallableStatementHandler(executor, ms, parameter, rowBounds, resultHandler, boundSql);    
      break;    
    default:    
      throw new ExecutorException("Unknown statement type: " + ms.getStatementType());    
  }    
}
    

BaseStatementHandler的三个实现类:
SimpleStatementHandler:解决不须要预编译的sql,不须要解决参数的sql
PreparedStatementHandler:解决须要预编译的sql
CallableStatementHandler:调用存储过程

在查问和更新时才会去获取StatementHandler对象,因而它是由Executor治理和创立的。
以SimpleExecutor为例剖析生成StatementHandler和Statement的源码
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);    
    //获取Statement对象
     ** stmt = prepareStatement(handler, ms.getStatementLog());    **      
      return handler.<E>query(stmt, resultHandler);    
    } finally {    
      closeStatement(stmt);    
    }    
}

获取Statement实例源码剖析

private Statement prepareStatement(StatementHandler handler, Log statementLog) throws SQLException {    
  Statement stmt;    
  Connection connection = getConnection(statementLog);    
  //获取Statement实例
  stmt = handler.prepare(connection, transaction.getTimeout());    
  //stmt进行参数解决    
  //SimpleStatementHandler中parameterize为空办法,因为它只解决简略sql,有参数的sql由
  PreparedStatementHandler解决
  handler.parameterize(stmt);    
  return stmt;    
}    

 //调用RoutingStatementHandler中的办法,本质是调用了属性delegate的prepare办法
 //下面提到了delegate的赋值
  public Statement prepare(Connection connection, Integer transactionTimeout) throws SQLException {    
    return delegate.prepare(connection, transactionTimeout);    
  }    
    
 //调用BaseStatementHandler中的prepare
  public Statement prepare(Connection connection, Integer transactionTimeout) throws SQLException {    
    ErrorContext.instance().sql(boundSql.getSql());    
    Statement statement = null;    
    try {
      //由子类提供实现    
      statement = instantiateStatement(connection);    
      setStatementTimeout(statement, transactionTimeout);    
      setFetchSize(statement);    
      return statement;    
    } 

//SimpleStatementHandler提供的实现
protected Statement instantiateStatement(Connection connection) throws SQLException {    
    if (mappedStatement.getResultSetType() = null) {    
      return connection.createStatement(mappedStatement.getResultSetType().getValue(), ResultSet.CONCUR_READ_ONLY);    
    } else {    
      return connection.createStatement();    
    }    
  }