MyBatis查询的源码分析

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如下为mybatis的一个集合查询： 

String resource = "mybatis.cfg.xml";  
Reader reader = Resources.getResourceAsReader(resource);  
SqlSessionFactory ssf = new SqlSessionFactoryBuilder().build(reader);  
	
SqlSession sqlSession = ssf.openSession();  
	
try {  
	List users = sqlSession.selectList("User.selectUser", "1");  //查询状态为1的用户
	for (User user : users) {
		System.out.println(user);  
	}
} catch (Exception e) {  
	e.printStackTrace();  
} finally {  
	sqlSession.close();
}

  

• 其中SqlSession默认为DefaultSqlSession：

public class DefaultSqlSession implements SqlSession {

  private Configuration configuration;
  private Executor executor;
... ...

  @Override
  public  List selectList(String statement) {
    return this.selectList(statement, null);
  }

  @Override
  public  List selectList(String statement, Object parameter) {
    return this.selectList(statement, parameter, RowBounds.DEFAULT);	//其中RowBounds.DEFAULT为new RowBounds();
  }

  @Override
  public  List selectList(String statement, Object parameter, RowBounds rowBounds) {
    try {
	//MappedStatement是mybatis根据创建的映射对象
      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();
    }
  }

... ...
} 
  
/**
  * 行数限制
  */
public class RowBounds {

  public static final int NO_ROW_OFFSET = 0;
  public static final int NO_ROW_LIMIT = Integer.MAX_VALUE;
  public static final RowBounds DEFAULT = new RowBounds();

  private int offset;
  private int limit;

  public RowBounds() {
    this.offset = NO_ROW_OFFSET;
    this.limit = NO_ROW_LIMIT;
  }

  public RowBounds(int offset, int limit) {
    this.offset = offset;
    this.limit = limit;
  }
... ...
}
 
  
  
 
  
 
   
 当我们启用缓存后查询执行器Executor为CachingExector： 
 
  
 
  
public class CachingExecutor implements Executor {

  private Executor delegate;
  private TransactionalCacheManager tcm = new TransactionalCacheManager();

  public CachingExecutor(Executor delegate) {
    this.delegate = delegate;//这里的delegate为abstract class BaseExecutor implements Executor
    delegate.setExecutorWrapper(this);
  }
... ...

  @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);//如果中设置flushCache="true"，则清除缓存
      if (ms.isUseCache() && resultHandler == null) {
        ensureNoOutParams(ms, parameterObject, boundSql);//确保查询不包括输出参数，因为缓存程序不支持输出参数
        @SuppressWarnings("unchecked")
        List list = (List) tcm.getObject(cache, key);//从缓存管理器中获取缓存（缓存管理器是对各区块缓存的统一管理工具）
        if (list == null) {	//如果从缓存中获取不到，则执行查询
          list = delegate. query(ms, parameterObject, rowBounds, resultHandler, key, boundSql);
          tcm.putObject(cache, key, list); //将查询到的结果放入缓存
        }
        return list;
      }
    }
    return delegate. query(ms, parameterObject, rowBounds, resultHandler, key, boundSql);
  }

... ...

}

 

• CachingExecutor中的执行器代理delegate一般为ReuseExecutor：

public class ReuseExecutor extends BaseExecutor {

... ...

  @Override
  public  List doQuery(MappedStatement ms, Object parameter, RowBounds rowBounds, ResultHandler resultHandler, BoundSql boundSql) throws SQLException {
    Configuration configuration = ms.getConfiguration();
    StatementHandler handler = configuration.newStatementHandler(wrapper, ms, parameter, rowBounds, resultHandler, boundSql);
    Statement stmt = prepareStatement(handler, ms.getStatementLog());
    return handler.query(stmt, resultHandler); //执行查询
  }

... ...

}

 

• StatementHandler一般为路由StatementHandler，即RoutingStatementHandler：

public class RoutingStatementHandler implements StatementHandler {

  private final StatementHandler delegate;

  public RoutingStatementHandler(Executor executor, MappedStatement ms, Object parameter, RowBounds rowBounds, ResultHandler resultHandler, BoundSql boundSql) {
/*这里的switch-case即是传说中的策略模式*/
    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());
    }

  }

 

• RoutingStatementHandler其中的StatementHandler代理delegate，通常为PreparedStatementHandler；其中的query(...)方法通过调用ResultHandler的handleResultSets(...)封装查询结果：

public class PreparedStatementHandler extends BaseStatementHandler {

... ...
  @Override
  public  List query(Statement statement, ResultHandler resultHandler) throws SQLException {
    PreparedStatement ps = (PreparedStatement) statement;
    ps.execute();	//执行查询
    return resultSetHandler. handleResultSets(ps);	//处理结果集
  }
... ...
}

 

• ResultSetHandler只有一个默认结果集处理器DefaultResultSetHandler：

public interface ResultSetHandler {
  /**
    * 处理结果集
    */
   List handleResultSets(Statement stmt) throws SQLException;

... ...

}

public class DefaultResultSetHandler implements ResultSetHandler {
  ... ...
  // nested resultmaps
  private final Map nestedResultObjects = new HashMap();//嵌套结果缓存
  private final Map ancestorObjects = new HashMap();//祖对象缓存，即最外层的对象缓存
  private final Map ancestorColumnPrefix = new HashMap();
  ... ...
  
  /**
    * 处理所有的结果集（一次查询可能有多个语句，所以可能有多个结果集）
    */
  @Override
  public List