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MyBatis-一次Select查询过程

MapperRegistry 的实例,有一个属性 Map<Class<?>, MapperProxyFactory<?>> knownMappers :

  • key: Mapper类,比如接口'com.xxx.yyy.model.UserMapper';
  • value: MapperProxyFactory 对象,是Mapper代理类 MapperProxy 的工厂,创建 MapperProxy 对象执行Mapper类中定义的方法。

1.2 mappedStatements

类型是 Map<String, MappedStatement>

MappedStatement
MappedStatement

2 查询过程

MyBatis分三个步骤执行查询过程:

DefaultSqlSession
session.getMapper(UserMapper.class)
userMapper.findList()

2.1 创建SqlSession

DefaultSqlSessionFactory.openSession :

public SqlSession openSession(boolean autoCommit) {
    // 第一步
    // Configuration中:defaultExecutorType = ExecutorType.SIMPLE
    return openSessionFromDataSource(configuration.getDefaultExecutorType(), null, autoCommit);
  }
  
    // 第二步
    //openSessionFromDataSource关键代码:
    final Executor executor = configuration.newExecutor(tx, execType);
    return new DefaultSqlSession(configuration, executor, autoCommit);
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Executor从 Configuration.newExecutor 方法中创建:

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;
  }
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newExecutor 的执行过程如下:

  • 根据不同的ExecutorType返回不同Executor;
  • 因Configuration中默认是ExecutorType.SIMPLE,所以默认使用 SimpleExecutor ;
  • 如果开启了缓存,则返回 CachingExecutor 对象;
  • 通过 interceptorChain.pluginAll 加入拦截器 Interceptor 列表。

2.2 获取Mapper

DefaultSqlSessionConfiguration.mapperRegistry 中获取Mapper的实现对象:

// 第一步
// DefaultSqlSesison:
 public <T> T getMapper(Class<T> type) {
    return configuration.<T>getMapper(type, this);
  }

// 第二步
// Configuration:
 public <T> T getMapper(Class<T> type, SqlSession sqlSession) {
    return mapperRegistry.getMapper(type, sqlSession);
  }
  
// 第三步
// MapperRegistry:
// 从属性knownMappers中获取MapperProxyFactory对象,获取到后执行newInstance获取MapperProxy,即Mapper类的实现对象。
public <T> T getMapper(Class<T> type, SqlSession sqlSession) {
    final MapperProxyFactory<T> mapperProxyFactory = (MapperProxyFactory<T>) 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);
    }
  }
  
 // 第四步
 // MapperProxyFactory 创建MapperProxy对象
   protected T newInstance(MapperProxy<T> mapperProxy) {
    return (T) Proxy.newProxyInstance(mapperInterface.getClassLoader(), new Class[] { mapperInterface }, mapperProxy);
  }
  public T newInstance(SqlSession sqlSession) {
    final MapperProxy<T> mapperProxy = new MapperProxy<T>(sqlSession, mapperInterface, methodCache);
    return newInstance(mapperProxy);
  }

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由上面 MapperProxyFactory 代码可知, 每次执行 session.getMapper 都会创建 MapperProxy 对象及其代理对象 ,所以应避免多次调用 session.getMapper

2.3 执行查询

MyBatis使用JDK代理方式, MapperProxy 实现了 InvocationHandler 接口,所以Mapper接口类的实现方法,是在 MapperProxy.invoke 方法里执行。

invoke 中,获取或创建一个 MapperMethod 对象,然后执行 MapperMethod.execute 方法。

//// 获取MapperMethod对象
  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;
  }
  
//// 执行execute
final MapperMethod mapperMethod = cachedMapperMethod(method);
return mapperMethod.execute(sqlSession, args);
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2.3.1 MapperMethod创建

MapperMethod 有两个属性:

private final SqlCommand command;
  private final MethodSignature method;
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1) SqlCommand

SqlCommand 有两个属性:

  • String name: MappedStatement的id;
  • SqlCommandType type: MappedStatement的sqlCommandType, UNKNOWN, INSERT, UPDATE, DELETE, SELECT, FLUSH。

SqlCommand创建时,会从Configuration中获取MappedStatement对象,获取到则用于name和type的赋值:

String statementId = mapperInterface.getName() + "." + methodName;

if (configuration.hasStatement(statementId)) {
    return configuration.getMappedStatement(statementId);
}
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2) MethodSignature

private final boolean returnsMany; // configuration.getObjectFactory().isCollection(this.returnType) || this.returnType.isArray()
    private final boolean returnsMap; // 
    private final boolean returnsVoid; // void.class.equals(this.returnType)
    private final boolean returnsCursor; // org.apache.ibatis.cursor.Cursor.class.equals(this.returnType)
    private final Class<?> returnType;
    //////
    private final ParamNameResolver paramNameResolver; //  new ParamNameResolver(configuration, method)
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2.3.2 MapperMethod.execute

根据 command.type ,判断执行的操作:

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 {
          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;
  }
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在execute调用的 executeFor* 方法中,最终调用的是 sqlSession.select* 方法。

2.3.3 session.selectList

public <E> List<E> selectList(String statement, Object parameter, RowBounds rowBounds) {
    try {
      // 第一步,从Configuration中取出MappedStatement对象
      // statement : interfaceName + "." + methodName
      MappedStatement ms = configuration.getMappedStatement(statement);
      
      // 第二步,执行查询
      // executor : 前面Configuration.newExecutor创建的executor,默认SimpleExecutor
      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();
    }
  }
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executor.query的调用链:

BaseExecutor.query -> BaseExecutor.queryFromDatabase -> SimpleExecutor.doQuery

SimpleExecutor.doQuery:

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();
      
      // 第一步 创建RoutingStatementHandler
      StatementHandler handler = configuration.newStatementHandler(wrapper, ms, parameter, rowBounds, resultHandler, boundSql);
      
      // 第二步 调用StatementHandler.prepare创建Statement
      stmt = prepareStatement(handler, ms.getStatementLog());
      
      // 第三步 执行StatementHandler.query
      return handler.<E>query(stmt, resultHandler);
    } finally {
      closeStatement(stmt);
    }
  }
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2.3.4 StatementHandler

1) RoutingStatementHandler

configuration.newStatementHandler中创建的是 RoutingStatementHandler ,然后设置拦截器:

public StatementHandler newStatementHandler(Executor executor, MappedStatement mappedStatement, Object parameterObject, RowBounds rowBounds, ResultHandler resultHandler, BoundSql boundSql) {
    StatementHandler statementHandler = new RoutingStatementHandler(executor, mappedStatement, parameterObject, rowBounds, resultHandler, boundSql);
    statementHandler = (StatementHandler) interceptorChain.pluginAll(statementHandler);
    return statementHandler;
  }
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RoutingStatementHandler 是一种委派模式,根据 MappedStatement.statementType 的不同,返回不同的 StatementHandler 实现类:

// 代理
  private final StatementHandler 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());
    }
  }
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2) StatementHandler.prepare

创建 java.sql.Statement 对象,调用链:

RoutingStatementHandler.prepare -> BaseStatementHandler.prepare -> PreparedStatementHandler.instantiateStatement

instantiateStatementBaseStatementHandler 的抽象方法,供子类实现。

// PreparedStatementHandler
  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();
    }
  }
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3) StatementHandler.query

有了 Statement 之后,就可以拿来进行查询了:

@Override
  public <E> List<E> query(Statement statement, ResultHandler resultHandler) throws SQLException {
    PreparedStatement ps = (PreparedStatement) statement;
    ps.execute();
    return resultSetHandler.<E> handleResultSets(ps);
  }
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原文  https://juejin.im/post/5ec24d255188256d4f7eea23
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