今天的我们已经没有欠下技术债了，所以我们来探讨下关于MyBatis的延迟加载。

首先我们来看官网的说明：

MyBatis 能够对嵌套查询进行延迟加载，因此可以将大量语句同时运行的开销分散开来。 然而，如果你加载记录列表之后立刻就遍历列表以获取嵌套的数据，就会触发所有的延迟加载查询，性能可能会变得很糟糕。

这里我需要着重说明一点：
延迟加载在Mybatis中不是可以针对每一种情况的，它必须是在嵌套返回的情况下才能够进行使用。

有了下面的说明后，我们可以返回在解析返回结果时，之前我们没有说明的嵌套解析部分：

  public void handleRowValues(ResultSetWrapper rsw, ResultMap resultMap, ResultHandler<?> resultHandler, RowBounds rowBounds, ResultMapping parentMapping) throws SQLException {
    //处理嵌套映射的情况
    if (resultMap.hasNestedResultMaps()) {
      ensureNoRowBounds();
      checkResultHandler();
      handleRowValuesForNestedResultMap(rsw, resultMap, resultHandler, rowBounds, parentMapping);
    } else {
      //处理简单映射情况
      handleRowValuesForSimpleResultMap(rsw, resultMap, resultHandler, rowBounds, parentMapping);
    }
  }

下面开始我们分析。

1. 嵌套映射处理分析

这里我们先来看有2个判断条件，比较简单，我们直接过一下就行：

  private void ensureNoRowBounds() {
    if (configuration.isSafeRowBoundsEnabled() && rowBounds != null && (rowBounds.getLimit() < RowBounds.NO_ROW_LIMIT || rowBounds.getOffset() > RowBounds.NO_ROW_OFFSET)) {
      throw new ExecutorException("Mapped Statements with nested result mappings cannot be safely constrained by RowBounds. "
          + "Use safeRowBoundsEnabled=false setting to bypass this check.");
    }
  }

  protected void checkResultHandler() {
    if (resultHandler != null && configuration.isSafeResultHandlerEnabled() && !mappedStatement.isResultOrdered()) {
      throw new ExecutorException("Mapped Statements with nested result mappings cannot be safely used with a custom ResultHandler. "
          + "Use safeResultHandlerEnabled=false setting to bypass this check "
          + "or ensure your statement returns ordered data and set resultOrdered=true on it.");
    }
  }

这里也不用太过关注，我们重点放在主要逻辑上：

private void handleRowValuesForNestedResultMap(ResultSetWrapper rsw, ResultMap resultMap, ResultHandler<?> resultHandler, RowBounds rowBounds, ResultMapping parentMapping) throws SQLException {
    final DefaultResultContext<Object> resultContext = new DefaultResultContext<>();
    ResultSet resultSet = rsw.getResultSet();
    skipRows(resultSet, rowBounds);
    Object rowValue = previousRowValue;
    while (shouldProcessMoreRows(resultContext, rowBounds) && !resultSet.isClosed() && resultSet.next()) {
      final ResultMap discriminatedResultMap = resolveDiscriminatedResultMap(resultSet, resultMap, null);
      final CacheKey rowKey = createRowKey(discriminatedResultMap, rsw, null);
      Object partialObject = nestedResultObjects.get(rowKey);
      // issue #577 && #542
      if (mappedStatement.isResultOrdered()) {
        if (partialObject == null && rowValue != null) {
          nestedResultObjects.clear();
          storeObject(resultHandler, resultContext, rowValue, parentMapping, resultSet);
        }
        rowValue = getRowValue(rsw, discriminatedResultMap, rowKey, null, partialObject);
      } else {
        rowValue = getRowValue(rsw, discriminatedResultMap, rowKey, null, partialObject);
        if (partialObject == null) {
          storeObject(resultHandler, resultContext, rowValue, parentMapping, resultSet);
        }
      }
    }
    if (rowValue != null && mappedStatement.isResultOrdered() && shouldProcessMoreRows(resultContext, rowBounds)) {
      storeObject(resultHandler, resultContext, rowValue, parentMapping, resultSet);
      previousRowValue = null;
    } else if (rowValue != null) {
      previousRowValue = rowValue;
    }
  }

这里延迟加载的深度比较深，我们简单的过一下：
从getRowValue-> createResultObject->createParameterizedResultObject->getNestedQueryConstructorValue

这里的过程会比较的绕很烦，我目前是打算理解其意即可，所以我们重点放在getNestedQueryConstructorValue方法上：

 private Object getNestedQueryConstructorValue(ResultSet rs, ResultMapping constructorMapping, String columnPrefix) throws SQLException {
    final String nestedQueryId = constructorMapping.getNestedQueryId();
    final MappedStatement nestedQuery = configuration.getMappedStatement(nestedQueryId);
    final Class<?> nestedQueryParameterType = nestedQuery.getParameterMap().getType();
    final Object nestedQueryParameterObject = prepareParameterForNestedQuery(rs, constructorMapping, nestedQueryParameterType, columnPrefix);
    Object value = null;
    if (nestedQueryParameterObject != null) {
      final BoundSql nestedBoundSql = nestedQuery.getBoundSql(nestedQueryParameterObject);
      final CacheKey key = executor.createCacheKey(nestedQuery, nestedQueryParameterObject, RowBounds.DEFAULT, nestedBoundSql);
      final Class<?> targetType = constructorMapping.getJavaType();
      final ResultLoader resultLoader = new ResultLoader(configuration, executor, nestedQuery, nestedQueryParameterObject, targetType, key, nestedBoundSql);
      value = resultLoader.loadResult();
    }
    return value;
  }

这里会出现一个新的类ResultLoader，专门是和延迟加载相关的，我们来看下

2. ResultLoader解析

我们先来看属性和构造方法：

public class ResultLoader {

  protected final Configuration configuration;
  protected final Executor executor;
  protected final MappedStatement mappedStatement;
  protected final Object parameterObject;
  protected final Class<?> targetType;
  protected final ObjectFactory objectFactory;
  protected final CacheKey cacheKey;
  protected final BoundSql boundSql;
  protected final ResultExtractor resultExtractor;
  protected final long creatorThreadId;

  protected boolean loaded;
  protected Object resultObject;

  public ResultLoader(Configuration config, Executor executor, MappedStatement mappedStatement, Object parameterObject, Class<?> targetType, CacheKey cacheKey, BoundSql boundSql) {
    this.configuration = config;
    this.executor = executor;
    this.mappedStatement = mappedStatement;
    this.parameterObject = parameterObject;
    this.targetType = targetType;
    this.objectFactory = configuration.getObjectFactory();
    this.cacheKey = cacheKey;
    this.boundSql = boundSql;
    this.resultExtractor = new ResultExtractor(configuration, objectFactory);
    this.creatorThreadId = Thread.currentThread().getId();
  }

再来看下他的方法：

  public Object loadResult() throws SQLException {
    List<Object> list = selectList();
    resultObject = resultExtractor.extractObjectFromList(list, targetType);
    return resultObject;
  }


 private <E> List<E> selectList() throws SQLException {
    Executor localExecutor = executor;
    //如果当前线程不是创建线程，则调用 #newExecutor() 方法，创建 Executor 对象，因为 Executor 是非线程安全的。
    if (Thread.currentThread().getId() != this.creatorThreadId || localExecutor.isClosed()) {
      localExecutor = newExecutor();
    }
    try {
      return localExecutor.query(mappedStatement, parameterObject, RowBounds.DEFAULT, Executor.NO_RESULT_HANDLER, cacheKey, boundSql);
    } finally {
      if (localExecutor != executor) {
        localExecutor.close(false);
      }
    }
  }


  private Executor newExecutor() {
    final Environment environment = configuration.getEnvironment();
    if (environment == null) {
      throw new ExecutorException("ResultLoader could not load lazily.  Environment was not configured.");
    }
    final DataSource ds = environment.getDataSource();
    if (ds == null) {
      throw new ExecutorException("ResultLoader could not load lazily.  DataSource was not configured.");
    }
    final TransactionFactory transactionFactory = environment.getTransactionFactory();
    final Transaction tx = transactionFactory.newTransaction(ds, null, false);
    return configuration.newExecutor(tx, ExecutorType.SIMPLE);
  }

这里的逻辑就是额外使用localExecutor查询

接下来我们在进入另一个类ResultExtractor：

3.ResultExtractor解析

我们直接整体来看：

public class ResultExtractor {
  private final Configuration configuration;
  private final ObjectFactory objectFactory;

  public ResultExtractor(Configuration configuration, ObjectFactory objectFactory) {
    this.configuration = configuration;
    this.objectFactory = objectFactory;
  }

  public Object extractObjectFromList(List<Object> list, Class<?> targetType) {
    Object value = null;
    //情况一，targetType 就是 list ，直接返回
    if (targetType != null && targetType.isAssignableFrom(list.getClass())) {
      value = list;
      // 情况二，targetType 是集合，添加到其中
    } else if (targetType != null && objectFactory.isCollection(targetType)) {
      value = objectFactory.create(targetType);
      MetaObject metaObject = configuration.newMetaObject(value);
      metaObject.addAll(list);
      // 情况三，targetType 是数组
    } else if (targetType != null && targetType.isArray()) {
      Class<?> arrayComponentType = targetType.getComponentType();
      Object array = Array.newInstance(arrayComponentType, list.size());
      if (arrayComponentType.isPrimitive()) {
        for (int i = 0; i < list.size(); i++) {
          Array.set(array, i, list.get(i));
        }
        value = array;
      } else {
        value = list.toArray((Object[])array);
      }
      // 情况四，普通对象，取首个对象
    } else {
      if (list != null && list.size() > 1) {
        throw new ExecutorException("Statement returned more than one row, where no more than one was expected.");
      } else if (list != null && list.size() == 1) {
        value = list.get(0);
      }
    }
    return value;
  }
}

4. 今日总结

今天其实我自己也不是太搞懂，关于这块的封装有点深，而且相关地方也比较多，就大概看一下吧。具体没有深入了解MyBatis的运作机制。允许我今天的水~~~