在上一章内容中我们又欠下了这些技术债：

parameterHandler
resultSetHandler
KeyGenerator

今天我们就来着重解决一下关于KeyGenerator的源码。

1. KeyGenerator解析

首先我们需要了解这个类主要功能。我们来看官网介绍：
selectKey 元素中的语句将会首先或之后运行，然后插入语句会被调用。这可以提供给你一个与数据库中自动生成主键类似的行为，同时保持了 Java 代码的简洁。
我们再来看下有哪些属性：

selectKey 元素的属性

介绍完之后我们再来看下源码，先来看接口源码，其中的方法对应order属性的顺序：

public interface KeyGenerator {

  void processBefore(Executor executor, MappedStatement ms, Statement stmt, Object parameter);

  void processAfter(Executor executor, MappedStatement ms, Statement stmt, Object parameter);

}

KeyGennerator子类继承

我们以一个例子来进行说明：

@Options(useGeneratedKeys = true, keyProperty = "id")
@Insert({"insert into country (countryname,countrycode) values (#{countryname},#{countrycode})"})
int insertBean(Country country);

然后我们来看一处源码，看用的是KeyGenerator的哪个子类。在MappedStatement的Builder内部类中：

      mappedStatement.keyGenerator = configuration.isUseGeneratedKeys() && SqlCommandType.INSERT.equals(sqlCommandType) ? Jdbc3KeyGenerator.INSTANCE : NoKeyGenerator.INSTANCE;

我们可以看到用的是Jdbc3KeyGenerator这个类。

2. Jdbc3KeyGenerator解析

首先我们先来看类构造方法：

public class Jdbc3KeyGenerator implements KeyGenerator {

  /**
   * A shared instance.
   *
   * @since 3.4.3
   */
  public static final Jdbc3KeyGenerator INSTANCE = new Jdbc3KeyGenerator();

这里可以看到我们用了单例模式中的饿汉式。再来看他的实现方法：

  @Override
  public void processBefore(Executor executor, MappedStatement ms, Statement stmt, Object parameter) {
    // do nothing
  }

  @Override
  public void processAfter(Executor executor, MappedStatement ms, Statement stmt, Object parameter) {
    processBatch(ms, stmt, parameter);
  }

继续深入：

  public void processBatch(MappedStatement ms, Statement stmt, Object parameter) {
    // 获得主键属性的配置。如果为空，则直接返回，说明不需要主键
    final String[] keyProperties = ms.getKeyProperties();
    if (keyProperties == null || keyProperties.length == 0) {
      return;
    }
    try (ResultSet rs = stmt.getGeneratedKeys()) {
      // 获得返回的自增主键
      final ResultSetMetaData rsmd = rs.getMetaData();
      final Configuration configuration = ms.getConfiguration();
      if (rsmd.getColumnCount() < keyProperties.length) {
        // Error?
      } else {
        //赋值自增主键值
        assignKeys(configuration, rs, rsmd, keyProperties, parameter);
      }
    } catch (Exception e) {
      throw new ExecutorException("Error getting generated key or setting result to parameter object. Cause: " + e, e);
    }
  }

那么重点来了：

  private void assignKeys(Configuration configuration, ResultSet rs, ResultSetMetaData rsmd, String[] keyProperties,
      Object parameter) throws SQLException {
    if (parameter instanceof ParamMap || parameter instanceof StrictMap) {
      // Multi-param or single param with @Param
      assignKeysToParamMap(configuration, rs, rsmd, keyProperties, (Map<String, ?>) parameter);
    } else if (parameter instanceof ArrayList && !((ArrayList<?>) parameter).isEmpty()
        && ((ArrayList<?>) parameter).get(0) instanceof ParamMap) {
      // Multi-param or single param with @Param in batch operation
      assignKeysToParamMapList(configuration, rs, rsmd, keyProperties, ((ArrayList<ParamMap<?>>) parameter));
    } else {
      // Single param without @Param
      assignKeysToParam(configuration, rs, rsmd, keyProperties, parameter);
    }
  }

我们按照顺序来进行分析，首先便是多@Param参数情况下，会形成一个ParamMap，之后调用 assignKeysToParamMap方法：

  private void assignKeysToParamMap(Configuration configuration, ResultSet rs, ResultSetMetaData rsmd,
      String[] keyProperties, Map<String, ?> paramMap) throws SQLException {
    //如果参数为空直接返回
    if (paramMap.isEmpty()) {
      return;
    }
    Map<String, Entry<Iterator<?>, List<KeyAssigner>>> assignerMap = new HashMap<>();
    for (int i = 0; i < keyProperties.length; i++) {
      //获取组成形成KeyAssigner
      Entry<String, KeyAssigner> entry = getAssignerForParamMap(configuration, rsmd, i + 1, paramMap, keyProperties[i],
          keyProperties, true);
      Entry<Iterator<?>, List<KeyAssigner>> iteratorPair = assignerMap.computeIfAbsent(entry.getKey(),
          k -> entry(collectionize(paramMap.get(k)).iterator(), new ArrayList<>()));
      iteratorPair.getValue().add(entry.getValue());
    }
    long counter = 0;
    while (rs.next()) {
      for (Entry<Iterator<?>, List<KeyAssigner>> pair : assignerMap.values()) {
        if (!pair.getKey().hasNext()) {
          throw new ExecutorException(String.format(MSG_TOO_MANY_KEYS, counter));
        }
        Object param = pair.getKey().next();
        //反射赋值
        pair.getValue().forEach(x -> x.assign(rs, param));
      }
      counter++;
    }
  }

大概逻辑就是整理完之后挨个遍历赋值，这里重点便是getAssignerForParamMap方法，我们进入一起看下：

 private Entry<String, KeyAssigner> getAssignerForParamMap(Configuration config, ResultSetMetaData rsmd,
      int columnPosition, Map<String, ?> paramMap, String keyProperty, String[] keyProperties, boolean omitParamName) {
    //判断paramMap是否只有一个唯一key
    boolean singleParam = paramMap.values().stream().distinct().count() == 1;
    //获取keyProperty的.的位置
    int firstDot = keyProperty.indexOf('.');
    //如果是多个参数，但是keyProperty又不包含.，会抛出错误
    if (firstDot == -1) {
      if (singleParam) {
        return getAssignerForSingleParam(config, rsmd, columnPosition, paramMap, keyProperty, omitParamName);
      }
      throw new ExecutorException("Could not determine which parameter to assign generated keys to. "
          + "Note that when there are multiple parameters, 'keyProperty' must include the parameter name (e.g. 'param.id'). "
          + "Specified key properties are " + ArrayUtil.toString(keyProperties) + " and available parameters are "
          + paramMap.keySet());
    }
    //多参数，需要截取keyProperty的.之前部分，去paramMap拿对应值
    String paramName = keyProperty.substring(0, firstDot);
    if (paramMap.containsKey(paramName)) {
      String argParamName = omitParamName ? null : paramName;
      String argKeyProperty = keyProperty.substring(firstDot + 1);
      return entry(paramName, new KeyAssigner(config, rsmd, columnPosition, argParamName, argKeyProperty));
    } else if (singleParam) {
      return getAssignerForSingleParam(config, rsmd, columnPosition, paramMap, keyProperty, omitParamName);
    } else {
      throw new ExecutorException("Could not find parameter '" + paramName + "'. "
          + "Note that when there are multiple parameters, 'keyProperty' must include the parameter name (e.g. 'param.id'). "
          + "Specified key properties are " + ArrayUtil.toString(keyProperties) + " and available parameters are "
          + paramMap.keySet());
    }
  }

根据注释，我们能看懂个大概，之后就进入getAssignerForSingleParam方法：

 private Entry<String, KeyAssigner> getAssignerForSingleParam(Configuration config, ResultSetMetaData rsmd,
      int columnPosition, Map<String, ?> paramMap, String keyProperty, boolean omitParamName) {
    // Assume 'keyProperty' to be a property of the single param.
    String singleParamName = nameOfSingleParam(paramMap);
    String argParamName = omitParamName ? null : singleParamName;
    return entry(singleParamName, new KeyAssigner(config, rsmd, columnPosition, argParamName, keyProperty));
  }

还记得之前说的遍历赋值逻辑么，也就是

pair.getValue().forEach(x -> x.assign(rs, param));方法中赋值：

我们来看下这个方法：

protected void assign(ResultSet rs, Object param) {
      if (paramName != null) {
        // If paramName is set, param is ParamMap
        param = ((ParamMap<?>) param).get(paramName);
      }
      MetaObject metaParam = configuration.newMetaObject(param);
      try {
        if (typeHandler == null) {
          if (metaParam.hasSetter(propertyName)) {
            Class<?> propertyType = metaParam.getSetterType(propertyName);
            typeHandler = typeHandlerRegistry.getTypeHandler(propertyType,
                JdbcType.forCode(rsmd.getColumnType(columnPosition)));
          } else {
            throw new ExecutorException("No setter found for the keyProperty '" + propertyName + "' in '"
                + metaParam.getOriginalObject().getClass().getName() + "'.");
          }
        }
        if (typeHandler == null) {
          // Error?
        } else {
          Object value = typeHandler.getResult(rs, columnPosition);
          metaParam.setValue(propertyName, value);
        }
      } catch (SQLException e) {
        throw new ExecutorException("Error getting generated key or setting result to parameter object. Cause: " + e,
            e);
      }
    }
  }

这里就是借助一些辅助的类来进行反射赋值了，逻辑自己整理下不难。下面我们也过一下SelectKeyGenerator类。

3. SelectKeyGenerator解析

我们先继续来看构造方法：

public class SelectKeyGenerator implements KeyGenerator {

  public static final String SELECT_KEY_SUFFIX = "!selectKey";
  private final boolean executeBefore;
  private final MappedStatement keyStatement;

  public SelectKeyGenerator(MappedStatement keyStatement, boolean executeBefore) {
    this.executeBefore = executeBefore;
    this.keyStatement = keyStatement;
  }
}

之后我们整体再来看一下它的继承实现，我们只是简单的过一下，不会具体深入：

@Override
  public void processBefore(Executor executor, MappedStatement ms, Statement stmt, Object parameter) {
    if (executeBefore) {
      processGeneratedKeys(executor, ms, parameter);
    }
  }

  @Override
  public void processAfter(Executor executor, MappedStatement ms, Statement stmt, Object parameter) {
    if (!executeBefore) {
      processGeneratedKeys(executor, ms, parameter);
    }
  }

  private void processGeneratedKeys(Executor executor, MappedStatement ms, Object parameter) {
    try {
      if (parameter != null && keyStatement != null && keyStatement.getKeyProperties() != null) {
        String[] keyProperties = keyStatement.getKeyProperties();
        final Configuration configuration = ms.getConfiguration();
        final MetaObject metaParam = configuration.newMetaObject(parameter);
        if (keyProperties != null) {
          // Do not close keyExecutor.
          // The transaction will be closed by parent executor.
          Executor keyExecutor = configuration.newExecutor(executor.getTransaction(), ExecutorType.SIMPLE);
          List<Object> values = keyExecutor.query(keyStatement, parameter, RowBounds.DEFAULT, Executor.NO_RESULT_HANDLER);
          if (values.size() == 0) {
            throw new ExecutorException("SelectKey returned no data.");
          } else if (values.size() > 1) {
            throw new ExecutorException("SelectKey returned more than one value.");
          } else {
            MetaObject metaResult = configuration.newMetaObject(values.get(0));
            if (keyProperties.length == 1) {
              if (metaResult.hasGetter(keyProperties[0])) {
                setValue(metaParam, keyProperties[0], metaResult.getValue(keyProperties[0]));
              } else {
                // no getter for the property - maybe just a single value object
                // so try that
                setValue(metaParam, keyProperties[0], values.get(0));
              }
            } else {
              handleMultipleProperties(keyProperties, metaParam, metaResult);
            }
          }
        }
      }
    } catch (ExecutorException e) {
      throw e;
    } catch (Exception e) {
      throw new ExecutorException("Error selecting key or setting result to parameter object. Cause: " + e, e);
    }
  }

这里主要SQL之前还是之后的都会到相同的方法，方法内部就是新建一个Executor，然后向数据库发送一条简单的sql执行语句，返回内容则赋值到参数中。

最后还剩一个NoKeyGenerator

4. NoKeyGenerator解析

public class NoKeyGenerator implements KeyGenerator {

  /**
   * A shared instance.
   * @since 3.4.3
   */
  public static final NoKeyGenerator INSTANCE = new NoKeyGenerator();

  @Override
  public void processBefore(Executor executor, MappedStatement ms, Statement stmt, Object parameter) {
    // Do Nothing
  }

  @Override
  public void processAfter(Executor executor, MappedStatement ms, Statement stmt, Object parameter) {
    // Do Nothing
  }

}

这个类就比较简单了，没什么特殊作用。

5. 今日总结

今天我们接触了关于KeyGenerator的源码，理解了他们的作用，我们再来做下小结：

Jdbc3KeyGenerator：只能在执行SQL之后，它是一起的，通过业务SQL的执行从中获取数据库自增主键信息并匹配复制
SelectKeyGenerator： 可以在执行SQL之前或之后运行，都是通过另起访问一个简单的SQL查询到值并进行注入
NoKeyGenerator： 什么操作都没有