ParameterHandler是用來設定引數規則的，當StatementHandler呼叫prepare方法之後，接下來就是呼叫它來進行設定引數。

ParameterHandler介面：

public interface ParameterHandler {

  Object getParameterObject();

  void setParameters(PreparedStatement ps)
      throws SQLException;

}

ParameterHandler的預設實現類是DefaultParameterHandler，其實現了介面中定義的兩個方法。

getParameterObject是獲取引數，這個引數值就是你傳遞進來的值，可能是個實體、map或單個基本型別資料。

@Override
  public Object getParameterObject() {
    return parameterObject;
  }

//設定引數
  @Override
  public void setParameters(PreparedStatement ps) {
    ErrorContext.instance().activity("setting parameters").object(mappedStatement.getParameterMap().getId());
	//獲取所有引數，ParameterMapping是java型別和jdbc型別的對應關係
    List<ParameterMapping> parameterMappings = boundSql.getParameterMappings();
    if (parameterMappings != null) {
      for (int i = 0; i < parameterMappings.size(); i++) {
        ParameterMapping parameterMapping = parameterMappings.get(i);
        if (parameterMapping.getMode() != ParameterMode.OUT) {
		  //引數值
          Object value;
		  //獲取引數名稱
          String propertyName = parameterMapping.getProperty();
          if (boundSql.hasAdditionalParameter(propertyName)) { // issue #448 ask first for additional params
			//獲取引數值
            value = boundSql.getAdditionalParameter(propertyName);
          } else if (parameterObject == null) {
            value = null;
          } else if (typeHandlerRegistry.hasTypeHandler(parameterObject.getClass())) {
			 //如果是單個值則直接賦值
            value = parameterObject;
          } else {
            MetaObject metaObject = configuration.newMetaObject(parameterObject);
            value = metaObject.getValue(propertyName);
          }
         //獲取引數值對應的jdbc型別
          TypeHandler typeHandler = parameterMapping.getTypeHandler();
          JdbcType jdbcType = parameterMapping.getJdbcType();
          if (value == null && jdbcType == null) {
            jdbcType = configuration.getJdbcTypeForNull();
          }
          try {
	   //設定引數值和jdbc型別的對應關係
            typeHandler.setParameter(ps, i + 1, value, jdbcType);
          } catch (TypeException e) {
            throw new TypeException("Could not set parameters for mapping: " + parameterMapping + ". Cause: " + e, e);
          } catch (SQLException e) {
            throw new TypeException("Could not set parameters for mapping: " + parameterMapping + ". Cause: " + e, e);
          }
        }
      }
    }
  }
 

完整的DefaultParameterHandler原始碼如下：

/**
 * @author Clinton Begin
 * @author Eduardo Macarron
 */
public class DefaultParameterHandler implements ParameterHandler {

  private final TypeHandlerRegistry typeHandlerRegistry;

  private final MappedStatement mappedStatement;
  //所有的引數值
  private final Object parameterObject;
  private BoundSql boundSql;
  private Configuration configuration;

  public DefaultParameterHandler(MappedStatement mappedStatement, Object parameterObject, BoundSql boundSql) {
    this.mappedStatement = mappedStatement;
    this.configuration = mappedStatement.getConfiguration();
    this.typeHandlerRegistry = mappedStatement.getConfiguration().getTypeHandlerRegistry();
    this.parameterObject = parameterObject;
    this.boundSql = boundSql;
  }

  @Override
  public Object getParameterObject() {
    return parameterObject;
  }
  //設定引數
  @Override
  public void setParameters(PreparedStatement ps) {
    ErrorContext.instance().activity("setting parameters").object(mappedStatement.getParameterMap().getId());
	//獲取所有引數，ParameterMapping是java型別和jdbc型別的對應關係
    List<ParameterMapping> parameterMappings = boundSql.getParameterMappings();
    if (parameterMappings != null) {
      for (int i = 0; i < parameterMappings.size(); i++) {
        ParameterMapping parameterMapping = parameterMappings.get(i);
        if (parameterMapping.getMode() != ParameterMode.OUT) {
		  //引數值
          Object value;
		  //獲取引數名稱
          String propertyName = parameterMapping.getProperty();
          if (boundSql.hasAdditionalParameter(propertyName)) { // issue #448 ask first for additional params
			//獲取引數值
            value = boundSql.getAdditionalParameter(propertyName);
          } else if (parameterObject == null) {
            value = null;
          } else if (typeHandlerRegistry.hasTypeHandler(parameterObject.getClass())) {
			 //如果是單個值則直接賦值
            value = parameterObject;
          } else {
            MetaObject metaObject = configuration.newMetaObject(parameterObject);
            value = metaObject.getValue(propertyName);
          }
		  //獲取引數值對應的jdbc型別
          TypeHandler typeHandler = parameterMapping.getTypeHandler();
          JdbcType jdbcType = parameterMapping.getJdbcType();
          if (value == null && jdbcType == null) {
            jdbcType = configuration.getJdbcTypeForNull();
          }
          try {
			//設定引數值和jdbc型別的對應關係
            typeHandler.setParameter(ps, i + 1, value, jdbcType);
          } catch (TypeException e) {
            throw new TypeException("Could not set parameters for mapping: " + parameterMapping + ". Cause: " + e, e);
          } catch (SQLException e) {
            throw new TypeException("Could not set parameters for mapping: " + parameterMapping + ". Cause: " + e, e);
          }
        }
      }
    }
  }

}