MyBatis通用Mapper与分页PageHelper混淆报错问题

曾经终败给现在 2022-05-14 16:39 191阅读 0赞

### 背景 ###

当同时引入`通用Mapper`与`PageHelper`两款插件的时候，会存在报错的可能。

如果像这样，先执行通用Mapper,再执行分页插件就会出错

<plugin interceptor="com.github.abel533.mapperhelper.MapperInterceptor">
    			<property name="mappers" value="com.github.abel533.mapper.Mapper"/>
    		</plugin>
    
    		
    		<plugin interceptor="com.github.pagehelper.PageHelper">
    			<property name="dialect" value="mysql"/>
    		</plugin>

--------------------

### 原因解析 ###

Mybatis中存在一个拦截器的回调接口，可以在拦截Myabatis四大杀器`Executor、StatementHandler`,`ParameterHandler`,`ResultHandler`。为什么只能拦截这四个接口？，因为它们都调用了拦截器栈。  
在`Configuration`中实例化了这几个组件:

public Executor newExecutor(Transaction transaction, ExecutorType executorType) { 
            executorType = executorType == null ? this.defaultExecutorType : executorType;
            executorType = executorType == null ? ExecutorType.SIMPLE : executorType;
            Object 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 (this.cacheEnabled) { 
                executor = new CachingExecutor((Executor)executor);
            }
            Executor executor = (Executor)this.interceptorChain.pluginAll(executor);
            return executor;
        }

public ParameterHandler newParameterHandler(MappedStatement mappedStatement, Object parameterObject, BoundSql boundSql) { 
            ParameterHandler parameterHandler = mappedStatement.getLang().createParameterHandler(mappedStatement, parameterObject, boundSql);
            parameterHandler = (ParameterHandler)this.interceptorChain.pluginAll(parameterHandler);
            return parameterHandler;
        }

public ResultSetHandler newResultSetHandler(Executor executor, MappedStatement mappedStatement, RowBounds rowBounds, ParameterHandler parameterHandler, ResultHandler resultHandler, BoundSql boundSql) { 
            ResultSetHandler resultSetHandler = new DefaultResultSetHandler(executor, mappedStatement, parameterHandler, resultHandler, boundSql, rowBounds);
            ResultSetHandler resultSetHandler = (ResultSetHandler)this.interceptorChain.pluginAll(resultSetHandler);
            return resultSetHandler;
        }

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 = (StatementHandler)this.interceptorChain.pluginAll(statementHandler);
            return statementHandler;
        }

上面的四个方法都调用了`pluginAll`，interceptorChain是一种责任链的设计模式

当自定的`interceptor`加入到拦截器链中，就可以获取到上面四个对象，通过拿到对象的属性进一步扩展，例如修改SQL，大部分的情况下都是针对SQL进行自定义的扩展，`PageHelper`与`通用Mapper`就是这样处理的。

**下面我们来谈谈为什么出错？**  
这个问题出在`通用Mapper这里`  
`MapperInterceptor`在做拦截的时候会将`MappedStatement`的执行对象替换为`MapperProvider`对象，`MapperProvider`是通用`Mapper`自己定义的，在原有的基础上进行封装增强，为我们封装了一些常用的`CRUD`操作，所以我们一般只需要继承`通用Mapper`的接口就可以实现大部分逻辑。但是它的`实例化必须依赖带参数的构造函数`

public class MapperProvider extends MapperTemplate { 
      public MapperProvider(Class<?> mapperClass, MapperHelper mapperHelper) { 
           super(mapperClass, mapperHelper);
       }

而分页插件需要调用的原生的`MappedStatement`对象来完成一些操作，在创建`SqlSource`对象时，利用反射来获取`SQL`

private SqlSource createSqlSource(Object parameterObject) { 
            try { 
                String sql;
                if (this.providerTakesParameterObject) { 
                    sql = (String)this.providerMethod.invoke(this.providerType.newInstance(), parameterObject);
                } else { 
                    sql = (String)this.providerMethod.invoke(this.providerType.newInstance());
                }
    
                Class<?> parameterType = parameterObject == null ? Object.class : parameterObject.getClass();
                StaticSqlSource sqlSource = (StaticSqlSource)this.sqlSourceParser.parse(sql, parameterType, new HashMap());
                return new OrderByStaticSqlSource(sqlSource);
            } catch (Exception var5) { 
                throw new BuilderException("Error invoking SqlProvider method (" + this.providerType.getName() + "." + this.providerMethod.getName() + "). Cause: " + var5, var5);
            }
        }

重点在`sql = (String)this.providerMethod.invoke(this.providerType.newInstance(),`这个调用无参构造器来实例化的，现在通过拦截器链调用到这里就`providerType`就变为`MapperProvider`，前面提到过的`MapperProvider`没有默认的构造函数支持，所以创建失败。

--------------------

### 解决方案 ###

调整顺序先用分页插件，再用通用mapper

<plugin interceptor="com.github.pagehelper.PageHelper">
    			<property name="dialect" value="mysql"/>
    		</plugin>
    		
    		<plugin interceptor="com.github.abel533.mapperhelper.MapperInterceptor">
    			<property name="mappers" value="com.github.abel533.mapper.Mapper"/>
    		</plugin>