浅谈MyBatis Plus主键设置策略

根据一次插入失败报错来了解下MyBatis Plus主键设置策略
今天学习使用MyBatis Plus，发现使用代码生成器生成对应的实体类、Service和Mapper后，在保存数据时报错

com.baomidou.mybatisplus.exceptions.MybatisPlusException: java.lang.reflect.InvocationTargetException
    at com.baomidou.mybatisplus.MybatisSqlSessionTemplate$SqlSessionInterceptor.invoke(MybatisSqlSessionTemplate.java:405)
    at com.sun.proxy.$Proxy70.insert(Unknown Source)
    at com.baomidou.mybatisplus.MybatisSqlSessionTemplate.insert(MybatisSqlSessionTemplate.java:243)
    at com.baomidou.mybatisplus.activerecord.Model.insert(Model.java:56)
    at com.lemon.rabbit.common.base.city.CityInitial.printInfo(CityInitial.java:112)
    at com.lemon.rabbit.common.base.city.CityInitial.parseNextLevel(CityInitial.java:87)
    at com.lemon.rabbit.common.base.city.CityInitial.test(CityInitial.java:59)
    at com.lemon.rabbit.RabbitApplicationTests.contextLoads(RabbitApplicationTests.java:19)
    at sun.reflect.NativeMethodAccessorImpl.invoke0(Native Method)
    at sun.reflect.NativeMethodAccessorImpl.invoke(NativeMethodAccessorImpl.java:62)
    at sun.reflect.DelegatingMethodAccessorImpl.invoke(DelegatingMethodAccessorImpl.java:43)
    at java.lang.reflect.Method.invoke(Method.java:498)
    at org.junit.runners.model.FrameworkMethod$1.runReflectiveCall(FrameworkMethod.java:50)
    at org.junit.internal.runners.model.ReflectiveCallable.run(ReflectiveCallable.java:12)
    at org.junit.runners.model.FrameworkMethod.invokeExplosively(FrameworkMethod.java:47)
    at org.junit.internal.runners.statements.InvokeMethod.evaluate(InvokeMethod.java:17)
    at org.springframework.test.context.junit4.statements.RunBeforeTestExecutionCallbacks.evaluate(RunBeforeTestExecutionCallbacks.java:73)
    at org.springframework.test.context.junit4.statements.RunAfterTestExecutionCallbacks.evaluate(RunAfterTestExecutionCallbacks.java:83)
    at org.springframework.test.context.junit4.statements.RunBeforeTestMethodCallbacks.evaluate(RunBeforeTestMethodCallbacks.java:75)
    at org.springframework.test.context.junit4.statements.RunAfterTestMethodCallbacks.evaluate(RunAfterTestMethodCallbacks.java:86)
    at org.springframework.test.context.junit4.statements.SpringRepeat.evaluate(SpringRepeat.java:84)
    at org.junit.runners.ParentRunner.runLeaf(ParentRunner.java:325)
    at org.springframework.test.context.junit4.SpringJUnit4ClassRunner.runChild(SpringJUnit4ClassRunner.java:251)
    at org.springframework.test.context.junit4.SpringJUnit4ClassRunner.runChild(SpringJUnit4ClassRunner.java:97)
    at org.junit.runners.ParentRunner$3.run(ParentRunner.java:290)
    at org.junit.runners.ParentRunner$1.schedule(ParentRunner.java:71)
    at org.junit.runners.ParentRunner.runChildren(ParentRunner.java:288)
    at org.junit.runners.ParentRunner.access$000(ParentRunner.java:58)
    at org.junit.runners.ParentRunner$2.evaluate(ParentRunner.java:268)
    at org.springframework.test.context.junit4.statements.RunBeforeTestClassCallbacks.evaluate(RunBeforeTestClassCallbacks.java:61)
    at org.springframework.test.context.junit4.statements.RunAfterTestClassCallbacks.evaluate(RunAfterTestClassCallbacks.java:70)
    at org.junit.runners.ParentRunner.run(ParentRunner.java:363)
    at org.springframework.test.context.junit4.SpringJUnit4ClassRunner.run(SpringJUnit4ClassRunner.java:190)
    at org.junit.runner.JUnitCore.run(JUnitCore.java:137)
    at com.intellij.junit4.JUnit4IdeaTestRunner.startRunnerWithArgs(JUnit4IdeaTestRunner.java:68)
    at com.intellij.rt.execution.junit.IdeaTestRunner$Repeater.startRunnerWithArgs(IdeaTestRunner.java:47)
    at com.intellij.rt.execution.junit.JUnitStarter.prepareStreamsAndStart(JUnitStarter.java:242)
    at com.intellij.rt.execution.junit.JUnitStarter.main(JUnitStarter.java:70)
Caused by: java.lang.reflect.InvocationTargetException
    at sun.reflect.NativeMethodAccessorImpl.invoke0(Native Method)
    at sun.reflect.NativeMethodAccessorImpl.invoke(NativeMethodAccessorImpl.java:62)
    at sun.reflect.DelegatingMethodAccessorImpl.invoke(DelegatingMethodAccessorImpl.java:43)
    at java.lang.reflect.Method.invoke(Method.java:498)
    at com.baomidou.mybatisplus.MybatisSqlSessionTemplate$SqlSessionInterceptor.invoke(MybatisSqlSessionTemplate.java:401)
    ... 37 more
Caused by: org.apache.ibatis.exceptions.PersistenceException: 
### Error updating database.  Cause: org.apache.ibatis.reflection.ReflectionException: Could not set property 'id' of 'class com.lemon.rabbit.model.City' with value '1017367047558582273' Cause: java.lang.IllegalArgumentException: argument type mismatch
### Cause: org.apache.ibatis.reflection.ReflectionException: Could not set property 'id' of 'class com.lemon.rabbit.model.City' with value '1017367047558582273' Cause: java.lang.IllegalArgumentException: argument type mismatch
    at org.apache.ibatis.exceptions.ExceptionFactory.wrapException(ExceptionFactory.java:30)
    at org.apache.ibatis.session.defaults.DefaultSqlSession.update(DefaultSqlSession.java:200)
    at org.apache.ibatis.session.defaults.DefaultSqlSession.insert(DefaultSqlSession.java:185)
    ... 42 more
Caused by: org.apache.ibatis.reflection.ReflectionException: Could not set property 'id' of 'class com.lemon.rabbit.model.City' with value '1017367047558582273' Cause: java.lang.IllegalArgumentException: argument type mismatch
    at org.apache.ibatis.reflection.wrapper.BeanWrapper.setBeanProperty(BeanWrapper.java:185)
    at org.apache.ibatis.reflection.wrapper.BeanWrapper.set(BeanWrapper.java:59)
    at org.apache.ibatis.reflection.MetaObject.setValue(MetaObject.java:140)
    at com.baomidou.mybatisplus.MybatisDefaultParameterHandler.populateKeys(MybatisDefaultParameterHandler.java:217)
    at com.baomidou.mybatisplus.MybatisDefaultParameterHandler.processBatch(MybatisDefaultParameterHandler.java:156)
    at com.baomidou.mybatisplus.MybatisDefaultParameterHandler.<init>(MybatisDefaultParameterHandler.java:71)
    at com.baomidou.mybatisplus.MybatisXMLLanguageDriver.createParameterHandler(MybatisXMLLanguageDriver.java:37)
    at org.apache.ibatis.session.Configuration.newParameterHandler(Configuration.java:545)
    at org.apache.ibatis.executor.statement.BaseStatementHandler.<init>(BaseStatementHandler.java:69)
    at org.apache.ibatis.executor.statement.PreparedStatementHandler.<init>(PreparedStatementHandler.java:40)
    at org.apache.ibatis.executor.statement.RoutingStatementHandler.<init>(RoutingStatementHandler.java:46)
    at org.apache.ibatis.session.Configuration.newStatementHandler(Configuration.java:558)
    at org.apache.ibatis.executor.SimpleExecutor.doUpdate(SimpleExecutor.java:48)
    at org.apache.ibatis.executor.BaseExecutor.update(BaseExecutor.java:117)
    at org.apache.ibatis.executor.CachingExecutor.update(CachingExecutor.java:76)
    at org.apache.ibatis.session.defaults.DefaultSqlSession.update(DefaultSqlSession.java:198)
    ... 43 more
Caused by: java.lang.IllegalArgumentException: argument type mismatch
    at sun.reflect.NativeMethodAccessorImpl.invoke0(Native Method)
    at sun.reflect.NativeMethodAccessorImpl.invoke(NativeMethodAccessorImpl.java:62)
    at sun.reflect.DelegatingMethodAccessorImpl.invoke(DelegatingMethodAccessorImpl.java:43)
    at java.lang.reflect.Method.invoke(Method.java:498)
    at org.apache.ibatis.reflection.invoker.MethodInvoker.invoke(MethodInvoker.java:41)
    at org.apache.ibatis.reflection.wrapper.BeanWrapper.setBeanProperty(BeanWrapper.java:180)
    ... 58 more

实体类City的主键是Integer类型的，在进行insert操作时，MyBatis Plus自动生成了一个Long类型的主键id，导致参数类型不匹配，出现上述错误

经过查看日志和调试发现，MyBatis最终调用BeanWrapper的setBeanProperty方法，通过反射执行最终的插入操作（增删改查应该都是通过此处的反射，不过暂时只调试了insert方法）

private void setBeanProperty(PropertyTokenizer prop, Object object, Object value) {
    try {
        Invoker method = metaClass.getSetInvoker(prop.getName());
        Object[] params = {value};
        try {
            method.invoke(object, params);
        } catch (Throwable t) {
            throw ExceptionUtil.unwrapThrowable(t);
        }
    } catch (Throwable t) {
        throw new ReflectionException("Could not set property '" + prop.getName() + "' of '" + object.getClass() + "' with value '" + value + "' Cause: " + t.toString(), t);
    }
}

此处传入的object即为我们想要保存到数据库的实体信息（不带ID信息），value为主键信息，此时主键值已经是一个Long类型的值，我们接着向上看value是哪里传过来的

setBeanProperty是一个私有方法，在本类调用，查询看到set(PropertyTokenizer prop, Object value)方法调用了它

@Override
public void set(PropertyTokenizer prop, Object value) {
    if (prop.getIndex() != null) {
        Object collection = resolveCollection(prop, object);
        setCollectionValue(prop, collection, value);
    } else {
      setBeanProperty(prop, object, value);
    }
}

set方法中的value也是其他地方传入的

Caused by: org.apache.ibatis.reflection.ReflectionException: Could not set property 'id' of 'class com.lemon.rabbit.model.City' with value '1017367047558582273' Cause: java.lang.IllegalArgumentException: argument type mismatch
    at org.apache.ibatis.reflection.wrapper.BeanWrapper.setBeanProperty(BeanWrapper.java:185)
    at org.apache.ibatis.reflection.wrapper.BeanWrapper.set(BeanWrapper.java:59)
    at org.apache.ibatis.reflection.MetaObject.setValue(MetaObject.java:140)
    at com.baomidou.mybatisplus.MybatisDefaultParameterHandler.populateKeys(MybatisDefaultParameterHandler.java:217)
    at com.baomidou.mybatisplus.MybatisDefaultParameterHandler.processBatch(MybatisDefaultParameterHandler.java:156)
    at com.baomidou.mybatisplus.MybatisDefaultParameterHandler.<init>(MybatisDefaultParameterHandler.java:71)
    at com.baomidou.mybatisplus.MybatisXMLLanguageDriver.createParameterHandler(MybatisXMLLanguageDriver.java:37)
    at org.apache.ibatis.session.Configuration.newParameterHandler(Configuration.java:545)
    at org.apache.ibatis.executor.statement.BaseStatementHandler.<init>(BaseStatementHandler.java:69)
    at org.apache.ibatis.executor.statement.PreparedStatementHandler.<init>(PreparedStatementHandler.java:40)
    at org.apache.ibatis.executor.statement.RoutingStatementHandler.<init>(RoutingStatementHandler.java:46)
    at org.apache.ibatis.session.Configuration.newStatementHandler(Configuration.java:558)
    at org.apache.ibatis.executor.SimpleExecutor.doUpdate(SimpleExecutor.java:48)
    at org.apache.ibatis.executor.BaseExecutor.update(BaseExecutor.java:117)
    at org.apache.ibatis.executor.CachingExecutor.update(CachingExecutor.java:76)
    at org.apache.ibatis.session.defaults.DefaultSqlSession.update(DefaultSqlSession.java:198)
    ... 43 more

查看报错日志可以看到，BeanWrapper的上一层是MetaObject，我们找到MetaObject，看到在getValue方法中调用了BeanWrapper的set方法(BeanWrapper实现了ObjectWrapper)

public void setValue(String name, Object value) {
    PropertyTokenizer prop = new PropertyTokenizer(name);
    if (prop.hasNext()) {
        MetaObject metaValue = metaObjectForProperty(prop.getIndexedName());
        if (metaValue == SystemMetaObject.NULL_META_OBJECT) {
            if (value == null && prop.getChildren() != null) {
                // don't instantiate child path if value is null
                return;
            } else {
                metaValue = objectWrapper.instantiatePropertyValue(name, prop, objectFactory);
            }
        }
        metaValue.setValue(prop.getChildren(), value);
    } else {
        objectWrapper.set(prop, value);
    }
}

MetaObject中的主键值也是上层调用传入的，继续根据错误日志向上看：MybatisDefaultParameterHandler

    /**
     * <p>
     * 自定义元对象填充控制器
     * </p>
     *
     * @param metaObjectHandler 元数据填充处理器
     * @param tableInfo         数据库表反射信息
     * @param ms                MappedStatement
     * @param parameterObject   插入数据库对象
     * @return Object
     */
    protected static Object populateKeys(MetaObjectHandler metaObjectHandler, TableInfo tableInfo,
                                         MappedStatement ms, Object parameterObject) {
        if (null == tableInfo || StringUtils.isEmpty(tableInfo.getKeyProperty()) || null == tableInfo.getIdType()) {
            /* 不处理 */
            return parameterObject;
        }
        /* 自定义元对象填充控制器 */
        MetaObject metaObject = ms.getConfiguration().newMetaObject(parameterObject);
        if (ms.getSqlCommandType() == SqlCommandType.INSERT) {
            if (tableInfo.getIdType().getKey() >= 2) {
                Object idValue = metaObject.getValue(tableInfo.getKeyProperty());
                /* 自定义 ID */
                if (StringUtils.checkValNull(idValue)) {
                    if (tableInfo.getIdType() == IdType.ID_WORKER) {
                        metaObject.setValue(tableInfo.getKeyProperty(), IdWorker.getId());
                    } else if (tableInfo.getIdType() == IdType.ID_WORKER_STR) {
                        metaObject.setValue(tableInfo.getKeyProperty(), IdWorker.getIdStr());
                    } else if (tableInfo.getIdType() == IdType.UUID) {
                        metaObject.setValue(tableInfo.getKeyProperty(), IdWorker.get32UUID());
                    }
                }
            }
            // 插入填充
            if (metaObjectHandler.openInsertFill()) {
                metaObjectHandler.insertFill(metaObject);
            }
        } else if (ms.getSqlCommandType() == SqlCommandType.UPDATE && metaObjectHandler.openUpdateFill()) {
            // 更新填充
            metaObjectHandler.updateFill(metaObject);
        }
        return metaObject.getOriginalObject();
    }

在populateKeys方法中调用了metaObject.setValue()

                if (StringUtils.checkValNull(idValue)) {
                    if (tableInfo.getIdType() == IdType.ID_WORKER) {
                        metaObject.setValue(tableInfo.getKeyProperty(), IdWorker.getId());
                    } else if (tableInfo.getIdType() == IdType.ID_WORKER_STR) {
                        metaObject.setValue(tableInfo.getKeyProperty(), IdWorker.getIdStr());
                    } else if (tableInfo.getIdType() == IdType.UUID) {
                        metaObject.setValue(tableInfo.getKeyProperty(), IdWorker.get32UUID());
                    }
                }

可以看到，此处根据IdType生成不同类型的主键id，IdType是一个枚举类，定义了生成ID的类型

• AUTO 数据库ID自增
• INPUT 用户输入ID
• ID_WORKER 全局唯一ID，Long类型的主键
• ID_WORKER_STR 字符串全局唯一ID
• UUID 全局唯一ID，UUID类型的主键
• NONE 该类型为未设置主键类型

当IdType的类型为ID_WORKER、ID_WORKER_STR或者UUID时，主键由MyBatis Plus的IdWorker类生成

ID_WORKER

调用IdWorker的getId()方法，生成一个与时间相关的主键id

    public static long getId() {
        return worker.nextId();
    }

IdWorker的getId()方法引用了Sequence的nextId()方法

    public synchronized long nextId() {
        long timestamp = timeGen();
        if (timestamp < lastTimestamp) {//闰秒
            long offset = lastTimestamp - timestamp;
            if (offset <= 5) {
                try {
                    wait(offset << 1);
                    timestamp = timeGen();
                    if (timestamp < lastTimestamp) {
                        throw new RuntimeException(String.format("Clock moved backwards.  Refusing to generate id for %d milliseconds", offset));
                    }
                } catch (Exception e) {
                    throw new RuntimeException(e);
                }
            } else {
                throw new RuntimeException(String.format("Clock moved backwards.  Refusing to generate id for %d milliseconds", offset));
            }
        }

        if (lastTimestamp == timestamp) {
            // 相同毫秒内，序列号自增
            sequence = (sequence + 1) & sequenceMask;
            if (sequence == 0) {
                // 同一毫秒的序列数已经达到最大
                timestamp = tilNextMillis(lastTimestamp);
            }
        } else {
            // 不同毫秒内，序列号置为 1 - 3 随机数
            sequence = ThreadLocalRandom.current().nextLong(1, 3);
        }

        lastTimestamp = timestamp;

        return ((timestamp - twepoch) << timestampLeftShift)    // 时间戳部分
            | (datacenterId << datacenterIdShift)           // 数据中心部分
            | (workerId << workerIdShift)                   // 机器标识部分
            | sequence;                                     // 序列号部分
    }

ID_WORKER_STR

将worker.nextId()的返回值转化为字符串，和getId()方法相似

    public static String getIdStr() {
        return String.valueOf(worker.nextId());
    }

UUID

去除中划线的UUID字符串

    public static synchronized String get32UUID() {
        return UUID.randomUUID().toString().replace("-", "");
    }

此时，已经基本可以确认，问题出在IdType的配置上，那么这个IdType从哪里获取的呢？TableInfo中获取的！

TableInfo是数据库表反射信息实体类，此处由其他方法传入的，查看日志：

at com.baomidou.mybatisplus.MybatisDefaultParameterHandler.populateKeys(MybatisDefaultParameterHandler.java:217)
at com.baomidou.mybatisplus.MybatisDefaultParameterHandler.processBatch(MybatisDefaultParameterHandler.java:156)

通过日志可以看出，populateKeys()方法是在processBatch()方法中调用的，找到该方法

    /**
     * <p>
     * 批量（填充主键 ID）
     * </p>
     *
     * @param ms
     * @param parameterObject 插入数据库对象
     * @return
     */
    protected static Object processBatch(MappedStatement ms, Object parameterObject) {
        //检查parameterObject
        if (null == parameterObject) {
            return null;
        }
        boolean isFill = false;
        // 全局配置是否配置填充器
        MetaObjectHandler metaObjectHandler = GlobalConfigUtils.getMetaObjectHandler(ms.getConfiguration());
        /* 只处理插入或更新操作 */
        if (ms.getSqlCommandType() == SqlCommandType.INSERT) {
            isFill = true;
        } else if (ms.getSqlCommandType() == SqlCommandType.UPDATE
            && metaObjectHandler.openUpdateFill()) {
            isFill = true;
        }
        if (isFill) {
            Collection<Object> parameters = getParameters(parameterObject);
            if (null != parameters) {
                List<Object> objList = new ArrayList<>();
                for (Object parameter : parameters) {
                    TableInfo tableInfo = TableInfoHelper.getTableInfo(parameter.getClass());
                    if (null != tableInfo) {
                        objList.add(populateKeys(metaObjectHandler, tableInfo, ms, parameter));
                    } else {
                        /*
                         * 非表映射类不处理
                         */
                        objList.add(parameter);
                    }
                }
                return objList;
            } else {
                TableInfo tableInfo = null;
                if (parameterObject instanceof Map) {
                    Map map = (Map) parameterObject;
                    if (map.containsKey("et")) {
                        Object et = map.get("et");
                        if (et != null) {
                            if (et instanceof Map) {
                                Map realEtMap = (Map) et;
                                if (realEtMap.containsKey("MP_OPTLOCK_ET_ORIGINAL")) {//refer to OptimisticLockerInterceptor.MP_OPTLOCK_ET_ORIGINAL
                                    tableInfo = TableInfoHelper.getTableInfo(realEtMap.get("MP_OPTLOCK_ET_ORIGINAL").getClass());
                                }
                            } else {
                                tableInfo = TableInfoHelper.getTableInfo(et.getClass());
                            }
                        }
                    }
                } else {
                    tableInfo = TableInfoHelper.getTableInfo(parameterObject.getClass());
                }
                return populateKeys(metaObjectHandler, tableInfo, ms, parameterObject);
            }
        }
        return parameterObject;
    }

下面对TableInfo生成的这段代码做个说明

                TableInfo tableInfo = null;
                if (parameterObject instanceof Map) {
                    Map map = (Map) parameterObject;
                    if (map.containsKey("et")) {
                        Object et = map.get("et");
                        if (et != null) {
                            if (et instanceof Map) {
                                Map realEtMap = (Map) et;
                                if (realEtMap.containsKey("MP_OPTLOCK_ET_ORIGINAL")) {//refer to OptimisticLockerInterceptor.MP_OPTLOCK_ET_ORIGINAL
                                    tableInfo = TableInfoHelper.getTableInfo(realEtMap.get("MP_OPTLOCK_ET_ORIGINAL").getClass());
                                }
                            } else {
                                tableInfo = TableInfoHelper.getTableInfo(et.getClass());
                            }
                        }
                    }
                } else {
                    tableInfo = TableInfoHelper.getTableInfo(parameterObject.getClass());
                }

parameterObject：要保存到数据库中的实体类信息

我在保存数据时，参数形式并不是Map类型的，所以直接跳转到else中

tableInfo = TableInfoHelper.getTableInfo(parameterObject.getClass());

根据保存的实体类的类型去获取数据库表反射信息

我们看下getTableInfo()方法的实现

    public static TableInfo getTableInfo(Class<?> clazz) {
        return tableInfoCache.get(ClassUtils.getUserClass(clazz).getName());
    }

从tableInfoCache中获取指定类型的数据库反射信息。tableInfoCache是一个线程安全的私有静态Map，主要用于存放类型和数据库表的映射关系。断点调试到此处，看下tableInfoCache的内容

可以看到，idType的值为ID_WORKER，即生成一个与时间相关的Long类型的id。在放入到tableInfoCache的时候就已经指定了idType的值。

查看TableInfoHelper的源码可以得知，initTableInfo()方法负责initTableInfo的初始化

    public synchronized static TableInfo initTableInfo(MapperBuilderAssistant builderAssistant, Class<?> clazz) {
        //检测是否已存在
        TableInfo tableInfo = tableInfoCache.get(clazz.getName());
        if (StringUtils.checkValNotNull(tableInfo)) {
            if (StringUtils.checkValNotNull(builderAssistant)) {
                tableInfo.setConfigMark(builderAssistant.getConfiguration());
            }
            return tableInfo;
        }
        tableInfo = new TableInfo();
        GlobalConfiguration globalConfig;
        if (null != builderAssistant) {
            tableInfo.setCurrentNamespace(builderAssistant.getCurrentNamespace());
            tableInfo.setConfigMark(builderAssistant.getConfiguration());
            //获取全局配置，其中包括了idType
            globalConfig = GlobalConfigUtils.getGlobalConfig(builderAssistant.getConfiguration());
        } else {
            // 兼容测试场景
            globalConfig = GlobalConfigUtils.DEFAULT;
        }
        /* 表名 */
        TableName table = clazz.getAnnotation(TableName.class);
        String tableName = clazz.getSimpleName();
        if (table != null && StringUtils.isNotEmpty(table.value())) {
            tableName = table.value();
        } else {
            // 开启字段下划线申明
            if (globalConfig.isDbColumnUnderline()) {
                tableName = StringUtils.camelToUnderline(tableName);
            }
            // 大写命名判断
            if (globalConfig.isCapitalMode()) {
                tableName = tableName.toUpperCase();
            } else {
                // 首字母小写
                tableName = StringUtils.firstToLowerCase(tableName);
            }
            // 存在表前缀
            if (null != globalConfig.getTablePrefix()) {
                tableName = globalConfig.getTablePrefix() + tableName;
            }
        }
        tableInfo.setTableName(tableName);

        // 开启了自定义 KEY 生成器
        if (null != globalConfig.getKeyGenerator()) {
            tableInfo.setKeySequence(clazz.getAnnotation(KeySequence.class));
        }

        /* 表结果集映射 */
        if (table != null && StringUtils.isNotEmpty(table.resultMap())) {
            tableInfo.setResultMap(table.resultMap());
        }
        List<TableFieldInfo> fieldList = new ArrayList<>();
        List<Field> list = getAllFields(clazz);
        // 标记是否读取到主键
        boolean isReadPK = false;
        boolean existTableId = existTableId(list);
        for (Field field : list) {
            /*
             * 主键ID 初始化
             */
            if (!isReadPK) {
                if (existTableId) {
                    isReadPK = initTableId(globalConfig, tableInfo, field, clazz);
                } else {
                    isReadPK = initFieldId(globalConfig, tableInfo, field, clazz);
                }
                if (isReadPK) {
                    continue;
                }

            }
            /*
             * 字段初始化
             */
            if (initTableField(globalConfig, tableInfo, fieldList, field, clazz)) {
                continue;
            }

            /*
             * 字段, 使用 camelToUnderline 转换驼峰写法为下划线分割法, 如果已指定 TableField , 便不会执行这里
             */
            fieldList.add(new TableFieldInfo(globalConfig, tableInfo, field));
        }

        /* 字段列表 */
        tableInfo.setFieldList(globalConfig, fieldList);
        /*
         * 未发现主键注解，提示警告信息
         */
        if (StringUtils.isEmpty(tableInfo.getKeyColumn())) {
            logger.warn(String.format("Warn: Could not find @TableId in Class: %s.", clazz.getName()));
        }
        /*
         * 注入
         */
        tableInfoCache.put(clazz.getName(), tableInfo);
        return tableInfo;
    }

在existTableId方法中判断主键注解@TableId是否存在

    /**
     * <p>
     * 判断主键注解是否存在
     * </p>
     *
     * @param list 字段列表
     * @return
     */
    public static boolean existTableId(List<Field> list) {
        boolean exist = false;
        for (Field field : list) {
            TableId tableId = field.getAnnotation(TableId.class);
            if (tableId != null) {
                exist = true;
                break;
            }
        }
        return exist;
    }

当不存在主键注解时，会调用initFieldId()方法对主键属性进行初始化

    private static boolean initFieldId(GlobalConfiguration globalConfig, TableInfo tableInfo, Field field, Class<?> clazz) {
        String column = field.getName();
        if (globalConfig.isCapitalMode()) {
            column = column.toUpperCase();
        }
        if (DEFAULT_ID_NAME.equalsIgnoreCase(column)) {
            if (StringUtils.isEmpty(tableInfo.getKeyColumn())) {
                tableInfo.setIdType(globalConfig.getIdType());
                tableInfo.setKeyColumn(column);
                tableInfo.setKeyProperty(field.getName());
                return true;
            } else {
                throwExceptionId(clazz);
            }
        }
        return false;
    }

至此， 我们基本可以判断是因为在实体类City中id属性没有加@TableId注解，我们看下TableId的源码

@Documented
@Retention(RetentionPolicy.RUNTIME)
@Target(ElementType.FIELD)
public @interface TableId {

    /**
     * <p>
     * 字段值（驼峰命名方式，该值可无）
     * </p>
     */
    String value() default "";

    /**
     * <p>
     * 主键ID
     * </p>
     * {@link IdType}
     */
    IdType type() default IdType.NONE;

}

TableId的类型通过type来指定，默认是IdType.NONE（该类型为未设置主键类型），City表是通过数据库的自增序列实现的，所以设置为AUTO

@TableId(type = IdType.AUTO) private Integer id;

然后测试，程序正常运行，保存数据成功。

扩展

对于tableInfo默认的idType值配置，可以看出用的是全局配置的idType，全局配置的值是在initTableInfo()方法中获取的，有兴趣的话可以去看看全局配置的实现，此处暂不深入了

        GlobalConfiguration globalConfig;
        if (null != builderAssistant) {
            tableInfo.setCurrentNamespace(builderAssistant.getCurrentNamespace());
            tableInfo.setConfigMark(builderAssistant.getConfiguration());
            globalConfig = GlobalConfigUtils.getGlobalConfig(builderAssistant.getConfiguration());
        } else {
            // 兼容测试场景
            globalConfig = GlobalConfigUtils.DEFAULT;
        }

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