这篇文章主要介绍了MyBatis Plus主键设置策略是什么的相关知识，内容详细易懂，操作简单快捷，具有一定借鉴价值，相信大家阅读完这篇MyBatis Plus主键设置策略是什么文章都会有所收获，下面我们一起来看看吧。
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
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 }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()); }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。
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; }关于“MyBatis Plus主键设置策略是什么”这篇文章的内容就介绍到这里，感谢各位的阅读！相信大家对“MyBatis Plus主键设置策略是什么”知识都有一定的了解，大家如果还想学习更多知识，欢迎关注亿速云行业资讯频道。
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