Mybatis中selectKey源码分析
刚回答了一个问题这样一个问题，mybatis不能正常返回主键增加值 下面通过源码分析一下selectKey都具体实现；关于Mybatis 基于注解Mapper源码分析 可以看一下具体解析过程。
如果向数据库中插入一条数据，同时有希望返回该条记录的主键，该怎么处理了？有两种情况：
- 数据库主键不是自增列，需要预先生成
- 是自增列，插入后才能知道
这两种情况都可以通过SelectKey解决，第一个种就是before,第二张是after
@Insert("insert into table2 (name) values(#{name})")
@SelectKey(statement="call identity()", keyProperty="nameId", before=false, resultType=int.class)
int insertTable2(Name name);
在简单介绍了如何使用后，首先看一下KeyGenerator接口中都有那些方法
KeyGenerator接口
/**
* key生成器接口
*/
public interface KeyGenerator {
//在执行主SQL前执行selectKey
void processBefore(Executor executor, MappedStatement ms, Statement stmt, Object parameter);
//在主SQL执行后执行selectkey
void processAfter(Executor executor, MappedStatement ms, Statement stmt, Object parameter);
}
通过接口都源码可以发现在KeyGenerator接口中定义了 processBefore 和processAfter 两个方法，顾名思义就是在在执行SQL前执行和执行SQL后执行，那么下面通过时序图将其整体调用顺序进行一个概览
如何解析@SelectKey
before具体执行时机
protected BaseStatementHandler(Executor executor, MappedStatement mappedStatement, Object parameterObject, RowBounds rowBounds, ResultHandler resultHandler, BoundSql boundSql) {
this.configuration = mappedStatement.getConfiguration();
this.executor = executor;
this.mappedStatement = mappedStatement;
this.rowBounds = rowBounds;
this.typeHandlerRegistry = configuration.getTypeHandlerRegistry();
this.objectFactory = configuration.getObjectFactory();
if (boundSql == null) { // issue #435, get the key before calculating the statement
// //执行before
generateKeys(parameterObject);
boundSql = mappedStatement.getBoundSql(parameterObject);
}
this.boundSql = boundSql;
this.parameterHandler = configuration.newParameterHandler(mappedStatement, parameterObject, boundSql);
this.resultSetHandler = configuration.newResultSetHandler(executor, mappedStatement, rowBounds, parameterHandler, resultHandler, boundSql);
}
protected void generateKeys(Object parameter) {
KeyGenerator keyGenerator = mappedStatement.getKeyGenerator();
ErrorContext.instance().store();
keyGenerator.processBefore(executor, mappedStatement, null, parameter);
ErrorContext.instance().recall();
}
@Override
public void processBefore(Executor executor, MappedStatement ms, Statement stmt, Object parameter) {
//是否执行前执行，如果不是就不执行
if (executeBefore) {
processGeneratedKeys(executor, ms, parameter);
}
}
after执行时机
@Override
public void processAfter(Executor executor, MappedStatement ms, Statement stmt, Object parameter) {
//如果executeBefore配置为false则执行
if (!executeBefore) {
processGeneratedKeys(executor, ms, parameter);
}
}
具体执行源码
private void processGeneratedKeys(Executor executor, MappedStatement ms, Object parameter) {
try {
//如果parameter不为null同时keyStatement不为null且keyStatement 指定了keyProperties
if (parameter != null && keyStatement != null && keyStatement.getKeyProperties() != null) {
//获取keyProperties
String[] keyProperties = keyStatement.getKeyProperties();
//获取配置信息
final Configuration configuration = ms.getConfiguration();
//获取参数对象元数据
final MetaObject metaParam = configuration.newMetaObject(parameter);
//其实已经判断过了
if (keyProperties != null) {
//新建keyExecutor
Executor keyExecutor = configuration.newExecutor(executor.getTransaction(), ExecutorType.SIMPLE);
//执行查询
List<Object> values = keyExecutor.query(keyStatement, parameter, RowBounds.DEFAULT, Executor.NO_RESULT_HANDLER);
//如果查询结果为0个则抛出异常
if (values.size() == 0) {
throw new ExecutorException("SelectKey returned no data.");
} else if (values.size() > 1) {//查询的结果个数多余1个则抛出异常
throw new ExecutorException("SelectKey returned more than one value.");
} else {//只返了一个结果值
MetaObject metaResult = configuration.newMetaObject(values.get(0));
//如果keyProperty个数只有1个
if (keyProperties.length == 1) {
//如果查询结果对象存在这个属性的getter方法
if (metaResult.hasGetter(keyProperties[0])) {
//将属性值设置到param中
setValue(metaParam, keyProperties[0], metaResult.getValue(keyProperties[0]));
} else {
//如果没有getter方法就将当前值设置到属性中
setValue(metaParam, keyProperties[0], values.get(0));
}
} else {//处理指定多个key属性场景
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);
}
}
private void handleMultipleProperties(String[] keyProperties,
MetaObject metaParam, MetaObject metaResult) {
//获取所有key column
String[] keyColumns = keyStatement.getKeyColumns();
//如果key column不存在
if (keyColumns == null || keyColumns.length == 0) {
//没有指定key column则直接使用配置到 key property
for (String keyProperty : keyProperties) {
setValue(metaParam, keyProperty, metaResult.getValue(keyProperty));
}
} else {
//存在key column 但是数量不一致
if (keyColumns.length != keyProperties.length) {
throw new ExecutorException("If SelectKey has key columns, the number must match the number of key properties.");
}
//数量一致，要求keyColumn 和keyProperty一一对应
for (int i = 0; i < keyProperties.length; i++) {
setValue(metaParam, keyProperties[i], metaResult.getValue(keyColumns[i]));
}
}
}
