四、MyBatis 查询实现
- Employee empById = mapper.getEmpById(1);
- 首先会调用MapperProxy的invoke方法
@Override
public Object invoke(Object proxy, Method method, Object[] args) throws Throwable {
if (Object.class.equals(method.getDeclaringClass())) {
try {
return method.invoke(this, args);
} catch (Throwable t) {
throw ExceptionUtil.unwrapThrowable(t);
}
}
final MapperMethod mapperMethod = cachedMapperMethod(method);
return mapperMethod.execute(sqlSession, args);
}
- 实际上是调用的MapperMethod的execute(sqlSession, args)方法。该方法代码如下：
public Object execute(SqlSession sqlSession, Object[] args) {
Object result;
// 先判断command的type类型，是修改还是添加
switch (command.getType()) {
case INSERT: {
Object param = method.convertArgsToSqlCommandParam(args);
result = rowCountResult(sqlSession.insert(command.getName(), param));
break;
}
case UPDATE: {
// 解析成我们可以使用的参数。要么包装成map，要么返回。
Object param = method.convertArgsToSqlCommandParam(args);
// 查询单个
result = rowCountResult(sqlSession.update(command.getName(), param));
break;
}else {
Object param = method.convertArgsToSqlCommandParam(args);
result = sqlSession.selectOne(command.getName(), param);
}
....
return result;
}
- result = sqlSession.selectOne(command.getName(), param);。sqlSession的selectOne方法如下：
@Override
public <T> T selectOne(String statement, Object parameter) {
// Popular vote was to return null on 0 results and throw exception on too many.
List<T> list = this.<T>selectList(statement, parameter);
if (list.size() == 1) {
return list.get(0);
} else if (list.size() > 1) {
throw new TooManyResultsException("Expected one result (or null) to be returned by selectOne(), but found: " + list.size());
} else {
return null;
}
}
- 还是调用了DefaultSqlSession的selectList方法。
@Override
public <E> List<E> selectList(String statement, Object parameter, RowBounds rowBounds) {
try {
// 从全局配置中拿到 id；如：cn.guet.dao.EmployeeMapper.getEmpById
MappedStatement ms = configuration.getMappedStatement(statement);
return executor.query(ms, wrapCollection(parameter), rowBounds, Executor.NO_RESULT_HANDLER);
} catch (Exception e) {
throw ExceptionFactory.wrapException("Error querying database. Cause: " + e, e);
} finally {
ErrorContext.instance().reset();
}
}
@Override
public <E> List<E> query(MappedStatement ms, Object parameterObject, RowBounds rowBounds, ResultHandler resultHandler) throws SQLException {
// 获取绑定的SQL，如：select id,last_name lastName,email,gender from tb_employee where id = ?
// BoundSql 存了贺SQL相关的详细信息，SQL和参数等等
BoundSql boundSql = ms.getBoundSql(parameterObject);
// 创建一个缓存，方法id+SQL语句+等等。。
CacheKey key = createCacheKey(ms, parameterObject, rowBounds, boundSql);
return query(ms, parameterObject, rowBounds, resultHandler, key, boundSql);
}
@Override
public <E> List<E> query(MappedStatement ms, Object parameterObject, RowBounds rowBounds, ResultHandler resultHandler, CacheKey key, BoundSql boundSql)
throws SQLException {
// 先从MappedStatement 中获取缓存
Cache cache = ms.getCache();
if (cache != null) {
flushCacheIfRequired(ms);
if (ms.isUseCache() && resultHandler == null) {
ensureNoOutParams(ms, parameterObject, boundSql);
@SuppressWarnings("unchecked")
List<E> list = (List<E>) tcm.getObject(cache, key);
if (list == null) {
list = delegate.<E> query(ms, parameterObject, rowBounds, resultHandler, key, boundSql);
tcm.putObject(cache, key, list); // issue #578 and #116
}
return list;
}
}
return delegate.<E> query(ms, parameterObject, rowBounds, resultHandler, key, boundSql);
}
@SuppressWarnings("unchecked")
@Override
public <E> List<E> query(MappedStatement ms, Object parameter, RowBounds rowBounds, ResultHandler resultHandler, CacheKey key, BoundSql boundSql) throws SQLException {
ErrorContext.instance().resource(ms.getResource()).activity("executing a query").object(ms.getId());
if (closed) {
throw new ExecutorException("Executor was closed.");
}
if (queryStack == 0 && ms.isFlushCacheRequired()) {
clearLocalCache();
}
List<E> list;
try {
queryStack++;
// 从一级缓存中获取
list = resultHandler == null ? (List<E>) localCache.getObject(key) : null;
if (list != null) {
handleLocallyCachedOutputParameters(ms, key, parameter, boundSql);
} else {
//如果一级缓存中没有，则调用queryFromDatabase
list = queryFromDatabase(ms, parameter, rowBounds, resultHandler, key, boundSql);
}
} finally {
queryStack--;
}
if (queryStack == 0) {
for (DeferredLoad deferredLoad : deferredLoads) {
deferredLoad.load();
}
// issue #601
deferredLoads.clear();
if (configuration.getLocalCacheScope() == LocalCacheScope.STATEMENT) {
// issue #482
clearLocalCache();
}
}
return list;
}
private <E> List<E> queryFromDatabase(MappedStatement ms, Object parameter, RowBounds rowBounds, ResultHandler resultHandler, CacheKey key, BoundSql boundSql) throws SQLException {
List<E> list;
// 先给本地缓存放一个key，
localCache.putObject(key, EXECUTION_PLACEHOLDER);
try {
// 查出数据
list = doQuery(ms, parameter, rowBounds, resultHandler, boundSql);
} finally {
localCache.removeObject(key);
}
// 将数据放到本地缓存中去
localCache.putObject(key, list);
if (ms.getStatementType() == StatementType.CALLABLE) {
localOutputParameterCache.putObject(key, parameter);
}
return list;
}
- 然后是调用了SimpleExecutor的doQuery方法
@Override
public <E> List<E> doQuery(MappedStatement ms, Object parameter, RowBounds rowBounds, ResultHandler resultHandler, BoundSql boundSql) throws SQLException {
// 声明一个Statement
Statement stmt = null;
try {
// 拿到当前的配置信息
Configuration configuration = ms.getConfiguration();
// 创建一个StatementHandler ；四大对象之一
StatementHandler handler = configuration.newStatementHandler(wrapper, ms, parameter, rowBounds, resultHandler, boundSql);
// 拿到原生的Statement
stmt = prepareStatement(handler, ms.getStatementLog());
return handler.<E>query(stmt, resultHandler);
} finally {
closeStatement(stmt);
}
}
- 接着就到了ParameterHandler中的setParameters(PreparedStatement ps) 方法：
@Override
public void setParameters(PreparedStatement ps) {
ErrorContext.instance().activity("setting parameters").object(mappedStatement.getParameterMap().getId());
List<ParameterMapping> parameterMappings = boundSql.getParameterMappings();
if (parameterMappings != null) {
..........
TypeHandler typeHandler = parameterMapping.getTypeHandler();
JdbcType jdbcType = parameterMapping.getJdbcType();
typeHandler.setParameter(ps, i + 1, value, jdbcType);
查询流程总结
