mybatis - 执行 getById
1. getById 的执行
前面一篇 提到过, Mapper.java 创建的时候, 会通过 jdk 代理的方式来创建, 且代理处理类为: MapperProxy .
所以当执行 UserMapper 的 getById 方法的时候, 就会去 MapperProxy 中执行 invoke 方法.
//MapperProxy.java
@Override
public Object invoke(Object proxy, Method method, Object[] args) throws Throwable {
try {
if (Object.class.equals(method.getDeclaringClass())) {
return method.invoke(this, args);
} else if (isDefaultMethod(method)) {
return invokeDefaultMethod(proxy, method, args);
}
} catch (Throwable t) {
throw ExceptionUtil.unwrapThrowable(t);
}
final MapperMethod mapperMethod = cachedMapperMethod(method);
return mapperMethod.execute(sqlSession, args);
}
cachedMapperMethod 方法是做了一层缓存处理. 先从缓存中获取, 如果获取不到, 再创建 MapperMethod 对象.
private MapperMethod cachedMapperMethod(Method method) {
MapperMethod mapperMethod = methodCache.get(method);
if (mapperMethod == null) {
mapperMethod = new MapperMethod(mapperInterface, method, sqlSession.getConfiguration());
methodCache.put(method, mapperMethod);
}
return mapperMethod;
}
接下来看一下 MapperMethod.execute() 方法. 这个方法中, 会看到我们期待的东西.
public Object execute(SqlSession sqlSession, Object[] args) {
Object result;
switch (command.getType()) {
case INSERT: {
Object param = method.convertArgsToSqlCommandParam(args);
result = rowCountResult(sqlSession.insert(command.getName(), param));
break;
}
case UPDATE: {
Object param = method.convertArgsToSqlCommandParam(args);
result = rowCountResult(sqlSession.update(command.getName(), param));
break;
}
case DELETE: {
Object param = method.convertArgsToSqlCommandParam(args);
result = rowCountResult(sqlSession.delete(command.getName(), param));
break;
}
case SELECT:
if (method.returnsVoid() && method.hasResultHandler()) {
executeWithResultHandler(sqlSession, args);
result = null;
} else if (method.returnsMany()) {
result = executeForMany(sqlSession, args);
} else if (method.returnsMap()) {
result = executeForMap(sqlSession, args);
} else if (method.returnsCursor()) {
result = executeForCursor(sqlSession, args);
} else {
//对参数进行处理
Object param = method.convertArgsToSqlCommandParam(args);
result = sqlSession.selectOne(command.getName(), param);
}
break;
case FLUSH:
result = sqlSession.flushStatements();
break;
default:
throw new BindingException("Unknown execution method for: " + command.getName());
}
if (result == null && method.getReturnType().isPrimitive() && !method.returnsVoid()) {
throw new BindingException("Mapper method '" + command.getName()
+ " attempted to return null from a method with a primitive return type (" + method.getReturnType() + ").");
}
return result;
}
getById 在这里会调用 SqlSessionTemplate 的 selectOne 方法.
//SqlSessionTemplate.java
@Override
public <T> T selectOne(String statement, Object parameter) {
return this.sqlSessionProxy.<T> selectOne(statement, parameter);
}
前面解析过, sqlSessionProxy.selectOne 的时候, 会去 SqlSessionInterceptor 中执行 invoke() 方法.
private class SqlSessionInterceptor implements InvocationHandler {
@Override
public Object invoke(Object proxy, Method method, Object[] args) throws Throwable {
SqlSession sqlSession = getSqlSession(
SqlSessionTemplate.this.sqlSessionFactory,
SqlSessionTemplate.this.executorType,
SqlSessionTemplate.this.exceptionTranslator);
try {
Object result = method.invoke(sqlSession, args);
if (!isSqlSessionTransactional(sqlSession, SqlSessionTemplate.this.sqlSessionFactory)) {
// force commit even on non-dirty sessions because some databases require
// a commit/rollback before calling close()
sqlSession.commit(true);
}
return result;
} catch (Throwable t) {
Throwable unwrapped = unwrapThrowable(t);
if (SqlSessionTemplate.this.exceptionTranslator != null && unwrapped instanceof PersistenceException) {
// release the connection to avoid a deadlock if the translator is no loaded. See issue #22
closeSqlSession(sqlSession, SqlSessionTemplate.this.sqlSessionFactory);
sqlSession = null;
Throwable translated = SqlSessionTemplate.this.exceptionTranslator.translateExceptionIfPossible((PersistenceException) unwrapped);
if (translated != null) {
unwrapped = translated;
}
}
throw unwrapped;
} finally {
if (sqlSession != null) {
closeSqlSession(sqlSession, SqlSessionTemplate.this.sqlSessionFactory);
}
}
}
}
这里有两个方法需要重点关注, 一个是 getSqlSession() , 另一个是 invoke() 方法.
1.1 getSqlSession()
public static SqlSession getSqlSession(SqlSessionFactory sessionFactory, ExecutorType executorType, PersistenceExceptionTranslator exceptionTranslator) { notNull(sessionFactory, NO_SQL_SESSION_FACTORY_SPECIFIED); notNull(executorType, NO_EXECUTOR_TYPE_SPECIFIED); SqlSessionHolder holder = (SqlSessionHolder) TransactionSynchronizationManager.getResource(sessionFactory); SqlSession session = sessionHolder(executorType, holder); if (session != null) { return session; } if (LOGGER.isDebugEnabled()) { LOGGER.debug("Creating a new SqlSession"); } session = sessionFactory.openSession(executorType); registerSessionHolder(sessionFactory, executorType, exceptionTranslator, session); return session; }
这里的 SqlSessionFactory 就是配置类中创建的 默认的实现类: DefaultSqlSessionFactory
//DefaultSqlSessionFactory.java @Override public SqlSession openSession(ExecutorType execType) { return openSessionFromDataSource(execType, null, false); } | | \|/ private SqlSession openSessionFromDataSource(ExecutorType execType, TransactionIsolationLevel level, boolean autoCommit) { Transaction tx = null; try { final Environment environment = configuration.getEnvironment(); final TransactionFactory transactionFactory = getTransactionFactoryFromEnvironment(environment); tx = transactionFactory.newTransaction(environment.getDataSource(), level, autoCommit); final Executor executor = configuration.newExecutor(tx, execType); return new DefaultSqlSession(configuration, executor, autoCommit); } catch (Exception e) { closeTransaction(tx); // may have fetched a connection so lets call close() throw ExceptionFactory.wrapException("Error opening session. Cause: " + e, e); } finally { ErrorContext.instance().reset(); } }
这里先不管别的代码, 直接看返回值, 创建了一个 DefaultSqlSession 的实例返回.
1.2 method.invoke()
回到 SqlSessionInterceptor 中来, 继续看下面的执行代码:
Object result = method.invoke(sqlSession, args);
sqlSession 就是上面创建的 DefaultSqlSesion 实例. 我们知道 Method.invoke(obj, args) 是反射调用方法的一种方式.
此处的Method是 getById 的方法反射类型. 所以, 会调用 obj 的 getById() 方法, 即 DefaultSqlSession 的 getById() 方法.
@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; } } @Override public <E> List<E> selectList(String statement, Object parameter) { return this.selectList(statement, parameter, RowBounds.DEFAULT); } @Override public <E> List<E> selectList(String statement, Object parameter, RowBounds rowBounds) { try { 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(); } }
此例中, statement = "com.study.demo.mybatis.mapper.UserMapper.getById"
这里是通过statement 拿 MappedStatement, 而 MappedStatement 中, 就有 mapper.xml 中对应的 sql 语句.
后续过程中, 就可以拿着 sql 去数据库执行了.
到这里, 主体流程其实已经走通了.
