现在mybatis的用法
ibatis的用法
首先我们想看
sqlSession.getMapper(UserMapper.class);
但是sqlSession是一个接口，所以我们需要找到它的具体实现类
public class DefaultSqlSessionFactory implements SqlSessionFactory {
。。。
private SqlSession openSessionFromDataSource(ExecutorType execType, TransactionIsolationLevel level, boolean autoCommit) {
Transaction tx = null;
try {
/**
* 先获取需要的对象
*/
final Environment environment = configuration.getEnvironment();
final TransactionFactory transactionFactory = getTransactionFactoryFromEnvironment(environment);
tx = transactionFactory.newTransaction(environment.getDataSource(), level, autoCommit);
// 创建执行器
final Executor executor = configuration.newExecutor(tx, execType);
/**
* sqlSession实现类
*/
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();
}
}
。。。
}
这个类为我们返回SqlSession的具体实现。 DefaultSqlSession类其实也是从Configuration类中拿
public class DefaultSqlSession implements SqlSession {
。。。
@Override
public <T> T getMapper(Class<T> type) {
return configuration.getMapper(type, this);
}
。。。
}
Configuration类在解析xml时会将所有mapper放在MapperRegistry类里
public class Configuration {
。。。
public <T> T getMapper(Class<T> type, SqlSession sqlSession) {
return mapperRegistry.getMapper(type, sqlSession);
}
。。。
}
再深入MapperRegistry类发现里面有个HashMap专门保存我们的mapper类的对应工厂
public class MapperRegistry {
。。。
private final Map<Class<?>, MapperProxyFactory<?>> knownMappers = new HashMap<>();
。。。
public <T> T getMapper(Class<T> type, SqlSession sqlSession) {
final MapperProxyFactory<T> mapperProxyFactory = (MapperProxyFactory<T>) knownMappers.get(type);
if (mapperProxyFactory == null) {
throw new BindingException("Type " + type + " is not known to the MapperRegistry.");
}
try {
return mapperProxyFactory.newInstance(sqlSession);
} catch (Exception e) {
throw new BindingException("Error getting mapper instance. Cause: " + e, e);
}
}
。。。
}
获取到对应的mapper的创建工厂后在进入newInstance()方法，可以看到里面有个熟悉的jdk动态代理
public class MapperProxyFactory<T> {
private final Class<T> mapperInterface;
private final Map<Method, MapperMethod> methodCache = new ConcurrentHashMap<>();
public MapperProxyFactory(Class<T> mapperInterface) {
this.mapperInterface = mapperInterface;
}
public Class<T> getMapperInterface() {
return mapperInterface;
}
public Map<Method, MapperMethod> getMethodCache() {
return methodCache;
}
@SuppressWarnings("unchecked")
protected T newInstance(MapperProxy<T> mapperProxy) {
return (T) Proxy.newProxyInstance(mapperInterface.getClassLoader(), new Class[] { mapperInterface }, mapperProxy);
}
public T newInstance(SqlSession sqlSession) {
final MapperProxy<T> mapperProxy = new MapperProxy<>(sqlSession, mapperInterface, methodCache);
return newInstance(mapperProxy);
}
}
进入MapperProxy类就可以看到invoke()方法，我们再代码中调用mapper的方法实际都时调用这个方法
public class MapperProxy<T> implements InvocationHandler, Serializable {
。。。
@Override
public Object invoke(Object proxy, Method method, Object[] args) throws Throwable {
try {
// method的描述是否是一个类
if (Object.class.equals(method.getDeclaringClass())) {
return method.invoke(this, args);
// method是否是一个默认方法，有主体的方法
} else if (method.isDefault()) {
return invokeDefaultMethod(proxy, method, args);
}
} catch (Throwable t) {
throw ExceptionUtil.unwrapThrowable(t);
}
// 获取方法对应的MapperMethod
final MapperMethod mapperMethod = cachedMapperMethod(method);
return mapperMethod.execute(sqlSession, args);
}
private MapperMethod cachedMapperMethod(Method method) {
return methodCache.computeIfAbsent(method, k -> new MapperMethod(mapperInterface, method, sqlSession.getConfiguration()));
}
。。。
}
最后所有mapper的调用都会流向MpperMethod.execute()方法。MapperMethod的方法其实都是在组装sql语句和准备sql的参数，然后再调用SqlSession的方法，就如ibatis的用法一样。
public class MapperMethod {
。。。
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:
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);
Object param = method.convertArgsToSqlCommandParam(args);
result = sqlSession.selectOne(command.getName(), param);
if (method.returnsOptional()
&& (result == null || !method.getReturnType().equals(result.getClass()))) {
result = Optional.ofNullable(result);
}
}
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;
}
。。。
}
mybatis封装了ibatis为习惯面向对象编程的我们提供了适合我们使用的方式。
