前言
在開始動態(tài)代理的原理講解以前，我們先看一下集成mybatis以后dao層不使用動態(tài)代理以及使用動態(tài)代理的兩種實現(xiàn)方式，通過對比我們自己實現(xiàn)dao層接口以及mybatis動態(tài)代理可以更加直觀的展現(xiàn)出mybatis動態(tài)代理替我們所做的工作，有利于我們理解動態(tài)代理的過程，講解完以后我們再進行動態(tài)代理的原理解析，此講解基于mybatis的環(huán)境已經(jīng)搭建完成，并且已經(jīng)實現(xiàn)了基本的用戶類編寫以及用戶類的Dao接口的聲明，下面是Dao層的接口代碼
public interface UserDao { /* 查詢所有用戶信息 */ List<User> findAll(); /** * 保存用戶 * @param user */ void save(User user); /** * 更新用戶 * @return */ void update(User user); /** * 刪除用戶 */ void delete(Integer userId); /** * 查找一個用戶 * @param userId * @return */ User findOne(Integer userId); /** * 根據(jù)名字模糊查詢 * @param name * @return */ List<User> findByName(String name); /** * 根據(jù)組合對象進行模糊查詢 * @param vo * @return */ List<User> findByQueryVo(QueryVo vo);}一、Mybatis dao層兩種實現(xiàn)方式的對比
1.dao層不使用動態(tài)代理
dao層不使用動態(tài)代理的話，就需要我們自己實現(xiàn)dao層的接口，為了簡便起見，我只是實現(xiàn)了Dao接口中的findAll方法，以此方法為例子來展現(xiàn)我們自己實現(xiàn)Dao的方式的情況，讓我們來看代碼：
public class UserDaoImpl implements UserDao{ private SqlSessionFactory factory; public UserDaoImpl(SqlSessionFactory factory){ this.factory = factory; } public List<User> findAll() { //1.獲取sqlSession對象 SqlSession sqlSession = factory.openSession(); //2.調(diào)用selectList方法 List<User> list = sqlSession.selectList("com.example.dao.UserDao.findAll"); //3.關(guān)閉流 sqlSession.close(); return list; } public void save(User user) { } public void update(User user) { } public void delete(Integer userId) { } public User findOne(Integer userId) { return null; } public List<User> findByName(String name) { return null; } public List<User> findByQueryVo(QueryVo vo) { return null; }這里的關(guān)鍵代碼 List<User> list = sqlSession.selectList("com.example.dao.UserDao.findAll")，需要我們自己手動調(diào)用SqlSession里面的方法，基于動態(tài)代理的方式最后的目標(biāo)也是成功的調(diào)用到這里。
注意：如果是添加，更新或者刪除操作的話需要在方法中增加事務(wù)的提交。
2.dao層使用Mybatis的動態(tài)代理
使用動態(tài)代理的話Dao層的接口聲明完成以后只需要在使用的時候通過SqlSession對象的getMapper方法獲取對應(yīng)Dao接口的代理對象，關(guān)鍵代碼如下：
//3.獲取SqlSession對象SqlSession session = factory.openSession();//4.獲取dao的代理對象UserDao mapper = session.getMapper(UserDao.class);//5.執(zhí)行查詢所有的方法List<User> list = mapper.findAll();
獲取到dao層的代理對象以后通過代理對象調(diào)用查詢方法就可以實現(xiàn)查詢所有用戶列表的功能。
二、Mybatis動態(tài)代理實現(xiàn)方式的原理解析
動態(tài)代理中最重要的類：SqlSession、MapperProxy、MapperMethod，下面開始從入口方法到調(diào)用結(jié)束的過程分析。
1.調(diào)用方法的開始:
//4.獲取dao的代理對象
UserDao mapper = session.getMapper(UserDao.class); 因為SqlSesseion為接口，所以我們通過Debug方式發(fā)現(xiàn)這里使用的實現(xiàn)類為DefaultSqlSession。
2.找到DeaultSqlSession中的getMapper方法，發(fā)現(xiàn)這里沒有做其他的動作，只是將工作繼續(xù)拋到了Configuration類中，Configuration為類不是接口，可以直接進入該類的getMapper方法中
@Override public <T> T getMapper(Class<T> type) { return configuration.<T>getMapper(type, this); }3. 找到Configuration類的getMapper方法，這里也是將工作繼續(xù)交到MapperRegistry的getMapper的方法中，所以我們繼續(xù)向下進行。
public <T> T getMapper(Class<T> type, SqlSession sqlSession) { return mapperRegistry.getMapper(type, sqlSession); }4. 找到MapperRegistry的getMapper的方法,看到這里發(fā)現(xiàn)和以前不一樣了，通過MapperProxyFactory的命名方式我們知道這里將通過這個工廠生成我們所關(guān)注的MapperProxy的代理類，然后我們通過mapperProxyFactory.newInstance(sqlSession);進入MapperProxyFactory的newInstance方法中
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); } }5. 找到MapperProxyFactory的newIntance方法，通過參數(shù)類型SqlSession可以得知，上面的調(diào)用先進入第二個newInstance方法中并創(chuàng)建我們所需要重點關(guān)注的MapperProxy對象，第二個方法中再調(diào)用第一個newInstance方法并將MapperProxy對象傳入進去，根據(jù)該對象創(chuàng)建代理類并返回。這里已經(jīng)得到需要的代理類了，但是我們的代理類所做的工作還得繼續(xù)向下看MapperProxy類。
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<T>(sqlSession, mapperInterface, methodCache); return newInstance(mapperProxy); }6. 找到MapperProxy類，發(fā)現(xiàn)其確實實現(xiàn)了JDK動態(tài)代理必須實現(xiàn)的接口InvocationHandler，所以我們重點關(guān)注invoke()方法，這里看到在invoke方法里先獲取MapperMethod類，然后調(diào)用mapperMethod.execute()，所以我們繼續(xù)查看MapperMethod類的execute方法。
public class MapperProxy<T> implements InvocationHandler, Serializable { private static final long serialVersionUID = -6424540398559729838L; private final SqlSession sqlSession; private final Class<T> mapperInterface; private final Map<Method, MapperMethod> methodCache; public MapperProxy(SqlSession sqlSession, Class<T> mapperInterface, Map<Method, MapperMethod> methodCache) { this.sqlSession = sqlSession; this.mapperInterface = mapperInterface; this.methodCache = methodCache; } @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); } 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; } @UsesJava7 private Object invokeDefaultMethod(Object proxy, Method method, Object[] args) throws Throwable { final Constructor<MethodHandles.Lookup> constructor = MethodHandles.Lookup.class .getDeclaredConstructor(Class.class, int.class); if (!constructor.isAccessible()) { constructor.setAccessible(true); } final Class<?> declaringClass = method.getDeclaringClass(); return constructor .newInstance(declaringClass, MethodHandles.Lookup.PRIVATE | MethodHandles.Lookup.PROTECTED | MethodHandles.Lookup.PACKAGE | MethodHandles.Lookup.PUBLIC) .unreflectSpecial(method, declaringClass).bindTo(proxy).invokeWithArguments(args); } /** * Backport of java.lang.reflect.Method#isDefault() */ private boolean isDefaultMethod(Method method) { return ((method.getModifiers() & (Modifier.ABSTRACT | Modifier.PUBLIC | Modifier.STATIC)) == Modifier.PUBLIC) && method.getDeclaringClass().isInterface(); }}7. 找到類MapperMethod類的execute方法，發(fā)現(xiàn)execute中通過調(diào)用本類中的其他方法獲取并封裝返回結(jié)果，我們來看一下MapperMethod整個類。
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 { 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; }8. MapperMethod類是整個代理機制的核心類，對SqlSession中的操作進行了封裝使用。
該類里有兩個內(nèi)部類SqlCommand和MethodSignature。 SqlCommand用來封裝CRUD操作，也就是我們在xml中配置的操作的節(jié)點。每個節(jié)點都會生成一個MappedStatement類。
MethodSignature用來封裝方法的參數(shù)以及返回類型，在execute的方法中我們發(fā)現(xiàn)在這里又回到了SqlSession中的接口調(diào)用，和我們自己實現(xiàn)UerDao接口的方式中直接用SqlSession對象調(diào)用DefaultSqlSession的實現(xiàn)類的方法是一樣的，經(jīng)過一大圈的代理又回到了原地，這就是整個動態(tài)代理的實現(xiàn)過程了。
public class MapperMethod { private final SqlCommand command; private final MethodSignature method; public MapperMethod(Class<?> mapperInterface, Method method, Configuration config) { this.command = new SqlCommand(config, mapperInterface, method); this.method = new MethodSignature(config, mapperInterface, method); } 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 { 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; } private Object rowCountResult(int rowCount) { final Object result; if (method.returnsVoid()) { result = null; } else if (Integer.class.equals(method.getReturnType()) || Integer.TYPE.equals(method.getReturnType())) { result = rowCount; } else if (Long.class.equals(method.getReturnType()) || Long.TYPE.equals(method.getReturnType())) { result = (long)rowCount; } else if (Boolean.class.equals(method.getReturnType()) || Boolean.TYPE.equals(method.getReturnType())) { result = rowCount > 0; } else { throw new BindingException("Mapper method '" + command.getName() + "' has an unsupported return type: " + method.getReturnType()); } return result; } private void executeWithResultHandler(SqlSession sqlSession, Object[] args) { MappedStatement ms = sqlSession.getConfiguration().getMappedStatement(command.getName()); if (void.class.equals(ms.getResultMaps().get(0).getType())) { throw new BindingException("method " + command.getName() + " needs either a @ResultMap annotation, a @ResultType annotation," + " or a resultType attribute in XML so a ResultHandler can be used as a parameter."); } Object param = method.convertArgsToSqlCommandParam(args); if (method.hasRowBounds()) { RowBounds rowBounds = method.extractRowBounds(args); sqlSession.select(command.getName(), param, rowBounds, method.extractResultHandler(args)); } else { sqlSession.select(command.getName(), param, method.extractResultHandler(args)); } } private <E> Object executeForMany(SqlSession sqlSession, Object[] args) { List<E> result; Object param = method.convertArgsToSqlCommandParam(args); if (method.hasRowBounds()) { RowBounds rowBounds = method.extractRowBounds(args); result = sqlSession.<E>selectList(command.getName(), param, rowBounds); } else { result = sqlSession.<E>selectList(command.getName(), param); } // issue #510 Collections & arrays support if (!method.getReturnType().isAssignableFrom(result.getClass())) { if (method.getReturnType().isArray()) { return convertToArray(result); } else { return convertToDeclaredCollection(sqlSession.getConfiguration(), result); } } return result; } private <T> Cursor<T> executeForCursor(SqlSession sqlSession, Object[] args) { Cursor<T> result; Object param = method.convertArgsToSqlCommandParam(args); if (method.hasRowBounds()) { RowBounds rowBounds = method.extractRowBounds(args); result = sqlSession.<T>selectCursor(command.getName(), param, rowBounds); } else { result = sqlSession.<T>selectCursor(command.getName(), param); } return result; } private <E> Object convertToDeclaredCollection(Configuration config, List<E> list) { Object collection = config.getObjectFactory().create(method.getReturnType()); MetaObject metaObject = config.newMetaObject(collection); metaObject.addAll(list); return collection; } @SuppressWarnings("unchecked") private <E> Object convertToArray(List<E> list) { Class<?> arrayComponentType = method.getReturnType().getComponentType(); Object array = Array.newInstance(arrayComponentType, list.size()); if (arrayComponentType.isPrimitive()) { for (int i = 0; i < list.size(); i++) { Array.set(array, i, list.get(i)); } return array; } else { return list.toArray((E[])array); } } private <K, V> Map<K, V> executeForMap(SqlSession sqlSession, Object[] args) { Map<K, V> result; Object param = method.convertArgsToSqlCommandParam(args); if (method.hasRowBounds()) { RowBounds rowBounds = method.extractRowBounds(args); result = sqlSession.<K, V>selectMap(command.getName(), param, method.getMapKey(), rowBounds); } else { result = sqlSession.<K, V>selectMap(command.getName(), param, method.getMapKey()); } return result; } public static class ParamMap<V> extends HashMap<String, V> { private static final long serialVersionUID = -2212268410512043556L; @Override public V get(Object key) { if (!super.containsKey(key)) { throw new BindingException("Parameter '" + key + "' not found. Available parameters are " + keySet()); } return super.get(key); } } public static class SqlCommand { private final String name; private final SqlCommandType type; public SqlCommand(Configuration configuration, Class<?> mapperInterface, Method method) { final String methodName = method.getName(); final Class<?> declaringClass = method.getDeclaringClass(); MappedStatement ms = resolveMappedStatement(mapperInterface, methodName, declaringClass, configuration); if (ms == null) { if (method.getAnnotation(Flush.class) != null) { name = null; type = SqlCommandType.FLUSH; } else { throw new BindingException("Invalid bound statement (not found): " + mapperInterface.getName() + "." + methodName); } } else { name = ms.getId(); type = ms.getSqlCommandType(); if (type == SqlCommandType.UNKNOWN) { throw new BindingException("Unknown execution method for: " + name); } } } public String getName() { return name; } public SqlCommandType getType() { return type; } private MappedStatement resolveMappedStatement(Class<?> mapperInterface, String methodName, Class<?> declaringClass, Configuration configuration) { String statementId = mapperInterface.getName() + "." + methodName; if (configuration.hasStatement(statementId)) { return configuration.getMappedStatement(statementId); } else if (mapperInterface.equals(declaringClass)) { return null; } for (Class<?> superInterface : mapperInterface.getInterfaces()) { if (declaringClass.isAssignableFrom(superInterface)) { MappedStatement ms = resolveMappedStatement(superInterface, methodName, declaringClass, configuration); if (ms != null) { return ms; } } } return null; } } public static class MethodSignature { private final boolean returnsMany; private final boolean returnsMap; private final boolean returnsVoid; private final boolean returnsCursor; private final Class<?> returnType; private final String mapKey; private final Integer resultHandlerIndex; private final Integer rowBoundsIndex; private final ParamNameResolver paramNameResolver; public MethodSignature(Configuration configuration, Class<?> mapperInterface, Method method) { Type resolvedReturnType = TypeParameterResolver.resolveReturnType(method, mapperInterface); if (resolvedReturnType instanceof Class<?>) { this.returnType = (Class<?>) resolvedReturnType; } else if (resolvedReturnType instanceof ParameterizedType) { this.returnType = (Class<?>) ((ParameterizedType) resolvedReturnType).getRawType(); } else { this.returnType = method.getReturnType(); } this.returnsVoid = void.class.equals(this.returnType); this.returnsMany = (configuration.getObjectFactory().isCollection(this.returnType) || this.returnType.isArray()); this.returnsCursor = Cursor.class.equals(this.returnType); this.mapKey = getMapKey(method); this.returnsMap = (this.mapKey != null); this.rowBoundsIndex = getUniqueParamIndex(method, RowBounds.class); this.resultHandlerIndex = getUniqueParamIndex(method, ResultHandler.class); this.paramNameResolver = new ParamNameResolver(configuration, method); } public Object convertArgsToSqlCommandParam(Object[] args) { return paramNameResolver.getNamedParams(args); } public boolean hasRowBounds() { return rowBoundsIndex != null; } public RowBounds extractRowBounds(Object[] args) { return hasRowBounds() ? (RowBounds) args[rowBoundsIndex] : null; } public boolean hasResultHandler() { return resultHandlerIndex != null; } public ResultHandler extractResultHandler(Object[] args) { return hasResultHandler() ? (ResultHandler) args[resultHandlerIndex] : null; } public String getMapKey() { return mapKey; } public Class<?> getReturnType() { return returnType; } public boolean returnsMany() { return returnsMany; } public boolean returnsMap() { return returnsMap; } public boolean returnsVoid() { return returnsVoid; } public boolean returnsCursor() { return returnsCursor; } private Integer getUniqueParamIndex(Method method, Class<?> paramType) { Integer index = null; final Class<?>[] argTypes = method.getParameterTypes(); for (int i = 0; i < argTypes.length; i++) { if (paramType.isAssignableFrom(argTypes[i])) { if (index == null) { index = i; } else { throw new BindingException(method.getName() + " cannot have multiple " + paramType.getSimpleName() + " parameters"); } } } return index; } private String getMapKey(Method method) { String mapKey = null; if (Map.class.isAssignableFrom(method.getReturnType())) { final MapKey mapKeyAnnotation = method.getAnnotation(MapKey.class); if (mapKeyAnnotation != null) { mapKey = mapKeyAnnotation.value(); } } return mapKey; } }以上就是本文的全部內(nèi)容，希望對大家的學(xué)習(xí)有所幫助，也希望大家多多支持武林網(wǎng)。
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