MyBatis動態代理執行原理

前言

        大家使用MyBatis都知道，不管是單獨使用還是和Spring整合，我們都是使用介面定義的方式宣告資料庫的增刪改查方法。那麼我們只宣告一個介面，MyBatis是如何幫我們來實現SQL呢，對嗎，我們的sql是定義在/resources/mapper/mybatis下。每個單獨的xml檔案都有一個id和介面裡的方法一一對應。這裡的對應關係就是mybatis框架幫我們做的工作。

         這裡的關鍵類分為兩部分：SqlSessionFactory、SqlSession、MapperProxy。 

 

1、MapperProxy

       這個類乍一看有點陌生，那我們就先從呼叫入口出發來分析這個類。

 SqlSession sqlSession=sqlSessionFactory.openSession();
 StudentDao studentDao=sqlSession.getMapper(StudentDao.class);

 Student student=studentDao.getStudent(1);

　這裡看到呼叫sqlSession.getMapper

public <T> T getMapper(Class<T> type) {
     return this.configuration.getMapper(type, this);
}

　SqlSession本身不做任何事情，直接把任務甩給Configuration。

public <T> T getMapper(Class<T> type, SqlSession sqlSession) {
     return this.mapperRegistry.getMapper(type, sqlSession);
}

　Configuration又把任務甩給MapperRegistry。 從類名就能看出來它的作用是用來註冊介面和生成代理類例項的工具類。 從getMapper裡看到先獲取MapperProxyFactory，如果未空則拋異常。然後繼續呼叫mapperProxyFactory.newInstance()。

     這裡我們看到mybatis中變數太多的話可以使用var1、var2。。。是個好辦法哦。

public class MapperRegistry {
    private final Configuration config;
    private final Map<Class<?>, MapperProxyFactory<?>> knownMappers = new HashMap();

    public MapperRegistry(Configuration config) {
        this.config = config;
    }

    public <T> T getMapper(Class<T> type, SqlSession sqlSession) {
        MapperProxyFactory mapperProxyFactory = (MapperProxyFactory)this.knownMappers.get(type);
        if(mapperProxyFactory == null) {
            throw new BindingException("Type " + type + " is not known to the MapperRegistry.");
        } else {
            try {
                return mapperProxyFactory.newInstance(sqlSession);
            } catch (Exception var5) {
                throw new BindingException("Error getting mapper instance. Cause: " + var5, var5);
            }
        }
    }

    public <T> boolean hasMapper(Class<T> type) {
        return this.knownMappers.containsKey(type);
    }

    public <T> void addMapper(Class<T> type) {
        if(type.isInterface()) {
            if(this.hasMapper(type)) {
                throw new BindingException("Type " + type + " is already known to the MapperRegistry.");
            }

            boolean loadCompleted = false;

            try {
                this.knownMappers.put(type, new MapperProxyFactory(type));
                MapperAnnotationBuilder parser = new MapperAnnotationBuilder(this.config, type);
                parser.parse();
                loadCompleted = true;
            } finally {
                if(!loadCompleted) {
                    this.knownMappers.remove(type);
                }

            }
        }

    }

    public Collection<Class<?>> getMappers() {
        return Collections.unmodifiableCollection(this.knownMappers.keySet());
    }

    public void addMappers(String packageName, Class<?> superType) {
        ResolverUtil resolverUtil = new ResolverUtil();
        resolverUtil.find(new IsA(superType), packageName);
        Set mapperSet = resolverUtil.getClasses();
        Iterator var5 = mapperSet.iterator();

        while(var5.hasNext()) {
            Class mapperClass = (Class)var5.next();
            this.addMapper(mapperClass);
        }

    }

    public void addMappers(String packageName) {
        this.addMappers(packageName, Object.class);
    }
}

　　MapperProxyFactory是建立Mapper代理類的工廠類。 這個類裡看到兩個newInstance方法。第一個是直接建立一個代理類並返回。第二類是建立一個MapperProxy類然後呼叫第一個newInstance方法。這裡繞了一大圈，終於看到MapperProxy的影子了。呼叫鏈實在太長，繼續往下看。

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 this.mapperInterface;
    }

    public Map<Method, MapperMethod> getMethodCache() {
        return this.methodCache;
    }

    protected T newInstance(MapperProxy<T> mapperProxy) {
        return Proxy.newProxyInstance(this.mapperInterface.getClassLoader(), new Class[]{this.mapperInterface}, mapperProxy);
    }

    public T newInstance(SqlSession sqlSession) {
        MapperProxy mapperProxy = new MapperProxy(sqlSession, this.mapperInterface, this.methodCache);
        return this.newInstance(mapperProxy);
    }
}

　　MapperProxy這個類實現了jdk動態代理介面InvocationHandler。在invoke方法中實現代理方法呼叫的細節。 到這裡說它是關鍵類還沒看到有sql蛛絲馬跡的地方。那隻能繼續往下看，這裡看到在invoke方法裡先獲取MapperMethod類，然後呼叫mapperMethod.execute()。 

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;
    }

    public Object invoke(Object proxy, Method method, Object[] args) throws Throwable {
        try {
            if(Object.class.equals(method.getDeclaringClass())) {
                return method.invoke(this, args);
            }

            if(this.isDefaultMethod(method)) {
                return this.invokeDefaultMethod(proxy, method, args);
            }
        } catch (Throwable var5) {
            throw ExceptionUtil.unwrapThrowable(var5);
        }

        MapperMethod mapperMethod = this.cachedMapperMethod(method);
        return mapperMethod.execute(this.sqlSession, args);
    }

    private MapperMethod cachedMapperMethod(Method method) {
        MapperMethod mapperMethod = (MapperMethod)this.methodCache.get(method);
        if(mapperMethod == null) {
            mapperMethod = new MapperMethod(this.mapperInterface, method, this.sqlSession.getConfiguration());
            this.methodCache.put(method, mapperMethod);
        }

        return mapperMethod;
    }

    @UsesJava7
    private Object invokeDefaultMethod(Object proxy, Method method, Object[] args) throws Throwable {
        Constructor constructor = Lookup.class.getDeclaredConstructor(new Class[]{Class.class, Integer.TYPE});
        if(!constructor.isAccessible()) {
            constructor.setAccessible(true);
        }

        Class declaringClass = method.getDeclaringClass();
        return ((Lookup)constructor.newInstance(new Object[]{declaringClass, Integer.valueOf(2)})).unreflectSpecial(method, declaringClass).bindTo(proxy).invokeWithArguments(args);
    }

    private boolean isDefaultMethod(Method method) {
        return (method.getModifiers() & 1033) == 1 && method.getDeclaringClass().isInterface();
    }
}

　　MapperMethod類是整個代理機制的核心類，對SqlSession中的操作進行了封裝。 該類裡有兩個內部類SqlCommand和MethodSignature。 SqlCommand用來封裝增刪改查操作，也就是我們在xml中配置的select、update、delete、insert節點。每個節點都會生成一個MappedStatement類。MethodSignature用來封裝方法的引數，返回型別。

   

public class MapperMethod {
    private final MapperMethod.SqlCommand command;
    private final MapperMethod.MethodSignature method;

    public MapperMethod(Class<?> mapperInterface, Method method, Configuration config) {
        this.command = new MapperMethod.SqlCommand(config, mapperInterface, method);
        this.method = new MapperMethod.MethodSignature(config, mapperInterface, method);
    }

    public Object execute(SqlSession sqlSession, Object[] args) {
        Object param;
        Object result;
        switch(null.$SwitchMap$org$apache$ibatis$mapping$SqlCommandType[this.command.getType().ordinal()]) {
        case 1:
            param = this.method.convertArgsToSqlCommandParam(args);
            result = this.rowCountResult(sqlSession.insert(this.command.getName(), param));
            break;
        case 2:
            param = this.method.convertArgsToSqlCommandParam(args);
            result = this.rowCountResult(sqlSession.update(this.command.getName(), param));
            break;
        case 3:
            param = this.method.convertArgsToSqlCommandParam(args);
            result = this.rowCountResult(sqlSession.delete(this.command.getName(), param));
            break;
        case 4:
            if(this.method.returnsVoid() && this.method.hasResultHandler()) {
                this.executeWithResultHandler(sqlSession, args);
                result = null;
            } else if(this.method.returnsMany()) {
                result = this.executeForMany(sqlSession, args);
            } else if(this.method.returnsMap()) {
                result = this.executeForMap(sqlSession, args);
            } else if(this.method.returnsCursor()) {
                result = this.executeForCursor(sqlSession, args);
            } else {
                param = this.method.convertArgsToSqlCommandParam(args);
                result = sqlSession.selectOne(this.command.getName(), param);
            }
            break;
        case 5:
            result = sqlSession.flushStatements();
            break;
        default:
            throw new BindingException("Unknown execution method for: " + this.command.getName());
        }

        if(result == null && this.method.getReturnType().isPrimitive() && !this.method.returnsVoid()) {
            throw new BindingException("Mapper method `" + this.command.getName() + " attempted to return null from a method with a primitive return type (" + this.method.getReturnType() + ").");
        } else {
            return result;
        }
    }

    private Object rowCountResult(int rowCount) {
        Object result;
        if(this.method.returnsVoid()) {
            result = null;
        } else if(!Integer.class.equals(this.method.getReturnType()) && !Integer.TYPE.equals(this.method.getReturnType())) {
            if(!Long.class.equals(this.method.getReturnType()) && !Long.TYPE.equals(this.method.getReturnType())) {
                if(!Boolean.class.equals(this.method.getReturnType()) && !Boolean.TYPE.equals(this.method.getReturnType())) {
                    throw new BindingException("Mapper method `" + this.command.getName() + "` has an unsupported return type: " + this.method.getReturnType());
                }

                result = Boolean.valueOf(rowCount > 0);
            } else {
                result = Long.valueOf((long)rowCount);
            }
        } else {
            result = Integer.valueOf(rowCount);
        }

        return result;
    }

    private void executeWithResultHandler(SqlSession sqlSession, Object[] args) {
        MappedStatement ms = sqlSession.getConfiguration().getMappedStatement(this.command.getName());
        if(Void.TYPE.equals(((ResultMap)ms.getResultMaps().get(0)).getType())) {
            throw new BindingException("method " + this.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.");
        } else {
            Object param = this.method.convertArgsToSqlCommandParam(args);
            if(this.method.hasRowBounds()) {
                RowBounds rowBounds = this.method.extractRowBounds(args);
                sqlSession.select(this.command.getName(), param, rowBounds, this.method.extractResultHandler(args));
            } else {
                sqlSession.select(this.command.getName(), param, this.method.extractResultHandler(args));
            }

        }
    }

    private <E> Object executeForMany(SqlSession sqlSession, Object[] args) {
        Object param = this.method.convertArgsToSqlCommandParam(args);
        List result;
        if(this.method.hasRowBounds()) {
            RowBounds rowBounds = this.method.extractRowBounds(args);
            result = sqlSession.selectList(this.command.getName(), param, rowBounds);
        } else {
            result = sqlSession.selectList(this.command.getName(), param);
        }

        return !this.method.getReturnType().isAssignableFrom(result.getClass())?(this.method.getReturnType().isArray()?this.convertToArray(result):this.convertToDeclaredCollection(sqlSession.getConfiguration(), result)):result;
    }

    private <T> Cursor<T> executeForCursor(SqlSession sqlSession, Object[] args) {
        Object param = this.method.convertArgsToSqlCommandParam(args);
        Cursor result;
        if(this.method.hasRowBounds()) {
            RowBounds rowBounds = this.method.extractRowBounds(args);
            result = sqlSession.selectCursor(this.command.getName(), param, rowBounds);
        } else {
            result = sqlSession.selectCursor(this.command.getName(), param);
        }

        return result;
    }

    private <E> Object convertToDeclaredCollection(Configuration config, List<E> list) {
        Object collection = config.getObjectFactory().create(this.method.getReturnType());
        MetaObject metaObject = config.newMetaObject(collection);
        metaObject.addAll(list);
        return collection;
    }

    private <E> Object convertToArray(List<E> list) {
        Class arrayComponentType = this.method.getReturnType().getComponentType();
        Object array = Array.newInstance(arrayComponentType, list.size());
        if(!arrayComponentType.isPrimitive()) {
            return list.toArray((Object[])((Object[])array));
        } else {
            for(int i = 0; i < list.size(); ++i) {
                Array.set(array, i, list.get(i));
            }

            return array;
        }
    }

    private <K, V> Map<K, V> executeForMap(SqlSession sqlSession, Object[] args) {
        Object param = this.method.convertArgsToSqlCommandParam(args);
        Map result;
        if(this.method.hasRowBounds()) {
            RowBounds rowBounds = this.method.extractRowBounds(args);
            result = sqlSession.selectMap(this.command.getName(), param, this.method.getMapKey(), rowBounds);
        } else {
            result = sqlSession.selectMap(this.command.getName(), param, this.method.getMapKey());
        }

        return result;
    }

    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.TYPE.equals(this.returnType);
            this.returnsMany = configuration.getObjectFactory().isCollection(this.returnType) || this.returnType.isArray();
            this.returnsCursor = Cursor.class.equals(this.returnType);
            this.mapKey = this.getMapKey(method);
            this.returnsMap = this.mapKey != null;
            this.rowBoundsIndex = this.getUniqueParamIndex(method, RowBounds.class);
            this.resultHandlerIndex = this.getUniqueParamIndex(method, ResultHandler.class);
            this.paramNameResolver = new ParamNameResolver(configuration, method);
        }

        public Object convertArgsToSqlCommandParam(Object[] args) {
            return this.paramNameResolver.getNamedParams(args);
        }

        public boolean hasRowBounds() {
            return this.rowBoundsIndex != null;
        }

        public RowBounds extractRowBounds(Object[] args) {
            return this.hasRowBounds()?(RowBounds)args[this.rowBoundsIndex.intValue()]:null;
        }

        public boolean hasResultHandler() {
            return this.resultHandlerIndex != null;
        }

        public ResultHandler extractResultHandler(Object[] args) {
            return this.hasResultHandler()?(ResultHandler)args[this.resultHandlerIndex.intValue()]:null;
        }

        public String getMapKey() {
            return this.mapKey;
        }

        public Class<?> getReturnType() {
            return this.returnType;
        }

        public boolean returnsMany() {
            return this.returnsMany;
        }

        public boolean returnsMap() {
            return this.returnsMap;
        }

        public boolean returnsVoid() {
            return this.returnsVoid;
        }

        public boolean returnsCursor() {
            return this.returnsCursor;
        }

        private Integer getUniqueParamIndex(Method method, Class<?> paramType) {
            Integer index = null;
            Class[] argTypes = method.getParameterTypes();

            for(int i = 0; i < argTypes.length; ++i) {
                if(paramType.isAssignableFrom(argTypes[i])) {
                    if(index != null) {
                        throw new BindingException(method.getName() + " cannot have multiple " + paramType.getSimpleName() + " parameters");
                    }

                    index = Integer.valueOf(i);
                }
            }

            return index;
        }

        private String getMapKey(Method method) {
            String mapKey = null;
            if(Map.class.isAssignableFrom(method.getReturnType())) {
                MapKey mapKeyAnnotation = (MapKey)method.getAnnotation(MapKey.class);
                if(mapKeyAnnotation != null) {
                    mapKey = mapKeyAnnotation.value();
                }
            }

            return mapKey;
        }
    }

    public static class SqlCommand {
        private final String name;
        private final SqlCommandType type;

        public SqlCommand(Configuration configuration, Class<?> mapperInterface, Method method) {
            String statementName = mapperInterface.getName() + "." + method.getName();
            MappedStatement ms = null;
            if(configuration.hasStatement(statementName)) {
                ms = configuration.getMappedStatement(statementName);
            } else if(!mapperInterface.equals(method.getDeclaringClass())) {
                String parentStatementName = method.getDeclaringClass().getName() + "." + method.getName();
                if(configuration.hasStatement(parentStatementName)) {
                    ms = configuration.getMappedStatement(parentStatementName);
                }
            }

            if(ms == null) {
                if(method.getAnnotation(Flush.class) == null) {
                    throw new BindingException("Invalid bound statement (not found): " + statementName);
                }

                this.name = null;
                this.type = SqlCommandType.FLUSH;
            } else {
                this.name = ms.getId();
                this.type = ms.getSqlCommandType();
                if(this.type == SqlCommandType.UNKNOWN) {
                    throw new BindingException("Unknown execution method for: " + this.name);
                }
            }

        }

        public String getName() {
            return this.name;
        }

        public SqlCommandType getType() {
            return this.type;
        }
    }

    public static class ParamMap<V> extends HashMap<String, V> {
        private static final long serialVersionUID = -2212268410512043556L;

        public ParamMap() {
        }

        public V get(Object key) {
            if(!super.containsKey(key)) {
                throw new BindingException("Parameter `" + key + "` not found. Available parameters are " + this.keySet());
            } else {
                return super.get(key);
            }
        }
    }
}

　　OK。整個動態代理一次完整的呼叫就結束了，可以看到最後的核心還是回到sqlsession。

 

2、SqlSessionFactory、SqlSession

    從上面這個一大圈呼叫流程來看sqlsession才是關鍵嗎，它才是封裝sql操作的核心人物，那就來看看他是怎麼生成的，我們呼叫的實現類又是那個呢？

sqlSessionFactory=new SqlSessionFactoryBuilder().build(reader);

　通常我們是這樣先來獲取sqlsessionfactory，它是怎麼來的？ 

SqlSessionFacotryBuild()

public SqlSessionFactory build(Reader reader, String environment, Properties properties) {
        SqlSessionFactory var5;
        try {
            XMLConfigBuilder e = new XMLConfigBuilder(reader, environment, properties);
            var5 = this.build(e.parse());
        } catch (Exception var14) {
            throw ExceptionFactory.wrapException("Error building SqlSession.", var14);
        } finally {
            ErrorContext.instance().reset();

            try {
                reader.close();
            } catch (IOException var13) {
                ;
            }

        }

        return var5;
    }

public SqlSessionFactory build(Configuration config) {
    return new DefaultSqlSessionFactory(config);
}

 這裡看到先去解析xml配置檔案，然後儲存到Configuration物件裡。 最後建立一個DefaultSqlSessionFactory。  有了SqlSessionFactory再來看看SqlSession。

 

SqlSession sqlSession=sqlSessionFactory.openSession();

DefaultSqlSessionFactory

private SqlSession openSessionFromDataSource(ExecutorType execType, TransactionIsolationLevel level, boolean autoCommit) {
        Transaction tx = null;

        DefaultSqlSession var8;
        try {
            Environment e = this.configuration.getEnvironment();
            TransactionFactory transactionFactory = this.getTransactionFactoryFromEnvironment(e);
            tx = transactionFactory.newTransaction(e.getDataSource(), level, autoCommit);
            Executor executor = this.configuration.newExecutor(tx, execType);
            var8 = new DefaultSqlSession(this.configuration, executor, autoCommit);
        } catch (Exception var12) {
            this.closeTransaction(tx);
            throw ExceptionFactory.wrapException("Error opening session.  Cause: " + var12, var12);
        } finally {
            ErrorContext.instance().reset();
        }

        return var8;
    }

    private SqlSession openSessionFromConnection(ExecutorType execType, Connection connection) {
        DefaultSqlSession var8;
        try {
            boolean e;
            try {
                e = connection.getAutoCommit();
            } catch (SQLException var13) {
                e = true;
            }

            Environment environment = this.configuration.getEnvironment();
            TransactionFactory transactionFactory = this.getTransactionFactoryFromEnvironment(environment);
            Transaction tx = transactionFactory.newTransaction(connection);
            Executor executor = this.configuration.newExecutor(tx, execType);
            var8 = new DefaultSqlSession(this.configuration, executor, e);
        } catch (Exception var14) {
            throw ExceptionFactory.wrapException("Error opening session.  Cause: " + var14, var14);
        } finally {
            ErrorContext.instance().reset();
        }

        return var8;
    }

　　這裡看到兩個方法最後都返回了DefaultSqlSession類，也就是說最後你的工作都由DefaultSqlSession來完成。 

 

總結

        看了這兩個關鍵類的分析，其實我們還遺漏兩個比較低調的類Configuration、SimpleExecutor(實現了介面Executor)。通常我們說的SqlSession來完成我們的工作最後都要落實在執行器上，執行器是對Statement的封裝。SqlSession也就是個排程的角色吧。Configuration類幫我們解析xml檔案並儲存其中的配置資訊，在框架流轉的過程中使用。