Mybatis之工作原理

1.Mybatis的架构

1.1 Mybatis的框架分层

1.2 MyBatis的实现原理

mybatis底层还是采用原生jdbc来对数据库进行操作的，只是通过 SqlSessionFactory，SqlSession Executor,StatementHandler，ParameterHandler,ResultHandler和TypeHandler等几个处理器封装了这些过程

    执行器：Executor (update, query, flushStatements, commit, rollback, getTransaction, close, isClosed)
   参数处理器： ParameterHandler (getParameterObject, setParameters)
   结构处理器 ResultSetHandler (handleResultSets, handleOutputParameters)
    sql查询处理器：StatementHandler (prepare, parameterize, batch, update, query)

其中StatementHandler用通过ParameterHandler与ResultHandler分别进行参数预编译 与结果处理。而ParameterHandler与ResultHandler都使用TypeHandler进行映射。如下图：

2.Mybatis工作过程

通过读mybatis的源码进行分析mybatis的执行操作的整个过程，我们通过debug调试就可以知道Mybatis每一步做了什么事，我先把debug每一步结果 截图，然后在分析这个流程。
第一步:读取配置文件，形成InputStream

2.1 创建SqlSessionFacotry的过程

从debug调试看出 返回的 sqlSessionFactory 是DefaultSesssionFactory类型的，但是configuration此时已经被初始化了。查看源码后画如下创建DefaultSessionFactory的时序图：

2.2 创建SqlSession的过程

从debug调试 看出SqlSessinoFactory.openSession() 返回的sqlSession是 DefaultSession类型的，此SqlSession里包含一个Configuration的对象，和一个Executor对象。查看源码后画如下创建DefaultSession的时序图：

2.3 创建Mapper的过程

从debug调试可以看出，mapper是一个Mapper代理对象，而且初始化了Configuration对象，Executor的对象。查看源码后画如下创建Mapper的时序图：

2.4 执行CRUD过程

2.4.1 以select为例查看各步执行的源码

1.mapper.selectEmployeeList()其实是MapperProxy执行invoke方法，此方法显示是判断Method的方法是不是Object的toString等方法如果不是就执行MapperMethod

public Object invoke(Object proxy, Method method, Object[] args) throws Throwable {
// 判断Method的方法是不是Object的toString等方法 
    if(Object.class.equals(method.getDeclaringClass())) {
            try {
                return method.invoke(this, args);
            } catch (Throwable var5) {
                throw ExceptionUtil.unwrapThrowable(var5);
            }
        } else {
        //判断private final Map methodCache;这个map里面有没有这个方法的一级缓存，如果没
            MapperMethod mapperMethod = this.cachedMapperMethod(method);
            return mapperMethod.execute(this.sqlSession, args);
        }
    }
    //查询一级缓存和设置一级缓存

     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;
    }

经过上面的调用后进入MapperMethod里面执行

//判断sql命令类型
public Object execute(SqlSession sqlSession, Object[] args) {
        Object param;
        Object result;
        if(SqlCommandType.INSERT == this.command.getType()) {
            param = this.method.convertArgsToSqlCommandParam(args);
            result = this.rowCountResult(sqlSession.insert(this.command.getName(), param));
        } else if(SqlCommandType.UPDATE == this.command.getType()) {
            param = this.method.convertArgsToSqlCommandParam(args);
            result = this.rowCountResult(sqlSession.update(this.command.getName(), param));
        } else if(SqlCommandType.DELETE == this.command.getType()) {
            param = this.method.convertArgsToSqlCommandParam(args);
            result = this.rowCountResult(sqlSession.delete(this.command.getName(), param));
        } else if(SqlCommandType.SELECT == this.command.getType()) {
        //我们测试的是select类型，则再判断这个方法的返回类型
            if(this.method.returnsVoid() && this.method.hasResultHandler()) {
                this.executeWithResultHandler(sqlSession, args);
                result = null;
            } else if(this.method.returnsMany()) {
               //我们是查询列表，此方法执行
                result = this.executeForMany(sqlSession, args);
            } else if(this.method.returnsMap()) {
                result = this.executeForMap(sqlSession, args);
            } else {
                param = this.method.convertArgsToSqlCommandParam(args);
                result = sqlSession.selectOne(this.command.getName(), param);
            }
        } else {
            if(SqlCommandType.FLUSH != this.command.getType()) {
                throw new BindingException("Unknown execution method for: " + this.command.getName());
            }

            result = sqlSession.flushStatements();
        }

        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 executeForMany(SqlSession sqlSession, Object[] args) {
//将param做处理 自动处理为param1,param2..
        Object param = this.method.convertArgsToSqlCommandParam(args);
        List result;
        if(this.method.hasRowBounds()) {
            RowBounds rowBounds = this.method.extractRowBounds(args);
            //调用该对象的DefaultSqlSession的selectList方法
            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;
    }

//处理参数方法
 public Object convertArgsToSqlCommandParam(Object[] args) {
            int paramCount = this.params.size();
            if(args != null && paramCount != 0) {
                if(!this.hasNamedParameters && paramCount == 1) {
                    return args[((Integer)this.params.keySet().iterator().next()).intValue()];
                } else {
                    Map param = new MapperMethod.ParamMap();
                    int i = 0;

                    for(Iterator i$ = this.params.entrySet().iterator(); i$.hasNext(); ++i) {
                        Entry entry = (Entry)i$.next();
                        param.put(entry.getValue(), args[((Integer)entry.getKey()).intValue()]);
                        String genericParamName = "param" + String.valueOf(i + 1);
                        if(!param.containsKey(genericParamName)) {
                            param.put(genericParamName, args[((Integer)entry.getKey()).intValue()]);
                        }
                    }

                    return param;
                }
            } else {
                return null;
            }
        }

调用DefaultSqlSession的selectList的方法

   public  List selectList(String statement, Object parameter, RowBounds rowBounds) {
        List var5;
        try {
        //获取MappedStatement对象
            MappedStatement ms = this.configuration.getMappedStatement(statement);
            //调用cachingExecutor执行器的方法
            var5 = this.executor.query(ms, this.wrapCollection(parameter), rowBounds, Executor.NO_RESULT_HANDLER);
        } catch (Exception var9) {
            throw ExceptionFactory.wrapException("Error querying database.  Cause: " + var9, var9);
        } finally {
            ErrorContext.instance().reset();
        }

        return var5;
    }

//CachingExector的query方法
public  List query(MappedStatement ms, Object parameterObject, RowBounds rowBounds, ResultHandler resultHandler) throws SQLException {
    //
        BoundSql boundSql = ms.getBoundSql(parameterObject);
        CacheKey key = this.createCacheKey(ms, parameterObject, rowBounds, boundSql);
        //调用下2代码
        return this.query(ms, parameterObject, rowBounds, resultHandler, key, boundSql);
    }
    //2代码
 public  List query(MappedStatement ms, Object parameterObject, RowBounds rowBounds, ResultHandler resultHandler, CacheKey key, BoundSql boundSql) throws SQLException {
        Cache cache = ms.getCache();
        if(cache != null) {
            this.flushCacheIfRequired(ms);
            if(ms.isUseCache() && resultHandler == null) {
                this.ensureNoOutParams(ms, parameterObject, boundSql);
                List list = (List)this.tcm.getObject(cache, key);
                if(list == null) {
                //这里是调用Executor里的query方法 如果开启了缓存这掉CachingExecutor的 如果没有则是调用BaseExecutor的
                    list = this.delegate.query(ms, parameterObject, rowBounds, resultHandler, key, boundSql);
                    this.tcm.putObject(cache, key, list);
                }

                return list;
            }
        }

        return this.delegate.query(ms, parameterObject, rowBounds, resultHandler, key, boundSql);
    }

BaseExecutor的query方法

public  List 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(this.closed) {
            throw new ExecutorException("Executor was closed.");
        } else {
            if(this.queryStack == 0 && ms.isFlushCacheRequired()) {
                this.clearLocalCache();
            }

            List list;
            try {
                ++this.queryStack;
                list = resultHandler == null?(List)this.localCache.getObject(key):null;
                if(list != null) {
                    this.handleLocallyCachedOutputParameters(ms, key, parameter, boundSql);
                } else {
                //如果缓存中没有就从数据库中查询
                    list = this.queryFromDatabase(ms, parameter, rowBounds, resultHandler, key, boundSql);
                }
            } finally {
                --this.queryStack;
            }

            if(this.queryStack == 0) {
                Iterator i$ = this.deferredLoads.iterator();

                while(i$.hasNext()) {
                    BaseExecutor.DeferredLoad deferredLoad = (BaseExecutor.DeferredLoad)i$.next();
                    deferredLoad.load();
                }

                this.deferredLoads.clear();
                if(this.configuration.getLocalCacheScope() == LocalCacheScope.STATEMENT) {
                    this.clearLocalCache();
                }
            }

            return list;
        }
    }

//从数据库中查询
private  List queryFromDatabase(MappedStatement ms, Object parameter, RowBounds rowBounds, ResultHandler resultHandler, CacheKey key, BoundSql boundSql) throws SQLException {
        //放入缓存
        this.localCache.putObject(key, ExecutionPlaceholder.EXECUTION_PLACEHOLDER);

        List list;
        try {
        //此处是调用子Executor的方法，ExecutorType默认是使用的SimpleExecutor
            list = this.doQuery(ms, parameter, rowBounds, resultHandler, boundSql);
        } finally {
            this.localCache.removeObject(key);
        }

        this.localCache.putObject(key, list);
        if(ms.getStatementType() == StatementType.CALLABLE) {
            this.localOutputParameterCache.putObject(key, parameter);
        }

        return list;
    }

SimpleExecutor的doQuery方法

public  List doQuery(MappedStatement ms, Object parameter, RowBounds rowBounds, ResultHandler resultHandler, BoundSql boundSql) throws SQLException {
        Statement stmt = null;

        List var9;
        try {
            Configuration configuration = ms.getConfiguration();
            //创建StateMentHandler处理器
            StatementHandler handler = configuration.newStatementHandler(this.wrapper, ms, parameter, rowBounds, resultHandler, boundSql);
            //调用下3的方法
            stmt = this.prepareStatement(handler, ms.getStatementLog());
            //调用no4的方法
            var9 = handler.query(stmt, resultHandler);
        } finally {
            this.closeStatement(stmt);
        }

        return var9;
    }
    //下3方法
private Statement prepareStatement(StatementHandler handler, Log statementLog) throws SQLException {
        Connection connection = this.getConnection(statementLog);
        Statement stmt = handler.prepare(connection);
        //SatementHanlder 采用PreparedStatementHandler来实现此方法，而PreparedStatementHandler调用的是父接口ParameterHandler的方法
        handler.parameterize(stmt);
        return stmt;
    }

ParameterHandler参数处理器的方法

public interface ParameterHandler {
    Object getParameterObject();
    //此方法是用DefaultParameterHandler实现的
    void setParameters(PreparedStatement var1) throws SQLException;
}

DefaultParameterHandler默认参数处理器的方法

public void setParameters(PreparedStatement ps) {
        ErrorContext.instance().activity("setting parameters").object(this.mappedStatement.getParameterMap().getId());
        List parameterMappings = this.boundSql.getParameterMappings();
        if(parameterMappings != null) {
            for(int i = 0; i < parameterMappings.size(); ++i) {
                ParameterMapping parameterMapping = (ParameterMapping)parameterMappings.get(i);
                if(parameterMapping.getMode() != ParameterMode.OUT) {
                    String propertyName = parameterMapping.getProperty();
                    Object value;
                    if(this.boundSql.hasAdditionalParameter(propertyName)) {
                        value = this.boundSql.getAdditionalParameter(propertyName);
                    } else if(this.parameterObject == null) {
                        value = null;
                    } else if(this.typeHandlerRegistry.hasTypeHandler(this.parameterObject.getClass())) {
                        value = this.parameterObject;
                    } else {
                        MetaObject metaObject = this.configuration.newMetaObject(this.parameterObject);
                        value = metaObject.getValue(propertyName);
                    }
                   //这里用调用 TypeHandler类型映射处理器来映射
                    TypeHandler typeHandler = parameterMapping.getTypeHandler();
                    JdbcType jdbcType = parameterMapping.getJdbcType();
                    if(value == null && jdbcType == null) {
                        jdbcType = this.configuration.getJdbcTypeForNull();
                    }

                    try {
                    //类型处理器设置参数映射
                                               typeHandler.setParameter(ps, i + 1, value, jdbcType);
                    } catch (TypeException var10) {
                        throw new TypeException("Could not set parameters for mapping: " + parameterMapping + ". Cause: " + var10, var10);
                    } catch (SQLException var11) {
                        throw new TypeException("Could not set parameters for mapping: " + parameterMapping + ". Cause: " + var11, var11);
                    }
                }
            }
        }

    }

no4的方法

  public  List query(Statement statement, ResultHandler resultHandler) throws SQLException {
       //此处调用原生sql的处理器
        PreparedStatement ps = (PreparedStatement)statement;
        //发出原生sql命令
        ps.execute();
        //采用ResultHandler结果处理器对结果集封装
        return this.resultSetHandler.handleResultSets(ps);
    }

ResultHandler代码

public interface ResultSetHandler {
    //此处调用的是DefaultResultSetHandler的方法
     List handleResultSets(Statement var1) throws SQLException;

    void handleOutputParameters(CallableStatement var1) throws SQLException;
}

DefaultResultSetHandler的方法

public List