mybatis之执行器Executor

执行器Executor

Executor定义了数据库操作的基本方法，SqlSession接口中的功能都是基于Executor接口实现的，真正执行java和数据库交互的类，负责维护一级缓存和二级缓存，并提供事务管理的相关操作，会将数据库相关操作委托给StatementHandler完成

public enum ExecutorType {
  SIMPLE, // 对应SimpleExecutor 简易执行器，默认,每执行一次update或select，就开启一个Statement对象，用完立刻关闭
  REUSE, // 对应ReuseExecutor，重用预处理语句，执行update或select，以sql作为key查找Statement对象，存在就使用，不存在就创建，用完后，不关闭Statement对象，而是放置于map中
  BATCH //对应BatchExecutor，重用语句和批量更新，完成批处理
}

执行器的流程

• 创建： Executor通过Configuration对象中newExecutor()方法中选择相应的执行器生成

  Executor executor = configuration.newExecutor(tx, execType)

• 执行过程： Executor会先调用StatementHandler的prepare()方法预编译SQL语句，同时设置一些基本的运行参数，然后调用StatementHandler的parameterize()方法（实际上是启用了ParameterHandler设置参数）设置参数，resultHandler再组装查询结果返回调用者完成一次查询完成预编译，简单总结起来就是即先预编译SQL语句，之后设置参数（跟JDBC的prepareStatement过程类似）最后如果有查询结果就会组装返回

public interface Executor {
    ResultHandler NO_RESULT_HANDLER = null;
        // 执行insert、update、delete
    int update(MappedStatement var1, Object var2) throws SQLException;
        // 查询，带分页和缓存
    <E> List<E> query(MappedStatement var1, Object var2, RowBounds var3, ResultHandler var4, CacheKey var5, BoundSql var6) throws SQLException;
        // 查询，带分页
    <E> List<E> query(MappedStatement var1, Object var2, RowBounds var3, ResultHandler var4) throws SQLException;
    // 存储过程
    <E> Cursor<E> queryCursor(MappedStatement var1, Object var2, RowBounds var3) throws SQLException;
    // 刷新批处理语句
    List<BatchResult> flushStatements() throws SQLException;
// 事务提交
    void commit(boolean var1) throws SQLException;
// 事务回滚
    void rollback(boolean var1) throws SQLException;
        // 创建缓存中用到的CacheKey对象
    CacheKey createCacheKey(MappedStatement var1, Object var2, RowBounds var3, BoundSql var4);
        // 根据CacheKey对象查找缓存
    boolean isCached(MappedStatement var1, CacheKey var2);
        // 清空一级缓存
    void clearLocalCache();
        // 延迟加载一级缓存中的数据
    void deferLoad(MappedStatement var1, MetaObject var2, String var3, CacheKey var4, Class<?> var5);

    Transaction getTransaction();

    void close(boolean var1);

    boolean isClosed();

    void setExecutorWrapper(Executor var1);
}

Executor有两个实现类BaseExecutor和CachingExecutor

BaseExecutor

BaseExecutor是一个实现了Executor接口的抽象类，实现了Executor接口的大部分方法，主要提供了缓存管理和事务管理的基本功能，继承BaseExecutor需要实现四个基本方法来实现数据库的相关操作，doUpdate()、doQuery()、doQueryCursor()、doFlushStatement()方法。

public abstract class BaseExecutor implements Executor {
    private static final Log log = LogFactory.getLog(BaseExecutor.class);
      // 事务
    protected Transaction transaction;
      // 封装Executor对象
    protected Executor wrapper;
      // 延迟加载队列
    protected ConcurrentLinkedQueue<BaseExecutor.DeferredLoad> deferredLoads;
      // 一级缓存，用于缓存查询结果集映射得到的结果对象
    protected PerpetualCache localCache;
      // 一级缓存，用于缓存输出类型参数(存储过程)
    protected PerpetualCache localOutputParameterCache;
    protected Configuration configuration;
      // 用来记录嵌套查询层数
    protected int queryStack;
    private boolean closed;

    protected BaseExecutor(Configuration configuration, Transaction transaction) {
        this.transaction = transaction;
        this.deferredLoads = new ConcurrentLinkedQueue();
        this.localCache = new PerpetualCache("LocalCache");
        this.localOutputParameterCache = new PerpetualCache("LocalOutputParameterCache");
        this.closed = false;
        this.configuration = configuration;
        this.wrapper = this;
    }

    public int update(MappedStatement ms, Object parameter) throws SQLException {
        ErrorContext.instance().resource(ms.getResource()).activity("executing an update").object(ms.getId());
        if (this.closed) {
            throw new ExecutorException("Executor was closed.");
        } else {
              // 先清除一级缓存
            this.clearLocalCache();
            return this.doUpdate(ms, parameter);
        }
    }

    public <E> List<E> query(MappedStatement ms, Object parameter, RowBounds rowBounds, ResultHandler resultHandler) throws SQLException {
          // 获取BoundSql
        BoundSql boundSql = ms.getBoundSql(parameter);
          // 创建cacheKey
        CacheKey key = this.createCacheKey(ms, parameter, rowBounds, boundSql);
          // 执行查询
        return this.query(ms, parameter, rowBounds, resultHandler, key, boundSql);
    }

    public <E> List<E> 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 {
              //非嵌套查询且在<select>节点中配置flushCache属性时，清空一级缓存
            if (this.queryStack == 0 && ms.isFlushCacheRequired()) {
                this.clearLocalCache();
            }

            List list;
            try {
                  // 增加嵌套层数
                ++this.queryStack;
                  // 查询一级缓存
                list = resultHandler == null ? (List)this.localCache.getObject(key) : null;
                if (list != null) {
                      // 针对于存储过程调用处理，在一级缓存命中时，获取缓存中保存的输出类型参数，并设置到用户传入的实参中
                    this.handleLocallyCachedOutputParameters(ms, key, parameter, boundSql);
                } else {
                      // 缓存没有命中，调用doQuery查询数据库
                    list = this.queryFromDatabase(ms, parameter, rowBounds, resultHandler, key, boundSql);
                }
            } finally {
                  // 当前查询完成，查询层数减少
                --this.queryStack;
            }
                        
              // 延迟加载
              // 在外层查询结束时，所有嵌套查询已经完成，相关缓存项也已经完全加载，触发DeferredLoad加载一级缓存中记录的嵌套查询的结果对象
            if (this.queryStack == 0) {
                Iterator var8 = this.deferredLoads.iterator();

                while(var8.hasNext()) {
                    BaseExecutor.DeferredLoad deferredLoad = (BaseExecutor.DeferredLoad)var8.next();
                    deferredLoad.load();
                }
                                // 加载完成后，清空deferredLoads
                this.deferredLoads.clear();
                  // LocalCacheScope本地缓存的作用域
                if (this.configuration.getLocalCacheScope() == LocalCacheScope.STATEMENT) {
                    this.clearLocalCache();
                }
            }

            return list;
        }
    }

    public <E> Cursor<E> queryCursor(MappedStatement ms, Object parameter, RowBounds rowBounds) throws SQLException {
        BoundSql boundSql = ms.getBoundSql(parameter);
        return this.doQueryCursor(ms, parameter, rowBounds, boundSql);
    }

    public void deferLoad(MappedStatement ms, MetaObject resultObject, String property, CacheKey key, Class<?> targetType) {
        if (this.closed) {
            throw new ExecutorException("Executor was closed.");
        } else {
              // 创建延迟加载对象
            BaseExecutor.DeferredLoad deferredLoad = new BaseExecutor.DeferredLoad(resultObject, property, key, this.localCache, this.configuration, targetType);
              // 以及缓存中已经记录了指定查询的结果对象，直接从缓存中加载对象，设置到外层对象中
            if (deferredLoad.canLoad()) {
                deferredLoad.load();
            } else {
                  // 将DeferredLoad对象添加到deferredLoads队列中，待整个外层查询结束后，在加载该结果对象
                this.deferredLoads.add(new BaseExecutor.DeferredLoad(resultObject, property, key, this.localCache, this.configuration, targetType));
            }

        }
    }
        // cachekey由MappedStatement的id、offset、limit、SQL语句(包含？占位符)、用户传的实参以及Environment的id五部分组成
    public CacheKey createCacheKey(MappedStatement ms, Object parameterObject, RowBounds rowBounds, BoundSql boundSql) {
        if (this.closed) {
            throw new ExecutorException("Executor was closed.");
        } else {
              // 创建cacheKey
            CacheKey cacheKey = new CacheKey();
              // 将MappedStatement的id添加到CacheKey中
            cacheKey.update(ms.getId());
              // 将offset添加到cacheKey中
            cacheKey.update(rowBounds.getOffset());
              // 将limit添加到cacheKey中
            cacheKey.update(rowBounds.getLimit());
              // 将SQL语句添加到cacheKey中
            cacheKey.update(boundSql.getSql());
            List<ParameterMapping> parameterMappings = boundSql.getParameterMappings();
            TypeHandlerRegistry typeHandlerRegistry = ms.getConfiguration().getTypeHandlerRegistry();
            Iterator var8 = parameterMappings.iterator();

              // 获取用户传入的实参，添加到cacheKey中
            while(var8.hasNext()) {
                ParameterMapping parameterMapping = (ParameterMapping)var8.next();
                  // 过滤掉输出类型的参数
                if (parameterMapping.getMode() != ParameterMode.OUT) {
                    String propertyName = parameterMapping.getProperty();
                    Object value;
                    if (boundSql.hasAdditionalParameter(propertyName)) {
                        value = boundSql.getAdditionalParameter(propertyName);
                    } else if (parameterObject == null) {
                        value = null;
                    } else if (typeHandlerRegistry.hasTypeHandler(parameterObject.getClass())) {
                        value = parameterObject;
                    } else {
                        MetaObject metaObject = this.configuration.newMetaObject(parameterObject);
                        value = metaObject.getValue(propertyName);
                    }

                    cacheKey.update(value);
                }
            }
                        // 如果Environment不为空的话，把id添加到cachekey中
            if (this.configuration.getEnvironment() != null) {
                cacheKey.update(this.configuration.getEnvironment().getId());
            }

            return cacheKey;
        }
    }

    public boolean isCached(MappedStatement ms, CacheKey key) {
        return this.localCache.getObject(key) != null;
    }
  
      public List<BatchResult> flushStatements() throws SQLException {
          // 执行Executor中缓存的SQL语句
        return this.flushStatements(false);
    }

    public List<BatchResult> flushStatements(boolean isRollBack) throws SQLException {
        if (this.closed) {
            throw new ExecutorException("Executor was closed.");
        } else {
              // true表示不执行  false表示执行
            return this.doFlushStatements(isRollBack);
        }
    }

    public void commit(boolean required) throws SQLException {
        if (this.closed) {
            throw new ExecutorException("Cannot commit, transaction is already closed");
        } else {
              // 清空一级缓存
            this.clearLocalCache();
              // 执行缓存中的SQL语句
            this.flushStatements();
            if (required) {
                  // 提交事务
                this.transaction.commit();
            }

        }
    }

    public void rollback(boolean required) throws SQLException {
        if (!this.closed) {
            try {
                this.clearLocalCache();
                this.flushStatements(true);
            } finally {
                if (required) {
                    this.transaction.rollback();
                }

            }
        }

    }
  
   public Transaction getTransaction() {
        if (this.closed) {
            throw new ExecutorException("Executor was closed.");
        } else {
            return this.transaction;
        }
    }

    public void close(boolean forceRollback) {
        try {
            try {
                this.rollback(forceRollback);
            } finally {
                if (this.transaction != null) {
                    this.transaction.close();
                }

            }
        } catch (SQLException var11) {
            
        } finally {
            this.transaction = null;
            this.deferredLoads = null;
            this.localCache = null;
            this.localOutputParameterCache = null;
            this.closed = true;
        }

    }

    public boolean isClosed() {
        return this.closed;
    }



    public void clearLocalCache() {
        if (!this.closed) {
            this.localCache.clear();
            this.localOutputParameterCache.clear();
        }

    }

    protected abstract int doUpdate(MappedStatement var1, Object var2) throws SQLException;

    protected abstract List<BatchResult> doFlushStatements(boolean var1) throws SQLException;

    protected abstract <E> List<E> doQuery(MappedStatement var1, Object var2, RowBounds var3, ResultHandler var4, BoundSql var5) throws SQLException;

    protected abstract <E> Cursor<E> doQueryCursor(MappedStatement var1, Object var2, RowBounds var3, BoundSql var4) throws SQLException;

    protected void closeStatement(Statement statement) {
        if (statement != null) {
            try {
                statement.close();
            } catch (SQLException var3) {
            }
        }

    }

    protected void applyTransactionTimeout(Statement statement) throws SQLException {
        StatementUtil.applyTransactionTimeout(statement, statement.getQueryTimeout(), this.transaction.getTimeout());
    }

    private void handleLocallyCachedOutputParameters(MappedStatement ms, CacheKey key, Object parameter, BoundSql boundSql) {
        if (ms.getStatementType() == StatementType.CALLABLE) {
            Object cachedParameter = this.localOutputParameterCache.getObject(key);
            if (cachedParameter != null && parameter != null) {
                MetaObject metaCachedParameter = this.configuration.newMetaObject(cachedParameter);
                MetaObject metaParameter = this.configuration.newMetaObject(parameter);
                Iterator var8 = boundSql.getParameterMappings().iterator();

                while(var8.hasNext()) {
                    ParameterMapping parameterMapping = (ParameterMapping)var8.next();
                    if (parameterMapping.getMode() != ParameterMode.IN) {
                        String parameterName = parameterMapping.getProperty();
                        Object cachedValue = metaCachedParameter.getValue(parameterName);
                        metaParameter.setValue(parameterName, cachedValue);
                    }
                }
            }
        }

    }

  // 从数据库中查询数据
    private <E> List<E> queryFromDatabase(MappedStatement ms, Object parameter, RowBounds rowBounds, ResultHandler resultHandler, CacheKey key, BoundSql boundSql) throws SQLException {
          // ExecutionPlaceholder.EXECUTION_PLACEHOLDER 缓存的占位符
          // 在缓存中添加占位符
        this.localCache.putObject(key, ExecutionPlaceholder.EXECUTION_PLACEHOLDER);

        List list;
        try {
          // 查数据库
            list = this.doQuery(ms, parameter, rowBounds, resultHandler, boundSql);
        } finally {
              // 删除占位符
            this.localCache.removeObject(key);
        }
                // 将真正的结果对象放入一级缓存中
        this.localCache.putObject(key, list);
          // 是否为存储过程
        if (ms.getStatementType() == StatementType.CALLABLE) {
              // 缓存输出类型的参数
            this.localOutputParameterCache.putObject(key, parameter);
        }

        return list;
    }

    protected Connection getConnection(Log statementLog) throws SQLException {
        Connection connection = this.transaction.getConnection();
        return statementLog.isDebugEnabled() ? ConnectionLogger.newInstance(connection, statementLog, this.queryStack) : connection;
    }

    public void setExecutorWrapper(Executor wrapper) {
        this.wrapper = wrapper;
    }

    private static class DeferredLoad {
          // 外层对象对应的MetaObject
        private final MetaObject resultObject;
          // 延迟加载的属性名称
        private final String property;
          // 延迟加载的属性的类型
        private final Class<?> targetType;
          // 延迟加载的结果对象在一级缓存中相应的CacheKey
        private final CacheKey key;
          // 一级缓存，与BaseExecutor.localCache字段指向同一个对象
        private final PerpetualCache localCache;
        private final ObjectFactory objectFactory;
          // 负责结果对象的类型转换
        private final ResultExtractor resultExtractor;

        public DeferredLoad(MetaObject resultObject, String property, CacheKey key, PerpetualCache localCache, Configuration configuration, Class<?> targetType) {
            this.resultObject = resultObject;
            this.property = property;
            this.key = key;
            this.localCache = localCache;
            this.objectFactory = configuration.getObjectFactory();
            this.resultExtractor = new ResultExtractor(configuration, this.objectFactory);
            this.targetType = targetType;
        }
                // 检测缓存项是否已经完全加载到缓存中   ExecutionPlaceholder.EXECUTION_PLACEHOLDER为缓存的占位符
        public boolean canLoad() {
            return this.localCache.getObject(this.key) != null && this.localCache.getObject(this.key) != ExecutionPlaceholder.EXECUTION_PLACEHOLDER;
        }

        public void load() {
            List<Object> list = (List)this.localCache.getObject(this.key);
            Object value = this.resultExtractor.extractObjectFromList(list, this.targetType);
            this.resultObject.setValue(this.property, value);
        }
    }
}

一级缓存(默认开启)

一级缓存是会话级别的缓存，在mybatis中每创建一个SqlSession对象，就表示开启了一次数据库会话。在一次会话中，应用程序可能会在短时间内，如一次事务内，反复执行完全相同的查询语句，如果不对数据进行缓存，那么每次查询都会执行一次数据库查询操作，而多次完全相同的、时间间隔较短的查询语句得到的结果集极有可能完全相同，会造成数据库资源的浪费。

mybatis在Executor中会建立一个缓存，将每次查询的结果对象缓存起来。在执行查询操作时，会先查询一级缓存，如果其中存在完全一样的查询语句，则直接从一级缓存中取出相应的结果对象返给用户，减少数据库的压力。

一级缓存的生命周期与SqlSession相同（其实是与SqlSession中封装的Executor对象的生命周期相同），当调用Executor.close方法时，Executor所对应的一级缓存就会不可用。在调用Executor.update方法时，会清空一级缓存。

SimpleExecutor

SimpleExecutor继承了BaseExecutor抽象类，如果不进行配置的话，该执行器为默认执行器

public class SimpleExecutor extends BaseExecutor {
    public SimpleExecutor(Configuration configuration, Transaction transaction) {
        super(configuration, transaction);
    }

    public int doUpdate(MappedStatement ms, Object parameter) throws SQLException {
        Statement stmt = null;

        int var6;
        try {
            Configuration configuration = ms.getConfiguration();
            StatementHandler handler = configuration.newStatementHandler(this, ms, parameter, RowBounds.DEFAULT, (ResultHandler)null, (BoundSql)null);
            stmt = this.prepareStatement(handler, ms.getStatementLog());
            var6 = handler.update(stmt);
        } finally {
            this.closeStatement(stmt);
        }

        return var6;
    }

    public <E> List<E> 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对象，返回的为RoutingStatementHandler对象
            StatementHandler handler = configuration.newStatementHandler(this.wrapper, ms, parameter, rowBounds, resultHandler, boundSql);
              // 创建statement
            stmt = this.prepareStatement(handler, ms.getStatementLog());
              // 调用StatementHandler.query方法 执行sql语句  并使用resultHandler进行结果集的映射
            var9 = handler.query(stmt, resultHandler);
        } finally {
            this.closeStatement(stmt);
        }

        return var9;
    }

    protected <E> Cursor<E> doQueryCursor(MappedStatement ms, Object parameter, RowBounds rowBounds, BoundSql boundSql) throws SQLException {
        Configuration configuration = ms.getConfiguration();
        StatementHandler handler = configuration.newStatementHandler(this.wrapper, ms, parameter, rowBounds, (ResultHandler)null, boundSql);
        Statement stmt = this.prepareStatement(handler, ms.getStatementLog());
        Cursor<E> cursor = handler.queryCursor(stmt);
        stmt.closeOnCompletion();
        return cursor;
    }

    public List<BatchResult> doFlushStatements(boolean isRollback) {
        return Collections.emptyList();
    }

    private Statement prepareStatement(StatementHandler handler, Log statementLog) throws SQLException {
        Connection connection = this.getConnection(statementLog);
      // 预编译sql语句，设置一些基本的运行参数
        Statement stmt = handler.prepare(connection, this.transaction.getTimeout());
      // 设置参数
        handler.parameterize(stmt);
        return stmt;
    }
}

ReuseExecutor

重用statement对象是一种常用的优化手段，可以减少SQL预编译的开销以及创建和销毁Statement对象的开销，提高性能。

ReuseExecutor提供了Statement对象重用的功能，通过statementMap字段缓存使用过的Statement对象，key是SQL语句，value是sql所对应的Statement对象。

ReuseExecutor与SimpleExecutor的区别在于prepareStatement方法，SimpleExecutor每次都会通过connection创建新的Statement对象，而ReuseExecutor会先尝试从statementMap中获取。

public class ReuseExecutor extends BaseExecutor {
    private final Map<String, Statement> statementMap = new HashMap();

    public ReuseExecutor(Configuration configuration, Transaction transaction) {
        super(configuration, transaction);
    }

    public int doUpdate(MappedStatement ms, Object parameter) throws SQLException {
        Configuration configuration = ms.getConfiguration();
        StatementHandler handler = configuration.newStatementHandler(this, ms, parameter, RowBounds.DEFAULT, (ResultHandler)null, (BoundSql)null);
        Statement stmt = this.prepareStatement(handler, ms.getStatementLog());
        return handler.update(stmt);
    }

    public <E> List<E> doQuery(MappedStatement ms, Object parameter, RowBounds rowBounds, ResultHandler resultHandler, BoundSql boundSql) throws SQLException {
        Configuration configuration = ms.getConfiguration();
        StatementHandler handler = configuration.newStatementHandler(this.wrapper, ms, parameter, rowBounds, resultHandler, boundSql);
        Statement stmt = this.prepareStatement(handler, ms.getStatementLog());
        return handler.query(stmt, resultHandler);
    }

    protected <E> Cursor<E> doQueryCursor(MappedStatement ms, Object parameter, RowBounds rowBounds, BoundSql boundSql) throws SQLException {
        Configuration configuration = ms.getConfiguration();
        StatementHandler handler = configuration.newStatementHandler(this.wrapper, ms, parameter, rowBounds, (ResultHandler)null, boundSql);
        Statement stmt = this.prepareStatement(handler, ms.getStatementLog());
        return handler.queryCursor(stmt);
    }

    public List<BatchResult> doFlushStatements(boolean isRollback) {
        Iterator var2 = this.statementMap.values().iterator();

        while(var2.hasNext()) {
            Statement stmt = (Statement)var2.next();
              // 关闭statement
            this.closeStatement(stmt);
        }
                // 清空缓存
        this.statementMap.clear();
        return Collections.emptyList();
    }

    private Statement prepareStatement(StatementHandler handler, Log statementLog) throws SQLException {
        BoundSql boundSql = handler.getBoundSql();
        String sql = boundSql.getSql();
        Statement stmt;
          // 检测是否存储了该SQL
        if (this.hasStatementFor(sql)) {
            stmt = this.getStatement(sql);
              // 修改超时时间
            this.applyTransactionTimeout(stmt);
        } else {
            Connection connection = this.getConnection(statementLog);
              // 创建新的Statement对象
            stmt = handler.prepare(connection, this.transaction.getTimeout());
              // 缓存
            this.putStatement(sql, stmt);
        }
                // 处理占位符
        handler.parameterize(stmt);
        return stmt;
    }

    private boolean hasStatementFor(String sql) {
        try {
            Statement statement = (Statement)this.statementMap.get(sql);
            return statement != null && !statement.getConnection().isClosed();
        } catch (SQLException var3) {
            return false;
        }
    }

    private Statement getStatement(String s) {
        return (Statement)this.statementMap.get(s);
    }

    private void putStatement(String sql, Statement stmt) {
        this.statementMap.put(sql, stmt);
    }
}

BatchExecutor

在执行一条SQL语句，会将SQL语句以及相关参数通过网络发送到数据库系统。对于频繁操作数据库的应用来说，如果执行一条SQL就向数据库发送一次请求，就会在网络通信商浪费很多时间。使用批处理的优化方式可以在客户端缓存多条SQL语句，并将多条语句打包发送给数据库执行，从而减少网络开销，提高性能。

在批量执行多条SQL语句时，每次向数据库发送的SQL语句条数是有上限的，如果超过上限，数据库会拒绝执行这些SQL语句并抛出异常。

JDBC的批处理只支持insert、update和delete，不支持select

public class BatchExecutor extends BaseExecutor {
    public static final int BATCH_UPDATE_RETURN_VALUE = -2147482646;
      // 缓存多个statement，每个statement中都存储了多条SQL语句
    private final List<Statement> statementList = new ArrayList();
      // 记录批处理结果，BatchResult中通过updateCounts字段来记录每个statement执行的结果
    private final List<BatchResult> batchResultList = new ArrayList();
      // 记录当前执行的SQL
    private String currentSql;
      // 记录当前执行的MappedStatement
    private MappedStatement currentStatement;

    public BatchExecutor(Configuration configuration, Transaction transaction) {
        super(configuration, transaction);
    }

    public int doUpdate(MappedStatement ms, Object parameterObject) throws SQLException {
        Configuration configuration = ms.getConfiguration();
        StatementHandler handler = configuration.newStatementHandler(this, ms, parameterObject, RowBounds.DEFAULT, (ResultHandler)null, (BoundSql)null);
        BoundSql boundSql = handler.getBoundSql();
        String sql = boundSql.getSql();
        Statement stmt;
          // 如果当前执行的SQL与上次执行的相同，且对应的MappedStatement也相同
        if (sql.equals(this.currentSql) && ms.equals(this.currentStatement)) {
              // 获取集合中的最后一个
            int last = this.statementList.size() - 1;
            stmt = (Statement)this.statementList.get(last);
            this.applyTransactionTimeout(stmt);
              // 绑定实参，处理？占位符
            handler.parameterize(stmt);
            BatchResult batchResult = (BatchResult)this.batchResultList.get(last);
            batchResult.addParameterObject(parameterObject);
        } else {
            Connection connection = this.getConnection(ms.getStatementLog());
            stmt = handler.prepare(connection, this.transaction.getTimeout());
            handler.parameterize(stmt);
            this.currentSql = sql;
            this.currentStatement = ms;
            this.statementList.add(stmt);
            this.batchResultList.add(new BatchResult(ms, sql, parameterObject));
        }
                // 通过Statement.addBatch方法添加SQL语句
        handler.batch(stmt);
        return -2147482646;
    }

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

        List var10;
        try {
            this.flushStatements();
            Configuration configuration = ms.getConfiguration();
            StatementHandler handler = configuration.newStatementHandler(this.wrapper, ms, parameterObject, rowBounds, resultHandler, boundSql);
            Connection connection = this.getConnection(ms.getStatementLog());
            stmt = handler.prepare(connection, this.transaction.getTimeout());
            handler.parameterize(stmt);
            var10 = handler.query(stmt, resultHandler);
        } finally {
            this.closeStatement(stmt);
        }

        return var10;
    }

    protected <E> Cursor<E> doQueryCursor(MappedStatement ms, Object parameter, RowBounds rowBounds, BoundSql boundSql) throws SQLException {
        this.flushStatements();
        Configuration configuration = ms.getConfiguration();
        StatementHandler handler = configuration.newStatementHandler(this.wrapper, ms, parameter, rowBounds, (ResultHandler)null, boundSql);
        Connection connection = this.getConnection(ms.getStatementLog());
        Statement stmt = handler.prepare(connection, this.transaction.getTimeout());
        handler.parameterize(stmt);
        Cursor<E> cursor = handler.queryCursor(stmt);
        stmt.closeOnCompletion();
        return cursor;
    }

    public List<BatchResult> doFlushStatements(boolean isRollback) throws SQLException {
        boolean var17 = false;

        Statement stmt;
        ArrayList var21;
        Iterator var22;
        label179: {
            List var3;
            try {
                var17 = true;
                ArrayList results = new ArrayList();
                if (!isRollback) {
                    int i = 0;

                    for(int n = this.statementList.size(); i < n; ++i) {
                        stmt = (Statement)this.statementList.get(i);
                        this.applyTransactionTimeout(stmt);
                        BatchResult batchResult = (BatchResult)this.batchResultList.get(i);

                        try {
                              // 调用Statement.executeBatch来批量执行SQL 返回的int[] 来更新BatchResult的updateCounts字段
                            batchResult.setUpdateCounts(stmt.executeBatch());
                            MappedStatement ms = batchResult.getMappedStatement();
                            List<Object> parameterObjects = batchResult.getParameterObjects();
                            KeyGenerator keyGenerator = ms.getKeyGenerator();
                            if (Jdbc3KeyGenerator.class.equals(keyGenerator.getClass())) {
                                Jdbc3KeyGenerator jdbc3KeyGenerator = (Jdbc3KeyGenerator)keyGenerator;
                                jdbc3KeyGenerator.processBatch(ms, stmt, parameterObjects);
                            } else if (!NoKeyGenerator.class.equals(keyGenerator.getClass())) {
                                Iterator var10 = parameterObjects.iterator();

                                while(var10.hasNext()) {
                                    Object parameter = var10.next();
                                    keyGenerator.processAfter(this, ms, stmt, parameter);
                                }
                            }

                            this.closeStatement(stmt);
                        } catch (BatchUpdateException var18) {
                            StringBuilder message = new StringBuilder();
                            message.append(batchResult.getMappedStatement().getId()).append(" (batch index #").append(i + 1).append(")").append(" failed.");
                            if (i > 0) {
                                message.append(" ").append(i).append(" prior sub executor(s) completed successfully, but will be rolled back.");
                            }

                            throw new BatchExecutorException(message.toString(), var18, results, batchResult);
                        }

                        results.add(batchResult);
                    }

                    var21 = results;
                    var17 = false;
                    break label179;
                }

                var3 = Collections.emptyList();
                var17 = false;
            } finally {
                if (var17) {
                    Iterator var13 = this.statementList.iterator();

                    while(var13.hasNext()) {
                        Statement stmt = (Statement)var13.next();
                        this.closeStatement(stmt);
                    }

                    this.currentSql = null;
                    this.statementList.clear();
                    this.batchResultList.clear();
                }
            }

            var22 = this.statementList.iterator();

            while(var22.hasNext()) {
                stmt = (Statement)var22.next();
                this.closeStatement(stmt);
            }

            this.currentSql = null;
            this.statementList.clear();
            this.batchResultList.clear();
            return var3;
        }

        var22 = this.statementList.iterator();

        while(var22.hasNext()) {
            stmt = (Statement)var22.next();
            this.closeStatement(stmt);
        }

        this.currentSql = null;
        this.statementList.clear();
        this.batchResultList.clear();
        return var21;
    }
}

CachingExecutor

CachingExecutor提供了二级缓存的功能。

public class CachingExecutor implements Executor {
  // 底层的Executor
    private final Executor delegate;
      // 用于管理CachingExecutor使用的二级缓存对象  key为二级缓存对象  value为TransacionalCache对象
    private final TransactionalCacheManager tcm = new TransactionalCacheManager();

    public CachingExecutor(Executor delegate) {
        this.delegate = delegate;
        delegate.setExecutorWrapper(this);
    }

    public Transaction getTransaction() {
        return this.delegate.getTransaction();
    }

    public void close(boolean forceRollback) {
        try {
            if (forceRollback) {
                this.tcm.rollback();
            } else {
                this.tcm.commit();
            }
        } finally {
            this.delegate.close(forceRollback);
        }

    }

    public boolean isClosed() {
        return this.delegate.isClosed();
    }

    public int update(MappedStatement ms, Object parameterObject) throws SQLException {
        this.flushCacheIfRequired(ms);
        return this.delegate.update(ms, parameterObject);
    }

    public <E> Cursor<E> queryCursor(MappedStatement ms, Object parameter, RowBounds rowBounds) throws SQLException {
        this.flushCacheIfRequired(ms);
        return this.delegate.queryCursor(ms, parameter, rowBounds);
    }

    public <E> List<E> query(MappedStatement ms, Object parameterObject, RowBounds rowBounds, ResultHandler resultHandler) throws SQLException {
          // 获取BoundSql对象
        BoundSql boundSql = ms.getBoundSql(parameterObject);
          // 创建cacheKey对象
        CacheKey key = this.createCacheKey(ms, parameterObject, rowBounds, boundSql);
        return this.query(ms, parameterObject, rowBounds, resultHandler, key, boundSql);
    }

    public <E> List<E> query(MappedStatement ms, Object parameterObject, RowBounds rowBounds, ResultHandler resultHandler, CacheKey key, BoundSql boundSql) throws SQLException {
          // 获取查询有所在命名空间对应的二级缓存
        Cache cache = ms.getCache();
          // 是否开启了二级缓存
        if (cache != null) {
              // 根据<select>节点的配置，决定是否需要清空二级缓存
            this.flushCacheIfRequired(ms);
              // 检测SQL节点的useCache配置以及是否使用了resultHandler配置
            if (ms.isUseCache() && resultHandler == null) {
                  // 二级缓存不能保存输出类型的参数，如果查询操作调用了包含输出类型的存储过程，则报错
                this.ensureNoOutParams(ms, boundSql);
                  // 查询二级缓存
                List<E> list = (List)this.tcm.getObject(cache, key);
                if (list == null) {
                      // 二级缓存不存在，调用封装的executor对象的query方法
                    list = this.delegate.query(ms, parameterObject, rowBounds, resultHandler, key, boundSql);
                      // 将查询结果存储到TransactionalCache.entriesToAddOnCommit中
                    this.tcm.putObject(cache, key, list);
                }

                return list;
            }
        }
                // 如果没有启动二级缓存，则直接调用executor的query方法
        return this.delegate.query(ms, parameterObject, rowBounds, resultHandler, key, boundSql);
    }

    public List<BatchResult> flushStatements() throws SQLException {
        return this.delegate.flushStatements();
    }

    public void commit(boolean required) throws SQLException {
        this.delegate.commit(required);
        this.tcm.commit();
    }

    public void rollback(boolean required) throws SQLException {
        try {
            this.delegate.rollback(required);
        } finally {
            if (required) {
                this.tcm.rollback();
            }

        }

    }

    private void ensureNoOutParams(MappedStatement ms, BoundSql boundSql) {
        if (ms.getStatementType() == StatementType.CALLABLE) {
            Iterator var3 = boundSql.getParameterMappings().iterator();

            while(var3.hasNext()) {
                ParameterMapping parameterMapping = (ParameterMapping)var3.next();
                if (parameterMapping.getMode() != ParameterMode.IN) {
                    throw new ExecutorException("Caching stored procedures with OUT params is not supported.  Please configure useCache=false in " + ms.getId() + " statement.");
                }
            }
        }

    }

    public CacheKey createCacheKey(MappedStatement ms, Object parameterObject, RowBounds rowBounds, BoundSql boundSql) {
        return this.delegate.createCacheKey(ms, parameterObject, rowBounds, boundSql);
    }

    public boolean isCached(MappedStatement ms, CacheKey key) {
        return this.delegate.isCached(ms, key);
    }

    public void deferLoad(MappedStatement ms, MetaObject resultObject, String property, CacheKey key, Class<?> targetType) {
        this.delegate.deferLoad(ms, resultObject, property, key, targetType);
    }

    public void clearLocalCache() {
        this.delegate.clearLocalCache();
    }

    private void flushCacheIfRequired(MappedStatement ms) {
        Cache cache = ms.getCache();
        if (cache != null && ms.isFlushCacheRequired()) {
            this.tcm.clear(cache);
        }

    }

    public void setExecutorWrapper(Executor executor) {
        throw new UnsupportedOperationException("This method should not be called");
    }
}

TransactionalCacheManager

TransactionalCacheManager 用于管理 CachingExecutor 使用的二级缓存

public class TransactionalCacheManager {
      // 用来管理CachingExecutor使用的二级缓存
      // key为CachingExecutor使用的二级缓存对象，value为对应的TransactionalCache对象
    private final Map<Cache, TransactionalCache> transactionalCaches = new HashMap();

    public TransactionalCacheManager() {
    }

    public void clear(Cache cache) {
        this.getTransactionalCache(cache).clear();
    }

    public Object getObject(Cache cache, CacheKey key) {
        return this.getTransactionalCache(cache).getObject(key);
    }

    public void putObject(Cache cache, CacheKey key, Object value) {
        this.getTransactionalCache(cache).putObject(key, value);
    }

    public void commit() {
        Iterator var1 = this.transactionalCaches.values().iterator();

        while(var1.hasNext()) {
            TransactionalCache txCache = (TransactionalCache)var1.next();
            txCache.commit();
        }

    }

    public void rollback() {
        Iterator var1 = this.transactionalCaches.values().iterator();

        while(var1.hasNext()) {
            TransactionalCache txCache = (TransactionalCache)var1.next();
            txCache.rollback();
        }

    }
// 获取TransactionalCache对象，所有的调用都是使用的TransactionalCache来调用对应的方法
    private TransactionalCache getTransactionalCache(Cache cache) {
        return (TransactionalCache)this.transactionalCaches.computeIfAbsent(cache, TransactionalCache::new);
    }
}

TransactionalCache

TransactionalCache 实现了 Cache 接口，主要用于保存在某个 SqlSession 的某个事务中需要向某个二级缓存中添加的数据

public class TransactionalCache implements Cache {
    private static final Log log = LogFactory.getLog(TransactionalCache.class);
      // 底层封装的二级缓存对应的Cache对象
    private final Cache delegate;
      // true表示当前TransactionalCache不可查询，且提交事务时会将底层Cache清空
    private boolean clearOnCommit;
      // 暂时存储数据，提交事务后会存储到二级缓存
    private final Map<Object, Object> entriesToAddOnCommit;
      // 记录缓存未命中的CacheKey对象
    private final Set<Object> entriesMissedInCache;

    public TransactionalCache(Cache delegate) {
        this.delegate = delegate;
        this.clearOnCommit = false;
        this.entriesToAddOnCommit = new HashMap();
        this.entriesMissedInCache = new HashSet();
    }

    public String getId() {
        return this.delegate.getId();
    }

    public int getSize() {
        return this.delegate.getSize();
    }

    public Object getObject(Object key) {
          // 先查缓存中是否有
        Object object = this.delegate.getObject(key);
        if (object == null) {    
              // 如果缓存中没有，将key存入到entriesMissedInCache中
            this.entriesMissedInCache.add(key);
        }

        return this.clearOnCommit ? null : object;
    }
        // putObject不会直接将结果对象记录到二级缓存中，而是暂时保存在entriesToAddOnCommit集合中，在事务提交时才会将结果对象添加到二级缓存中
    public void putObject(Object key, Object object) {
          // 将缓存项暂存在entriesToAddOnCommit中
        this.entriesToAddOnCommit.put(key, object);
    }

    public Object removeObject(Object key) {
        return null;
    }

    public void clear() {
        this.clearOnCommit = true;
        this.entriesToAddOnCommit.clear();
    }

    public void commit() {
        if (this.clearOnCommit) {
            this.delegate.clear();
        }
                // 将entriesToAddOnCommit集合中的数据放入二级缓存中
        this.flushPendingEntries();
        this.reset();
    }

    public void rollback() {
      // 事务回滚会将未命中缓存的数据清除掉
        this.unlockMissedEntries();
        this.reset();
    }

    private void reset() {
        this.clearOnCommit = false;
        this.entriesToAddOnCommit.clear();
        this.entriesMissedInCache.clear();
    }

  // 将entriesToAddOnCommit集合中的数据放入二级缓存中
    private void flushPendingEntries() {
        Iterator var1 = this.entriesToAddOnCommit.entrySet().iterator();

        while(var1.hasNext()) {
            Entry<Object, Object> entry = (Entry)var1.next();
          // 放入二级缓存
            this.delegate.putObject(entry.getKey(), entry.getValue());
        }

        var1 = this.entriesMissedInCache.iterator();

        while(var1.hasNext()) {
            Object entry = var1.next();
            if (!this.entriesToAddOnCommit.containsKey(entry)) {
                this.delegate.putObject(entry, (Object)null);
            }
        }

    }

    private void unlockMissedEntries() {
        Iterator var1 = this.entriesMissedInCache.iterator();

        while(var1.hasNext()) {
            Object entry = var1.next();

            try {
                this.delegate.removeObject(entry);
            } catch (Exception var4) {
                
            }
        }

    }
}

二级缓存

mybatis中的二级缓存是应用级别的缓存，生命周期与应用程序的生命周期相同。

二级缓存相关配置

• Mybatis-config.xml配置文件中的cacheEnabled，是二级缓存的总开关 只有配置为true时，二级缓存的配置才会生效，默认为true

  <settings>
      <setting name="cacheEnabled" value="true"></setting>
  </settings>

• mapper文件中配置cache节点或cache-ref节点 可以在命名空间粒度上管理二级缓存的开启和关闭
  配置了<cache>节点，在解析时会为该配置文件所对应的命名空间创建相应的Cache对象作为二级缓存，默认为PerpetualCache，可以使用type属性指定自定义的Cache对象。
  配置<cache-ref>可以使得多个命名空间共享一个Cache对象

• <select>节点的useCache属性，表示查询产生的结果是否要保存到二级缓存中。默认为true