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  还是以第一篇博客中给出的例子,根据代码实例来入手分析。

 1     static {
 2         InputStream inputStream = MybatisTest.class.getClassLoader().getResourceAsStream("mybatis-configuration.xml");
 3         sqlSessionFactory = new SqlSessionFactoryBuilder().build(inputStream);
 4     }
 5 
 6     /**
 7      * 查询单个记录
 8      */
 9     @Test
10     public void testSelectOne() {
11         SqlSession session = sqlSessionFactory.openSession();
12         User user = session.selectOne(NAME_SPACE + ".selectUserById", 1);
13         System.out.println(user);
14         session.close();
15     }

  如何加载配置文件前面也已经介绍了,通过配置文件产生SqlSessionFactory,追溯源码可以发现其实现是 DefaultSqlSessionFactory。

1   public SqlSessionFactory build(Configuration config) {
2     return new DefaultSqlSessionFactory(config);
3   }

  得到 SqlSessionFactory 之后,就可以通过 SqlSessionFactory 去获取 SqlSession 对象。源码如下:

 1     @Override
 2     public SqlSession openSession() {
 3         return openSessionFromDataSource(configuration.getDefaultExecutorType(), null, false);
 4     }
 5 
 6 
 7     private SqlSession openSessionFromDataSource(ExecutorType execType, TransactionIsolationLevel level, boolean autoCommit) {
 8         Transaction tx = null;
 9         try {
10             //Environment对象封装了配置文件中对于数据源和事务的配置
11             final Environment environment = configuration.getEnvironment();
12             final TransactionFactory transactionFactory = getTransactionFactoryFromEnvironment(environment);
13             tx = transactionFactory.newTransaction(environment.getDataSource(), level, autoCommit);
14             //获取Executor对象,用来执行sql语句
15             final Executor executor = configuration.newExecutor(tx, execType);
16             return new DefaultSqlSession(configuration, executor, autoCommit);
17         } catch (Exception e) {
18             closeTransaction(tx); // may have fetched a connection so lets call close()
19             throw ExceptionFactory.wrapException("Error opening session.  Cause: " + e, e);
20         } finally {
21             ErrorContext.instance().reset();
22         }
23     }

  这里我们重点看一下第 15 行代码:

 1   public Executor newExecutor(Transaction transaction, ExecutorType executorType) {
 2     executorType = executorType == null ? defaultExecutorType : executorType;
 3     executorType = executorType == null ? ExecutorType.SIMPLE : executorType;
 4     Executor executor;
 5     if (ExecutorType.BATCH == executorType) {
 6       executor = new BatchExecutor(this, transaction);
 7     } else if (ExecutorType.REUSE == executorType) {
 8       executor = new ReuseExecutor(this, transaction);
 9     } else {
10       executor = new SimpleExecutor(this, transaction);
11     }
12     if (cacheEnabled) {
13       executor = new CachingExecutor(executor);
14     }
15     executor = (Executor) interceptorChain.pluginAll(executor);
16     return executor;
17   }

  根据执行器类型这里有多种不同的执行器Executor。

  注意第 12 行代码,如果我们开启了缓存,即 cacheEnabled = true(这里是一级缓存,默认是开启的),第13行代码使用了装饰器模式,在原有的 Executor 上装饰了缓存功能。

  第 15 行用于设置插件。

  这时候已经得到SqlSession对象了,实际类型是 DefaultSqlSession。接下来我们就可以通过该对象来执行sql语句了。

 1、insert 操作

 1     /**
 2      * 插入一条记录
 3      */
 4     @Test
 5     public void testInsert() {
 6         SqlSession session = sqlSessionFactory.openSession();
 7         User user = new User(2, "zhangsan", 22);
 8         session.insert(NAME_SPACE + ".insertUser", user);
 9         session.commit();
10         session.close();
11     }

  通过第8行代码,我们进入到 insert 方法:

1   @Override
2   public int insert(String statement, Object parameter) {
3     return update(statement, parameter);
4   }

  注意:这里通过 insert 方法,调用的是 update 方法。

 1   public int update(String statement, Object parameter) {
 2     try {
 3       dirty = true;
 4       MappedStatement ms = configuration.getMappedStatement(statement);
 5       return executor.update(ms, wrapCollection(parameter));
 6     } catch (Exception e) {
 7       throw ExceptionFactory.wrapException("Error updating database.  Cause: " + e, e);
 8     } finally {
 9       ErrorContext.instance().reset();
10     }
11   }

  第4行根据给的statement参数,获取配置的所有如下信息,并将其封装到 MappedStatement 对象中,关于这个对象后面会详细介绍。

1     <!-- 向 user 表插入一条数据 -->
2     <insert id="insertUser" parameterType="com.ys.po.User" >
3         insert into
4         user(<include refid="Base_Column_List" />)
5         value(#{id,jdbcType=INTEGER},#{name,jdbcType=VARCHAR},#{age,jdbcType=INTEGER})
6     </insert>

  ①、接着我们看第 5 行代码,首先看 wrapCollection(parameter) 方法:

 1     private Object wrapCollection(final Object object) {
 2         if (object instanceof Collection) {
 3             DefaultSqlSession.StrictMap<Object> map = new DefaultSqlSession.StrictMap<Object>();
 4             map.put("collection", object);
 5             if (object instanceof List) {
 6                 map.put("list", object);
 7             }
 8             return map;
 9         } else if (object != null && object.getClass().isArray()) {
10             DefaultSqlSession.StrictMap<Object> map = new DefaultSqlSession.StrictMap<Object>();
11             map.put("array", object);
12             return map;
13         }
14         return object;
15     }

  通过这段代码的if-else if 语句主要做了如下两个操作:

  1、如果传入的参数是集合 Collection,在 map 集合中放入一个key为"collection"、value为参数的键值对,接着判断该集合是不是 List 类型,如果是,那么在 map 集合中在放入一个key为"list"、value为参数的键值对。

  2、如果传入的参数是数组类型,那么在 map 中放入一个key为"array"、value为参数的键值对。

  注意:这里的 StrictMap ,其实就是一个 HashMap。

 1   public static class StrictMap<V> extends HashMap<String, V> {
 2 
 3     private static final long serialVersionUID = -5741767162221585340L;
 4 
 5     @Override
 6     public V get(Object key) {
 7       if (!super.containsKey(key)) {
 8         throw new BindingException("Parameter '" + key + "' not found. Available parameters are " + this.keySet());
 9       }
10       return super.get(key);
11     }
12 
13   }
14 
15 }
View Code

  ②、wrapCollection(parameter) 方法介绍完了。接着我们看 executor.update()方法:

  这里需要说明的是 Executor 对象上面我们已经介绍了,由于默认是开启一级缓存的,这时候我们进入 CachingExecutor 类的 update() 方法:

1   public int update(MappedStatement ms, Object parameterObject) throws SQLException {
2     flushCacheIfRequired(ms);
3     return delegate.update(ms, parameterObject);
4   }

  首先我们看这里的第 2 行代码:

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

  这里表示的意思是是否清除缓存。看我们是否在配置文件中配置了 <cache> 标签,以及我们是否在 <insert /> 标签中是否增加了 flushCache="true"属性。如果有其中任何一个,此次操作都会清除缓存。

  接着我们再看第3行代码,这里的delegate 是 Executor,但是这是一个接口,其真实类型是SimpleExecutor,经过装饰器模式,调用 CachingExecutor 的 update 方法,经过处理后,最后最后调用 SimpleExecutor的update方法:

  具体调用:

  首先调用 BaseExecutor 的 update 方法

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

  然后调用 doUpdate 方法,由于 SimpleExecutor 继承 BaseExecutor 类,并重写了 doUpdate 方法,我们看 SimpleExecutor 类的 doUpdate 方法:

 1   public int doUpdate(MappedStatement ms, Object parameter) throws SQLException {
 2     Statement stmt = null;
 3     try {
 4       Configuration configuration = ms.getConfiguration();
 5       StatementHandler handler = configuration.newStatementHandler(this, ms, parameter, RowBounds.DEFAULT, null, null);
 6       stmt = prepareStatement(handler, ms.getStatementLog());
 7       return handler.update(stmt);
 8     } finally {
 9       closeStatement(stmt);
10     }
11   }

  看到这里,Statement 对象,看到我们熟悉的 JDBC 操作数据库的对象了吧。我们直接看第 6 行代码:

1   private Statement prepareStatement(StatementHandler handler, Log statementLog) throws SQLException {
2     Statement stmt;
3     Connection connection = getConnection(statementLog);
4     stmt = handler.prepare(connection, transaction.getTimeout());
5     handler.parameterize(stmt);
6     return stmt;
7   }

  第 3 行代码获取数据库连接,是根据前面配置的数据源来获取。接着我们看 handler.update(stemt) 方法:

 1   public int update(Statement statement) throws SQLException {
 2     String sql = boundSql.getSql();
 3     Object parameterObject = boundSql.getParameterObject();
 4     KeyGenerator keyGenerator = mappedStatement.getKeyGenerator();
 5     int rows;
 6     if (keyGenerator instanceof Jdbc3KeyGenerator) {
 7       statement.execute(sql, Statement.RETURN_GENERATED_KEYS);
 8       rows = statement.getUpdateCount();
 9       keyGenerator.processAfter(executor, mappedStatement, statement, parameterObject);
10     } else if (keyGenerator instanceof SelectKeyGenerator) {
11       statement.execute(sql);
12       rows = statement.getUpdateCount();
13       keyGenerator.processAfter(executor, mappedStatement, statement, parameterObject);
14     } else {
15       statement.execute(sql);
16       rows = statement.getUpdateCount();
17     }
18     return rows;
19   }

  这里就都是我们熟悉的 JDBC 操作了。

2、update 和 delete 操作

 1     /**
 2      * 更新一条记录
 3      */
 4     @Test
 5     public void testUpdate() {
 6         SqlSession session = sqlSessionFactory.openSession();
 7         User user = new User(2, "lisi", 22);
 8         session.update(NAME_SPACE + ".updateUserById", user);
 9         session.commit();
10         session.close();
11     }
12 
13     /**
14      * 删除一条记录
15      */
16     @Test
17     public void testDelete() {
18         SqlSession session = sqlSessionFactory.openSession();
19         session.delete(NAME_SPACE + ".deleteUserById", 2);
20         session.commit();
21         session.close();
22     }

  进入到上述第 8 行和 第 19 行代码,我们发现都是进入到和 上面 insert 操作一样的代码:

  第 8 行:

  public int update(String statement, Object parameter) {
    try {
      dirty = true;
      MappedStatement ms = configuration.getMappedStatement(statement);
      return executor.update(ms, wrapCollection(parameter));
    } catch (Exception e) {
      throw ExceptionFactory.wrapException("Error updating database.  Cause: " + e, e);
    } finally {
      ErrorContext.instance().reset();
    }
  }

  第 19 行:

  @Override
  public int delete(String statement, Object parameter) {
    return update(statement, parameter);
  }

  之后的 update 也是上面的代码。这也和我们理解的应该保持一致。

  结论:

insert、update、delete都是属于对数据库的行进行更新操作

  所以这三种语句的执行都是采用的同种逻辑处理。最终都可以调用 executeUpdate() 方法来处理。唯一不同的是 select 操作,必须要调用 executeQuery() 来执行。

3、select 操作

 1     /**
 2      * 查询单个记录
 3      */
 4     @Test
 5     public void testSelectOne() {
 6         SqlSession session = sqlSessionFactory.openSession();
 7         User user = session.selectOne(NAME_SPACE + ".selectUserById", 1);
 8         System.out.println(user);
 9         session.close();
10     }
11 
12     /**
13      * 查询多个记录
14      */
15     @Test
16     public void testSelectList() {
17         SqlSession session = sqlSessionFactory.openSession();
18         List<User> listUser = session.selectList(NAME_SPACE + ".selectUserAll");
19         if (listUser != null) {
20             System.out.println(listUser.size());
21         }
22         session.close();
23     }

  首先看第 7 行代码:

 1   public <T> T selectOne(String statement, Object parameter) {
 2     // Popular vote was to return null on 0 results and throw exception on too many.
 3     List<T> list = this.<T>selectList(statement, parameter);
 4     if (list.size() == 1) {
 5       return list.get(0);
 6     } else if (list.size() > 1) {
 7       throw new TooManyResultsException("Expected one result (or null) to be returned by selectOne(), but found: " + list.size());
 8     } else {
 9       return null;
10     }
11   }

  看到上面的第 3 行代码,我们可能马上就明白了,其实selectOne() 和 selectList() 也都是调用的 selectList() 方法,只不过 selectOne() 是获取集合的第一个元素而已。

  接着看 selectList() 源码:

 1   @Override
 2   public <E> List<E> selectList(String statement, Object parameter) {
 3     return this.selectList(statement, parameter, RowBounds.DEFAULT);
 4   }
 5 
 6   @Override
 7   public <E> List<E> selectList(String statement, Object parameter, RowBounds rowBounds) {
 8     try {
 9       MappedStatement ms = configuration.getMappedStatement(statement);
10       return executor.query(ms, wrapCollection(parameter), rowBounds, Executor.NO_RESULT_HANDLER);
11     } catch (Exception e) {
12       throw ExceptionFactory.wrapException("Error querying database.  Cause: " + e, e);
13     } finally {
14       ErrorContext.instance().reset();
15     }
16   }

  看第10的 query 方法:

1   public <E> List<E> query(MappedStatement ms, Object parameterObject, RowBounds rowBounds, ResultHandler resultHandler) throws SQLException {
2     BoundSql boundSql = ms.getBoundSql(parameterObject);
3     CacheKey key = createCacheKey(ms, parameterObject, rowBounds, boundSql);
4     return query(ms, parameterObject, rowBounds, resultHandler, key, boundSql);
5   }
 1   public <E> List<E> query(MappedStatement ms, Object parameterObject, RowBounds rowBounds, ResultHandler resultHandler, CacheKey key, BoundSql boundSql)
 2       throws SQLException {
 3     Cache cache = ms.getCache();
 4     if (cache != null) {
 5       flushCacheIfRequired(ms);
 6       if (ms.isUseCache() && resultHandler == null) {
 7         ensureNoOutParams(ms, parameterObject, boundSql);
 8         @SuppressWarnings("unchecked")
 9         List<E> list = (List<E>) tcm.getObject(cache, key);
10         if (list == null) {
11           list = delegate.<E> query(ms, parameterObject, rowBounds, resultHandler, key, boundSql);
12           tcm.putObject(cache, key, list); // issue #578 and #116
13         }
14         return list;
15       }
16     }
17     return delegate.<E> query(ms, parameterObject, rowBounds, resultHandler, key, boundSql);
18   }

  最后我们来到doQuery() 方法:

  public <E> List<E> doQuery(MappedStatement ms, Object parameter, RowBounds rowBounds, ResultHandler resultHandler, BoundSql boundSql) throws SQLException {
    Statement stmt = null;
    try {
      Configuration configuration = ms.getConfiguration();
      StatementHandler handler = configuration.newStatementHandler(wrapper, ms, parameter, rowBounds, resultHandler, boundSql);
      stmt = prepareStatement(handler, ms.getStatementLog());
      return handler.<E>query(stmt, resultHandler);
    } finally {
      closeStatement(stmt);
    }
  }
1   @Override
2   public <E> List<E> query(Statement statement, ResultHandler resultHandler) throws SQLException {
3     PreparedStatement ps = (PreparedStatement) statement;
4     ps.execute();
5     return resultSetHandler.<E> handleResultSets(ps);
6   }

  至此,select 操作也执行完毕了。