设计模式_代理模式_在SqlSessionTemplate(Spring)中的应用

1.SqlSessionTemplate的构造函数，根据传入的SqlSessionFactory和ExecutorType创建一个Spring管理的SqlSession，并生成SqlSession的动态代理对象

 public SqlSessionTemplate(SqlSessionFactory sqlSessionFactory, ExecutorType executorType,
       PersistenceExceptionTranslator exceptionTranslator) {

     notNull(sqlSessionFactory, "Property 'sqlSessionFactory' is required");
     notNull(executorType, "Property 'executorType' is required");

     this.sqlSessionFactory = sqlSessionFactory;
     this.executorType = executorType;
     this.exceptionTranslator = exceptionTranslator;
     //这里使用了jdk的动态代理，为SqlSession创建了代理对象
     this.sqlSessionProxy = (SqlSession) newProxyInstance(
         SqlSessionFactory.class.getClassLoader(),
         new Class[] { SqlSession.class },
         new SqlSessionInterceptor());
 }

2.创建一个名为SqlSessionInterceptor的InvocationHandler实现类，当调用SqlSession的代理对象的方法时，会调用这个 InvocationHandler实现类的invokd方法，这里，SqlSessionInterceptor中实现了对SqlSession的释放

 private class SqlSessionInterceptor implements InvocationHandler {
     @Override
     public Object invoke(Object proxy, Method method, Object[] args) throws Throwable {
       SqlSession sqlSession = getSqlSession(
           SqlSessionTemplate.this.sqlSessionFactory,
           SqlSessionTemplate.this.executorType,
           SqlSessionTemplate.this.exceptionTranslator);
       try {
         Object result = method.invoke(sqlSession, args);
         if (!isSqlSessionTransactional(sqlSession, SqlSessionTemplate.this.sqlSessionFactory)) {
           // force commit even on non-dirty sessions because some databases require
           // a commit/rollback before calling close()
           sqlSession.commit(true);
         }
         return result;
       } catch (Throwable t) {
         Throwable unwrapped = unwrapThrowable(t);
         if (SqlSessionTemplate.this.exceptionTranslator != null && unwrapped instanceof PersistenceException) {
           // release the connection to avoid a deadlock if the translator is no loaded. See issue #22
        　 //如果遇到异常，释放SqlSession
           closeSqlSession(sqlSession, SqlSessionTemplate.this.sqlSessionFactory);
           sqlSession = null;
           Throwable translated = SqlSessionTemplate.this.exceptionTranslator.translateExceptionIfPossible((PersistenceException) unwrapped);
           if (translated != null) {
             unwrapped = translated;
           }
         }
         throw unwrapped;
       } finally {
         //最后，如果SqlSession不为空，释放SqlSesssion
         if (sqlSession != null) {
           closeSqlSession(sqlSession, SqlSessionTemplate.this.sqlSessionFactory);
         }
       }
     }
   }

3.通过SqlSession的代理对象来执行相关的增删改查方法

 @Override
   public int insert(String statement) {
     //通过SqlSessionTemplate构造函数中生成的SqlSession的动态代理对象来执行 insert 操作
     return this.sqlSessionProxy.insert(statement);
 }

 @Override
   public int delete(String statement) {
     //通过SqlSessionTemplate构造函数中生成的SqlSession的动态代理对象来执行 delete 操作
     return this.sqlSessionProxy.delete(statement);
 }

 @Override
   public int update(String statement) {
     //通过SqlSessionTemplate构造函数中生成的SqlSession的动态代理对象来执行 update 操作
     return this.sqlSessionProxy.update(statement);
 }

 @Override
   public <T> T selectOne(String statement, Object parameter) {
     //通过SqlSessionTemplate构造函数中生成的SqlSession的动态代理对象来执行 selectOne 操作
     return this.sqlSessionProxy.<T> selectOne(statement, parameter);
 }

4.由于SqlSessionTemplate通过代理的方式帮我们维护了SqlSession的生命周期(close,rollback,comit这些)，所以我们不能显示的调用close方法

 @Override
   public void close() {
     //不可以对Spring管理的SqlSession执行close方法
     throw new UnsupportedOperationException("Manual close is not allowed over a Spring managed SqlSession");
 }

5.在SpringBoot项目中实测SqlSessionTemplate的方法执行，执行完方法后，会释放相应的SqlSession资源