spring jdbc 源码 分析

 2019-11-02 21:07  阅读(685)
文章分类:Spring boot

Collections.unmodifiableMap(map)————————-返回一个不可修改的map

转自:http://www.iteye.com/topic/11738

1 如何获得连接
看DataSourceUtils代码

Java代码

20191102100583\_1.png

  1. protectedstatic Connection doGetConnection(DataSource dataSource, boolean allowSynchronization);

  2. throws SQLException {

  3. ConnectionHolder conHolder = (ConnectionHolder); TransactionSynchronizationManager.getResource(dataSource);;

  4. if (conHolder != null); {

  5. conHolder.requested();;

  6. return conHolder.getConnection();;

  7. }

  8. Connection con = dataSource.getConnection();;

  9. if (allowSynchronization && TransactionSynchronizationManager.isSynchronizationActive();); {

  10. conHolder = new ConnectionHolder(con);;

  11. TransactionSynchronizationManager.bindResource(dataSource, conHolder);;

  12. TransactionSynchronizationManager.registerSynchronization(new ConnectionSynchronization(conHolder, dataSource););;

  13. conHolder.requested();;

  14. }

  15. return con;

  16. }

    protected static Connection doGetConnection(DataSource dataSource, boolean allowSynchronization); throws SQLException {

            ConnectionHolder conHolder = (ConnectionHolder); TransactionSynchronizationManager.getResource(dataSource);;
            if (conHolder != null); {
                conHolder.requested();;
                return conHolder.getConnection();;
            }
    
            Connection con = dataSource.getConnection();;
            if (allowSynchronization && TransactionSynchronizationManager.isSynchronizationActive();); {
                            conHolder = new ConnectionHolder(con);;
                TransactionSynchronizationManager.bindResource(dataSource, conHolder);;
                TransactionSynchronizationManager.registerSynchronization(new ConnectionSynchronization(conHolder, dataSource););;
                conHolder.requested();;
            }
    
            return con;
        }
    

原来连接是从TransactionSynchronizationManager中获取,如果TransactionSynchronizationManager中已经有了,那么拿过来然后调用conHolder.requested()。否则从原始的DataSource这创建一个连接,放到一个ConnectionHolder,然后再调用TransactionSynchronizationManager.bindResource绑定。
好,我们又遇到两个新的类TransactionSynchronizationManager和ConnectionHolder和。继续跟踪

2 TransactionSynchronizationManager
看其中的一些代码

Java代码

20191102100583\_2.png

  1. privatestatic ThreadLocal resources = new ThreadLocal();;

  2. publicstatic Object getResource(Object key); {

  3. Map map = (Map); resources.get();;

  4. if (map == null); {

  5. returnnull;

  6. }

  7. Object value = map.get(key);;

  8. return value;

  9. }

  10. publicstaticvoid bindResource(Object key, Object value); throws IllegalStateException {

  11. Map map = (Map); resources.get();;

  12. if (map == null); {

  13. map = new HashMap();;

  14. resources.set(map);;

  15. }

  16. map.put(key, value);;

  17. }

    private static ThreadLocal resources = new ThreadLocal();; public static Object getResource(Object key); { Map map = (Map); resources.get();; if (map == null); { return null; } Object value = map.get(key);; return value; } public static void bindResource(Object key, Object value); throws IllegalStateException { Map map = (Map); resources.get();; if (map == null); { map = new HashMap();; resources.set(map);; } map.put(key, value);; }

原来TransactionSynchronizationManager内部建立了一个ThreadLocal的resources,这个resources又是和一个map联系在一起的,这个map在某个线程第一次调用bindResource时生成。
联系前面的DataSourceUtils代码,我们可以总结出来。
某个线程使用DataSourceUtils,当第一次要求创建连接将在TransactionSynchronizationManager中创建出一个ThreadLocal的map。然后以DataSource作为键,ConnectionHolder为值放到map中。等这个线程下一次再请求的这个DataSource的时候,就从这个map中获取对应的ConnectionHolder。用map是为了解决同一个线程上多个DataSource。
然后我们来看看ConnectionHolder又是什么?

3 对连接进行引用计数
看ConnectionHolder代码,这个类很简单,看不出个所以然,只好再去看父类代码ResourceHolderSupport,我们感兴趣的是这两个方法

Java代码

20191102100583\_3.png

  1. publicvoid requested(); {

  2. this.referenceCount++;

  3. }

  4. publicvoid released(); {

  5. this.referenceCount–;

  6. }

    public void requested(); { this.referenceCount++; }

        public void released(); {
            this.referenceCount--;
        }
    

看得出这是一个引用计数的技巧。原来Spring中对Connection是竟量使用已创建的对象,而不是每次都创建一个新对象。这就是DataSourceUtils中

Java代码

20191102100583\_4.png

  1. if (conHolder != null); {

  2. conHolder.requested();;

  3. return conHolder.getConnection();;

  4. }

    if (conHolder != null); { conHolder.requested();; return conHolder.getConnection();; }

的原因

4 释放连接
完成事物后DataSourceTransactionManager有这样的代码

Java代码

20191102100583\_5.png

  1. protectedvoid doCleanupAfterCompletion(Object transaction); {

  2. DataSourceTransactionObject txObject = (DataSourceTransactionObject); transaction;

  3. // Remove the connection holder from the thread.

  4. TransactionSynchronizationManager.unbindResource(this.dataSource);;

  5. txObject.getConnectionHolder();.clear();;

  6. //… DataSourceUtils.closeConnectionIfNecessary(con, this.dataSource);;

  7. }

    protected void doCleanupAfterCompletion(Object transaction); { DataSourceTransactionObject txObject = (DataSourceTransactionObject); transaction;

            // Remove the connection holder from the thread.
            TransactionSynchronizationManager.unbindResource(this.dataSource);;
            txObject.getConnectionHolder();.clear();;
    
            //...       DataSourceUtils.closeConnectionIfNecessary(con, this.dataSource);;
        }
    

DataSourceUtils

Java代码

20191102100583\_6.png

  1. protectedstaticvoid doCloseConnectionIfNecessary(Connection con, DataSource dataSource); throws SQLException {

  2. if (con == null); {

  3. return;

  4. }

  5. ConnectionHolder conHolder = (ConnectionHolder); TransactionSynchronizationManager.getResource(dataSource);;

  6. if (conHolder != null && con == conHolder.getConnection();); {

  7. // It’s the transactional Connection: Don’t close it.

  8. conHolder.released();;

  9. return;

  10. }

  11. // Leave the Connection open only if the DataSource is our

  12. // special data source, and it wants the Connection left open.

  13. if (!(dataSource instanceof SmartDataSource); || ((SmartDataSource); dataSource);.shouldClose(con);); {

  14. logger.debug(“Closing JDBC connection”);;

  15. con.close();;

  16. }

  17. }

    protected static void doCloseConnectionIfNecessary(Connection con, DataSource dataSource); throws SQLException { if (con == null); { return; }

            ConnectionHolder conHolder = (ConnectionHolder); TransactionSynchronizationManager.getResource(dataSource);;
            if (conHolder != null && con == conHolder.getConnection();); {
                // It's the transactional Connection: Don't close it.
                conHolder.released();;
                return;
            }
    
            // Leave the Connection open only if the DataSource is our
            // special data source, and it wants the Connection left open.
            if (!(dataSource instanceof SmartDataSource); || ((SmartDataSource); dataSource);.shouldClose(con);); {
                logger.debug("Closing JDBC connection");;
                con.close();;
            }
        }
    

恍然大悟。如果事物完成,那么就
TransactionSynchronizationManager.unbindResource(this.dataSource);将当前的ConnectionHolder
从TransactionSynchronizationManager上脱离,然后doCloseConnectionIfNecessary。最后会把连接关闭掉。

5 两个辅助类JdbcTemplate和TransactionAwareDataSourceProxy
JdbcTemplate中的execute方法的第一句和最后一句

Java代码

20191102100583\_7.png

  1. public Object execute(PreparedStatementCreator psc, PreparedStatementCallback action);

  2. throws DataAccessException {

  3. Connection con = DataSourceUtils.getConnection(getDataSource(););;

  4. //其他代码

  5. DataSourceUtils.closeConnectionIfNecessary(con, getDataSource(););;

  6. }

  7. }

    public Object execute(PreparedStatementCreator psc, PreparedStatementCallback action); throws DataAccessException {

            Connection con = DataSourceUtils.getConnection(getDataSource(););;
            //其他代码
        DataSourceUtils.closeConnectionIfNecessary(con, getDataSource(););;
            }
        }
    

作用不言自明了吧

从TransactionAwareDataSourceProxy中获取的连接是这个样子的

Java代码

20191102100583\_8.png

  1. public Connection getConnection(); throws SQLException {

  2. Connection con = DataSourceUtils.doGetConnection(getTargetDataSource();, true);;

  3. return getTransactionAwareConnectionProxy(con, getTargetDataSource(););;

  4. }

    public Connection getConnection(); throws SQLException { Connection con = DataSourceUtils.doGetConnection(getTargetDataSource();, true);; return getTransactionAwareConnectionProxy(con, getTargetDataSource(););; }

万变不离其宗,不过我们还是看看getTransactionAwareConnectionProxy

Java代码

20191102100583\_9.png

  1. protected Connection getTransactionAwareConnectionProxy(Connection target, DataSource dataSource); {

  2. return (Connection); Proxy.newProxyInstance(

  3. ConnectionProxy.class.getClassLoader();,

  4. new Class[] {ConnectionProxy.class},

  5. new TransactionAwareInvocationHandler(target, dataSource););;

  6. }

    protected Connection getTransactionAwareConnectionProxy(Connection target, DataSource dataSource); { return (Connection); Proxy.newProxyInstance( ConnectionProxy.class.getClassLoader();, new Class[] {ConnectionProxy.class}, new TransactionAwareInvocationHandler(target, dataSource););; }

原来返回的是jdk的动态代理。继续看TransactionAwareInvocationHandler

Java代码

20191102100583\_10.png

  1. public Object invoke(Object proxy, Method method, Object[] args); throws Throwable {

  2. //… if (method.getName();.equals(CONNECTION_CLOSE_METHOD_NAME);); {

  3. if (this.dataSource != null); {

  4. DataSourceUtils.doCloseConnectionIfNecessary(this.target, this.dataSource);;

  5. }

  6. returnnull;

  7. }

  8. }

    public Object invoke(Object proxy, Method method, Object[] args); throws Throwable { //... if (method.getName();.equals(CONNECTION_CLOSE_METHOD_NAME);); { if (this.dataSource != null); { DataSourceUtils.doCloseConnectionIfNecessary(this.target, this.dataSource);; } return null; }

                        }
    

TransactionAwareDataSourceProxy会先从DataSourceUtils获取连接。然后将这个连接用jdk的动态代理包一下返回。外部代码如果调用的这个冒牌的Connection,就会先调用TransactionAwareInvocationHandler的invoke,在这个invoke 中,完成原来调用DataSourceUtils的功能。

总结上面的流程
Spring 对DataSource进行事务管理的关键在于ConnectionHolder和TransactionSynchronizationManager。
0.先从TransactionSynchronizationManager中尝试获取连接
1.如果前一步失败则在每个线程上,对每个DataSouce只创建一个Connection
2.这个Connection用ConnectionHolder包装起来,由TransactionSynchronizationManager管理
3.再次请求同一个连接的时候,从TransactionSynchronizationManager返回已经创建的ConnectionHolder,然后调用ConnectionHolder的request将引用计数+1
4.释放连接时要调用ConnectionHolder的released,将引用计数-1
5.当事物完成后,将ConnectionHolder从TransactionSynchronizationManager中解除。当谁都不用,这个连接被close

以上所有都是可以调用DataSourceUtils化简代码,而JdbcTemplate又是调用DataSourceUtils的。所以在Spring文档中要求尽量首先使用JdbcTemplate,其次是用DataSourceUtils来获取和释放连接。至于TransactionAwareDataSourceProxy,那是下策的下策。不过可以将Spring事务管理和遗留代码无缝集成。

所以如某位朋友说要使用Spring的事务管理,但是又不想用JdbcTemplate,那么可以考虑TransactionAwareDataSourceProxy。这个类是原来DataSource的代理。
其次,想使用Spring事物,又不想对Spring进行依赖是不可能的。与其试图自己模拟DataSourceUtils,不如直接使用现成的。


来源:http://ddrv.cn

点赞(0)
版权归原创作者所有,任何形式转载请联系作者; Java 技术驿站 >> spring jdbc 源码 分析

相关推荐