分库分表思路

总体思路和切入点：

1.在spring数据访问封装层通过动态代理无侵入的扩展代码加入分库分表策略。


（1）分库：通过动态代理扩展SqlSession的代码并传入分库参数来选择sqlSessionTemplate的数据源的方式实现分库策略

public abstract class SqlSessionDaoSupport implements InitializingBean {
	private SqlSessionFactoryBean sqlSessionFactoryBean;

	private Map<DataSource, SqlSessionTemplate> dataSourceMap;

	private SqlSession sqlSession;

	{
		//通过动态代理扩展SqlSession的代码并传入分库参数来选择sqlSessionTemplate的数据源的方式实现分库策略
		sqlSession = (SqlSession) Proxy.newProxyInstance(SqlSessionDaoSupport.class.getClassLoader(),
				new Class[] { SqlSession.class }, new SessionHandler());
	}

	@Autowired(required = false)
	public final void setSqlSessionFactory(SqlSessionFactoryBean sqlSessionFactoryBean) {
		this.sqlSessionFactoryBean = sqlSessionFactoryBean;
	}

	public final SqlSession getSqlSession() {
		return sqlSession;
	}

	@Override
	public final void afterPropertiesSet() throws Exception {
		sqlSessionFactoryBean.afterPropertiesSet();
		//
		dataSourceMap = new LinkedHashMap<DataSource, SqlSessionTemplate>();
		dataSourceMap.put(sqlSessionFactoryBean.getMainDataSource(),
				new SqlSessionTemplate(sqlSessionFactoryBean.getMainSqlSessionFactory()));

		Map<String, DataSource> shardDataSources = sqlSessionFactoryBean.getShardDataSources();
		if (shardDataSources != null) {
			for (Entry<String, DataSource> entry : shardDataSources.entrySet()) {
				SqlSessionFactory sqlSessionFactory = sqlSessionFactoryBean.getShardSqlSessionFactory().get(
						entry.getKey());
				dataSourceMap.put(entry.getValue(), new SqlSessionTemplate(sqlSessionFactory));
			}
		}
	}

	private class SessionHandler implements InvocationHandler {

		@Override
		public Object invoke(Object proxy, Method method, Object[] args) throws Throwable {
			try {
				// 默认DataSource为MainDataSource
				DataSource targetDS = sqlSessionFactoryBean.getMainDataSource();

				//
				if (args == null || args.length == 0) {
					// 准备事务
					prepareTx(targetDS);
					//
					return method.invoke(dataSourceMap.get(sqlSessionFactoryBean.getMainDataSource()), args);
				}
				if (!(args[0] instanceof String)) {
					// 准备事务
					prepareTx(targetDS);
					//
					return method.invoke(dataSourceMap.get(sqlSessionFactoryBean.getMainDataSource()), args);
				}
				ShardParam shardParam = (args.length > 1 && args[1] instanceof ShardParam) ? (ShardParam) args[1]
						: null;
				if (shardParam == null) {
					// 准备事务
					prepareTx(targetDS);
					//
					return method.invoke(dataSourceMap.get(sqlSessionFactoryBean.getMainDataSource()), args);
				} else {
					args[1] = shardParam.getParams();
				}
				
				//
				String statement;
				String shardStrategyName;
				ShardStrategy shardStrategy;

				statement = (String) args[0];
				shardStrategyName = shardParam.getName();

				shardStrategy = sqlSessionFactoryBean.getShardStrategyMap().get(shardStrategyName);
				if (shardStrategy == null) {
					shardStrategy = NoShardStrategy.INSTANCE;
				}

				Configuration configuration = sqlSessionFactoryBean.getMainSqlSessionFactory().getConfiguration();
				MappedStatement mappedStatement = configuration.getMappedStatement(statement);
				BoundSql boundSql = mappedStatement.getBoundSql(wrapCollection(shardParam.getParams()));

				shardStrategy.setMainDataSource(sqlSessionFactoryBean.getMainDataSource());
				shardStrategy.setShardDataSources(sqlSessionFactoryBean.getShardDataSources());
				shardStrategy.setShardParam(shardParam);
				shardStrategy.setSql(boundSql.getSql());
				//
				StrategyHolder.setShardStrategy(shardStrategy);
				// 重新指定目标DataSource
				targetDS = shardStrategy.getTargetDataSource();
				SqlSessionTemplate sqlSessionTemplate = null;
				if (targetDS == null || (sqlSessionTemplate = dataSourceMap.get(targetDS)) == null) {
					targetDS = sqlSessionFactoryBean.getMainDataSource();
					sqlSessionTemplate = dataSourceMap.get(targetDS);
				}

				// 准备事务
				prepareTx(targetDS);

				return method.invoke(sqlSessionTemplate, args);
			} finally {
				StrategyHolder.removeShardStrategy();
			}
		}

		/**
		 * 、 准备事务
		 * 
		 * @param targetDS
		 */
		private void prepareTx(DataSource targetDS) {
			//
			TransactionHolder.setDataSource(targetDS);

			// for transaction
			TransactionInfoWrap txInfo = TransactionHolder.getTransactionInfo();
			if (txInfo != null) {
				TransactionAttribute attr = txInfo.getTransactionAttribute();
				if (attr != null) {
					createTxIfAbsent(targetDS, txInfo);
				}
			}
		}

		/**
		 * 如果不存在则创建事务
		 * 
		 * @param targetDS
		 * @param txInfo
		 */
		private void createTxIfAbsent(DataSource targetDS, TransactionInfoWrap txInfo) {
			Map<DataSource, LinkedList<TransactionInfoWrap>> txTree = TransactionHolder.getTxTree();
			if (txTree == null || !txTree.containsKey(targetDS)) {
				createTx(targetDS, txInfo);
			}
		}

		private void createTx(DataSource targetDS, TransactionInfoWrap txInfo) {
			TransactionStatus txStatus = txInfo.getTransactionManager()
					.getTransaction(txInfo.getTransactionAttribute());
			// txStatus = new TransactionStatusWrap((DefaultTransactionStatus)
			// txStatus);
			TransactionHolder.addStatusDS(txStatus, targetDS);
			//
			TransactionInfoWrap txInfoCopy = txInfo.newCopy();
			txInfoCopy.newTransactionStatus(txStatus);
			//
			TransactionHolder.addTxInfo2Tree(targetDS, txInfoCopy);
		}

		private Object wrapCollection(final Object object) {
			if (object instanceof List) {
				return new HashMap<String, Object>() {
					private static final long serialVersionUID = -2533602760878803345L;
					{
						put("list", object);
					}
				};
			} else if (object != null && object.getClass().isArray()) {
				return new HashMap<String, Object>() {
					private static final long serialVersionUID = 8371167260656531195L;
					{
						put("array", object);
					}
				};
			}
			return object;
		}
	}
}

(2)分表：通过自定义Configuration拦截器并传入分表参数来替换BoundSql的sql的方式实现分表策略

public class SqlSessionFactoryBean implements ApplicationContextAware, MultiDataSourceSupport {

	private final Logger logger = LoggerFactory.getLogger(getClass());

	private ApplicationContext applicationContext;

	private DataSource mainDataSource;

	private SqlSessionFactory mainSqlSessionFactory;

	private Map<String, DataSource> shardDataSources;

	private Map<String, SqlSessionFactory> shardSqlSessionFactory;

	private List<DataSource> shardDataSourceList;

	private Resource[] mapperLocations;

	private Map<String, ShardStrategy> shardStrategyMap = new HashMap<String, ShardStrategy>();
	private Map<String, Class<?>> shardStrategyConfig = new HashMap<String, Class<?>>();

	private SqlConverter sqlConverter = new DefaultSqlConverter();

	public DataSource getMainDataSource() {
		return mainDataSource;
	}

	public void setMainDataSource(DataSource mainDataSource) {
		if (mainDataSource instanceof TransactionAwareDataSourceProxy) {
			// If we got a TransactionAwareDataSourceProxy, we need to perform
			// transactions for its underlying target DataSource, else data
			// access code won't see properly exposed transactions (i.e.
			// transactions for the target DataSource).
			this.mainDataSource = ((TransactionAwareDataSourceProxy) mainDataSource).getTargetDataSource();
		} else {
			this.mainDataSource = mainDataSource;
		}
	}

	public void setShardDataSourceList(List<DataSource> shardDataSourceList) {
		this.shardDataSourceList = shardDataSourceList;
	}

	public Map<String, DataSource> getShardDataSources() {
		return shardDataSources;
	}

	public void setMapperLocations(Resource[] mapperLocations) {
		this.mapperLocations = mapperLocations;
	}

	public void setShardStrategy(Map<String, Class<?>> shardStrategyMap) {
		this.shardStrategyConfig = shardStrategyMap;
	}

	public SqlSessionFactory getMainSqlSessionFactory() {
		return mainSqlSessionFactory;
	}

	public Map<String, SqlSessionFactory> getShardSqlSessionFactory() {
		return shardSqlSessionFactory;
	}

	public Map<String, ShardStrategy> getShardStrategyMap() {
		return shardStrategyMap;
	}

	public void afterPropertiesSet() throws Exception {
		if (mainDataSource == null && (shardDataSourceList == null || shardDataSourceList.size() == 0)) {
			throw new RuntimeException(
					" Property 'mainDataSource' and property 'shardDataSourceList' can not be null together! ");
		}
		if (shardDataSourceList != null && shardDataSourceList.size() > 0) {
			shardDataSources = new LinkedHashMap<String, DataSource>();
			Map<String, DataSource> dataSourceMap = applicationContext.getBeansOfType(DataSource.class);
			for (Entry<String, DataSource> entry : dataSourceMap.entrySet()) {
				for (int i = 0; i < shardDataSourceList.size(); i++) {
					DataSource ds = shardDataSourceList.get(i);
					if (entry.getValue() == ds) {
						DataSource dataSource = entry.getValue();
						if (dataSource instanceof TransactionAwareDataSourceProxy) {
							dataSource = ((TransactionAwareDataSourceProxy) dataSource).getTargetDataSource();
						}
						shardDataSources.put(entry.getKey(), dataSource);
					}
				}
			}
		}
		if (mainDataSource == null) {
			if (shardDataSourceList.get(0) instanceof TransactionAwareDataSourceProxy) {
				this.mainDataSource = ((TransactionAwareDataSourceProxy) shardDataSourceList.get(0))
						.getTargetDataSource();
			} else {
				mainDataSource = shardDataSources.get(0);
			}
		}

		this.mainSqlSessionFactory = buildSqlSessionFactory(getMainDataSource());
		if (getShardDataSources() != null && getShardDataSources().size() > 0) {
			shardSqlSessionFactory = new LinkedHashMap<String, SqlSessionFactory>(getShardDataSources().size());
			for (Entry<String, DataSource> entry : getShardDataSources().entrySet()) {
				shardSqlSessionFactory.put(entry.getKey(), buildSqlSessionFactory(entry.getValue()));
			}
		}
		//
		if (shardStrategyConfig != null) {
			shardStrategyMap = new HashMap<String, ShardStrategy>();
			for (Map.Entry<String, Class<?>> entry : shardStrategyConfig.entrySet()) {
				Class<?> clazz = entry.getValue();
				if (!ShardStrategy.class.isAssignableFrom(clazz)) {
					throw new IllegalArgumentException("class " + clazz.getName()
							+ " is illegal, subclass of ShardStrategy is required.");
				}
				try {
					shardStrategyMap.put(entry.getKey(), (ShardStrategy) (entry.getValue().newInstance()));
				} catch (Exception e) {
					throw new RuntimeException("new instance for class " + clazz.getName() + " failed, error:"
							+ e.getMessage());
				}
			}
			//
			shardStrategyConfig = null;
		}
	}

	private SqlSessionFactory buildSqlSessionFactory(DataSource dataSource) throws IOException {
		ShardPlugin plugin = new ShardPlugin();
		plugin.setSqlConverter(sqlConverter);

		Configuration configuration = null;
		SpringManagedTransactionFactory transactionFactory = null;
		//
		configuration = new Configuration();
		configuration.addInterceptor(plugin);//通过自定义Configuration拦截器并传入分表参数来替换BoundSql的sql的方式实现分表策略
		//
		transactionFactory = new SpringManagedTransactionFactory(dataSource);

		Environment environment = new Environment(SqlSessionFactoryBean.class.getSimpleName(), transactionFactory,
				dataSource);
		configuration.setEnvironment(environment);

		if (!ObjectUtils.isEmpty(this.mapperLocations)) {
			for (Resource mapperLocation : this.mapperLocations) {
				if (mapperLocation == null) {
					continue;
				}
				// this block is a workaround for issue
				// http://code.google.com/p/mybatis/issues/detail?id=235
				// when running MyBatis 3.0.4. But not always works.
				// Not needed in 3.0.5 and above.
				String path;
				if (mapperLocation instanceof ClassPathResource) {
					path = ((ClassPathResource) mapperLocation).getPath();
				} else {
					path = mapperLocation.toString();
				}

				try {
					XMLMapperBuilder xmlMapperBuilder = new XMLMapperBuilder(mapperLocation.getInputStream(),
							configuration, path, configuration.getSqlFragments());
					xmlMapperBuilder.parse();
				} catch (Exception e) {
					throw new NestedIOException("Failed to parse mapping resource: '" + mapperLocation + "'", e);
				} finally {
					ErrorContext.instance().reset();
				}

				if (this.logger.isDebugEnabled()) {
					this.logger.debug("Parsed mapper file: '" + mapperLocation + "'");
				}
			}
		} else {
			if (this.logger.isDebugEnabled()) {
				this.logger.debug("Property 'mapperLocations' was not specified or no matching resources found");
			}
		}

		return new SqlSessionFactoryBuilder().build(configuration);
	}

	@Override
	public void setApplicationContext(ApplicationContext applicationContext) throws BeansException {
		this.applicationContext = applicationContext;
	}

}

3.为了便于理清思路，上传一张mybatis执行SQL的时序图：