项目中可以根据注解声明的数据库，在特定的方法中切换数据源。下面就看看它是怎么做到的（尽管我不知道这么做有什么用）
首先看 Springboot 的 config 类有没有数据源相关的，从RenrenApplication找一下子就找到
/**
* 配置多数据源
* @author chenshun
* @email [email protected]
* @date 2017/8/19 0:41 */ @Configuration public class DynamicDataSourceConfig { @Bean @ConfigurationProperties("spring.datasource.druid.first") public DataSource firstDataSource(){ return DruidDataSourceBuilder.create().build(); } @Bean @ConfigurationProperties("spring.datasource.druid.second") public DataSource secondDataSource(){ return DruidDataSourceBuilder.create().build(); } @Bean @Primary public DynamicDataSource dataSource(DataSource firstDataSource, DataSource secondDataSource) { Map<Object, Object> targetDataSources = new HashMap<>(); targetDataSources.put(DataSourceNames.FIRST, firstDataSource); targetDataSources.put(DataSourceNames.SECOND, secondDataSource); return new DynamicDataSource(firstDataSource, targetDataSources); } }根据数据源的配置，找到datasources包有个DynamicDataSource类
/**
* 动态数据源
* @author chenshun
* @email [email protected]
* @date 2017/8/19 1:03 */ public class DynamicDataSource extends AbstractRoutingDataSource { private static final ThreadLocal<String> contextHolder = new ThreadLocal<>(); public DynamicDataSource(DataSource defaultTargetDataSource, Map<Object, Object> targetDataSources) { super.setDefaultTargetDataSource(defaultTargetDataSource); super.setTargetDataSources(targetDataSources); super.afterPropertiesSet(); } @Override protected Object determineCurrentLookupKey() { return getDataSource(); } public static void setDataSource(String dataSource) { contextHolder.set(dataSource); } public static String getDataSource() { return contextHolder.get(); } public static void clearDataSource() { contextHolder.remove(); } }
光是看这个类其实看不出什么东西，只知道有个ThreadLocal常量，并且可以用静态方法配置它。那么我们看看父类AbstractRoutingDataSource
/** * Abstract {@link javax.sql.DataSource} implementation that routes {@link #getConnection()} * calls to one of various target DataSources based on a lookup key. The latter is usually * (but not necessarily) determined through some thread-bound transaction context. * * @author Juergen Hoeller * @since 2.0.1 * @see #setTargetDataSources * @see #setDefaultTargetDataSource * @see #determineCurrentLookupKey() */ public abstract class AbstractRoutingDataSource extends AbstractDataSource implements InitializingBean { @Nullable private Map<Object, Object> targetDataSources; @Nullable private Object defaultTargetDataSource; private boolean lenientFallback = true; private DataSourceLookup dataSourceLookup = new JndiDataSourceLookup(); @Nullable private Map<Object, DataSource> resolvedDataSources; @Nullable private DataSource resolvedDefaultDataSource; /** * Specify the map of target DataSources, with the lookup key as key. * The mapped value can either be a corresponding {@link javax.sql.DataSource} * instance or a data source name String (to be resolved via a * {@link #setDataSourceLookup DataSourceLookup}). * <p>The key can be of arbitrary type; this class implements the * generic lookup process only. The concrete key representation will * be handled by {@link #resolveSpecifiedLookupKey(Object)} and * {@link #determineCurrentLookupKey()}. */ public void setTargetDataSources(Map<Object, Object> targetDataSources) { this.targetDataSources = targetDataSources; } /** * Specify the default target DataSource, if any. * <p>The mapped value can either be a corresponding {@link javax.sql.DataSource} * instance or a data source name String (to be resolved via a * {@link #setDataSourceLookup DataSourceLookup}). * <p>This DataSource will be used as target if none of the keyed * {@link #setTargetDataSources targetDataSources} match the * {@link #determineCurrentLookupKey()} current lookup key. */ public void setDefaultTargetDataSource(Object defaultTargetDataSource) { this.defaultTargetDataSource = defaultTargetDataSource; } /** * Specify whether to apply a lenient fallback to the default DataSource * if no specific DataSource could be found for the current lookup key. * <p>Default is "true", accepting lookup keys without a corresponding entry * in the target DataSource map - simply falling back to the default DataSource * in that case. * <p>Switch this flag to "false" if you would prefer the fallback to only apply * if the lookup key was {@code null}. Lookup keys without a DataSource * entry will then lead to an IllegalStateException. * @see #setTargetDataSources * @see #setDefaultTargetDataSource * @see #determineCurrentLookupKey() */ public void setLenientFallback(boolean lenientFallback) { this.lenientFallback = lenientFallback; } /** * Set the DataSourceLookup implementation to use for resolving data source * name Strings in the {@link #setTargetDataSources targetDataSources} map. * <p>Default is a {@link JndiDataSourceLookup}, allowing the JNDI names * of application server DataSources to be specified directly. */ public void setDataSourceLookup(@Nullable DataSourceLookup dataSourceLookup) { this.dataSourceLookup = (dataSourceLookup != null ? dataSourceLookup : new JndiDataSourceLookup()); } @Override public void afterPropertiesSet() { if (this.targetDataSources == null) { throw new IllegalArgumentException("Property 'targetDataSources' is required"); } this.resolvedDataSources = new HashMap<>(this.targetDataSources.size()); this.targetDataSources.forEach((key, value) -> { Object lookupKey = resolveSpecifiedLookupKey(key); DataSource dataSource = resolveSpecifiedDataSource(value); this.resolvedDataSources.put(lookupKey, dataSource); }); if (this.defaultTargetDataSource != null) { this.resolvedDefaultDataSource = resolveSpecifiedDataSource(this.defaultTargetDataSource); } } /** * Resolve the given lookup key object, as specified in the * {@link #setTargetDataSources targetDataSources} map, into * the actual lookup key to be used for matching with the * {@link #determineCurrentLookupKey() current lookup key}. * <p>The default implementation simply returns the given key as-is. * @param lookupKey the lookup key object as specified by the user * @return the lookup key as needed for matching */ protected Object resolveSpecifiedLookupKey(Object lookupKey) { return lookupKey; } /** * Resolve the specified data source object into a DataSource instance. * <p>The default implementation handles DataSource instances and data source * names (to be resolved via a {@link #setDataSourceLookup DataSourceLookup}). * @param dataSource the data source value object as specified in the * {@link #setTargetDataSources targetDataSources} map * @return the resolved DataSource (never {@code null}) * @throws IllegalArgumentException in case of an unsupported value type */ protected DataSource resolveSpecifiedDataSource(Object dataSource) throws IllegalArgumentException { if (dataSource instanceof DataSource) { return (DataSource) dataSource; } else if (dataSource instanceof String) { return this.dataSourceLookup.getDataSource((String) dataSource); } else { throw new IllegalArgumentException( "Illegal data source value - only [javax.sql.DataSource] and String supported: " + dataSource); } } @Override public Connection getConnection() throws SQLException { return determineTargetDataSource().getConnection(); } @Override public Connection getConnection(String username, String password) throws SQLException { return determineTargetDataSource().getConnection(username, password); } @Override @SuppressWarnings("unchecked") public <T> T unwrap(Class<T> iface) throws SQLException { if (iface.isInstance(this)) { return (T) this; } return determineTargetDataSource().unwrap(iface); } @Override public boolean isWrapperFor(Class<?> iface) throws SQLException { return (iface.isInstance(this) || determineTargetDataSource().isWrapperFor(iface)); } /** * Retrieve the current target DataSource. Determines the * {@link #determineCurrentLookupKey() current lookup key}, performs * a lookup in the {@link #setTargetDataSources targetDataSources} map, * falls back to the specified * {@link #setDefaultTargetDataSource default target DataSource} if necessary. * @see #determineCurrentLookupKey() */ protected DataSource determineTargetDataSource() { Assert.notNull(this.resolvedDataSources, "DataSource router not initialized"); Object lookupKey = determineCurrentLookupKey(); DataSource dataSource = this.resolvedDataSources.get(lookupKey); if (dataSource == null && (this.lenientFallback || lookupKey == null)) { dataSource = this.resolvedDefaultDataSource; } if (dataSource == null) { throw new IllegalStateException("Cannot determine target DataSource for lookup key [" + lookupKey + "]"); } return dataSource; } /** * Determine the current lookup key. This will typically be * implemented to check a thread-bound transaction context. * <p>Allows for arbitrary keys. The returned key needs * to match the stored lookup key type, as resolved by the * {@link #resolveSpecifiedLookupKey} method. */ @Nullable protected abstract Object determineCurrentLookupKey(); }
根据注释可以看出，这个类重写了getConnection()方法。而getConnection()方法实现的逻辑就是从determineCurrentLookupKey()中获取数据源，然后打开连接。结合自己实现的DynamicDataSource我们不难得出一个结论：项目是根据ThreadLocal常量来获取数据库连接的。这个我们后面再证实，先来看看项目中是什么时候切换数据库的。
datasources包下还有个aspect包，一看就让人觉得是通过 aop 来切换数据源的了。直接来看代码
DataSourceAspect
/**
* 多数据源，切面处理类
* @author chenshun
* @email [email protected] * @date 2017/9/16 22:20 */ @Aspect @Component public class DataSourceAspect implements Ordered { protected Logger logger = LoggerFactory.getLogger(getClass()); @Pointcut("@annotation(io.renren.datasources.annotation.DataSource)") public void dataSourcePointCut() { } @Around("dataSourcePointCut()") public Object around(ProceedingJoinPoint point) throws Throwable { MethodSignature signature = (MethodSignature) point.getSignature(); Method method = signature.getMethod(); DataSource ds = method.getAnnotation(DataSource.class); if(ds == null){ DynamicDataSource.setDataSource(DataSourceNames.FIRST); logger.debug("set datasource is " + DataSourceNames.FIRST); }else { DynamicDataSource.setDataSource(ds.name()); logger.debug("set datasource is " + ds.name()); } try { return point.proceed(); } finally { DynamicDataSource.clearDataSource(); logger.debug("clean datasource"); } } @Override public int getOrder() { return 1; } }
果不其然，是根据 aop 来取出调用的方法的@DataSource中的数据源字符串值，决定用什么数据源的。结合上面的代码，可以得出结论：是在调用有@DataSource修饰的方法时，修改的当前线程数据源。印象中 Servlet 默认是一个请求一条线程处理，所以是这个请求内的数据源都改变了。
全局搜索一下@DataSource，发现只有一个测试的 service 有用到
/**
* 测试多数据源
*
* @author Mark [email protected]
* @since 3.1.0 2018-01-28
*/
@Service public class DataSourceTestService { @Autowired private SysUserService sysUserService; public SysUserEntity queryUser(Long userId){ return sysUserService.selectById(userId); } @DataSource(name = DataSourceNames.SECOND) public SysUserEntity queryUser2(Long userId){ return sysUserService.selectById(userId); } }
那么切换数据源这部分，在项目源码中就到此为止了。
我们探究一下在 Mybatis 中是什么时候根据数据源来创建连接的。
首先我们要知道 Mybatis 的工作流程。这个读一读官网就知道，Mybatis-Spring 先通过SqlSessionFactoryBean创建SqlSessionFactory实例，然后在适当的时候创建SqlSession，创建SqlSession的时候应该已经打开数据库连接了，因为要管理事务的。所以我们其实可以直接找到SqlSessionFactory的实现类：DefaultSqlSessionFactory
public class DefaultSqlSessionFactory implements SqlSessionFactory { private final Configuration configuration; public DefaultSqlSessionFactory(Configuration configuration) { this.configuration = configuration; } @Override public SqlSession openSession() { return openSessionFromDataSource(configuration.getDefaultExecutorType(), null, false); } @Override public SqlSession openSession(boolean autoCommit) { return openSessionFromDataSource(configuration.getDefaultExecutorType(), null, autoCommit); } @Override public SqlSession openSession(ExecutorType execType) { return openSessionFromDataSource(execType, null, false); } @Override public SqlSession openSession(TransactionIsolationLevel level) { return openSessionFromDataSource(configuration.getDefaultExecutorType(), level, false); } @Override public SqlSession openSession(ExecutorType execType, TransactionIsolationLevel level) { return openSessionFromDataSource(execType, level, false); } @Override public SqlSession openSession(ExecutorType execType, boolean autoCommit) { return openSessionFromDataSource(execType, null, autoCommit); } @Override public SqlSession openSession(Connection connection) { return openSessionFromConnection(configuration.getDefaultExecutorType(), connection); } @Override public SqlSession openSession(ExecutorType execType, Connection connection) { return openSessionFromConnection(execType, connection); } @Override public Configuration getConfiguration() { return configuration; } private SqlSession openSessionFromDataSource(ExecutorType execType, TransactionIsolationLevel level, boolean autoCommit) { Transaction tx = null; try { final Environment environment = configuration.getEnvironment(); final TransactionFactory transactionFactory = getTransactionFactoryFromEnvironment(environment); tx = transactionFactory.newTransaction(environment.getDataSource(), level, autoCommit); final Executor executor = configuration.newExecutor(tx, execType); return new DefaultSqlSession(configuration, executor, autoCommit); } catch (Exception e) { closeTransaction(tx); // may have fetched a connection so lets call close() throw ExceptionFactory.wrapException("Error opening session. Cause: " + e, e); } finally { ErrorContext.instance().reset(); } } private SqlSession openSessionFromConnection(ExecutorType execType, Connection connection) { try { boolean autoCommit; try { autoCommit = connection.getAutoCommit(); } catch (SQLException e) { // Failover to true, as most poor drivers // or databases won't support transactions autoCommit = true; } final Environment environment = configuration.getEnvironment(); final TransactionFactory transactionFactory = getTransactionFactoryFromEnvironment(environment); final Transaction tx = transactionFactory.newTransaction(connection); final Executor executor = configuration.newExecutor(tx, execType); return new DefaultSqlSession(configuration, executor, autoCommit); } catch (Exception e) { throw ExceptionFactory.wrapException("Error opening session. Cause: " + e, e); } finally { ErrorContext.instance().reset(); } } private TransactionFactory getTransactionFactoryFromEnvironment(Environment environment) { if (environment == null || environment.getTransactionFactory() == null) { return new ManagedTransactionFactory(); } return environment.getTransactionFactory(); } private void closeTransaction(Transaction tx) { if (tx != null) { try { tx.close(); } catch (SQLException ignore) { // Intentionally ignore. Prefer previous error. } } } }
从这个 factory 的源码可以读出，创建SqlSession调用的是openSessionFromDataSource()方法。在这个方法中：
- 先是从
configuration中拿到enviroment，熟悉 Mybatis 的朋友都知道我们配置数据源就是在enviroment中 - 然后调用
getTransactionFactoryFromEnvironment()创建了TransactionFactory实例 - 根据
TransactionFactory开启了事务tx - 根据这个事务
tx创建了执行类executor - 最后创建了一个
DefaultSqlSession，也就是我们用来做对数据库查询操作的类。
一步步来看是什么时候打开的连接吧
创建 TransactionFactory 实例
直接看getTransactionFactoryFromEnvironment()方法源码。
private TransactionFactory getTransactionFactoryFromEnvironment(Environment environment) { if (environment == null || environment.getTransactionFactory() == null) { return new ManagedTransactionFactory(); } return environment.getTransactionFactory(); }
如果没有配置 transactionFactory，那么返回ManagedTransactionFactory
ManagedTransactionFactory.java
public class ManagedTransactionFactory implements TransactionFactory { private boolean closeConnection = true; @Override public void setProperties(Properties props) { if (props != null) { String closeConnectionProperty = props.getProperty("closeConnection"); if (closeConnectionProperty != null) { closeConnection = Boolean.valueOf(closeConnectionProperty); } } } @Override public Transaction newTransaction(Connection conn) { return new ManagedTransaction(conn, closeConnection); } @Override public Transaction newTransaction(DataSource ds, TransactionIsolationLevel level, boolean autoCommit) { // Silently ignores autocommit and isolation level, as managed transactions are entirely // controlled by an external manager. It's silently ignored so that // code remains portable between managed and unmanaged configurations. return new ManagedTransaction(ds, level, closeConnection); } }根据 TransactionFactory 开启了事务
ManagedTransaction.java
/**
* {@link Transaction} that lets the container manage the full lifecycle of the transaction.
* Delays connection retrieval until getConnection() is called.
* Ignores all commit or rollback requests.
* By default, it closes the connection but can be configured not to do it.
*
* @author Clinton Begin
*
* @see ManagedTransactionFactory */ public class ManagedTransaction implements Transaction { private static final Log log = LogFactory.getLog(ManagedTransaction.class); private DataSource dataSource; private TransactionIsolationLevel level; private Connection connection; private final boolean closeConnection; public ManagedTransaction(Connection connection, boolean closeConnection) { this.connection = connection; this.closeConnection = closeConnection; } public ManagedTransaction(DataSource ds, TransactionIsolationLevel level, boolean closeConnection) { this.dataSource = ds; this.level = level; this.closeConnection = closeConnection; } @Override public Connection getConnection() throws SQLException { if (this.connection == null) { openConnection(); } return this.connection; } @Override public void commit() throws SQLException { // Does nothing } @Override public void rollback() throws SQLException { // Does nothing } @Override public void close() throws SQLException { if (this.closeConnection && this.connection != null) { if (log.isDebugEnabled()) { log.debug("Closing JDBC Connection [" + this.connection + "]"); } this.connection.close(); } } protected void openConnection() throws SQLException { if (log.isDebugEnabled()) { log.debug("Opening JDBC Connection"); } this.connection = this.dataSource.getConnection(); if (this.level != null) { this.connection.setTransactionIsolation(this.level.getLevel()); } } @Override public Integer getTimeout() throws SQLException { return null; } }
这里有个openConnection()方法根据dataSource打开了数据库连接。这跟我们最初自己配的动态数据源连接上了，接下来就看何时调用了。
根据这个事务创建了执行类 executor
public Executor newExecutor(Transaction transaction, ExecutorType executorType) { executorType = executorType == null ? defaultExecutorType : executorType; executorType = executorType == null ? ExecutorType.SIMPLE : executorType; Executor executor; if (ExecutorType.BATCH == executorType) { executor = new BatchExecutor(this, transaction); } else if (ExecutorType.REUSE == executorType) { executor = new ReuseExecutor(this, transaction); } else { executor = new SimpleExecutor(this, transaction); } if (cacheEnabled) { executor = new CachingExecutor(executor); } executor = (Executor) interceptorChain.pluginAll(executor); return executor; }可以看出这个executor还用interceptorChain添加了 plugins，这跟常用的分页插件也有关，这里就不多说了。创建 DefaultSqlSession
这个类代码太多了，我就仅贴出关键部分
@Override
public Connection getConnection() { try { return executor.getTransaction().getConnection(); } catch (SQLException e) { throw ExceptionFactory.wrapException("Error getting a new connection. Cause: " + e, e); } }这里调用了我们之前创建的事务类的getConnection()方法，在那个方法中打开了我们自定义的数据源的连接。而在我们自定义的DynamicDataSource中，getConnection()又是根据ThreadLocal常量来获取数据源的，所以只要当前请求的线程中被自定义切面类修改了数据源，那么等到这个线程要用 Mybatis打开数据源的连接的时候，就会打开切换过后的数据源的连接了。
本篇文章内容也不多，不过根据上述的 Mybatis 的工作流程来读它的源码，很容易就可以读出其他功能是怎么实现的，比如说 Mybatis-Spring 中的注入 Mapper，实际上是通过 Mybatis 的动态代理，解析调用的 Mapper 接口的方法对象，获取注解、方法名、参数等等信息，再用SqlSession来调用。