一、AbstractRoutingDataSource简介
AbstractRoutingDataSource是Spring（package:org.springframework.jdbc.datasource.lookup）提供的一个多数据源解决方案。 1.多租户支持：根据租户选取对应的数据源，从而达到租户隔离的目的 2.分库分表：根据分片规则选取不同的数据源 3.读写分离：根据读写类型选取合适的数据源 4.数据源负载均衡：根据负载均衡的策略选取合适的数据源 5.多数据库支持：根据业务需求选择不同类型的数据源，如同时使用MySQL和Oracle
二、AbstractRoutingDataSource原理
1.过程描述
AbstractRoutingDataSource继承自AbstractDataSource，同时AbstractDataSource实现了DataSource接口。 我们可以把AbstractRoutingDataSource看成是一堆DataSource实例的集合，当需要使用的时候，从中取出对应的Connection，然后再执行相应的sql。
2.源码解析
AbstractRoutingDataSource
public abstract class AbstractRoutingDataSource extends AbstractDataSource implements InitializingBean {
……
}
可以看到AbstractRoutingDataSource继承自AbstractDataSource，进一步还能看到AbstractDataSource实现了DataSource接口，整体的继承实现关系如下：
getConnection()
getConnection()是通过一个determineTargetDataSource()的方法获取的Connection
@Override
public Connection getConnection() throws SQLException {
return this.determineTargetDataSource().getConnection();
}
@Override
public Connection getConnection(String username, String password) throws SQLException {
return this.determineTargetDataSource().getConnection(username, password);
}
determineTargetDataSource
/**
* 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;
}
其中有一个名为resolvedDataSources的Map，我们看下这个Map的定义：
@Nullable
private Map<Object, DataSource> resolvedDataSources;
可以知道这个Map是实际上存放DataSource的容器。 那么Map的Key是如何获取的呢，从源码可以知道key=lookupKey，通过determineCurrentLookupKey()方法获取的。
/**
* 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();
这个方法是一个抽象方法，所以这就是我们需要实现的业务相关的核心方法。
afterPropertiesSet()
方法对targetDataSources和defaultTargetDataSource稍作加工和校验之后，分别赋值给resolvedDataSources和resolvedDefaultDataSource
@Override
public void afterPropertiesSet() {
if (this.targetDataSources == null) {
throw new IllegalArgumentException("Property 'targetDataSources' is required");
}
this.resolvedDataSources = CollectionUtils.newHashMap(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);
}
}
/**
* 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;
}
三、其他的AbstractRoutingDataSource
@DS简介
@DS是baomidou封装的mybatis-plus的注解，内部有其自身的AbstractRoutingDataSource，并用DynamicRoutingDataSource做了相关的实现
初始化
在实例化的时候，通过DynamicRoutingDataSource的afterPropertiesSet()实现dataSource的初始化：
@Override
public void afterPropertiesSet() {
// 检查开启了配置但没有相关依赖
checkEnv();
// 添加并分组数据源
Map<String, DataSource> dataSources = new HashMap<>(16);
for (DynamicDataSourceProvider provider : providers) {
Map<String, DataSource> dsMap = provider.loadDataSources();
if (dsMap != null) {
dataSources.putAll(dsMap);
}
}
for (Map.Entry<String, DataSource> dsItem : dataSources.entrySet()) {
addDataSource(dsItem.getKey(), dsItem.getValue());
}
// 检测默认数据源是否设置
if (groupDataSources.containsKey(primary)) {
log.info("dynamic-datasource initial loaded [{}] datasource,primary group datasource named [{}]", dataSources.size(), primary);
} else if (dataSourceMap.containsKey(primary)) {
log.info("dynamic-datasource initial loaded [{}] datasource,primary datasource named [{}]", dataSources.size(), primary);
} else {
log.warn("dynamic-datasource initial loaded [{}] datasource,Please add your primary datasource or check your configuration", dataSources.size());
}
}
其中provider会默认从yml文件加载所有的数据源，我们也可以通过实现DynamicDataSourceProvider接口loadDataSources()方法的方式，实现从其他地方加载数据源的功能并通过addDataSource()方法放入一个叫dataSourceMap的Map中
数据源导入
与spring提供的AbstractRoutingDataSource不同的是，@DS直接提供了路由到具体数据源的方法，即通过AOP拦截的方式。 数据操作会被DynamicDataSourceAnnotationInterceptor拦截，并把数据源名放入DynamicDataSourceContextHolder的ThreadLocal
获取指定数据源
进行数据操作的时候，会调用AbstractRoutingDataSource的getConnection()方法。 DynamicRoutingDataSource会从DynamicDataSourceContextHolder获取之前拦截器放入ThreadLocal的数据源名称再通过数据源名称到dataSourceMap中获取指定数据源
