mybatis-plus@DS实现动态切换数据源原理

2022-11-13 12:11:51 mybatis

1、mybatis-plus @DS实现动态切换数据源原理

首先mybatis-plus使用com.baomidou.dynamic.datasource.AbstractRoutingDataSource继承 AbstractDataSource接管数据源；具体实现类为com.baomidou.dynamic.datasource.DynamicRoutingDataSource。项目初始化调用public synchronized void aDDDataSource(String ds, DataSource dataSource)加载数据源，数据源存进dataSourceMap中。
private Map<String, DataSource> dataSourceMap = new LinkedHashMap<>();

private Map<String, DynamicGroupDataSource> groupDataSources = new ConcurrentHashMap<>();

public synchronized void addDataSource(String ds, DataSource dataSource) {
    if (p6spy) {
      dataSource = new P6DataSource(dataSource);
    }
    dataSourceMap.put(ds, dataSource);
    if (ds.contains(UNDERLINE)) {
      String group = ds.split(UNDERLINE)[0];
      if (groupDataSources.containsKey(group)) {
        groupDataSources.get(group).addDatasource(dataSource);
      } else {
        try {
          DynamicGroupDataSource groupDatasource = new DynamicGroupDataSource(group,
              strategy.newInstance());
          groupDatasource.addDatasource(dataSource);
          groupDataSources.put(group, groupDatasource);
        } catch (Exception e) {
          log.error("dynamic-datasource - add the datasource named [{}] error", ds, e);
          dataSourceMap.remove(ds);
        }
      }
    }
    log.info("dynamic-datasource - load a datasource named [{}] success", ds);
  }

进行数据操作时，方法会被com.baomidou.dynamic.datasource.aop.DynamicDataSourceAnnotationInterceptor拦截，

public class DynamicDataSourceAnnotationInterceptor implements MethodInterceptor {

  
  private static final String DYNAMIC_PREFIX = "#";
  private static final DynamicDataSourceClassResolver RESOLVER = new DynamicDataSourceClassResolver();
  @Setter
  private DsProcessor dsProcessor;

  @Override
  public Object invoke(MethodInvocation invocation) throws Throwable {
    try {
      DynamicDataSourceContextHolder.push(determineDatasource(invocation));
      return invocation.proceed();
    } finally {
      DynamicDataSourceContextHolder.poll();
    }
  }

  private String determineDatasource(MethodInvocation invocation) throws Throwable {
    Method method = invocation.getMethod();
    DS ds = method.isAnnotationPresent(DS.class)
        ? method.getAnnotation(DS.class)
        : AnnotationUtils.findAnnotation(RESOLVER.targetClass(invocation), DS.class);
    String key = ds.value();
    return (!key.isEmpty() && key.startsWith(DYNAMIC_PREFIX)) ? dsProcessor
        .determineDatasource(invocation, key) : key;
  }
}

拦截器首先从被拦截的方法或者类（一般@DS注解用于Service，也可用于Mapper和Controller）上寻找@DS注解，获取到@DS注解的值后将其存入com.baomidou.dynamic.datasource.toolkit.DynamicDataSourceContextHolder；DynamicDataSourceContextHolder使用ThreadLocal存储当前线程的数据源名。

public final class DynamicDataSourceContextHolder {

  
  @SuppressWarnings("unchecked")
  private static final ThreadLocal<Deque<String>> LOOKUP_KEY_HOLDER = new ThreadLocal() {
    @Override
    protected Object initialValue() {
      return new ArrayDeque();
    }
  };

  private DynamicDataSourceContextHolder() {
  }

  
  public static String peek() {
    return LOOKUP_KEY_HOLDER.get().peek();
  }

  
  public static void push(String ds) {
    LOOKUP_KEY_HOLDER.get().push(StringUtils.isEmpty(ds) ? "" : ds);
  }

  
  public static void poll() {
    Deque<String> deque = LOOKUP_KEY_HOLDER.get();
    deque.poll();
    if (deque.isEmpty()) {
      LOOKUP_KEY_HOLDER.remove();
    }
  }

  
  public static void clear() {
    LOOKUP_KEY_HOLDER.remove();
  }
}

进行数据操作时，会调用org.springframework.jdbc.datasource.getConnection()方法；getConnection()方法最终调用了com.baomidou.dynamic.datasource.AbstractRoutingDataSource的getConnection()方法；

  @Override
  public Connection getConnection() throws sqlException {
    return determineDataSource().getConnection();
  }

determineDataSource()由子类com.baomidou.dynamic.datasource.DynamicRoutingDataSource实现，可以看到DynamicRoutingDataSource从DynamicDataSourceContextHolder获取数据源名称，这个在之前拦截器处理存进ThreadLocal中，如果有数据源名称则从dataSourceMap中获取，没有则获取默认的primary数据源。

public DataSource determineDataSource() {
    return getDataSource(DynamicDataSourceContextHolder.peek());
}

public DataSource getDataSource(String ds) {
    if (StringUtils.isEmpty(ds)) {
        return determinePrimaryDataSource();
    } else if (!groupDataSources.isEmpty() && groupDataSources.containsKey(ds)) {
        log.debug("dynamic-datasource switch to the datasource named [{}]", ds);
        return groupDataSources.get(ds).determineDataSource();
    } else if (dataSourceMap.containsKey(ds)) {
        log.debug("dynamic-datasource switch to the datasource named [{}]", ds);
        return dataSourceMap.get(ds);
    }
    if (strict) {
        throw new RuntimeException("dynamic-datasource could not find a datasource named" + ds);
    }
    return determinePrimaryDataSource();
}

private DataSource determinePrimaryDataSource() {
    log.debug("dynamic-datasource switch to the primary datasource");
    return groupDataSources.containsKey(primary) ? groupDataSources.get(primary)
        .determineDataSource() : dataSourceMap.get(primary);
}

此时的数据源已经切换成了我们需要的数据源。

数据操作完成后，方法返回第二步中的拦截器，执行DynamicDataSourceContextHolder.poll();清除掉此次的数据源，避免影响后续数据操作。

附上动态数据源相关配置

spring:
  application:
    name: 
  datasource:
    dynamic:
      primary: dataSource1
      datasource:
        dataSource1:
          type: com.alibaba.druid.pool.DruidDataSource
          driverClassName: com.microsoft.sqlserver.jdbc.SQLServerDriver
          url: jdbc:sqlserver://localhost:1433;database=dataSource1
          username: 
          passWord: 
        dataSource2:
          type: com.alibaba.druid.pool.DruidDataSource
          driverClassName: com.microsoft.sqlserver.jdbc.SQLServerDriver
          url: jdbc:sqlserver://localhost:1433;instanceName=sqlserver2017;DatabaseName=dataSource2
          username: 
          password:

pom.xml

<dependency>
    <groupId>com.baomidou</groupId>
    <artifactId>dynamic-datasource-spring-boot-starter</artifactId>
    <version>2.5.6</version>
</dependency>

相应类

@Service
//@DS("dataSource2") 放在类上就是类下所有方法都使用这个数据源。
public class XXXServiceImpl extends BaseServiceImpl<XXXMapper, XXXBean> implements XXXService {

    @DS("dataSource1")
    public void selectDataFromSource1() {
       // do somethinng;
    }
    
    @DS("dataSource2")
    public void selectDataFromSource1() {
       // do somethinng;
    }
}

**注意：**不可在事务中切换数据库，保证事务需要方法使用同一连接，使用@DS(dataSource1)方法调用@DS(dataSource2)无法切换连接，会导致方法报错。

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