1. 预备知识
这里默认大家都会SSM框架了，使用时我们要往sqlSessionFactory里注入数据源。那么猜测：1、可以往sqlSessionFactory里注入多数据源来实现切换；2、将多个数据源封装成一个总源，再把这个总源注入到sqlSessionFactory里实现切换。答案是使用后者，即封装成总源的形式。Spring提供了动态切换数据源的功能，那么我们来看看其实现原理
2. 实现原理
笔者是根据源码讲解的，这些步骤讲完会贴出源码内容
一、
Spring提供了AbstractRoutingDataSource抽象类，其继承了AbstractDataSource。而AbstractDataSource又实现了DataSource。因此我们可以将AbstractRoutingDataSource的实现类注入到sqlSessionFactory中来实现切换数据源
二、
刚才我们将多个数据源封装成总源的想法在AbstractRoutingDataSource中有体现，其内部用一个Map集合封装多个数据源，即 private Map<Object, DataSource> resolvedDataSources; ，那么要使用时从该Map集合中获取即可
三、
AbstractRoutingDataSource中有个determineTargetDataSource()方法，其作用是决定使用哪个数据源。我们通过determineTargetDataSource()方法从Map集合中获取数据源，那么必须有个key值指定才行。所以determineTargetDataSource()方法内部通过调用determineCurrentLookupKey()方法来获取key值，Spring将determineCurrentLookupKey()方法抽象出来给用户实现，从而让用户决定使用哪个数据源
四、
既然知道我们需要重写determineCurrentLookupKey()方法，那么就开始把。实现时发现该方法没有参数，我们无法传参来决定返回的key值，又不能改动方法（因为是重写），所以方法内部调用我们自定义类的静态方法即可解决问题
public class DynamicDataSource extends AbstractRoutingDataSource {
@Override
protected Object determineCurrentLookupKey() {
return DynamicDataSourceHolder.getDataSourceKey();
}
}
五、
自定义类，作用是让我们传入key值来决定使用哪个key
public class DynamicDataSourceHolder {
// ThreadLocal没什么好说的，绑定当前线程
private static final ThreadLocal<String> dataSourceKey = new ThreadLocal<String>();
public static String getDataSourceKey(){
return dataSourceKey.get();
}
public static void setDataSourceKey(String key){
dataSourceKey.set(key);
}
public static void clearDataSourceKey(){
dataSourceKey.remove();
}
}
六、
AbstractRoutingDataSource抽象类源码（不喜可跳）
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;
public AbstractRoutingDataSource() {
}
public void setTargetDataSources(Map<Object, Object> targetDataSources) {
this.targetDataSources = targetDataSources;
}
public void setDefaultTargetDataSource(Object defaultTargetDataSource) {
this.defaultTargetDataSource = defaultTargetDataSource;
}
public void setLenientFallback(boolean lenientFallback) {
this.lenientFallback = lenientFallback;
}
public void setDataSourceLookup(@Nullable DataSourceLookup dataSourceLookup) {
this.dataSourceLookup = (DataSourceLookup)(dataSourceLookup != null ? dataSourceLookup : new JndiDataSourceLookup());
}
public void afterPropertiesSet() {
if (this.targetDataSources == null) {
throw new IllegalArgumentException("Property 'targetDataSources' is required");
} else {
this.resolvedDataSources = new HashMap(this.targetDataSources.size());
this.targetDataSources.forEach((key, value) -> {
Object lookupKey = this.resolveSpecifiedLookupKey(key);
DataSource dataSource = this.resolveSpecifiedDataSource(value);
this.resolvedDataSources.put(lookupKey, dataSource);
});
if (this.defaultTargetDataSource != null) {
this.resolvedDefaultDataSource = this.resolveSpecifiedDataSource(this.defaultTargetDataSource);
}
}
}
protected Object resolveSpecifiedLookupKey(Object lookupKey) {
return lookupKey;
}
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);
}
}
public Connection getConnection() throws SQLException {
return this.determineTargetDataSource().getConnection();
}
public Connection getConnection(String username, String password) throws SQLException {
return this.determineTargetDataSource().getConnection(username, password);
}
public <T> T unwrap(Class<T> iface) throws SQLException {
return iface.isInstance(this) ? this : this.determineTargetDataSource().unwrap(iface);
}
public boolean isWrapperFor(Class<?> iface) throws SQLException {
return iface.isInstance(this) || this.determineTargetDataSource().isWrapperFor(iface);
}
protected DataSource determineTargetDataSource() {
Assert.notNull(this.resolvedDataSources, "DataSource router not initialized");
Object lookupKey = this.determineCurrentLookupKey();
DataSource 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 + "]");
} else {
return dataSource;
}
}
@Nullable
protected abstract Object determineCurrentLookupKey();
}
3. 配置
3.1 配置db.properties
这里配置两个数据库，一个评论库，一个用户库
# 问题库
howl.comments.driverClassName = com.mysql.jdbc.Driver
howl.comments.url = jdbc:mysql://127.0.0.1:3306/comment
howl.comments.username = root
howl.comments.password =
# 用户库
howl.users.driverClassName = com.mysql.jdbc.Driver
howl.users.url = jdbc:mysql://127.0.0.1:3306/user
howl.users.username = root
howl.users.password =
3.2 配置applicationContext.xml
<!-- 加载properties文件 -->
<context:property-placeholder location="classpath:db.properties"></context:property-placeholder>
<!-- 问题的数据源 -->
<bean id="commentsDataSource" class="org.springframework.jdbc.datasource.DriverManagerDataSource">
<property name="driverClassName" value="${howl.comments.driverClassName}"></property>
<property name="url" value="${howl.comments.url}"></property>
<property name="username" value="${howl.comments.username}"></property>
<property name="password" value="${howl.comments.password}"></property>
</bean>
<!-- 用户的数据源 -->
<bean id="usersDataSource" class="org.springframework.jdbc.datasource.DriverManagerDataSource">
<property name="driverClassName" value="${howl.users.driverClassName}"></property>
<property name="url" value="${howl.users.url}"></property>
<property name="username" value="${howl.users.username}"></property>
<property name="password" value="${howl.users.password}"></property>
</bean>
<!-- 通过setter方法，往DynamicDataSource的Map集合中注入数据 -->
<!-- 具体参数，看名字可以明白 -->
<bean id="dynamicDataSource" class="com.howl.util.DynamicDataSource">
<property name="targetDataSources">
<map key-type="java.lang.String">
<entry key="cds" value-ref="commentsDataSource"/>
<entry key="uds" value-ref="usersDataSource"/>
</map>
</property>
<property name="defaultTargetDataSource" ref="commentsDataSource"></property>
</bean>
<!-- 将`总源`注入SqlSessionFactory工厂 -->
<bean id="sqlSessionFactory" class="org.mybatis.spring.SqlSessionFactoryBean">
<property name="configLocation" value="classpath:mybatis-config.xml"></property>
<property name="dataSource" ref="dynamicDataSource"></property>
</bean>
因为dynamicDataSource是继承AbstractRoutingDataSource，所以setter注入方法得去父类里面去找，开始笔者也是懵了一下
3.3 切换数据源
数据源是在Service层切换的
UserService
@Service
public class UserService {
@Autowired
private UserDao userDao;
public User selectUserById(int id) {
// 表明使用usersDataSource库
DynamicDataSourceHolder.setDataSourceKey("uds");
return userDao.selectUserById(id);
}
}
CommentService
@Service
public class CommentService {
@Autowired
CommentDao commentDao;
public List<Comment> selectCommentById(int blogId) {
// 表明使用评论库
DynamicDataSourceHolder.setDataSourceKey("cds");
return commentDao.selectCommentById(blogId, -1);
}
}
3.4 自动切换
手动切换容易忘记，我们学了AOP可以使用AOP来切换，这里使用注解实现
<!-- 开启AOP注解支持 -->
<aop:aspectj-autoproxy></aop:aspectj-autoproxy>
切面类
@Component
@Aspect
public class DataSourceAspect {
@Pointcut("execution(* com.howl.service.impl.*(..))")
private void pt1() {
}
@Around("pt1()")
public Object around(ProceedingJoinPoint pjp) {
Object rtValue = null;
try {
String name = pjp.getTarget().getClass().getName();
if (name.equals("com.howl.service.UserService")) {
DynamicDataSourceHolder.setDataSourceKey("uds");
}
if (name.equals("com.howl.service.CommentService")){
DynamicDataSourceHolder.setDataSourceKey("cds");
}
// 调用业务层方法
rtValue = pjp.proceed();
System.out.println("后置通知");
} catch (Throwable t) {
System.out.println("异常通知");
t.printStackTrace();
} finally {
System.out.println("最终通知");
}
return rtValue;
}
}
使用环绕通知实现切入com.howl.service.impl里的所有方法，在遇到UserService、CommentService时，前置通知动态切换对应的数据源
4. 总结
- 以前笔者认为Service层多了impl包和接口是多余的，现在要用到AOP的时候后悔莫及，所以默认结构如此肯定有道理的
- 出bug的时候，才知道分步测试哪里出问题了，如果TDD推动那么能快速定位报错地方，日志也很重要
参考