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源码分析
位于sharding-jdbc-core模块下的包com.dangdang.ddframe.rdb.sharding.hint中，核心类HintManagerHolder的部分源码如下：
/**
* Hint manager holder.
* <p>Use thread-local to manage hint.</p>
* @author zhangliang
*/
@NoArgsConstructor(access = AccessLevel.PRIVATE)
public final class HintManagerHolder {
// hint特性保存数据的核心变量，即保存一个HintManager类型对象到ThreadLocal中
private static final ThreadLocal<HintManager> HINT_MANAGER_HOLDER = new ThreadLocal<>();
/**
* Set hint manager.
* @param hintManager hint manager instance
*/
public static void setHintManager(final HintManager hintManager) {
Preconditions.checkState(null == HINT_MANAGER_HOLDER.get(), "HintManagerHolder has previous value, please clear first.");
HINT_MANAGER_HOLDER.set(hintManager);
}
public static boolean isUseShardingHint() {
// 判断当前线程中是否使用了sharding hint--即HintManager中的shardingHint为true
return null != HINT_MANAGER_HOLDER.get() && HINT_MANAGER_HOLDER.get().isShardingHint();
}
public static Optional<ShardingValue<?>> getDatabaseShardingValue(final ShardingKey shardingKey) {
// 如果使用了sharding hint，那么从ThreadLocal中取数据库的sharding值
return isUseShardingHint() ? Optional.<ShardingValue<?>>fromNullable(HINT_MANAGER_HOLDER.get().getDatabaseShardingValue(shardingKey)) : Optional.<ShardingValue<?>>absent();
}
public static Optional<ShardingValue<?>> getTableShardingValue(final ShardingKey shardingKey) {
// 如果使用了sharding hint，那么从ThreadLocal中取表的sharding值
return isUseShardingHint() ? Optional.<ShardingValue<?>>fromNullable(HINT_MANAGER_HOLDER.get().getTableShardingValue(shardingKey)) : Optional.<ShardingValue<?>>absent();
}
public static boolean isMasterRouteOnly() {
// 是否强制路由主库--sharding-jdbc的特性之一：强制路由
return null != HINT_MANAGER_HOLDER.get() && HINT_MANAGER_HOLDER.get().isMasterRouteOnly();
}
public static boolean isDatabaseShardingOnly() {
// 是否只是数据库sharding
return null != HINT_MANAGER_HOLDER.get() && HINT_MANAGER_HOLDER.get().isDatabaseShardingOnly();
}
/**
* Clear hint manager for current thread-local.
*/
public static void clear() {
// ThreadLocal用完需要清理
HINT_MANAGER_HOLDER.remove();
}
}
ThreadLocal中管理的HintManager定义如下：
@NoArgsConstructor(access = AccessLevel.PRIVATE)
public final class HintManager implements AutoCloseable {
// 数据库强制路由的值
private final Map<ShardingKey, ShardingValue<?>> databaseShardingValues = new HashMap<>();
// 表强制路由的值
private final Map<ShardingKey, ShardingValue<?>> tableShardingValues = new HashMap<>();
// 即是否使用了强制路由特性
@Getter
private boolean shardingHint;
// 是否强制路由到主数据库
@Getter
private boolean masterRouteOnly;
@Getter
private boolean databaseShardingOnly;
... ...
@Override
public void close() {
HintManagerHolder.clear();
}
}
sharding值保存在ThreadLocal中，所以需要在操作结束时调用HintManager.close()来清除ThreadLocal中的内容。HintManager实现了AutoCloseable接口，推荐使用
try with resource（JDK7新特性，参考Java 7中的Try-with-resources）自动关闭清理ThreadLocl线程中的数据。
如何使用
分析了sharding-jdbc的强制路由实现的源码，接下来说说如何使用这一niubility特性，假定数据源定义如下：
private static ShardingDataSource getShardingDataSource() throws SQLException {
DataSourceRule dataSourceRule = new DataSourceRule(createDataSourceMap());
TableRule orderTableRule = TableRule
.builder("t_order")
.actualTables(Arrays.asList("t_order_0", "t_order_1"))
.dataSourceRule(dataSourceRule)
.build();
TableRule orderItemTableRule = TableRule
.builder("t_order_item")
.actualTables(Arrays.asList("t_order_item_0", "t_order_item_1"))
.dataSourceRule(dataSourceRule)
.build();
ShardingRule shardingRule = ShardingRule.builder()
.dataSourceRule(dataSourceRule)
.tableRules(Arrays.asList(orderTableRule, orderItemTableRule))
.bindingTableRules(Collections.singletonList(new BindingTableRule(Arrays.asList(orderTableRule, orderItemTableRule))))
.databaseShardingStrategy(new DatabaseShardingStrategy("user_id", new ModuloDatabaseShardingAlgorithm()))
.tableShardingStrategy(new TableShardingStrategy("order_id", new ModuloTableShardingAlgorithm())).build();
return new ShardingDataSource(shardingRule);
}
根据数据源定义可知，数据库的sharding column为user_id，表的sharding column为order_id；
1、强制路由数据库
- 如何使用
private static void printHintSimpleSelect(final DataSource dataSource) throws SQLException {
// SQL语句并不涉及任何数据库路由和表路由信息（即where语句中没有user_id条件和order_id条件）
String sql = "SELECT i.* FROM t_order o JOIN t_order_item i ON o.order_id=i.order_id";
try (
HintManager hintManager = HintManager.getInstance();
Connection conn = dataSource.getConnection();
PreparedStatement preparedStatement = conn.prepareStatement(sql)) {
// 强制路由：数据库路由sharding column即user_id的值为10
hintManager.addDatabaseShardingValue("t_order", "user_id", 10);
try (ResultSet rs = preparedStatement.executeQuery()) {
//todo do something
}
}
}
由于指定了强制路由数据库的值user_id=10，所以只会输出
ds_jdbc_0这个库中符合条件的数据。而ds_jdbc_1会被过滤；
- 实现原理
private Collection<String> routeDataSources(final TableRule tableRule) {
// 首先得到数据库sharding策略，例如：数据库按照列user_id进行sharding
DatabaseShardingStrategy strategy = shardingRule.getDatabaseShardingStrategy(tableRule);
// 然后从ThreadLocal中取出sharding的值
List<ShardingValue<?>> shardingValues = HintManagerHolder.isUseShardingHint() ? getDatabaseShardingValuesFromHint(strategy.getShardingColumns())
: getShardingValues(strategy.getShardingColumns());
Collection<String> result = strategy.doStaticSharding(tableRule.getActualDatasourceNames(), shardingValues);
Preconditions.checkState(!result.isEmpty(), "no database route info");
return result;
}
2、强制路由表
- 如何使用
private static void printHintSimpleSelect(final DataSource dataSource) throws SQLException {
// SQL语句并不涉及任何数据库路由和表路由信息（即where语句中没有user_id条件和order_id条件）
String sql = "SELECT i.* FROM t_order o JOIN t_order_item i ON o.order_id=i.order_id";
try (
HintManager hintManager = HintManager.getInstance();
Connection conn = dataSource.getConnection();
PreparedStatement preparedStatement = conn.prepareStatement(sql)) {
// 强制路由：表路由sharding column即order_id的值为1000
hintManager.addTableShardingValue("t_order", "order_id", 1000);
try (ResultSet rs = preparedStatement.executeQuery()) {
//todo do something
}
}
}
由于指定了强制路由表的值order_id=1000，所以只会输出所有库中与
t_order_0匹配的数据。而与t_order_1匹配的数据会被过滤；
- 实现原理
private Collection<String> routeTables(final TableRule tableRule, final String routedDataSource) {
// 首先得到表的sharding策略，例如：表按照列order_id进行sharding
TableShardingStrategy strategy = shardingRule.getTableShardingStrategy(tableRule);
// 然后从ThreadLocal中取出sharding的值
List<ShardingValue<?>> shardingValues = HintManagerHolder.isUseShardingHint() ? getTableShardingValuesFromHint(strategy.getShardingColumns())
: getShardingValues(strategy.getShardingColumns());
Collection<String> result = tableRule.isDynamic() ? strategy.doDynamicSharding(shardingValues) : strategy.doStaticSharding(tableRule.getActualTableNames(routedDataSource), shardingValues);
Preconditions.checkState(!result.isEmpty(), "no table route info");
return result;
}
3、强制路由主库
- 如何使用
HintManager hintManager = HintManager.getInstance();
hintManager.setMasterRouteOnly();
