基于mybatis 手写分表

Mybatis并不提供分表的功能，但是可以通过自定义插件来实现分表。

以下是一个简单的分表插件示例：

首先，创建一个自定义插件类TableShardPlugin：

@Intercepts({@Signature(type = Executor.class, method = "update", args = {MappedStatement.class, Object.class})})
public class TableShardPlugin implements Interceptor {

    @Override
    public Object intercept(Invocation invocation) throws Throwable {
        MappedStatement mappedStatement = (MappedStatement) invocation.getArgs()[0];
        Object parameter = invocation.getArgs()[1];

        // 获取注解
        TableShard tableShard = mappedStatement.getMethod().getAnnotation(TableShard.class);

        if (tableShard == null) {
            return invocation.proceed();
        }

        // 获取分表字段的值
        String shardValue = "";
        try {
            Field field = parameter.getClass().getDeclaredField(tableShard.shardField());
            field.setAccessible(true);
            shardValue = String.valueOf(field.get(parameter));
        } catch (Exception e) {
            throw new RuntimeException("get shardValue error", e);
        }

        // 获取分表规则
        TableShardStrategy strategy = tableShard.shardStrategy().newInstance();

        // 获取分表后缀
        String suffix = strategy.getSuffix(shardValue);

        // 修改sql语句
        BoundSql boundSql = mappedStatement.getBoundSql(parameter);
        String sql = boundSql.getSql().replace(tableShard.tableName(), tableShard.tableName() + "_" + suffix);
        BoundSql newBoundSql = new BoundSql(mappedStatement.getConfiguration(), sql, boundSql.getParameterMappings(), boundSql.getParameterObject());
        MappedStatement newMappedStatement = copyFromMappedStatement(mappedStatement, new BoundSqlSqlSource(newBoundSql));
        invocation.getArgs()[0] = newMappedStatement;

        return invocation.proceed();
    }

    @Override
    public Object plugin(Object target) {
        return Plugin.wrap(target, this);
    }

    @Override
    public void setProperties(Properties properties) {}

    private MappedStatement copyFromMappedStatement(MappedStatement ms, SqlSource newSqlSource) {
        MappedStatement.Builder builder = new MappedStatement.Builder(ms.getConfiguration(), ms.getId(), newSqlSource, ms.getSqlCommandType());
        builder.resource(ms.getResource());
        builder.fetchSize(ms.getFetchSize());
        builder.statementType(ms.getStatementType());
        builder.keyGenerator(ms.getKeyGenerator());
        builder.timeout(ms.getTimeout());
        builder.parameterMap(ms.getParameterMap());
        builder.resultMaps(ms.getResultMaps());
        builder.resultSetType(ms.getResultSetType());
        builder.cache(ms.getCache());
        builder.flushCacheRequired(ms.isFlushCacheRequired());
        builder.useCache(ms.isUseCache());
        return builder.build();
    }

    private static class BoundSqlSqlSource implements SqlSource {

        private BoundSql boundSql;

        public BoundSqlSqlSource(BoundSql boundSql) {
            this.boundSql = boundSql;
        }

        @Override
        public BoundSql getBoundSql(Object parameterObject) {
            return boundSql;
        }
    }

}

然后，创建一个注解TableShard：

@Retention(RetentionPolicy.RUNTIME)
@Target(ElementType.METHOD)
public @interface TableShard {

    String tableName();

    String shardField();

    Class<? extends TableShardStrategy> shardStrategy();

}

最后，实现分表策略接口TableShardStrategy：

public interface TableShardStrategy {

    String getSuffix(String shardValue);

}

使用示例：

@Mapper
public interface UserMapper {

    @TableShard(tableName = "user", shardField = "id", shardStrategy = UserIdModShardStrategy.class)
    int insert(User user);

}

其中，UserIdModShardStrategy是一个分表策略实现类，根据用户id取模得到分表后缀。

public class UserIdModShardStrategy implements TableShardStrategy {

    @Override
    public String getSuffix(String shardValue) {
        int mod = Integer.parseInt(shardValue) % 10;
        return String.valueOf(mod);
    }

}

以上就是一个简单的手写Mybatis分表插件实现。