与4.1.1不同，本次debug的项目没有使用mybatis，为了去除第三方框架带来的源码干扰，本次采用的原生JDBC操作原生ShardingSphereAPI的读写分离+分片。
以下的Debug会深刻的了解为啥ShardingJDBC是嵌入式代码，其究竟如何复写的JDBC代码。

设置断点

随着路由信息收集的完成，debug来到了SQLRewriteResult rewriteResult = rewrite(logicSQL, metaData, props, routeContext);将断点设置到这里，显然，这里就是SQL语句该写的开始。其源码如下：

  1.     private SQLRewriteResult rewrite(final LogicSQL logicSQL, final ShardingSphereMetaData metaData, final ConfigurationProperties props, final RouteContext routeContext) {
  2.         return new SQLRewriteEntry(
  3.                 metaData.getSchema(), props, metaData.getRuleMetaData().getRules()).rewrite(logicSQL.getSql(), logicSQL.getParameters(), logicSQL.getSqlStatementContext(), routeContext);
  4.     }

开始重写

rewrite中做的第一件事情就是SQLRewriteEntry对象的创建：

  1.     public SQLRewriteEntry(final ShardingSphereSchema schema, final ConfigurationProperties props, final Collection<ShardingSphereRule> rules) {
  2.         this.schema = schema;
  3.         this.props = props;
  4.         decorators = OrderedSPIRegistry.getRegisteredServices(rules, SQLRewriteContextDecorator.class);
  5.     }

进行一些初始化后调用其rewrite方法：

  1.     public SQLRewriteResult rewrite(final String sql, final List<Object> parameters, final SQLStatementContext<?> sqlStatementContext, final RouteContext routeContext) {
  2.         SQLRewriteContext sqlRewriteContext = createSQLRewriteContext(sql, parameters, sqlStatementContext, routeContext);
  3.         return routeContext.getRouteUnits().isEmpty()
  4.                 ? new GenericSQLRewriteEngine().rewrite(sqlRewriteContext) : new RouteSQLRewriteEngine().rewrite(sqlRewriteContext, routeContext);
  5.     }

创建改写上下文

  1.     private SQLRewriteContext createSQLRewriteContext(final String sql, final List<Object> parameters, final SQLStatementContext<?> sqlStatementContext, final RouteContext routeContext) {
  2.         SQLRewriteContext result = new SQLRewriteContext(schema, sqlStatementContext, sql, parameters);
  3.         decorate(decorators, result, routeContext);
  4.         result.generateSQLTokens();
  5.         return result;
  6.     }

追踪：

  1.     public SQLRewriteContext(final ShardingSphereSchema schema, final SQLStatementContext<?> sqlStatementContext, final String sql, final List<Object> parameters) {
  2.         this.schema = schema;
  3.         this.sqlStatementContext = sqlStatementContext;
  4.         this.sql = sql;
  5.         this.parameters = parameters;
  6.         addSQLTokenGenerators(new DefaultTokenGeneratorBuilder().getSQLTokenGenerators());
  7.         parameterBuilder = sqlStatementContext instanceof InsertStatementContext
  8.                 ? new GroupedParameterBuilder(
  9.                         ((InsertStatementContext) sqlStatementContext).getGroupedParameters(), ((InsertStatementContext) sqlStatementContext).getOnDuplicateKeyUpdateParameters())
  10.                 : new StandardParameterBuilder(parameters);
  11.     }

快进到获取完上下文：
image.png

执行重写

根据路由上下文和路径信息开始执行路由重写引擎的rewrite方法，其源码如下：

  1.     public RouteSQLRewriteResult rewrite(final SQLRewriteContext sqlRewriteContext, final RouteContext routeContext) {
  2.         Map<RouteUnit, SQLRewriteUnit> result = new LinkedHashMap<>(routeContext.getRouteUnits().size(), 1);
  3.         for (RouteUnit each : routeContext.getRouteUnits()) {
  4.             result.put(each, new SQLRewriteUnit(new RouteSQLBuilder(sqlRewriteContext, each).toSQL(), getParameters(sqlRewriteContext.getParameterBuilder(), routeContext, each)));
  5.         }
  6.         return new RouteSQLRewriteResult(result);
  7.     }

这里的具体逻辑位于toSQL方法中：

  1.     @Override
  2.     public final String toSQL() {
  3.         if (context.getSqlTokens().isEmpty()) {
  4.             return context.getSql();
  5.         }
  6.         Collections.sort(context.getSqlTokens());
  7.         StringBuilder result = new StringBuilder();
  8.         result.append(context.getSql(), 0, context.getSqlTokens().get(0).getStartIndex());
  9.         for (SQLToken each : context.getSqlTokens()) {
  10.             result.append(getSQLTokenText(each));
  11.             result.append(getConjunctionText(each));
  12.         }
  13.         return result.toString();
  14.     }

具体的路由获取位于getSQLTokenText和getConjunctionText：

  1.     protected String getSQLTokenText(final SQLToken sqlToken) {
  2.         if (sqlToken instanceof RouteUnitAware) {
  3.             return ((RouteUnitAware) sqlToken).toString(routeUnit);
  4.         }
  5.         return sqlToken.toString();
  6.     }

这样真实的SQL语句就放入到了SQLRewriteResult：
image.png